Selecting a bearing

12. Preload

The purpose of applying preload to a bearing is to improve the runout precision of the rotating axis, and to reduce vibration and noise. It is important to select the proper amount of preload and method for each application. Otherwise, bearing performance such as life, noise, and vibration will be degraded. Excessive heat could be generated also.

The purpose of preload

Figure 12-1

Optimum preload

Minebea recommends an optimum preload based on the calculation of the optimum surface stress. When the preload is applied to the ball bearing, a contact ellipse is generated as a result of elastic deformation of the contact areas between balls and raceways. The surface stress is given by dividing the loads Q (ball load), which are generated in the perpendicular direction at the contacts between balls and raceways, by the surface areas of the contact ellipses.

In figure12-1
The contact ellipse area (S) between the balls and raceways is formulated as
   S = πab
(a: the major axis of the contact ellipse area, b: the minor axis of the contact ellipse area)
   P = Q / S [MPa]
P represents the average surface stress, and Q represents the loads generated in the perpendicular direction at contact part between balls and raceways.

If the preload is the dominant load applied to the bearing, the guideline for preload, which focuses on noise life, is as follows.

Over 10,000 hours noise life requirement

The specific preload should not generate an average surface contact stress (P) higher than 800Mpa.

5,000 - 10,000 hours noise life requirement (general products)

The specific preload should be generating an average surface contact stress (P) of roughly 1000MPa.

Less than 5,000 hours noise life requirement (critical stiffness application)

The specific preload should be generating an average surface contact stress (P) of roughly 1500MPa.

Simple calculation of the preloads by using dynamic load rating (Cr)

Maximum permissible load

If the average surface stress generated on high carbon chromium steel is more than 2700 Mpa, permanent deformation will occur. Even if the loads are applied for very short time, the loads should not generate more than 2700 Mpa of average surface stress. From experience, we recommend applying loads which do not generate more than 1600MPa of average surface stress. Besides preload, other types of loads should also be considered because they could generate surface stress.

Preload and stiffness

There are two basic methods of preloading: solid preload (Figure12-2) and spring preload (Figure12-3).

Solid preload can be obtained by mechanically locking all of the rings in position. The advantages of this type of design are simplicity and high stiffness. However, expansion and shrinkage of the components due to temperature change occasionally can cause changes in preload. The components could also wear out, and eventually the preloads could be reduced.

Spring Preload (Constant pressure preload) can be applied by using a coil spring, a spring wave washer, and so on. An advantage of spring preload is a stable preload despite temperature variation. The disadvantages are complexity and low stiffness.

The preload can be applied in two directions: Duplex face to face (DF) (Figure 12-4) and duplex back to back (DB) (Figure 12-5).The stiffness is higher in DB.

Figure 12-2 Solid Preload

Figure 12-3 Spring Preload

Figure 12-4 Duplex face to face (DF)

Figure 12-5 Duplex back to back (DB)