How to Use Acceptance Curves for Slewing Bearing Selection: A Practical Engineering Guide

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What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. It handles axial loads, radial loads, and tilting moments simultaneously. These bearings are essential in cranes, excavators, wind turbines, tunnel boring machines, and other heavy machinery.

The defining characteristic of a slewing bearing is its ability to carry three load types at once. Axial load acts vertically along the bearing axis. Radial load acts horizontally, perpendicular to the axis. Tilting moment is the overturning force created when loads act at a distance from the bearing center. In real applications, these three loads combine in different proportions. A crane lifting a load at a long radius experiences high tilting moment and moderate axial load. An excavator digging at close range experiences high axial load and moderate moment. The bearing must handle the specific combination that the application produces.

Why Slewing Bearing Selection Requires More Than Static Load Ratings

Selecting a slewing bearing is not as simple as comparing a single load value against a catalog rating. Catalog ratings are typically given as separate values for axial load, radial load, and tilting moment. But these loads do not act independently. They act together, and the bearing’s capacity under combined loading is not simply the sum of its individual capacities.

The limitation of single-value ratings

A bearing rated for 1,000 kN axial load and 500 kNm tilting moment cannot necessarily handle 800 kN axial load plus 400 kNm moment. The interaction between loads creates complex stress states in the raceway. Some rolling elements experience higher contact stresses than others. The load distribution changes as the ratio of axial load to moment changes. A single rating number cannot capture this complexity.

The limitation of traditional methods

Traditional selection methods, such as the Rumbarger method, have served the industry for decades. But these methods assume zero clearance and rigid bearing rings. They work reasonably well for bearings with uniform roller sizes, but they cannot handle configurations where the upper and lower axial roller rows have different diameters. This is a significant limitation because many modern three-row roller bearings use asymmetrical roller arrangements to optimize performance for predominantly unidirectional axial loads.

Engineers need a method that covers the entire load space—one that shows safe and unsafe combinations of axial load and tilting moment across a continuous range, not just at isolated rating points.

What Is an Acceptance Curve for Slewing Bearing Selection?

An acceptance curve is a two-dimensional curve in the axial load (Fa) and tilting moment (M) plane. The curve represents the boundary between safe and unsafe static loading for a specific slewing bearing.

How the curve works

Every point on the acceptance curve represents a load combination that brings the bearing to the ISO-defined static failure point. The static failure point is the load at which the maximum Hertzian contact stress reaches the material’s yield limit. At this point, the raceway experiences permanent plastic deformation.

If the application’s load combination (Fa, M) falls below the curve, the bearing is safe for static loading. The contact stresses remain below the yield limit, and no plastic deformation occurs. If the load combination falls above the curve, the bearing is at risk of plastic deformation. A larger bearing or a different configuration is required.

Why the curve is useful

The acceptance curve provides a visual, intuitive tool for bearing selection. Instead of performing iterative calculations for each candidate bearing, the engineer can plot the application’s load point and compare it against the curves for several bearings. The bearing whose curve lies above the load point is a safe choice. The one whose curve lies below is not.

This method offers convenience in engineering implementation. It does not require solving nonlinear equations for each load combination. It does not require specialized software. It provides a clear, graphical answer that supports decision-making.

How Acceptance Curves Are Derived for Slewing Bearings

The derivation of an acceptance curve follows a systematic process based on ISO 76, the international standard for static load ratings of rolling bearings.

Step 1: Define the bearing geometry

The process begins with the bearing’s internal geometry: the number of roller rows, the roller diameter and length in each row, the raceway diameters, and the contact angles. For a three-row roller bearing, the upper axial row, lower axial row, and radial row each have their own geometry.

Step 2: Establish load equilibrium

The applied axial load and tilting moment must be balanced by the contact forces at the rolling elements. The upper axial rollers carry load when the moment acts in one direction. The lower axial rollers carry load when the moment acts in the opposite direction. The radial rollers carry the radial load, if present. The distribution of load among the rollers depends on the bearing’s internal clearance and the stiffness of the rings.

Step 3: Calculate contact stresses

Each roller-raceway contact produces a contact stress. The stress depends on the load carried by that roller and the contact geometry. The maximum stress occurs at the most heavily loaded roller.

Step 4: Determine the failure point

The static failure point is reached when the maximum contact stress equals the allowable stress for the bearing material. This allowable stress is defined in ISO 76 and depends on the material and heat treatment. For standard bearing steels with raceway hardness of 55–62 HRC, the allowable stress is a known value.

Step 5: Repeat for multiple load ratios

The process is repeated for different ratios of axial load to tilting moment. At each ratio, the load magnitude is increased until the failure stress is reached. The resulting pairs of (Fa, M) values define points on the acceptance curve. Connecting these points produces the complete curve.

Handling asymmetrical roller arrangements

When the upper and lower axial roller rows have different roller diameters—a common design for predominantly unidirectional axial loads—the derivation must account for this asymmetry. The curve is not symmetric about the M axis. The safe zone extends further in the direction of the predominant load. The acceptance curve method handles this naturally because it calculates the load distribution for each roller row separately.

How to Read and Use an Acceptance Curve

Using an acceptance curve for bearing selection involves five practical steps.

Step 1: Determine the application loads

Calculate the axial load (Fa) and tilting moment (M) for the worst-case operating condition. Include the weight of the structure, the payload, and any dynamic amplification factors. For a crane, the worst case may occur at maximum load and maximum radius. For an excavator, it may occur during digging with the bucket at full extension.

Step 2: Plot the load point

On a graph with axial load on the horizontal axis and tilting moment on the vertical axis, plot the point (Fa, M) that represents the application’s load. If multiple load cases exist, plot all of them. The most demanding case determines the required bearing capacity.

Step 3: Compare against the acceptance curve

Overlay the acceptance curve for the candidate bearing on the same graph. If the load point lies below the curve, the bearing is safe. If it lies above, the bearing is overloaded.

Step 4: Apply a safety factor

The acceptance curve represents the theoretical static failure point. For real applications, a safety factor is applied. The safety factor accounts for uncertainties in load estimation, material variability, and dynamic effects. The table below shows typical safety factors for different application conditions.

Application ConditionRecommended Safety Factor
Normal operation, well-defined loads1.5 – 2.0
Moderate shock loads, some load uncertainty2.0 – 2.5
Heavy shock loads, significant eccentricity2.5 – 4.0
Safety-critical applications≥ 4.0

To apply the safety factor, multiply the applied loads by the factor before plotting. Alternatively, compare the load point against a reduced curve that represents the allowable load rather than the failure load.

Step 5: Compare multiple candidates

Plot the acceptance curves for several candidate bearings on the same graph. The optimal bearing is the smallest one whose curve lies above the factored load point. This approach avoids over-specifying (choosing a larger, more expensive bearing than necessary) and under-specifying (choosing a bearing that may fail).

Advantages and Limitations of the Acceptance Curve Method

The acceptance curve method offers clear advantages for slewing bearing selection.

Advantages

The method is fast. It provides an immediate visual answer without iterative calculation. It is intuitive. The two-dimensional graph clearly shows the safe operating zone. It is general. It applies to different roller counts, diameters, and arrangements. It is practical. The method does not require specialized software or complex numerical methods.

Limitations

The method assumes rigid bearing rings and zero clearance. In reality, the bearing rings and mounting structure deform under load. This deformation changes the load distribution and can reduce the effective capacity. For critical applications, finite element analysis (FEA) should be used to verify the acceptance curve results.

The method is based on static capacity. It does not predict fatigue life under dynamic loading. For applications with many load cycles, a separate fatigue life calculation is required.

The method requires accurate load data. If the estimated axial load or tilting moment is wrong, the selection will be wrong. Load determination should be performed carefully, with appropriate safety margins.

The acceptance curve should be used as a preliminary screening tool. It narrows the candidate list quickly and reliably. For the final selection, especially in safety-critical applications, FEA validation and consultation with the bearing manufacturer are recommended.

How LDB Bearing Supports Slewing Bearing Selection

LDB Bearing designs and manufactures high-quality slewing bearings for heavy machinery applications. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC and gear teeth hardened to 50–60 HRC.

LDB’s selection support:

  • Comprehensive product range: Single-row four-point contact ball bearings, double-row ball bearings, crossed roller bearings, and three-row roller bearings. Internal, external, or gearless configurations. Sizes from 108mm to over 2,000mm.
  • Engineering support: Application engineering for load calculations, acceptance curve analysis, and finite element verification. The engineering team helps customers interpret load data and select the optimal bearing for their application.
  • Quality assurance: ISO 9001-certified manufacturing with documented inspection reports and full material traceability. Dimensional records are retained for every bearing sold.
  • Global reach: Serving 73 countries with over 500,000 units in service.

LDB understands that bearing selection is a critical engineering decision. The company provides the technical support and product quality that equipment manufacturers need to make informed choices. Whether you are selecting a bearing for a new design or replacing a failed bearing, LDB offers the expertise and documentation to support your decision.

Contact LDB Bearing today to discuss your slewing bearing selection requirements.

FAQs

1. What is the difference between static load rating and acceptance curve?
A static load rating is a single value for one load type. An acceptance curve shows the safe combinations of axial load and tilting moment across a continuous range. The curve provides more complete information for combined loading.

2. How do I use an acceptance curve for a specific application?
Plot the application’s axial load and tilting moment on a graph. Compare the load point against the bearing’s acceptance curve. If the point is below the curve, the bearing is safe. Apply a safety factor to account for uncertainties.

3. What safety factor should I apply when using an acceptance curve?
For normal operation, 1.5 to 2.0 is typical. For shock loads or significant eccentricity, 2.5 to 4.0 may be required. The safety factor depends on the application’s criticality and the accuracy of load estimation.

4. Can acceptance curves be used for dynamic load selection?
No. Acceptance curves are based on static capacity. For dynamic loading, a separate fatigue life calculation is required. The acceptance curve should be used as a preliminary screening tool.

5. How accurate are acceptance curves compared to FEA?
Acceptance curves assume rigid rings and zero clearance. FEA accounts for structural deformation and clearance effects. For critical applications, FEA should be used to verify acceptance curve results.

Slewing Bearings in Amusement Rides: Safety-Critical Requirements for Ferris Wheels and Carousels

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What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. It handles axial loads, radial loads, and tilting moments simultaneously. These bearings are essential in cranes, excavators, wind turbines, and other heavy machinery.

In amusement rides, slewing bearings serve a different purpose. They are not just load-bearing components—they are safety-critical components. A failed bearing in a crane stops work. A failed bearing in a Ferris wheel or carousel puts passengers at risk. This distinction drives every design, manufacturing, and maintenance decision.

Why Amusement Rides Demand Safety-Critical Slewing Bearings

Amusement rides operate under a unique combination of conditions. They carry passengers—people who have not been trained in machinery operation and who cannot protect themselves if something goes wrong. They operate continuously during park hours, often with minimal downtime between cycles. They are exposed to weather, temperature changes, and environmental contaminants. And they are subject to strict safety regulations that vary by country and region.

The consequence of failure

When a slewing bearing fails in an amusement ride, the result is not just equipment downtime. A seized bearing can trap passengers in the air. Excessive play can cause uncontrolled movement. Structural failure can lead to catastrophic collapse. These outcomes are unacceptable, which is why amusement ride bearings must meet requirements far more stringent than those for general industrial applications.

Regulatory oversight

Amusement rides fall under specific safety standards in most countries. In Europe, EN 13814 covers the design, manufacture, and operation of amusement rides and amusement devices. In the United States, ASTM F2291 provides similar guidance. These standards specify requirements for structural integrity, load capacity, fatigue life, and safety factors. Slewing bearings used in amusement rides must comply with these requirements, which often exceed general industrial bearing standards.

Precision Requirements for Amusement Ride Slewing Bearings

Precision is not optional in amusement ride bearings. It affects passenger comfort, ride dynamics, and safety.

Runout and rotational accuracy

Slewing bearings in Ferris wheels and carousels must maintain low runout to ensure smooth rotation. Excessive runout causes vibration, noise, and uneven load distribution. It can also affect the ride experience, creating motion that feels unstable or uncomfortable. Precision grades of P5 or better are typically specified, with P4 used for the most demanding applications.

Gear precision

Many amusement ride slewing bearings include integral gear teeth for driven rotation. The gear teeth must be manufactured to precise tolerances to ensure proper meshing with the drive pinion. Backlash must be controlled within specification—too much backlash causes impact loads and noise, too little causes binding and accelerated wear. Gear teeth are typically surface-hardened to 50–60 HRC to resist wear and maintain precision over millions of cycles.

Clearance control

Internal clearance in the slewing bearing affects load distribution and rotational accuracy. For amusement ride applications, clearance is typically specified on the tight side of the standard range. This ensures even load distribution across the rolling elements and minimizes play that could affect ride dynamics.

Sealing Systems for Outdoor Amusement Ride Slewing Bearings

Amusement rides operate outdoors. They are exposed to rain, dust, temperature extremes, and sometimes coastal salt air. Sealing systems must protect the bearing from these environmental threats.

Multi-lip and labyrinth seals

Multi-lip seals provide redundant protection. Each lip creates a separate barrier against contaminant ingress. Labyrinth seals use a complex path that contaminants cannot easily traverse. Both designs are used in amusement ride applications, with the choice depending on the specific exposure conditions.

Seal material selection

Seal materials must resist UV degradation, ozone, temperature extremes, and moisture. Nitrile rubber (NBR) is suitable for moderate conditions. Fluororubber (FKM) offers superior resistance to temperature and chemicals. For coastal installations, seal materials must also resist salt air corrosion.

Seal replacement schedule

Seals are wear items. Even the best seals degrade over time. A preventive replacement schedule should be established based on operating hours and environmental conditions. Seals should be inspected regularly and replaced immediately if damage is found.

Corrosion Protection for Coastal and Outdoor Installations

Amusement rides in coastal areas face aggressive corrosion from salt air. Rides in other outdoor locations face corrosion from rain, humidity, and temperature cycling. Corrosion protection is essential for both safety and longevity.

Material options

Standard bearing steel with corrosion-resistant coatings is the most common choice. High-density chromium plating or zinc-nickel plating provides a barrier against moisture and salt. For more severe exposure, stainless steel rings may be specified, though this reduces load capacity.

Coating integrity

Coatings must remain intact to provide protection. Damage to the coating creates a starting point for corrosion. Coating inspection should be part of routine maintenance. Any damage should be repaired or the bearing replaced if the damage is extensive.

Drainage and water management

The bearing mounting arrangement should provide drainage so water does not accumulate around the bearing. Standing water accelerates corrosion and can penetrate seals. Drain holes, sloped mounting surfaces, and protective covers help manage water exposure.

Material Quality for Safety-Critical Slewing Bearings

Material quality directly affects bearing life and safety. For amusement ride applications, material selection and heat treatment must be carefully controlled.

Steel grade

Forged alloy steel such as 42CrMo is the standard for safety-critical slewing bearings. This material provides the strength, toughness, and hardenability needed for high-load, high-cycle applications. The forging process aligns the grain structure to the component shape, improving fatigue resistance.

Heat treatment

Raceway surfaces are induction-hardened to 55–62 HRC with a hardened layer depth of 3–5mm. This hardening creates a wear-resistant surface while maintaining a tough core that can absorb shock loads. Gear teeth are hardened to 50–60 HRC. Heat treatment records should be documented and traceable.

Material traceability

For safety-critical applications, material traceability is essential. The steel grade, heat number, and heat treatment records should be traceable from raw material through finished bearing. This documentation supports quality assurance and provides a record for future inspection or replacement.

Safety Factors and Load Capacity for Amusement Ride Slewing Bearings

Safety factors for amusement ride bearings are higher than those for general industrial applications. The consequences of failure justify the additional conservatism.

Static safety factor

The static safety factor is the ratio of the bearing’s static load rating to the maximum static load it will experience. For amusement ride applications, a static safety factor of 2.0 or higher is typically specified. This provides margin against unexpected loads, shock loading, and material variability.

Dynamic load rating

The dynamic load rating determines the bearing’s fatigue life under cyclic loading. Amusement rides operate for millions of cycles over their service life. The bearing must be selected so that the equivalent dynamic load does not exceed the dynamic load rating divided by an appropriate life factor.

Load spectrum analysis

Accurate load determination requires a load spectrum analysis. The loads on the bearing vary as the ride moves through its cycle. The analysis must consider the maximum load, the duration of each load level, and the number of cycles. This information feeds into the fatigue life calculation.

Maintenance and Inspection of Amusement Ride Slewing Bearings

Regular maintenance and inspection are essential for safety. A structured program should include several key activities.

Visual inspection

Inspect the bearing for signs of damage, corrosion, or leakage. Check the seals for cracks, tears, or gaps. Look for grease leakage, which indicates seal failure. Inspect the gear teeth for wear, pitting, or breakage.

Clearance measurement

Measure the axial and radial clearance periodically. Clearance increases as the bearing wears. When clearance exceeds the specified limit, the bearing should be replaced. Clearance measurement provides an objective indicator of bearing condition.

Vibration monitoring

Vibration monitoring can detect developing damage before it becomes visible. Changes in vibration signature—increased amplitude, new frequency components—indicate raceway or rolling element damage. For amusement rides, vibration monitoring can be integrated into the ride control system.

Bolt torque checks

Bolts that mount the bearing to the structure can loosen over time. Loose bolts create uneven load distribution and can lead to bearing failure. Bolt torque should be checked on a regular schedule and retightened to specification.

Lubrication

Proper lubrication reduces friction and wear. Grease should be replenished at specified intervals. The grease type must be compatible with the operating environment and the seal material. In coastal or humid environments, lubrication intervals should be shortened.

How LDB Bearing Supports Amusement Ride Applications

LDB Bearing designs and manufactures high-quality slewing bearings for safety-critical applications. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC and gear teeth hardened to 50–60 HRC.

LDB’s amusement ride capabilities:

  • Precision grades: P5, P4, and P2 standards with documented runout measurements
  • Seal options: Heavy-duty rubber, labyrinth, and multi-lip seals for outdoor and coastal environments
  • Corrosion protection: Coatings and material options for coastal installations
  • Quality assurance: ISO 9001-certified manufacturing with documented inspection reports and full material traceability
  • Engineering support: Application engineering for load calculations, safety factor determination, and custom design
  • Global reach: Serving 73 countries with over 500,000 units in service

LDB understands that amusement ride bearings are not just components—they are safety-critical elements. The company’s engineering team works with ride manufacturers to ensure that every bearing meets the required safety factors, precision standards, and environmental protection. Documentation and traceability support regulatory compliance and long-term maintenance planning.

Contact LDB Bearing today to discuss your amusement ride slewing bearing requirements.

FAQs

1. What makes amusement ride slewing bearings different from industrial bearings?
Amusement ride bearings are safety-critical. They must meet higher safety factors, tighter precision standards, and more stringent documentation requirements than general industrial bearings.

2. What safety factor is required for amusement ride slewing bearings?
A static safety factor of 2.0 or higher is typically specified. This provides margin against unexpected loads and material variability.

3. What precision grade is used for Ferris wheel slewing bearings?
P5 or better is typically specified, with P4 used for the most demanding applications. Precision affects ride smoothness, vibration, and passenger comfort.

4. How often should amusement ride slewing bearings be inspected?
Inspection frequency depends on ride usage and environmental conditions. Visual inspections should be performed regularly, with clearance measurement and vibration monitoring on a scheduled basis.

5. Can standard slewing bearings be used in amusement rides?
Standard bearings may lack the precision, safety factors, and documentation required for amusement ride applications. Safety-critical bearings should be specified for these applications.

Corrosion-Resistant Slewing Bearings for Water Treatment and Chemical Plants

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What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. It handles axial loads, radial loads, and tilting moments simultaneously. These bearings are essential in cranes, excavators, wind turbines, and other heavy machinery.

In water treatment and chemical plants, slewing bearings appear in rotating filter screens, gate opening mechanisms, clarifier scrapers, and mixing equipment. These environments attack standard bearing steel in ways that most industrial applications do not. Understanding the corrosion challenge is the first step toward reliable equipment operation.

Why Water Treatment and Chemical Plants Demand Corrosion-Resistant Slewing Bearings

Water treatment and chemical processing facilities expose steel structures to aggressive corrosion. The ISO 12944 standard classifies these environments by corrosivity. Chemical plants fall under category C4 (high corrosivity), which also covers coastal shipyards and areas with moderate salinity . Immersed structures in fresh water—such as hydroelectric plants and river installations—fall under category Im1, while structures immersed in sea or brackish water fall under Im2 .

Standard slewing bearing steel—typically 42CrMo or 50Mn—contains iron that oxidizes readily when exposed to moisture and chemical agents. In a water treatment plant, humidity is constant. In a chemical plant, airborne acids or caustic vapors accelerate the corrosion process. Without adequate protection, the raceway surfaces corrode, the seals degrade, and the bearing fails prematurely.

The consequences are not just bearing replacement. A corroded slewing bearing can seize, preventing critical equipment from operating. In a water treatment plant, a failed gate mechanism can disrupt flow control. In a chemical plant, a seized mixer can halt production. The cost of downtime far exceeds the cost of the bearing itself.

Material Selection for Corrosion-Resistant Slewing Bearings

Material selection is the primary defense against corrosion. Three main approaches exist, each suited to different exposure levels.

Standard alloy steel with corrosion-resistant coatings

This approach applies a protective coating to standard bearing steel. The underlying material retains its full load capacity—typically 55–62 HRC raceway hardness and verified 42CrMo forged alloy steel. Coatings provide the corrosion barrier while the steel provides the strength.

High-density chromium plating applied through electrodeposition creates a true molecular bond with the base metal. This coating does not flake under contact stress, and coating buildup is typically less than 0.005mm, preserving bearing geometry. Zinc-nickel plating offers sacrificial protection—if the coating is scratched, the surrounding zinc continues to protect the exposed steel electrochemically.

Stainless steel rings

Stainless steel options include 440C martensitic stainless and 316 austenitic stainless. 440C provides good corrosion resistance with moderate load capacity, suitable for intermittent moisture exposure. 316 stainless offers superior corrosion resistance but reduces load capacity significantly—typically 40–60% lower than standard bearing steel. This trade-off must be evaluated against actual exposure conditions.

Ceramic rolling elements

Ceramic materials such as silicon nitride eliminate corrosion at the rolling element surfaces because they contain no iron. However, the raceways remain steel and still require protection. Ceramic elements are typically used in combination with coated or stainless steel rings for maximum corrosion resistance.

The selection logic is straightforward: match the material to the actual exposure. A bearing in a dry chemical processing area may only need coating protection. A bearing immersed in wastewater requires either stainless steel or a combination of coating and enhanced sealing.

Surface Treatment Technologies for Slewing Bearings in Chemical Environments

Surface treatments extend bearing life by creating a barrier between the steel and the corrosive environment.

Electrodeposited high-density chromium—sometimes called Endurakote—provides a hard, smooth surface that resists both corrosion and wear. The coating bonds at the molecular level, so it does not separate under the high contact stresses present in slewing bearing raceways.

Zinc-nickel plating offers a different mechanism. The zinc in the coating acts as a sacrificial anode. If the coating is damaged, the zinc corrodes preferentially, protecting the steel beneath. This self-healing characteristic makes zinc-nickel valuable in applications where mechanical damage to the coating is likely.

Thermal spray zinc-aluminum coatings provide cathodic protection for larger surfaces. The coating is applied by spraying molten metal onto the prepared surface. While effective, thermal spray coatings are typically thicker than electroplated coatings and may require machining after application to maintain bearing tolerances.

Phosphate treatments and solid film lubricants offer moderate protection for less severe environments. These treatments are often used in combination with other coatings rather than as standalone solutions.

The coating must work with the sealing system. A coating that protects the raceway but allows moisture past the seals will still fail. Conversely, a seal that blocks contaminants but allows the raceway to corrode from condensation will also fail. The two systems must function together.

Sealing Systems for Wet and Chemical Environments

Seals are the first line of defense against water and chemical ingress. In water treatment and chemical plants, seals must block liquid water, vapor, and dissolved chemicals from reaching the raceway.

Multi-lip seal designs provide redundant protection. A triple-lip seal, for example, has three separate sealing lips in sliding contact with the bearing ring. If the first lip is damaged or worn, the second and third lips continue to protect the interior . This redundancy is valuable in environments where seal replacement is difficult or expensive.

Deflectors add a mechanical barrier upstream of the seals. A deflector is a ring mounted on the bearing that creates a narrow passage between the deflector and the rotating component . This passage blocks the flow of water carrying contaminating particles before they reach the seal lips. Deflectors made from polyether ether ketone (PEEK) resist water and marine organisms, making them suitable for the most aggressive environments .

Seal material selection is critical. Nitrile rubber (NBR) works well for general water exposure but has limited chemical resistance. Fluororubber (FKM) offers superior resistance to chemicals and higher operating temperatures, making it the preferred choice for chemical plant applications . The seal material must be compatible with both the operating environment and the lubricant it retains.

Lubrication Strategies for Corrosion Protection

Lubrication in wet and chemical environments must do more than reduce friction. The grease must resist water washout, protect against corrosion, and maintain its structure despite contamination.

Calcium sulfonate greases are widely specified for these applications. These greases are inherently water-resistant—they do not wash out even when exposed to significant water contamination . Calcium sulfonate greases also provide excellent corrosion protection, with Emcor rust test ratings of 0-0 (no rust) even in the presence of salt water . The thickener structure resists breakdown, maintaining lubricant consistency over extended service intervals.

Calcium sulfonate complex greases add extreme pressure (EP) properties. These formulations handle heavy loads and shock loading while maintaining their water resistance . The base oil viscosity is typically high—around 460–680 cSt at 40°C—to ensure adequate film thickness in slow-rotating slewing bearing applications.

Lubricant sampling provides insight into bearing condition. Grease samples taken from near the main load-bearing area can be analyzed for wear particles, water content, and additive depletion . Metal particles in the grease indicate active wear. Water content above specification indicates seal failure. Additive depletion signals that the grease is reaching the end of its useful life.

Lubrication intervals should be shortened in wet environments. The standard interval of 100–200 operating hours may be reduced to 50–100 hours when water exposure is constant. Fresh grease purges contaminated grease from the raceway and replenishes the corrosion-inhibiting additives.

Inspection and Maintenance of Slewing Bearings in Corrosive Service

Regular inspection detects corrosion before it causes failure. A structured inspection program should include several key checks.

Seal inspection is the most frequent task. Seals should be checked monthly for cracks, tears, hardening, or gaps between the seal and the ring. Damaged seals allow water and chemicals to reach the raceway. Replacement should be immediate .

Grease condition should be assessed at every lubrication interval. Grease that appears discolored, gritty, or emulsified indicates contamination or degradation. If metal particles are visible, the bearing may already be damaged .

Vibration monitoring can detect corrosion-related wear before it becomes visible. As raceway surfaces corrode and spall, vibration signatures change. Increased vibration amplitude or new frequency components indicate developing damage.

Bolt torque checks should be performed on a schedule appropriate to the environment. Corrosive environments can accelerate bolt loosening, and loose bolts create uneven load distribution that accelerates bearing wear .

Clearance measurement tracks wear over time. Axial clearance increases as the raceway wears. When clearance reaches 1.5 times the initial value, bearing replacement should be considered. At twice the initial value, replacement is mandatory .

Freshwater rinsing can remove accumulated salts and chemicals from external bearing surfaces in coastal or chemical exposure applications. This simple step reduces the concentration of corrosive agents at the seal interface.

How LDB Bearing Supports Corrosion-Resistant Applications

LDB Bearing designs and manufactures high-quality slewing bearings for demanding environments. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC and gear teeth hardened to 50–60 HRC.

LDB’s corrosion-resistant capabilities:

  • Material options: Standard alloy steel with corrosion-resistant coatings for moderate environments; stainless steel variants available for more severe exposure
  • Seal configurations: Heavy-duty and multi-lip seal options for moisture and water exposure; seal materials matched to chemical compatibility requirements
  • Custom engineering: Application-specific designs for water treatment, chemical processing, and other corrosive environments
  • Quality manufacturing: ISO 9001-certified manufacturing with documented inspection reports and full material traceability
  • Global reach: Serving 73 countries with over 500,000 units in service

LDB understands that corrosion resistance is not a single feature but a system. Material, coating, seal, and lubricant must work together to protect the bearing. The company’s engineering team provides application support to match the bearing configuration to the specific exposure conditions of each installation.

Contact LDB Bearing today to discuss your corrosion-resistant slewing bearing requirements.

FAQs

1. What is the most common material for slewing bearings in water treatment plants?
Standard alloy steel with corrosion-resistant coatings is the most common choice for moderate exposure. Stainless steel options are used when direct water contact is constant or chemical exposure is severe.

2. Is coated alloy steel better than stainless steel for chemical plants?
Coated alloy steel retains full load capacity and is typically more cost-effective. Stainless steel offers superior corrosion resistance but reduces load capacity by 40–60%. The choice depends on the specific chemical exposure and load requirements.

3. What grease is best for wet and chemical environments?
Calcium sulfonate greases are widely specified for their water resistance and corrosion protection. They maintain structure even with significant water contamination and provide Emcor rust test ratings of 0-0.

4. How often should seals be inspected in corrosive environments?
Seals should be inspected monthly in water treatment and chemical plant applications. Immediate replacement is recommended if any damage is found.

5. How long can a slewing bearing last in an immersed application?
Service life depends on the material, sealing system, lubrication, and maintenance program. With proper protection and regular inspection, bearings can provide many years of service even in aggressive environments.

50Mn vs. 42CrMo4: Choosing the Right Material and Heat Treatment for Slewing Bearings

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In the heavy machinery industry, replacing a failed slewing bearing is a logistical nightmare. The direct cost of the bearing is often dwarfed by the astronomical expenses of equipment downtime, crane mobilization, and intensive labor required to dismantle the machine. Therefore, the lifespan and load-bearing capacity of a slewing bearing must be guaranteed from the very beginning.

These operational limits are fundamentally dictated by two underlying factors: the raw steel grade and the precision of its heat treatment. While external dimensions and internal geometries determine how a bearing fits into a machine, the metallurgical properties dictate how long it will survive under continuous stress, friction, and extreme tilting moments. For engineers and procurement managers, understanding the differences in forging materials and hardening processes is the most effective way to evaluate true manufacturing quality and optimize the Total Cost of Ownership (TCO).

Core Material Properties: 50Mn vs. 42CrMo4

The forged rings of large slewing bearings are predominantly manufactured from two distinct grades of structural steel. The choice between them directly impacts the bearing’s mechanical limits, crack resistance, and overall manufacturing cost. Before the raceways are even hardened, the raw forgings undergo a baseline thermal treatment—usually Normalizing or Quenching and Tempering (Q&T)—to establish their core toughness.

50Mn (Medium Carbon Structural Steel)

50Mn relies primarily on its approximately 0.50% carbon content to achieve hardness, coupled with elevated manganese levels (usually 0.70% to 1.00%) to improve tensile strength and wear resistance.

  • Metallurgical Profile: It offers excellent machinability and a highly balanced profile of yield strength and ductility. Because it does not rely on expensive alloying elements, it is highly cost-effective for large-volume production. After a baseline normalizing treatment, it provides a solid structural foundation, though its hardenability depth is somewhat limited compared to alloy steels.
  • Best Applications: 50Mn is the industry standard for light-to-medium duty equipment with predictable, stable load profiles and low-impact operational cycles. This includes standard excavator slewing rings, small truck-mounted cranes, water treatment clarifiers, packaging machinery, and industrial automation turntables.

42CrMo4 (Chromium-Molybdenum Alloy Steel)

42CrMo4 is a high-strength, low-alloy steel specifically formulated for severe engineering environments. It is the European equivalent of the widely used 4140 grade.

  • Metallurgical Profile: The addition of chromium (Cr) significantly increases deep hardenability and resistance to abrasive wear. More importantly, molybdenum (Mo) boosts high-temperature strength, enhances impact toughness, and crucially prevents “temper brittleness.” When subjected to a rigorous Quenching and Tempering (Q&T) process, the core of a 42CrMo4 forging acts like an incredibly tough, shock-absorbing spring, resisting crack propagation even under extreme fatigue loading.
  • Best Applications: This material is the mandatory standard for heavy-duty, highly dynamic, and life-critical environments. It is heavily specified for offshore deck cranes (where wave dynamics create unpredictable shock loads), wind turbine pitch and yaw bearings, heavy crawler cranes, and tunnel boring machines (TBMs). It also performs exceptionally well in sub-zero environments where standard carbon steels become dangerously brittle.

Raceway Induction Hardening: Case Depth and Transition Zones

Regardless of whether 50Mn or 42CrMo4 is selected, the raceways—the specific tracks where the steel balls or cylindrical rollers make direct, concentrated contact with the rings—must undergo specialized localized heat treatment. The raw forged steel, even when tempered, is too soft to withstand the severe Hertzian contact stress generated by the rolling elements under heavy payloads.

Manufacturers utilize medium-frequency induction hardening. In this process, a custom-shaped copper coil passes over the raceway, using high-frequency alternating current to generate eddy currents in the steel. This rapidly heats the surface to its austenitizing temperature (around 850°C – 900°C), which is immediately followed by a rapid quench using CNC-controlled water or polymer fluid jets.

  • Target Hardness: The rapid cooling locks the steel’s molecular structure into a highly wear-resistant phase known as martensite. The raceway surface must achieve a precise hardness of HRC 55 to HRC 62. This dense, glass-hard layer prevents abrasive wear, micro-pitting, and surface fatigue (spalling).
  • Effective Case Depth: Hardening just the micro-surface is insufficient. If the hardened layer is too shallow, heavy axial loads will push right through it, crushing the hardened shell into the softer core material beneath—a catastrophic failure known as “core crushing.” The required case depth typically ranges from 3mm to 10mm, calculated meticulously based on the rolling element diameter and the maximum dynamic tilting moment the bearing will face.
  • The Transition Zone: A hallmark of premium heat treatment is a smooth, gradual hardness gradient from the HRC 60 surface down to the HRC 30 core. If the transition from hard martensite to the softer core is too abrupt, the entire hardened case can peel off (exfoliation) under heavy stress.

Understanding the “Soft Zone” (The Unhardened Gap)

A critical, yet often misunderstood, technical feature of all induction-hardened slewing bearings is the soft zone, often referred to as the unhardened gap.

Because the raceway is a continuous circle, the moving induction heating coil must eventually meet its starting point to complete the 360-degree sweep. Overlapping the heat treatment is physically impossible without disastrous consequences: reheating the already-quenched martensitic steel would essentially over-temper it, creating extreme tensile stress and making that junction dangerously brittle and prone to immediate cracking. Therefore, manufacturers intentionally shut off the induction power just before completing the circle, leaving a small gap (usually 10mm to 20mm) unhardened.

  • Identification: This soft zone is the weakest structural point of the raceway. It is permanently marked on the outer and inner rings, usually with a stamped “S” or a distinct painted line (often red or green). Furthermore, the insertion plug (the hole through which the steel balls or rollers are loaded into the bearing) is placed precisely within this unhardened gap to consolidate the structural weak points into a single location.
  • Installation Protocol: During equipment assembly, the soft zone must be strictly positioned in the non-load zone (or the zone of minimum continuous load). For example, on a crane, the “S” mark should be placed at a 90-degree angle to the primary lifting plane (the boom axis). This ensures that the heavy, continuous compressive forces and tilting moments never press the rolling elements directly into the softer, unhardened steel, preventing premature localized denting (brinelling).

Gear Surface Treatment: Matching Wear to Duty Cycles

A vast majority of slewing bearings integrate an internal or external gear to drive the rotation of the equipment. Just like the raceway, the heat treatment required for these gear teeth depends entirely on the machine’s operational duty cycle and the torque applied by the pinion gear.

  1. Standard Unhardened Gears: For applications with very slow rotational speeds, manual slewing, or highly infrequent duty cycles, the natural hardness of the 50Mn or 42CrMo4 normalized forging (typically around 180–220 HB) is more than sufficient.
  2. Tooth Flank Hardening (Surface Hardening): In high-cycle applications where the pinion gear engages the bearing constantly—such as excavators or automated robotic positioners—the gear teeth will wear out rapidly if left unhardened. Induction hardening the flanks (sides) of the teeth to HRC 45–55 drastically reduces friction and prevents tooth pitting.
  3. Contour (Root) Hardening: For the most extreme applications, hardening just the face of the tooth is not enough. When a machine experiences severe shock loads (for instance, an excavator bucket striking bedrock, which sends a shockwave back through the slew drive), the bending moment can snap standard gear teeth right at the base. Contour hardening treats the entire tooth profile, sweeping down into the gear root. This significantly increases the tooth’s bending fatigue strength and prevents catastrophic root shear.

LDB Bearing: Leading Custom Slewing Bearing Manufacturer in China

When standard off-the-shelf components fall short in demanding, high-moment applications, heavy equipment OEMs and aftermarket operators turn to LDB Bearing. Located in Luoyang—the heart of China’s heavy bearing manufacturing industry—LDB has established itself as a premier global supplier of high-precision slewing bearings, slew drives, and custom rotational solutions.

Understanding the critical nature of material science in lifting and rotating operations, our engineering team ensures that every slewing ring is tailored to its operational environment. From specifying the exact balance of 50Mn or 42CrMo4 steel forgings to executing precision CNC induction hardening for exact case depths, LDB maintains absolute control over the metallurgical process. We utilize comprehensive Non-Destructive Testing (NDT), including ultrasonic and magnetic particle inspections, to guarantee the internal integrity of every bearing before it leaves our facility.

Backed by advanced CNC manufacturing facilities, rigorous quality control protocols, and deep industry expertise, LDB Bearing delivers reliable, cost-effective, and highly durable slewing rings that meet stringent international standards. Partner with LDB Bearing to ensure your heavy machinery rotates safely, smoothly, and dependably under the most punishing loads.

Demystifying Tilting Moments in Crane Slewing Bearings: A Comprehensive Guide

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For engineers designing or maintaining cranes, the slewing bearing is arguably the most critical structural component. It acts as the sole connection between the rotating upper structure and the stationary undercarriage. While these bearings must support the immense downward axial force of the machine’s weight, the true test of a crane slewing bearing lies in its ability to withstand the tilting moment (also known as the overturning moment).

Understanding how tilting moments are generated, how they affect internal bearing geometry, and how to select the right bearing to counteract them is essential for ensuring crane safety, operational stability, and component longevity.

What is a Tilting Moment in Crane Operations?

In mechanical terms, a moment is a rotational force generated when a load is applied at a distance from a pivot point. For a crane, the pivot point is the central axis of the slewing bearing.

When a crane extends its boom to lift a payload, the weight of the load multiplied by its horizontal distance from the bearing’s center creates a massive leverage effect. This leverage is the tilting moment. It essentially acts as a prying force that attempts to tip the rotating upper structure off the chassis.

However, the payload is not the only factor. A complete tilting moment calculation must account for:

  • The suspended payload: The weight of the lifted object and the lifting tackle.
  • The boom structure: The dead weight of the extended boom and its center of gravity.
  • The counterweight: Situated at the rear of the crane, the counterweight generates a reverse tilting moment to balance the load.
  • Environmental and dynamic forces: Wind pressure acting on the boom and the load, as well as the centrifugal forces generated during rotation or sudden braking.

In an unloaded state, the counterweight heavily biases the tilting moment toward the rear. During a maximum capacity lift, the moment shifts violently toward the front. The slewing bearing must gracefully handle these continuous, alternating extremes.

How Tilting Moments Affect Internal Bearing Mechanics

Unlike a simple thrust bearing that distributes weight evenly across all rolling elements, a slewing bearing subjected to a high tilting moment experiences severe, asymmetrical loading.

Because of the overturning force, the load distribution across the bearing’s diameter becomes highly unequal. One side of the bearing is pushed downward with extreme compressive force, squeezing the rolling elements against the raceways. Conversely, the diametrically opposite side experiences a prying action, transferring intense tension to the mounting bolts and altering the contact angle between the rolling elements and the raceway.

If a bearing is undersized for the applied tilting moment, several failure modes can occur:

  • Edge Loading: Rolling elements may be pushed out of their optimal contact track, causing stress concentrations at the edges of the raceway.
  • Raceway Spalling: Concentrated pressure exceeds the surface fatigue limit of the hardened steel, causing the raceway material to flake or pit.
  • Deformation: The rings themselves can flex, leading to uneven rotation, increased friction, and eventual structural failure.

The Static Load Limit Curve: The Engineer’s Toolkit

To determine if a slewing bearing can survive a specific crane application, engineers rely on the Static Load Capacity Curve (or Limit Load Curve) provided by the bearing manufacturer.

This graph plots the Axial Load on the X-axis against the Tilting Moment on the Y-axis.

  • The curve represents the absolute maximum safe operating threshold for that specific bearing geometry and material hardness.
  • To ensure safety, the engineer calculates the maximum possible axial load and tilting moment for the crane under worst-case scenarios (including dynamic multipliers and safety factors).
  • This calculated operational point must fall safely below the limit curve. If the plotted point falls on or above the curve, permanent plastic deformation of the raceways or rolling elements is imminent, and a larger or more robust bearing must be selected.

Selecting the Right Bearing Structure for High-Moment Applications

Different slewing bearing designs handle tilting moments with varying degrees of efficiency. Selecting the correct internal geometry is directly tied to the severity of the crane’s load profile.

Single-Row Four-Point Contact Ball Bearings

These are widely used in light-to-medium duty applications, such as small truck-mounted cranes or utility derricks. The raceways are machined with a gothic arch profile, allowing each steel ball to contact the raceway at four points. While excellent for handling combined loads in a compact space, point-contact elements have lower capacity for extreme tilting moments compared to rollers.

Double-Row Ball Slewing Bearings

Featuring two independent rows of balls, this design separates the load paths. It offers a larger safety margin and greater stiffness than single-row designs, making it suitable for medium-duty mobile cranes and tower cranes where tilting moments fluctuate significantly.

Three-Row Roller Slewing Bearings

For heavy-duty applications—such as offshore deck cranes, massive crawler cranes, and port equipment—the three-row roller bearing is the undisputed industry standard.

  • Instead of balls, this design uses cylindrical rollers. Because rollers provide line contact rather than point contact, they can absorb drastically higher compressive forces without deforming.
  • The design physically separates the forces: two rows of horizontal rollers handle the massive downward axial loads and tilting moments, while a third row of vertical rollers exclusively handles radial loads.
  • This specialization gives three-row roller bearings the highest possible tilting moment capacity for a given diameter.

Operational Best Practices to Prevent Overload

Even the most precisely engineered slewing bearing can fail if crane operations ignore the realities of tilting moments. To maximize bearing lifespan, operators and maintenance teams must ensure:

  1. Strict Adherence to Load Charts: Crane load charts are explicitly designed around the slewing bearing’s tilting moment capacity. Extending a boom too far with a heavy load exponentially increases the moment, even if the total weight is within limits.
  2. Smooth Operation: Sudden stops during rotation (slewing) or harsh braking while lowering a load injects massive dynamic shock loads into the bearing, momentarily spiking the tilting moment far beyond static calculations.
  3. Foundation Stiffness: The mounting structure (the crane chassis) must be perfectly flat and incredibly rigid. If the chassis flexes under a heavy tilting moment, that distortion is transferred directly into the slewing bearing, causing it to warp and leading to rapid internal wear.

By understanding the mechanics of tilting moments and selecting the appropriate bearing geometry—such as transitioning to three-row roller designs for heavy-lifting environments—crane manufacturers can ensure their equipment operates safely, reliably, and efficiently for decades.

LDB Bearing: Leading Custom Slewing Bearing Manufacturer in China

When standard off-the-shelf components fall short in demanding, high-moment applications, heavy equipment OEMs and aftermarket operators turn to LDB Bearing. Located in Luoyang—the heart of China’s heavy bearing manufacturing industry—LDB has established itself as a premier global supplier of high-precision slewing bearings, slew drives, and custom rotational solutions.

Understanding the critical nature of tilting moments in crane and lifting operations, our engineering team specializes in application-specific structural designs. Whether you require a massive three-row roller slewing bearing to handle the extreme overturning forces of an offshore deck crane, or a compact double-row ball bearing for a mobile utility truck, LDB provides comprehensive technical support. We work closely with our clients through every stage of development, from initial static load limit curve calculations and internal geometry optimization to material forging and gear machining.

Backed by advanced CNC manufacturing facilities, rigorous quality control protocols, and deep industry expertise, LDB Bearing delivers reliable, cost-effective, and highly durable slewing rings that meet stringent international standards. Partner with LDB Bearing to ensure your heavy machinery rotates safely, smoothly, and dependably under the most punishing loads.

Slewing Bearings for Underwater and Hydropower Applications: Design Challenges and Solutions

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What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. These bearings handle axial loads, radial loads, and tilting moments simultaneously. They are essential in cranes, excavators, wind turbines, and other heavy machinery.

Most slewing bearings operate in air. Some operate in environments where water, moisture, or humidity is present. A smaller number operate fully submerged underwater. These underwater applications present unique engineering challenges that do not exist in conventional installations.

Why Underwater and Hydropower Applications Demand Special Slewing Bearings

Underwater and hydropower applications place extreme demands on slewing bearings. The bearing must function reliably in an environment that attacks every component of its design.

The pressure problem

Water pressure increases with depth. A tidal power plant positioned 15 meters below the water surface experiences pressure that is three times stronger than the pressure on a wind turbine. This pressure acts on every surface of the bearing and tries to force water past seals and into internal cavities.

The corrosion problem

Seawater contains chloride ions that penetrate metal surfaces and break down protective oxide layers. This triggers electrochemical corrosion. In the splash zone and tidal zone, corrosion rates can reach three to ten times the rate of fully submerged areas. Marine organisms such as barnacles and algae attach to bearing surfaces, increasing rotational resistance and creating conditions for localized corrosion.

The maintenance problem

Underwater maintenance is expensive and difficult. A seal that fails at the surface can be replaced with relative ease. A seal that fails 15 meters underwater requires specialized diving equipment, extended downtime, and significant cost. The patent literature identifies this as a key problem: seals in underwater slewing bearings cannot be replaced underwater, and the maintenance operation is expensive and requires frequent fixing of the assembled machine.

The load problem

Hydropower and tidal applications involve high loads. Tidal turbine rotor bearings must withstand the force of moving water. Hydropower gate bearings must support the weight of water-control structures. These loads combine with the constant pressure and corrosion of the underwater environment.

Design Challenges for Underwater Slewing Bearings

Sealing Challenges

Seals are the first line of defense against water ingress. In underwater applications, they face conditions that do not exist in air.

Multiple seal layers

A single seal is not sufficient for underwater service. Patent designs use multiple adjacent seals on the bearing face that contacts water directly. Three or more seals may be arranged in series to provide redundant protection. Adjacent seal lips can be oriented in different directions to block water intrusion from multiple angles.

Deflector elements

Advanced designs include a deflector mounted on the bearing ring. The deflector partially surrounds the seal lips and creates a narrow passage that blocks the flow of water carrying contaminating particles. This prevents particles from reaching the seal lips and extends seal life. Deflectors made from polyether ether ketone (PEEK) resist seawater and marine organisms.

Seal material requirements

Standard rubber seals degrade in seawater. Underwater bearings require specialized materials. Fluororubber seals offer superior resistance to seawater, ozone, and aging. The sealing surface itself may be made from stainless steel to resist corrosion.

The seal replacement problem

The most challenging design constraint is that seals cannot be replaced underwater. This means the seal system must be designed for the full service life of the bearing. Every component must be engineered for maximum durability because there is no opportunity for intervention.

Corrosion Protection Challenges

Every exposed surface of an underwater slewing bearing must resist corrosion. The bearing rings, gear teeth, mounting surfaces, and fasteners are all vulnerable.

Surface coating technologies

Thermal spray zinc-aluminum alloy coatings provide cathodic protection. The zinc acts as a sacrificial anode and protects the underlying steel even if the coating is scratched or damaged. Polytetrafluoroethylene (PTFE) composite coatings reduce friction while providing a corrosion barrier.

Material selection

Stainless steel or duplex stainless steel rings offer superior corrosion resistance compared to standard bearing steels. For rolling elements, ceramic materials such as silicon nitride eliminate corrosion entirely because they do not contain iron that can oxidize.

Cathodic protection

Impressed current or sacrificial anode cathodic protection systems can be used for submerged bearings. These systems shift the metal surface potential to a range where corrosion does not occur. They work in combination with coatings and seals to provide comprehensive protection.

Lubrication Challenges

Lubricant must protect the bearing from friction and wear while resisting water contamination. In underwater applications, lubricant selection and retention become critical.

Lubricant selection

The lubricant must resist water washout, maintain film strength under pressure, and protect against corrosion. Specialized greases with corrosion inhibitors are used. The lubricant must also be compatible with the seal materials and the operating temperature range.

Lubricant retention

The sealed chamber must retain the lubricant for extended periods. Every seal must perform its function without failure. The annular chamber between the inner and outer rings is protected by the seal system to prevent lubricant loss and water ingress.

Centralized lubrication

Large slewing rings may require centralized lubrication systems. These systems deliver measured quantities of grease to multiple points on the bearing at defined intervals. For underwater applications, the lubrication system itself must be protected from corrosion.

Load and Structural Challenges

Underwater slewing bearings must handle loads that are often higher than comparable surface applications.

Tidal turbine loads

A tidal turbine rotor bearing supports the weight of the rotor assembly and the thrust force from water flow. The three-row roller bearing design is used for this application because it handles high axial and radial loads simultaneously. The bearing must also withstand the cyclic loading from tidal flow reversals.

Hydropower gate loads

Hydropower gate bearings support the weight of large steel gates and the water pressure acting on them. These bearings must maintain precise positioning to control water flow accurately.

Structural integration

The bearing must integrate with the surrounding structure in a way that maintains alignment and prevents distortion. For underwater installations, the structural interface must also resist corrosion and marine growth.

Solutions and Design Approaches

Multi-Layer Seal Systems with Deflectors

The most effective solution combines multiple seals with a mechanical deflector. The deflector creates a physical barrier that blocks large particles and reduces water velocity at the seal lip. The multiple seals provide redundant protection. The innermost seal sees the cleanest environment because the outer seals and deflector have removed most contaminants.

Corrosion-Resistant Material Selection

Designers can choose from several material options:

  • Stainless steel rings for maximum corrosion resistance
  • Duplex stainless steel for higher strength and corrosion resistance
  • Ceramic rolling elements to eliminate corrosion at contact points
  • PEEK deflectors to resist seawater and marine organisms

Surface Treatment Combinations

No single coating provides complete protection. A combination approach is most effective:

  • Thermal spray zinc-aluminum coating for cathodic protection
  • Organic sealant over the thermal spray for additional barrier protection
  • PTFE coating on sliding surfaces to reduce friction
  • Corrosion-resistant fasteners and mounting hardware

Design for Maintenance-Free Operation

Because underwater maintenance is so difficult, the design must minimize the need for intervention:

  • Seals designed for full service life
  • Lubricant selected for long-term stability
  • Corrosion protection systems that do not require replacement
  • Materials that resist marine growth

Condition Monitoring

Where maintenance access is limited, condition monitoring becomes essential. Vibration sensors can detect early bearing damage. Lubricant sampling, where possible, can reveal wear particles and contamination. Temperature sensors can detect abnormal operating conditions. For fully submerged bearings, remote monitoring systems transmit data to surface operators.

Applications in Hydropower and Tidal Energy

Tidal Power Plants

Tidal power plants use slewing bearings to support and adjust rotor blades. A novel rotor blade adjustment bearing supports rotors of a tidal power plant positioned 15 meters below the water surface. The three-row roller bearing withstands extraordinary loads under water pressure that is three times stronger than wind turbine pressure.

Hydropower Gates

Hydropower facilities use slewing bearings to rotate and position water-control gates. These bearings must operate reliably for decades with minimal maintenance. The underwater environment accelerates corrosion, making material selection and sealing critical.

Underwater Turbines

Underwater turbines for tidal or river current energy use slewing bearings for blade pitch control. These bearings must resist corrosion, water pressure, and the constant flow of water carrying abrasive particles.

How LDB Bearing Supports Underwater Applications

LDB Bearing designs and manufactures high-quality slewing bearings for heavy machinery applications, including those requiring corrosion resistance. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC.

LDB’s capabilities:

  • Material options: Standard alloy steel with corrosion-resistant coatings for moderate environments
  • Seal configurations: Heavy-duty and multi-lip seal options for moisture and water exposure
  • Custom designs: Application-specific engineering for demanding environments
  • Quality manufacturing: ISO 9001-certified manufacturing with documented inspection reports and full material traceability
  • Global reach: Serving 73 countries with over 500,000 units in service

For underwater and hydropower applications requiring specialized corrosion protection and sealing beyond standard configurations, LDB offers engineering consultation to assess application requirements. The company’s experience in heavy machinery and renewable energy applications provides a foundation for addressing demanding operating environments.

Contact LDB Bearing today to discuss your slewing bearing requirements.

Understanding Slewing Bearing Soft Zones: Causes, Risks, and Mitigation

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What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. These bearings handle axial loads, radial loads, and tilting moments simultaneously. They are essential in cranes, excavators, wind turbines, and other heavy machinery.

The raceway of a slewing bearing must be hard enough to resist wear and indentation. Manufacturers achieve this through induction hardening, which heats the raceway surface and quenches it rapidly. However, this process creates a section of the raceway that does not receive full hardening. This unhardened section is called the soft zone.

What Is a Slewing Bearing Soft Zone?

A soft zone is the unhardened section of a slewing bearing raceway. It occurs where the induction hardening process begins and ends. When the heating coil starts at one point and travels around the raceway, the start and end points overlap imperfectly. The result is a narrow band that does not reach the required hardness.

Why soft zones form

Induction hardening works by passing a high-frequency current through a coil near the raceway surface. The current induces eddy currents in the steel, heating the surface rapidly. A quench follows immediately to harden the heated zone. When the coil completes a full revolution, the starting point has already cooled. The overlap between the start and end of the hardening pass creates a transition zone that receives less heat or less quench. This transition zone remains softer than the rest of the raceway.

How soft zones are marked

Manufacturers mark the soft zone with an “S” on the bearing ring. The mark appears on the interior or exterior diameter, depending on the bearing design. This mark tells the installer where the soft zone is located so they can position it correctly during mounting.

Why Soft Zones Matter for Slewing Bearing Performance

The soft zone is the weakest point on the raceway. Under load, the unhardened steel deforms more easily than the hardened sections. This deformation creates several problems.

Reduced load capacity at the soft zone

The soft zone has lower hardness and lower yield strength than the hardened raceway. When a rolling element passes over the soft zone under heavy load, the surface can indent permanently. This indentation creates a stress concentration that accelerates fatigue and spalling.

Crack initiation

Research on a 6-ton excavator slewing bearing identified cracks at the soft zone. Metallurgical analysis showed that cracks initiated at the edge of the soft zone where hardness transitioned abruptly. The blockage hole edge and the heat treatment transition zone were identified as crack initiation sites.

Accelerated wear

The soft zone wears faster than the hardened raceway. As the bearing rotates, rolling elements pass over the soft zone repeatedly. The softer surface wears down, increasing clearance and reducing rotational accuracy. Over time, the wear can become severe enough to require bearing replacement.

Seal damage risk

Excessive wear at the soft zone can create a step or ledge on the raceway. This step can damage the seal as it passes over the worn area. Once the seal is damaged, contaminants enter the bearing and accelerate degradation.

Industry Standards for Slewing Bearing Soft Zones

Industry standards recognize the soft zone as an unavoidable feature of induction-hardened slewing bearings. Standards specify limits on soft zone width to ensure acceptable performance.

Chinese standard JB/T 2300-2011

This standard specifies requirements for slewing bearing soft zones. For rings with blockage holes, the soft zone width should not exceed the blockage hole diameter plus 35mm. The blockage hole is a hole drilled into the ring to allow the hardening coil to pass through. The soft zone extends around this hole.

Other standards

Similar requirements exist in other industry standards. The goal is to limit the soft zone width so that it does not significantly reduce the bearing’s load-carrying capacity. A wider soft zone means a larger area of the raceway is not hardened, reducing the bearing’s overall performance.

How to Mitigate Soft Zone Risks in Slewing Bearings

While soft zones cannot be eliminated entirely, their impact can be minimized through proper design, installation, and operation.

Positioning the soft zone outside the load zone

The most important mitigation step is proper positioning. When installing a slewing bearing, the “S” mark should be placed outside the main load area. In most applications, the primary load direction is known. For a crane, the load acts downward on one side of the bearing. For an excavator, the digging force acts in a specific direction. By positioning the soft zone away from this loaded area, the risk of indentation and cracking is significantly reduced.

Optimizing the hardening process

Manufacturers can reduce soft zone width by optimizing the induction hardening process. Using multiple inductors with oscillating movements and preheating can eliminate the seam or reduce its width. Advanced techniques create a more continuous hardened zone with minimal soft zone width.

Material selection

Higher-quality steel with better hardenability can reduce the depth of the soft zone. Steel grades like 42CrMo have good hardenability, meaning the hardness transition is sharper and the soft zone is narrower. Lower-grade steels may have wider transition zones.

Surface treatment after hardening

Some manufacturers apply additional surface treatments after induction hardening to improve soft zone properties. These treatments may include nitriding or other surface modification processes. However, these treatments add cost and may not be suitable for all applications.

Inspection and quality control

Hardness testing verifies that the soft zone meets specification. Manufacturers should test hardness at multiple points along the raceway, including the soft zone, to ensure it meets minimum requirements. If the soft zone is too wide or too soft, the bearing should be rejected.

How to Inspect for Slewing Bearing Soft Zone Problems

Detecting soft zone problems early allows corrective action before catastrophic failure occurs.

Hardness testing

The most direct method is hardness testing. A portable hardness tester can measure the raceway hardness at various points. The soft zone should be tested specifically to verify it meets minimum hardness requirements. If the soft zone hardness is below specification, the bearing may be at risk.

Visual inspection

Visual inspection can reveal signs of soft zone damage. Look for:

  • Indentations or depressions on the raceway
  • Discoloration indicating overheating or wear
  • Cracks, particularly at the edges of the soft zone
  • Uneven wear patterns

Vibration monitoring

Vibration monitoring can detect soft zone problems during operation. As rolling elements pass over a damaged soft zone, they generate vibration signatures. Increased vibration amplitude or changes in frequency patterns can indicate soft zone degradation.

Grease analysis

Grease analysis can detect wear particles from the soft zone. Metal particles in the grease indicate active wear. If the particles are concentrated in a specific size range, they may originate from the soft zone.

Practical Recommendations for Slewing Bearing Users

For equipment manufacturers and operators, understanding soft zones helps prevent premature failure.

During installation

Always identify the “S” mark before mounting the bearing. Position the soft zone outside the main load area. If the load direction is not fixed, position the soft zone in the least loaded area. Document the soft zone position for future maintenance reference.

During operation

Avoid overloading the bearing, especially when the load is positioned over the soft zone. Monitor for signs of soft zone damage, including unusual noise, vibration, or grease contamination. If soft zone damage is suspected, reduce load or schedule inspection.

During maintenance

Include soft zone inspection in routine maintenance. Check for indentations, cracks, or uneven wear at the soft zone location. If damage is found, assess whether the bearing can continue in service or needs replacement. In some cases, the bearing can be rotated to move the soft zone to a different position relative to the load.

How LDB Bearing Manages Soft Zones in Manufacturing

LDB Bearing designs and manufactures high-quality slewing bearings for heavy machinery applications. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC and gear teeth hardened to 50–60 HRC.

LDB’s soft zone management:

  • Process control: In-house induction hardening with documented hardness records and controlled soft zone width
  • Quality inspection: Hardness testing at multiple points including soft zone verification
  • Marking: Clear “S” mark on every bearing to identify soft zone location
  • Engineering support: Application engineering to help customers position the soft zone correctly during installation
  • Quality certification: ISO 9001-certified manufacturing with documented inspection reports and full material traceability

Serving 73 countries with over 500,000 units in service, LDB delivers the reliability that heavy machinery applications demand. Understanding soft zones—why they form, how they affect performance, and how to mitigate their risks—helps equipment manufacturers and operators achieve maximum bearing life. LDB offers the technical expertise and quality assurance that demanding applications require.

Contact LDB Bearing today to discuss your slewing bearing requirements.

FAQs

1. What is a soft zone in a slewing bearing?
A soft zone is the unhardened section of a slewing bearing raceway. It occurs where the induction hardening process begins and ends, creating a narrow band that does not reach the required hardness.

2. Why does a soft zone form during induction hardening?
When the heating coil completes a full revolution, the starting point has already cooled. The overlap between the start and end of the hardening pass creates a transition zone that receives less heat or less quench.

3. How is the soft zone identified on a slewing bearing?
Manufacturers mark the soft zone with an “S” on the interior or exterior diameter. This mark tells the installer where the soft zone is located.

4. Where should the soft zone be positioned during installation?
The soft zone should be positioned outside the main load area. For most applications, the primary load direction is known, so the “S” mark can be placed away from the loaded area.

5. What are the risks of ignoring soft zone positioning?
Ignoring soft zone positioning can lead to raceway indentation, crack initiation, accelerated wear, and premature bearing failure. The soft zone is the weakest point on the raceway and requires careful handling.

Segmented Slewing Bearings: Design and Applications for Ultra-Large Diameters

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What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. These bearings handle axial loads, radial loads, and tilting moments simultaneously. They are essential in cranes, excavators, wind turbines, and other heavy machinery.

Most slewing bearings are manufactured as single-piece rings. However, when the required diameter exceeds manufacturing or transportation limits, a different approach becomes necessary. Segmented slewing bearings solve this problem by dividing the bearing into multiple sections that are assembled on site.

Why Segmented Slewing Bearings Exist

The demand for ever-larger machinery has pushed slewing bearing diameters beyond what traditional manufacturing and logistics can handle. Offshore installation vessels, ultra-large tunnel boring machines, and giant mining equipment require bearings that measure 10 meters or more in diameter.

Manufacturing limits

Single-piece slewing bearings can be manufactured up to certain diameters depending on the manufacturer’s equipment. Beyond a certain size, manufacturing becomes impractical or impossible with conventional processes.

Transportation constraints

Even if a bearing could be manufactured as a single piece, transporting it presents another challenge. A 15-meter diameter bearing cannot be shipped by standard road or rail. Specialized transport would be prohibitively expensive and logistically complex. Segmented designs solve this problem by allowing shipment in standard containers or on conventional trucks.

Site access limitations

Many project sites—offshore platforms, remote mining locations, underground tunnels—have limited access for large equipment. Segmented bearings can be transported in manageable sections and assembled on site using available lifting equipment.

The industry has responded to these challenges. In 2024, a segmented roller slewing bearing with an outer diameter of 23.4 meters was delivered, consisting of 68 individual segments, for a 2,000-tonne offshore installation crane. This bearing is designed for a service life of 25 years.

How Segmented Slewing Bearings Are Designed

Designing a segmented slewing bearing requires solving problems that do not exist with single-piece bearings. The joints between segments must maintain structural integrity, load-carrying capacity, and sealing performance.

Segmentation strategy

The number of segments depends on the bearing diameter and transportation constraints. Smaller segmented bearings may have 4 to 8 segments, while the largest designs can have 60 or more. Each segment is manufactured with precision to ensure proper fit and alignment during assembly.

Joint design

The joints between segments are critical to performance. They must transfer loads across the connection without creating stress concentrations. Bolted connections are common, with precision-machined mating surfaces to ensure proper load transfer. Some designs use additional keyways or dowels to handle shear forces.

Raceway continuity

The raceway must be continuous across segment joints to allow smooth rolling element passage. Any discontinuity would cause impact loads, noise, and accelerated wear. Manufacturing tolerances are extremely tight to ensure proper alignment when segments are assembled.

Sealing continuity

Seals must also be continuous across joints. Segmented seal designs allow the seal to bridge the gap between segments while maintaining contact with the raceway. This prevents contaminant ingress and lubricant loss at the joints.

Induction-hardened raceways

Modern segmented bearings use induction-hardened raceways rather than replaceable raceway plates. This design eliminates the need for screwed raceway segments, reducing weight and improving cross-section efficiency. The hardening process is performed on each segment before assembly, with careful control to ensure uniform hardness across the entire assembled raceway.

Manufacturing Segmented Slewing Bearings

Manufacturing segmented slewing bearings requires capabilities beyond those needed for single-piece bearings. Each segment must be manufactured to exacting tolerances, and the assembled bearing must perform as a single unit.

Segment manufacturing

Each segment is forged, machined, and heat-treated individually. The raceway is induction-hardened to achieve 55-62 HRC surface hardness with adequate case depth. Gear teeth, if present, are also hardened. After heat treatment, segments are precision-ground to final dimensions.

Quality control

Dimensional inspection verifies that each segment meets specifications. The curvature of the raceway, the flatness of mounting surfaces, and the position of bolt holes are all critical. Any deviation would cause misalignment during assembly, leading to premature failure.

Pre-assembly testing

Before shipment, segments may be pre-assembled at the factory to verify fit and function. This identifies any issues before the bearing reaches the project site, where correction would be far more difficult and expensive.

Applications for Segmented Slewing Bearings

Segmented slewing bearings serve applications where single-piece bearings are impractical or impossible. These are typically the largest and most demanding machines in operation.

Offshore installation cranes

A 23.4-meter segmented bearing delivered in 2024 was for a 2,000-tonne offshore installation crane. These cranes lift massive offshore wind components and oil platform modules. The bearing must handle enormous loads while maintaining precise control in a marine environment.

Tunnel boring machines

The largest tunnel boring machines require main bearings that exceed single-piece manufacturing limits. Segmented designs allow these machines to be built with cutter head diameters of 15 meters or more. The bearing must support the cutter head while withstanding extreme thrust and tilting moments.

Mining equipment

Large mining excavators and draglines use segmented slewing bearings when their size exceeds manufacturing capabilities. These machines operate in harsh conditions with heavy shock loads.

Wind turbine test stands

Test facilities for large wind turbine drivetrains may use segmented bearings to support the rotor assembly during testing. The bearing must handle the full rotor weight and operating loads.

Installation and Assembly of Segmented Slewing Bearings

Assembling a segmented slewing bearing on site requires careful planning and execution. The process differs significantly from installing a single-piece bearing.

Site preparation

The mounting surface must be flat, level, and rigid. For large bearings, the foundation is often constructed with embedded anchor bolts or mounting plates. The surface flatness is critical—any deviation would cause uneven load distribution when the bearing is assembled.

Segment positioning

Segments are lifted into position using cranes or other lifting equipment. Each segment is aligned with its neighbors and temporarily secured. Guide pins or alignment fixtures help achieve proper positioning.

Bolting and torqueing

Once all segments are in position, the connection bolts are tightened to specified torque values. The tightening sequence is critical to ensure even load distribution across the joints. Hydraulic tensioners may be used for large bolts.

Raceway alignment

After assembly, the raceway continuity is checked. Any steps or misalignment at the joints must be corrected before the bearing is put into service. This may require adjustment of the segment positions or additional machining on site.

Seal installation

Seals are installed across the joints to maintain continuity. The seal material must be compatible with the operating environment and lubricant.

Maintenance Considerations for Segmented Slewing Bearings

Segmented bearings require maintenance similar to single-piece bearings, with additional attention to the joints.

Joint inspection

The segment joints are potential weak points. Regular inspection should check for bolt loosening, joint opening, or seal damage at the connections. Any issues should be addressed promptly to prevent escalation.

Lubrication

The lubrication system must ensure that all segments receive adequate lubricant. Grease channels and fittings are distributed around the bearing circumference. The joints may require additional attention to ensure proper lubricant distribution.

Monitoring

Condition monitoring systems can track bearing performance over time. Vibration, temperature, and rotational resistance are key indicators. Any change from baseline performance warrants investigation.

How LDB Bearing Supports Ultra-Large Diameter Applications

LDB Bearing designs and manufactures high-quality slewing bearings for heavy machinery applications. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC and gear teeth hardened to 50–60 HRC.

LDB’s capabilities:

  • Custom design: Application-specific bearings engineered for individual project requirements
  • Quality manufacturing: ISO 9001-certified manufacturing with documented inspection reports and full material traceability
  • Precision engineering: Capabilities to meet the tight tolerances required for demanding applications
  • Global reach: Serving 73 countries with over 500,000 units in service

While LDB specializes in a comprehensive range of slewing bearing sizes and types, the company understands the challenges of ultra-large diameter applications. For projects requiring segmented bearings, LDB offers engineering support and quality assurance to help ensure successful outcomes.

Contact LDB Bearing today to discuss your slewing bearing requirements.

FAQs

1. What is a segmented slewing bearing?
A segmented slewing bearing is a large-diameter bearing manufactured in multiple sections that are assembled on site. This approach overcomes manufacturing and transportation limits for bearings exceeding 10 meters in diameter.

2. Why use a segmented design instead of a single-piece bearing?
Segmented designs solve manufacturing limits (equipment cannot produce larger single pieces), transportation constraints (large bearings cannot be shipped conventionally), and site access limitations (segments can be transported in standard containers).

3. How large can segmented slewing bearings be?
Segmented slewing bearings have been produced with outer diameters exceeding 23 meters. A 23.4-meter bearing with 68 segments was delivered in 2024.

4. What are the challenges in segmented slewing bearing design?
Key challenges include joint design (transferring loads across connections), raceway continuity (maintaining smooth rolling element passage), and sealing continuity (preventing contamination at joints).

5. How are segmented slewing bearings assembled?
Segments are positioned with cranes, aligned with guide pins, bolted to specified torque, and checked for raceway continuity. Seals are installed across joints to maintain protection.

Slewing Bearings for Tunnel Boring Machines: Cutter Head and Erector Applications

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What Is a Slewing Bearing?

A slewing bearing is a large rotating component that supports heavy loads while enabling rotation between two structures. These bearings handle axial loads, radial loads, and tilting moments simultaneously. They are essential in cranes, excavators, wind turbines, and tunnel boring machines.

Unlike standard bearings that operate at high speeds, slewing bearings in tunnel boring machines work under extreme slow-speed, heavy-load conditions. They must withstand enormous forces while maintaining precise rotational control. The bearing serves as the critical connection between the stationary shield structure and the rotating cutter head.

Why Tunnel Boring Machines Need Slewing Bearings

Tunnel boring machines (TBMs) operate under some of the most demanding conditions in heavy engineering. These massive machines excavate tunnels through rock, soil, and mixed ground conditions. The cutter head must rotate continuously while supporting the entire thrust load of the machine.

Extreme loads and slow rotation

The TBM main bearing is a typical large, low-speed, and heavy-load slewing bearing. It supports the cutter head’s weight, the thrust force from the machine’s hydraulic system, and the tilting moments created when the cutter head encounters uneven rock conditions. The bearing must maintain rotational accuracy while carrying loads measured in thousands of tonnes.

Unique design requirements

Since TBM are designed specifically for geological conditions, bore diameter, and tunnel type, the slewing bearings used must also be designed and manufactured individually for each machine. Each bearing design must be based on individual service life requirements and specifications. This is not a standard product—it is a custom-engineered component for each project.

Critical reliability

The dismounting and repair of a defective main bearing is expensive and involves massive effort, or may not be an option at all. For major projects like the Gotthard Base Tunnel, the reliability of the bearings and smooth operation of the machines is crucial. Four rothe erde slewing bearings were used in that project alone.

How Slewing Bearings Work in Tunnel Boring Machines

Tunnel boring machines use two primary types of slewing bearings: main shield bearings and erector bearings. Each serves a distinct function.

Main shield bearings: connecting cutter head to thrust system

Main bearings connect the cutter head with the thrust system and carry all the cutter head loads for secure and continuous operation. They ensure that the cutter head rotates reliably, allowing it to fight through rock and withstand the loads acting on it.

The main bearing must handle three load types simultaneously:

  • Axial load: Thrust force from the machine’s hydraulic cylinders pushing the cutter head forward
  • Radial load: Forces perpendicular to the tunnel axis from steering and ground conditions
  • Tilting moment: Overturning forces when the cutter head encounters uneven resistance

Erector bearings: positioning tunnel segments

Erector bearings allow for complete rotation of the erector, which positions pre-manufactured concrete segments against the tunnel wall, providing it with stable support. The concrete elements primarily secure the tunnel, and in the second step, these elements support the TBM when moving forward.

These bearings are generally used as single-row four-point bearings or crossed roller bearings. Each type offers application-specific advantages: four-point bearings are extremely resistant to deformations of the adjacent construction, while crossed roller bearings provide low torque resistance for low consumption values and longer service life.

Types of Slewing Bearings for TBM Applications

Different TBM applications require different bearing configurations. The choice depends on load requirements, precision needs, and structural constraints.

Three-row cylindrical roller bearings (main bearings)

The most common configuration for TBM main bearings is the three-row cylindrical roller bearing. This design has three rows of cylindrical rollers arranged axially: two rows of thrust rollers and one row of radial rollers between them. The axial and radial loads are mainly borne by the main thrust rollers, radial rollers, and corresponding raceways, while the tilting moment is comprehensively supported by all three rows.

The challenge with this design is that under load, the inner ring’s axis tilts while the outer ring remains normal. This creates localized contact between the inner ring and radial rollers, causing stress concentration that accelerates wear and shortens service life. Researchers have addressed this by designing radial roller raceways with arc-shaped cross-sections and matching curved rollers, which improves contact posture and reduces stress concentration.

Double-row tapered roller bearings (main bearings)

Alternatively, double-row tapered-roller bearings may be used as main bearings. These bearings handle combined radial and axial loads with high rigidity. The tapered design allows for precise clearance adjustment and good load distribution.

Single-row four-point contact ball bearings (erector bearings)

Four-point contact bearings are specifically engineered for erector applications. Each ball contacts the raceway at four points, enabling the bearing to handle bidirectional axial loads and tilting moments. These bearings are extremely resistant to deformations of the adjacent construction, making them suitable for the variable conditions encountered during tunnel construction.

Crossed roller bearings (erector bearings)

Crossed roller bearings use cylindrical rollers arranged at 90-degree angles to each other. This design provides line contact between rollers and raceways, resulting in higher rigidity than ball bearings. Thanks to their low torque resistance, crossed roller bearings make for low consumption values and longer service life in erector applications.

How Tunnel Boring Machine Slewing Bearings Are Manufactured

Manufacturing TBM slewing bearings requires specialized capabilities. These are not off-the-shelf products—they are custom-engineered for each machine.

Size and manufacturing capabilities

Slewing bearings can be manufactured with a diameter of up to 9.6 meters as a single piece. Segmented slewing bearings are produced with a diameter of up to 18 meters. The main bearing diameter can reach 8.61 meters. China has achieved full product spectrum coverage from 3 meters to 8.61 meters in diameter.

Material and heat treatment

TBM main bearings require high-quality forged alloy steel with induction-hardened raceways. The bearing must achieve 55-62 HRC surface hardness with adequate hardened layer depth to withstand the extreme contact stresses. Industry standards for TBM main bearing steel are being developed, with YB/T 6360-2025 published in 2025.

Lubrication challenges

The main bearing of a TBM is a typical large, low-speed, and heavy-load slewing bearing. Lubrication and cooling usually occur via an oil bath and circulation to ensure good lubrication and heat dissipation. The immersion height of the oil bath is usually 1/2 to 2/3 of the height of the main bearing, and an oil supply system achieves oil circulation. Understanding the lubrication flow field is critical to optimizing lubrication and cooling methods.

Common Challenges and Failure Modes

TBM main bearings face unique challenges due to their large size, complex structure, and harsh operating conditions.

Uneven load distribution

The complex working environment makes the main bearing subject to huge and uneven loads. This causes abnormal peeling and wear of the raceway and rollers, affecting service life. The TBM main bearing often contends with uneven loads due to complex geological conditions.

Thrust side defects

Existing research predominantly concentrates on raceway radial side defects or roller defects. However, defects of main bearings occur mainly on the thrust side. The derivation of defect frequency for thrust side defects remains an area requiring further study.

Lubrication flow field complexity

The main bearings of TBMs are often larger than normal bearings in both size and number of components. They are subject to demanding working conditions and complex behavior in their internal lubrication flow field. The oil-air two-phase distribution in the main bearing is directly related to the lubrication condition and heat dissipation performance.

How LDB Bearing Supports Tunnel Boring Machine Applications

LDB Bearing designs and manufactures high-quality slewing bearings for heavy machinery applications, including tunnel boring machines. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC and gear teeth hardened to 50–60 HRC.

LDB’s TBM capabilities:

  • Custom design: Application-specific bearings engineered for individual TBM specifications
  • Quality manufacturing: ISO 9001-certified manufacturing with documented inspection reports and full material traceability
  • Precision engineering: Capabilities to meet the tight tolerances required for TBM main and erector bearings
  • Global reach: Serving 73 countries with over 500,000 units in service

Serving the heavy machinery industry for over two decades, LDB understands the critical nature of TBM bearings. When a tunnel boring machine stops, the project stops. LDB offers the technical expertise and quality assurance that tunnel construction projects demand.

Contact LDB Bearing today to discuss your tunnel boring machine slewing bearing requirements.

FAQs

1. What is the difference between a TBM main bearing and an erector bearing?
The main bearing connects the cutter head to the thrust system and carries all cutter head loads. The erector bearing positions precast concrete segments against the tunnel wall during construction.

2. What type of slewing bearing is used as a TBM main bearing?
Three-row cylindrical roller bearings are the most common configuration, with two thrust roller rows and one radial roller row. Double-row tapered roller bearings are used as an alternative.

3. How large can TBM slewing bearings be?
Single-piece slewing bearings can be manufactured up to 9.6 meters in diameter. Segmented designs can reach 18 meters. Main bearing diameters can reach 8.61 meters.

4. Why is lubrication challenging for TBM main bearings?
TBM main bearings are large, low-speed, and heavy-load components. The oil-air two-phase distribution directly affects lubrication and heat dissipation. The immersion height is typically 1/2 to 2/3 of the bearing height.

5. What causes TBM main bearing failures?
Uneven load distribution from complex geological conditions causes abnormal peeling and wear. Thrust side defects are particularly common but have received less research attention than radial side defects.

Repair and Remanufacturing of Large Slewing Bearings: A Cost-Effective Alternative

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What Is a Slewing Bearing?

A slewing bearing—also called a slewing ring or turntable bearing—is a large-diameter rotational bearing that supports heavy loads while enabling rotation between two structures. It handles axial loads, radial loads, and tilting moments simultaneously. These bearings are essential in cranes, excavators, wind turbines, tunnel boring machines, and other heavy equipment where large structures must rotate.

Large-diameter slewing bearings often reach several meters in diameter with wide raceways. They are precision-engineered components that are expensive to manufacture and time-consuming to replace. When a slewing bearing fails, a new bearing can take weeks or months to deliver due to the complex manufacturing process and long supply chains. This creates significant downtime for equipment operators.

Why Consider Repair and Remanufacturing for Slewing Bearings?

Replacing a large slewing bearing is costly and time-consuming. The bearing itself is expensive—often costing tens of thousands of dollars or more. The replacement process requires specialized equipment, skilled labor, and extended downtime. For equipment operators, every day of downtime means lost production and revenue.

Repair and remanufacturing offer a practical alternative. Depending on the level of repair, savings can range from 40% to 70% compared to the cost of a new bearing. Kaydon reports that remanufactured bearings typically cost 50% less than new OEM bearings. The lead time for remanufacturing is significantly shorter—Kaydon offers four-week turnaround for bearings up to 8 feet in diameter. Trasmec reports delivery time reductions of up to 80% compared to new production.

A slewing bearing with only 30% of its calculated service life remaining can still be well worth remanufacturing. SKF notes that remanufacturing can extend bearing life cycle by 50% or more while reducing total lifecycle costs. Some bearings can even be remanufactured multiple times without compromising performance.

Environmental benefits are another consideration. Remanufacturing uses significantly less energy and materials than producing a new bearing, resulting in lower CO₂ emissions. This makes remanufacturing a more sustainable choice for environmentally conscious operators.

When Is a Slewing Bearing Eligible for Remanufacturing?

Not every damaged slewing bearing can be repaired. The bearing must meet certain criteria to be eligible for remanufacturing.

Bearing condition: The best candidates are bearings removed before serious degradation occurs. Once the damage progresses beyond a certain point, repair becomes impractical or impossible. Bearings that have experienced severe cracking, broken teeth, or extensive raceway spalling may not be repairable.

Hardened case depth: The remaining hardened layer depth is critical. If regrinding the raceway would remove too much of the hardened case, the bearing cannot support the required loads. RBC Bearings applies a design rule of 11% of the rolling element diameter as the minimum effective case depth required for most applications.

Structural integrity: The bearing rings must be free of cracks or other structural damage. Non-destructive testing (NDT) methods such as magnetic particle inspection and ultrasonic testing detect hidden defects.

Most bearings can be remanufactured: Kaydon states that it can remanufacture any bearing up to 26.25 feet (8 meters) in diameter, regardless of design, configuration, or original manufacturer. High-quality bearings with stable performance are particularly good candidates.

The Slewing Bearing Remanufacturing Process

The remanufacturing process for slewing bearings follows a systematic approach. The specific steps depend on the condition of the bearing and the level of repair required.

1. Initial Inspection and Assessment

The process begins with a thorough inspection. The bearing is cleaned and examined for damage. Key measurements include:

  • Turning torque: Measures rotational resistance
  • Free-state clearance: Checks internal clearance
  • Gear condition: Inspects teeth for wear, pitting, or breakage
  • External features: Documents overall condition

Non-destructive testing identifies hidden defects. Magnetic particle inspection detects surface cracks, while ultrasonic testing checks for subsurface defects and measures hardened case depth. Hardness readings are recorded to verify that the raceway still meets specifications.

An engineering analysis determines whether the bearing can be remanufactured and at what level. A formal inspection report is provided to the customer with recommendations.

2. Disassembly and Cleaning

The bearing is fully disassembled. All components—rings, rolling elements, spacers, cages, and seals—are separated and cleaned. This allows thorough inspection of each part.

3. Raceway Repair (Level 3 and 4 Repairs)

For bearings with worn or damaged raceways, the damaged surface is removed and rebuilt. Several methods are used:

Plasma-Transferred Arc (PTA) Welding: This advanced welding process applies a new cladding material to the damaged raceway. PTA welding offers a small heat-affected zone, high automation, and deposition rates up to 15 kg/h. The cladding material bonds metallurgically to the base material. Researchers have developed a complete repair process chain using PTA welding to restore damaged slewing bearing raceways.

The process chain involves:

  1. Removing the entire damaged raceway (milling out the worn surface)
  2. Applying repair welding using a specially developed alloy as cladding
  3. Heat treatment to improve service life properties
  4. Machining to reach nominal dimensions
  5. Incremental forming to generate residual compressive stresses

Precision grinding: In some cases, the raceway is reground to restore the proper geometry and surface finish. This optimizes load-carrying capability. Care must be taken to ensure sufficient hardened case remains.

New induction hardening: For bearings with high wear, the raceway may undergo a new induction hardening process after repair.

4. Component Replacement

Rolling elements (balls or rollers) are always replaced with new ones during remanufacturing. Cages, spacers, and seals are also replaced as needed. Some bearings require replacement of a major component such as an inner or outer ring—this is the highest level of remanufacturing (Level 4).

5. Reassembly and Final Inspection

The bearing is reassembled with new rolling elements and seals, then packed with fresh lubricant. Final inspection verifies all critical features meet specifications. A test report documents clearance and torque measurements. Remanufactured bearings typically come with a one-year warranty, just like new bearings.

Levels of Slewing Bearing Remanufacturing

Bearing manufacturers classify repairs by level, depending on the extent of work required:

LevelDescriptionTypical Candidates
Level 1Clean, inspect, lubricate, repackageBearings with storage damage only
Level 2Clean, inspect, polish raceways, replace seals, reassembleBearings at ~50% of theoretical life
Level 3Full race grinding, replace rolling elements and cages, new sealsEnd-of-life bearings, abnormal clearance
Level 4Replace major components (inner or outer ring), plus Level 3 workHeavily damaged bearings

Warning Signs That a Slewing Bearing Needs Attention

Identifying slewing bearing problems early increases the chances of successful remanufacturing. Watch for these warning signs:

Metal particles in grease: Metal flakes or dark discoloration in the expelled grease indicates active internal wear. This is the most reliable indicator of raceway damage. Clean grease should be smooth and uniform. Metal particles mean the bearing surfaces are deteriorating.

Abnormal noise: Grinding, clicking, or rhythmic popping during rotation indicates raceway or gear damage. A rhythmic knock repeating at regular intervals often indicates brinelling—localized raceway deformation. Continuous grinding usually means raceway damage.

Uneven or jerky rotation: Hesitation, binding, or uneven movement during rotation suggests worn rolling elements or raceways. Resistance at specific points often points to a localized defect such as a crack or deformed raceway.

Excessive play: Visible rocking of the upper structure—especially with the boom extended—means the bearing clearance has exceeded its wear limit. For excavators, axial clearance exceeding 2.0mm to 3.0mm is considered the red line.

Grease leakage: Visible grease leaking around the seal indicates seal failure. Once the seal is compromised, contaminants enter the bearing cavity and accelerate wear rapidly.

How LDB Bearing Supports Cost-Effective Solutions

LDB Bearing designs and manufactures high-quality slewing bearings and slew drives for heavy machinery applications. Products use verified 42CrMo forged alloy steel with induction-hardened raceways achieving 55–62 HRC and gear teeth hardened to 50–60 HRC.

LDB’s commitment to value:

  • Quality manufacturing: ISO 9001-certified manufacturing with documented inspection reports and full material traceability
  • Comprehensive product range: All major slewing bearing types with internal, external, or gearless configurations
  • Engineering support: Application engineering for load calculations, finite element analysis, and custom designs
  • Global reach: Serving 73 countries with over 500,000 units in service

While LDB specializes in new slewing bearing manufacturing, the company understands the importance of lifecycle management. Proper selection, installation, and maintenance extend bearing life and maximize return on investment. When replacement becomes necessary, LDB offers quality alternatives to help equipment operators maintain productivity with minimal downtime.

Contact LDB Bearing today to discuss your slewing bearing requirements and explore the most cost-effective solution for your application.

FAQs

1. How much can I save by remanufacturing a slewing bearing instead of buying new?
Savings typically range from 40% to 70% compared to the cost of a new bearing. Kaydon reports savings of approximately 50%, while Trasmec reports 40-70% savings.

2. How long does slewing bearing remanufacturing take?
Lead time is significantly shorter than new production. Kaydon offers four-week turnaround for bearings up to 8 feet in diameter, with slightly longer lead times for larger bearings. This compares to weeks or months for new bearings.

3. What types of slewing bearings can be remanufactured?
Most slewing bearing types can be remanufactured, including four-point and eight-point contact ball, crossed roller, and three-row roller bearings. Diameters up to 26 feet (8 meters) can be handled.

4. How many times can a slewing bearing be remanufactured?
Some bearings can be remanufactured multiple times without compromising performance. Each remanufacturing cycle extends the bearing’s service life. The practical limit depends on the remaining material and hardened case depth.

5. What is the warranty on remanufactured slewing bearings?
Reputable manufacturers like Kaydon offer a full one-year warranty on remanufactured bearings, the same as new bearings.