Bearings are used with a slight internal clearance under operating conditions. However, in some applications, bearings are given an initial load; means that the bearings internal clearance is negative before operation. This is called preload. In all bearing arrangements where preload is deemed necessary, it should be applied as lightly as possible to achieve the desired result, avoiding excessive heat generation, which reduces speed capability and bearing life. it is commonly applied to tapered roller bearings and angular ball bearings.
Purpose of preload
The purpose of preload in a bearing system is to eliminate clearance that is designed into standard ball bearings. Almost all radial ball bearings are made with a clearance or space between their components to allow for free movement. This space, if not taken up by a preload can allow the rolling elements to slide rather than roll, or even allow for races to misaligned.
Application of preload
Giving preload to bearing results in the rolling element and raceway surfaces being under constant elastic compressive forces at their contact points. This has the effect of making the bearing extremely rigid so that even when load is applied to the bearing, radial or axial shaft displacement does not occur. Thus, the natural frequency of the shaft is increased, which is suitable for high speeds. The most common method of preloading is to apply an axial load to two duplex bearings so that the inner and outer rings are displaced axially in relation to each other.
The constant pressure preload is a method that uses any of coil spring, plate spring, or board spring to apply a proper amount of preload to bearing. Because the rigidity of preload springs is generally and sufficiently smaller than that of bearing, the preloads are kept almost constant although bearing.
The Position preload allow a bearing to keep the constant relative position regardless of its operation status.
Preloading and axial adjustment is a method of removing or controlling the internal clearance of a bearing. This is important because the degree of internal clearance within a bearing can influence noise, vibration, heat buildup and fatigue life. When applied correctly, preloading reduces or overcomes the causes of these problems.
Great care and accuracy are required in order to avoid excessive take up of internal clearance, which may occur during setup by overloading the bearings, or during operation due to thermal expansion. it controls radial and axial play; provides predictable system rigidity; reduces non-repetitive run-out; reduces the difference in contact angles between the inner and outer rings at very high speeds; and controls ball skidding under very high acceleration.
The best preload for a system should be individually determined based on a bearings size and the system's required stiffness, starting torque, running torque, life, and loading parameters. Excessive preload can cause increased heat, fatigue, and torque.
