1.3 Bearing Selection Factors

Bearings are basically antifriction devices. For this reason, the friction characteristics of different bearing types have to be examined.

In addition to the rolling resistance, other factors which contribute to the friction are as follows:

1. Sliding between the rolling elements and the race. When the rolling elements are curved, all points in contact do not have the same linear velocity, because of their differing radii of rotation. In Fig. 1-5, for example, a point A on the ball will have a definite linear velocity if no sliding occurs. However, a second point B on the ball will have less linear velocity than A because of its smaller radius of rotation. But point B on the race actually has a slightly greater linear velocity than A. This introduces sliding in both backward and forward directions. Other factors which introduce sliding are the inevitable inaccuracies in geometry and other deviations from true rolling.

2. The sliding action between the rolling element and the separator. Although contact takes place at the poles, where the velocity is lowest, some sliding action is present.

3. In roller bearings, the sliding action between the rolling elements and the guide flanges.

4. The losses between the bearing parts and the lubricant and between the different particles of the lubricant.

Palmgren1 gives the following frictional coefficients for antifriction bearings:

Self-aligning ball bearings       f = 0.0010
Cylindrical roller bearing         f = 0.0011
Thrust ball bearings                     f = 0.0013
Single-row deep-groove ball bearings     f = 0.0015
Tapered and spherical roller bearings       f = 0.0018
Needle bearings                    f = 0.0045

All these coefficients are referred to the bearing bore. They are for run-in bearings, under normal conditions, with good lubrication. When determining the total losses in a given application, the seal friction must not be ignored since it may be considerable.

In addition to considerations related to friction, attention must be given to speed requirements. Permissible speeds are influenced by bearing size, properties, lubrication detail and operating temperatures. The permissible speed varies inversely with mean bearing diameter.

Some guidelines for selecting bearings can be summarized as follows:

• Ball bearings are the less expensive choice in the smaller sizes and under lighter loads, while roller bearings are less expensive for larger sizes and heavier loads.

• Roller bearings are more satisfactory under shock or impact loading than ball bearings.

• Ball-thrust bearings are for pure thrust loading only. At high speeds, a deep-groove or angular-contact ball bearing usually will be a better choice, even for pure thrust loads.

• Self-aligning ball bearings and cylindrical roller bearings have very low friction coefficients.

• Deep-groove ball bearings are available with seals built into the bearing so that the bearing can be prelubricated to operate for long periods without attention.

The following Table 1-1 attempts to summarize and tabulate various considerations which influence the selection of the appropriate bearings:

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