Precise Control and Measurement Methods of Bearing Preload

Bearing preload is the core parameter to ensure the smooth operation of high-precision equipment. This article systematically analyzes the working principle, control method and measurement technology of bearing preload, and combines industrial cases and manufacturer standards (such as NSK, SKF) to provide practical guidance for engineers.

1. The role and importance of preload

Bearing preload eliminates axial clearance and makes the rolling element and raceway in close contact, thereby improving the equipment stiffness, rotation accuracy and vibration resistance. Its core value is reflected in:

Eliminate reverse error: Preload can reduce the displacement error of the bearing under variable load, which is suitable for high-precision scenarios such as machine tool spindles.

Extend life: Reasonable preload makes the load evenly distributed and avoids local wear (such as pitting caused by concentrated force on the rolling element).

Temperature and efficiency balance: Excessive preload will increase the friction temperature rise, while too little will cause vibration, and precise control is required to achieve optimal performance.

II. Preload control method

1. Mechanical structure design
Spacer adjustment method: Axial displacement control is achieved by grinding the length and tolerance of the inner and outer spacers. For example:
NSK recommends segmented control of the length tolerance of the inner and outer spacers (-0.0025~-0.00125mm, -0.00125~0mm) to match the shaft matching requirements.
SKF adjusts the preload level by the difference in spacer tolerance (such as A/B/C/D levels corresponding to different preload amounts).
Elastic element preload: A constant load is applied using a spring or a torque wrench, which is suitable for dynamic working conditions (such as high-speed spindles).
2. Assembly process optimization
End cover clearance control: NSK recommends that the clearance between the end cover and the bearing seat is 0.01~0.05mm to avoid deformation of the outer ring caused by the locking screws.
Friction torque detection: By measuring the rotational torque of the paired bearings, it is indirectly inferred whether the preload meets the standard (such as the paired installation of angular contact ball bearings).
3. Manufacturer standard practice
Material and processing requirements: SKF stipulates that the end face parallelism of the spacer is ≤2μm and the hardness is 45-60HRC. NSK uses SUJ2 high-carbon chromium steel to ensure stability.
Tolerance grading management: For example, ABEC9 grade inner diameter tolerance (-0.0025mm~0mm) is used for dental turbine bearings to achieve ultra-precision matching.

III. Preload measurement technology

1. Starting torque method
The torque value is calculated by measuring the tangential tension at the start of the bearing (torque = force × shaft radius). This method is suitable for large preload scenarios (such as the fixed end of the ball screw), but the error of small preload is large.

2. Axial static stiffness method
Apply axial load to the bearing and measure the displacement to draw a stiffness curve. This method is highly sensitive to ball bearings and can reflect the nonlinear relationship between preload and stiffness.

3. Natural frequency method
The axial resonance frequency is detected through vibration excitation to indirectly evaluate the preload state. The advantage is non-contact measurement, but it is significantly affected by the assembly fixture.

IV. Industrial application cases and lessons

Failure analysis of low-temperature bearing box: The preload of a certain device was out of control due to improper positioning of the spring sheet, and the bearing was stuck after running for 1 hour. Finally, the problem was solved by adjusting the tolerance of the spacer (end face parallelism <0.002mm).

Dental turbine bearing optimization: NSK uses ES1 stainless steel and ceramic balls, combined with ABEC9 tolerance, to achieve stable preload at 500,000 rpm.

V. Conclusion

The control and measurement of bearing preload is a core topic in precision mechanical design. From spacer grinding to torque detection, each step must take into account theoretical calculations and process practices. In the future, with the popularization of intelligent sensors and simulation technology, dynamic monitoring and adaptive adjustment of preload will become a trend. Engineers need to continue to pay attention to manufacturer technology updates (such as NSK’s ES1 material and SKF’s preload classification standards) to meet the challenges of higher precision and reliability.

References:
[1] NSK Dental Handpiece Bearing Technology White Paper
[2] “Control Methods for Rolling Bearing Installation Preload”
[3] “Practical Guide for Bearing Preload Control and Measurement”

Disclaimer: Some cases and data in this article are quoted from public industrial literature, and reprinting requires authorization.