Selection of Ball and Roller Bearings

As compared with sleeve bearings, ball and roller bearings offer the following advantages:
1) Starting friction is low;
2) Less axial space is required;
3) Relatively accurate shaft alignment can be maintained;
4 ) Both radial and axial loads can be carried by certain types;
5) Angle of load application is not restricted;
6) Replacement is relatively easy;
7) Comparatively heavy overloads can be carried momentarily;
8) Lubrication is simple; and
9) Design and application can be made with the assistance of bearing supplier engineers.

In selecting a ball or roller bearing for a specific application five choices must be made:

1) the bearing series;
2) the type of bearing;
3) the size of bearing;
4) the method of lubrication; and
5) the type of mounting.
Naturally these considerations are modified or affected by the anticipated operating conditions, expected life, cost, and overhaul philosophy.

It is well to review the possible history of the bearing and its function in the machine it will be applied to, thus:
1) Will it be expected to endure removal and reapplication?;
2) Must it be free from maintenance attention during its useful life?;
3) Can wear of the housing or shaft be tolerated during the overhaul period?;
4) Must it be adjustable to take up wear, or to change shaft location?;
5) How accurately can the load spectrum be estimated? and;
6) Will it be relatively free from abuse in operation?.

Though many cautions could be pointed out, it should always be remembered that inadequate design approaches limit the utilization of rolling element bearings, reduce customersatisfaction, and reduce reliability. Time spent in this stage of design is the most rewarding effort of the bearing engineer, and here again he can depend on the bearing manufacturers field organization for assistance.

Type: Where loads are low, ball bearings are usually less expensive than roller bearings in terms of unit-carrying capacity. Where loads are high, the reverse is usually true.

For a purely radial load, almost any type of radial bearing can be used, the actual choice being determined by other factors. To support a combination of thrust and radial loads, several types of bearings may be considered. If the thrust load component is large, it may be most economical to provide a separate thrust bearing. When a separate thrust bearing cannot be used due to high speed, lack of space, or other factors, the following types may be considered: angular contact ball bearing, deep groove ball bearing without filling slot, tapered roller bearing with steep contact angle, and self-aligning bearing of the wide type. If movement or deflection in an axial direction must be held to a minimum, then a separate thrust bearing or a preloaded bearing capable of taking considerable thrust load is required.

To minimize deflection due to a moment in an axial plane, a rigid bearing such as a double row angular contact type with outwardly converging load lines is required. In such cases,the resulting stresses must be taken into consideration in determining the proper size of the
bearing.

For shock loads or heavy loads of short duration, roller bearings are usually preferred.

Special bearing designs may be required where accelerations are usually high as in planetary or crank motions.

Where the problem of excessive shaft deflection or misalignment between shaft and
housing is present, a self-aligning type of bearing may be a satisfactory solution.
It should be kept in mind that a great deal of difficulty can be avoided if standard types of bearings are used in preference to special designs, wherever possible.

Size: The size of bearing required for a given application is determined by the loads that are to be carried and, in some cases, by the amount of rigidity that is necessary to limit deflection to some specified amount.

The forces to which a bearing will be subjected can be calculated by the laws of engineering mechanics from the known loads, power, operating pressure, etc. Where loads are irregular, varying, or of unknown magnitude, it may be difficult to determine the actual forces. In such cases, empirical determination of such forces, based on extensive experience in bearing design, may be needed to attack the problem successfully. Where such experience is lacking, the bearing manufacturer should be consulted or the services of a bearing expert obtained.

If a ball or roller bearing is to be subjected to a combination of radial and thrust loads, an equivalent radial load is computed in the case of radial or angular type bearings and an equivalent thrust load is computed in the case of thrust bearings.