2025-10-14 03:20:43PRS Bearings
In the field of precision machinery, crossed roller bearings are highly sought after for their high precision, high rigidity, and compact design. They are widely used in critical equipment such as industrial robots, medical devices, CNC machine tools, and rotary tables. However, to fully realize the potential of these bearings, a crucial step is preload. Proper preload calculation and application not only significantly improves bearing operating accuracy and rigidity, but also effectively extends its service life.
Simply put, bearing preload refers to the creation of an initial, predetermined contact stress within the bearing during installation, through a specific force or displacement. For rolling element bearings, this means that a certain amount of compression already exists between the rolling elements and raceways, even without external load.
Increase stiffness: Preload eliminates internal clearance (play) within the bearing. Once this clearance is eliminated, the bearing experiences less deformation when subjected to external loads, significantly increasing overall system stiffness. This is crucial for applications requiring high-precision positioning.
Improve running accuracy: Eliminating play means that the rolling elements maintain close contact with the raceways. This helps reduce runout and runout during movement, ensuring the bearing's rotational and positioning accuracy.
Reduce vibration and noise: Preload effectively dampens vibration and shock from rolling elements under high speeds or variable loads, thereby reducing operating noise and improving equipment stability. Prolonging Service Life: Proper preload helps optimize load distribution between the rolling elements and raceways, avoiding localized stress concentrations and, to a certain extent, extending bearing fatigue life.
Preventing Slippage: In certain high-speed or light-load applications, preload can ensure that the rolling elements consistently roll rather than slip, preventing surface damage.
Unlike traditional deep groove ball bearings or tapered roller bearings, the rolling elements (typically cylindrical rollers) in crossed roller bearings are staggered 90 degrees between the inner and outer rings. This unique design allows a single bearing to simultaneously withstand radial, axial, and tilting moment loads.
Therefore, the preload application method and calculation are also unique.
Crossed Roller Bearing Preload Classification
Crossed roller bearing preload is generally classified into several levels:
Micro preload: Suitable for applications requiring high rotational accuracy and moderate rigidity, with low external loads.
Light preload: The most commonly used level, offering a good balance between rigidity, precision, and temperature rise.
Medium preload: Suitable for applications with heavy loads, shock loads, or where extremely high rigidity is required. Heavy preload: Used only in extreme operating conditions involving extreme shock and heavy loads. Temperature rise and lifespan are key considerations.
The catalogs provided by crossed roller bearing manufacturer Presun typically clearly specify these preload levels and their corresponding preload forces or preload displacements.
When calculating the preload for crossed roller bearings, multiple factors must be considered to ensure that the preload achieves the desired effect without being excessive, leading to premature bearing failure. Key considerations include:
Bearing Model and Size: Bearings of different models and sizes vary in their internal structure, number of rollers, raceway geometry, and other factors, all of which directly affect the relationship between preload and bearing stiffness.
Material Properties: Material parameters such as the elastic modulus of the bearing steel form the basis for calculating deflection.
Load Type and Magnitude: The radial, axial, and moment loads the bearing will experience during actual operation. The preload must be sufficient to resist these external loads and ensure that the bearing does not develop negative clearance during operation. Operating Speed: High-speed operation may require more precise preload control to avoid heat and wear.
Operating Temperature: Temperature fluctuations cause bearing components to expand and contract, affecting the actual preload. Thermal expansion coefficients must be accounted for to compensate.
Required Stiffness Requirement: The required preload can be inferred based on the specific stiffness requirements of the equipment.
Mounting Method: The bearing mounting method (for example, bolted or press-fit) also affects the application and stability of the preload.
Empirical Formula Method: For some standard applications and common models, the manufacturer may provide recommended preload values or empirically calculated formulas. This is generally a quick but potentially inaccurate method.
Stiffness Matching Method: Based on the required stiffness target for the system, the required preload can be inferred using bearing stiffness curves or calculation models provided by the bearing manufacturer. Bearing stiffness is typically nonlinear and increases with preload.
Finite Element Analysis (FEA): For particularly complex or demanding applications, finite element analysis (FEA) software can be used to model the bearing and its mounting structure, simulating stress distribution, deformation, and stiffness characteristics under varying preload conditions. This provides the most accurate analysis results, but is computationally expensive.
Theoretical Formulas and Iterative Calculations: Based on Hertzian contact theory, combined with bearing geometry and material properties, a theoretical formula for the contact deformation between rolling elements and raceways is derived. Then, through iterative calculations, the displacement or force required to generate the desired contact stress (i.e., preload) within the bearing is determined. This typically involves considering:
Contact Angle Change: Preload causes slight changes in the contact angle between the roller and raceway.
Roller Deformation: The elastic deformation of each roller under preload.
Inner and Outer Ring Deformation: The elastic deformation of the bearing's inner and outer rings under preload.
Preload is typically achieved through the following methods:
Bolt Tightening Method: Preload is achieved by uniformly tightening the fixing bolts, causing axial relative displacement between the inner and outer rings of the bearing. This requires strict control of the bolt tightening torque, typically using a torque wrench.
Shim Adjustment Method: Preload is achieved by inserting precision shims between the bearing mounting surfaces and adjusting the shim thickness to control the axial clearance.
Press Fit Method: The bearing is pressed into the seat or onto the journal with an interference fit, using the interference to create radial or axial deformation, thereby achieving preload.
Key Parameters and Considerations for Preload Calculation
Bearing Model and Size: This is the basis for determining the preload amount/preload force.
Working Load: The internal load F0 generated by preload must be considered together with the external working load Fa. The total load is the vector sum of these two factors. Impact of Temperature Rise:
Bearings generate heat during operation, causing differential thermal expansion of the inner and outer rings, shaft, and bearing seat.
For preload, temperature rise can lead to increased preload (if the shaft's coefficient of thermal expansion is greater than the bearing seat), exacerbating the temperature rise and even causing bearing seizure. A thermal balance analysis is essential.
Fatigue Life:
Preload significantly affects bearing fatigue life. According to the Lundberg-Palmgren theory, bearing life L10 is proportional to the equivalent load P raised to the power of -10/3.
Preload increases the internal equivalent load P.
A rule of thumb: The reduction in life due to the internal load F0 generated by preload should not exceed 50% of the rated life. In other words, ensure that the external operating load is the primary load.
Preload calculation for crossed roller bearings is critical to ensuring high performance and long life. It requires more than a simple numerical value; it requires a deep understanding of the bearing's operating principles, material properties, and application conditions.
Providing professional preload calculation services or guidance will undoubtedly enhance customer trust in your products and solutions. We encourage engineers to not only master theoretical calculation methods in practical applications, but also to repeatedly verify and optimize them based on specific operating conditions and actual test data.
As precision manufacturing continues to advance, in-depth research and precise control of cross-roller bearing preload will continue to push the performance envelope in areas such as robotics and automated equipment.
