Normally, NTN bearings are used with a slight internal clearance under operating conditions. However, in some applications, NTN bearings are given an initial load; this means that the bearings' internal clearance is negative before operation. This is called "preload" and is commonly applied to NTN angular ball bearings and tapered roller bearings.

Purpose of preload

The following results are obtained by constant elastic compressive force applied to the contact points of rolling elements and raceway by providing preload.

1. NTN bearing's rigidity increases, internal clearance tends not to be produced even when heavy load is applied.
2. The particular frequency of the NTN bearing increases and is becomes suitable for high-speed rotation.
3. Shaft runout is suppressed; rotation and position precision are enhanced.
4. Vibration and noise are controlled.
5. Sliding of rolling elements by turning, spinning, or pivoting, is controlled and smearing is reduced.
6. Fretting produced by external vibration is prevented.

Applying excessive preload could result in reduction of life, abnormal heating, or increase in turning torque. You should therefore consider the objectives before determining the amount of preload.