Introduction
Bearing preload and axial adjustment are critical yet often misunderstood aspects of precision bearing engineering. When applied correctly, preload minimizes vibration, reduces heat build-up, lowers noise, and extends bearing fatigue life. For industries where accuracy and reliability are paramount—such as aerospace, machine tools, and high-speed rotating equipment—understanding preload is essential for performance optimization and cost savings.
What Is Bearing Preload?
Preload is the intentional application of an axial force to remove or control the internal clearance of a bearing. Internal clearance directly affects key performance factors, including:
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Noise and vibration levels
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Heat generation during operation
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Axial and radial rigidity
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Rolling element stability under acceleration
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Overall fatigue life of the bearing
When preload is properly applied, it:
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Eliminates unnecessary axial and radial play
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Improves system stiffness and positional accuracy
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Reduces non-repetitive runout at high speeds
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Balances contact angles between inner and outer rings
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Prevents ball skidding under rapid acceleration
Understanding Bearing Yield
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Axial Yield: The deflection between inner and outer rings when preload or thrust load is applied.
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Radial Yield: The deflection caused by radial loading.
Both are influenced by bearing design, contact angle, and applied loads. Preload should always be as light as possible to achieve the desired rigidity while avoiding excessive heat, which reduces speed capability and service life.
Main Methods of Bearing Preload
1. Spring Preload
Spring preloading is often the simplest method and is widely used due to its flexibility.
Advantages:
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Maintains relatively constant preload under varying thermal conditions
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Compensates for minor misalignment
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No need for specially ground preload bearings
Limitations:
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Not ideal for reversing thrust loads
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Requires additional space for spring travel
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May induce misalignment in the loaded ring
Typical Designs: Coil springs, Belleville washers, wave springs, and finger spring washers. Springs are usually applied to the non-rotating part (outer ring) with clearance fits to ensure effective force transmission.
2. Axial Adjustment
Axial adjustment achieves preload by mounting at least two bearings in opposition and offsetting them axially using threaded members, shims, or spacers.
Advantages:
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Provides rigid preload with no added friction
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Preferred for very low-torque applications
Best Practices:
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Use precision lapped shims rather than threaded members to avoid misalignment
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Maintain shim tolerances within 1–2 micrometres
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Ensure extreme cleanliness during assembly
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Align bearing ring faces accurately for consistent performance
Consideration: Incorrect adjustment can lead to excessive preload, overheating, or premature failure, particularly if thermal expansion is not accounted for.
3. Duplex Bearings
Duplex bearings are matched bearing pairs supplied with factory-ground preload offsets. When clamped together, the offset faces establish a permanent preload.
Advantages:
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Simplifies installation for end users
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Provides predictable radial and axial rigidity
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Handles bi-directional or heavy uni-directional thrust loads
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Ensures minimal runout and precise alignment
Limitations:
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Generates more heat, reducing speed capability
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Sensitive to misalignment and thermal expansion
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Higher torque compared to spring or axial adjustment
Mounting Configurations:
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DB (Back-to-Back): High moment rigidity, suitable when shafts run warmer than housings.
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DF (Face-to-Face): Accommodates misalignment but offers lower speed capability.
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DT (Tandem): Shares heavy thrust loads in one direction but requires additional preload (often via springs) since no internal preload exists.
Best Practices for Preload Application
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Always apply the minimum preload necessary to achieve performance requirements.
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Consider thermal effects—especially in high-speed or high-temperature environments.
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Select the preload method based on application needs: flexibility (springs), precision (axial adjustment), or rigidity (Duplex bearings).
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Ensure high assembly cleanliness and precision alignment to prevent early failures.
Conclusion
Bearing preload and axial adjustment are essential techniques for ensuring accuracy, reliability, and longevity in rotating machinery. Choosing the correct preload method—spring, axial adjustment, or Duplex bearings—depends on the application’s demands for rigidity, load direction, speed, and thermal behavior. By applying preload carefully and strategically, engineers can significantly improve bearing performance and extend service life.
