1. Excessive preload: Excessive preload can have a sharp reduction in life due to increased load on the rollers which in turn increases friction, damages the lubricant and can cause high operating temperatures.
2. Variation between target preload value and actual preload value: When adjusting preload in a bearing system, it is also important that the established preload value, determined either by calculation or from experience, is attained with the least possible variation. To reduce variation when mounting tapered roller bearings, for example, the shaft must be turned several times so that the rollers are not skewed and the roller ends are in contact with the guide flange of the inner ring. When the rollers are not fully settled into position, this results in a much smaller preload than the requisite value.
3. Variation between design and test conditions: SKF recommends calculating the preload force and checking if it’s appropriate by testing the application. Generally, it is not always possible to identify all influencing factors in operation fully during the design stage and adjustments may be necessary. The accuracy of the calculated results depends on how closely the estimated operating temperature and elastic behavior of the associated components – most importantly the housing – match the actual operating conditions.
4. Variation between ambient and operating temperature: During operation, if the shaft temperature is higher than the housing temperature, the preload, which was adjusted at ambient temperature during mounting, will change.
5. Thermal expansion: Thermal expansion of the inner ring in the radial direction leads to an increase of the preload. Thermal expansion in the axial direction increases the preload when the bearings are face-to-face, but is reduced for back-to-back arrangements. Depending on the distance between the bearings, and provided the coefficient of thermal expansion is the same for the bearings and associated components, thermal expansion in both the radial and axial directions can cancel each other out so that preload remains unchanged for back-to-back arrangements independent of operating temperature.
6. Stiffness: When selecting the preload for a bearing arrangement, the degree of stiffness increases marginally once preload exceeds a given optimum value. When exceeding this optimum value, friction and the resulting increase in heat can substantially reduce bearing service life. Excessive preload includes a risk that the operational reliability of a bearing arrangement is compromised. However, bear in mind that stiffness is also influenced by the elasticity of the shaft and housing, the shaft and housing fits and the elastic deformation of all other components adjacent to the bearings, including abutments. Each of these factors has a considerable impact on the resilience of the total bearing system. The axial and radial resiliencies of a bearing depend on its internal design, contact conditions (point or line contact), the number and diameter of rolling elements and the contact angle. The greater the contact angle, the higher the degree of stiffness in the axial direction.
7. Additional factors like excessive axial clamping of a ring and misalignment can further increase preload and subsequently reduce bearing life.
