General Considerations. In a bearing with rolling contact, the shaft is supported on rollers or balls. A bearing of this type has the theoretical advantage of reduced friction, but to be practical it must fulfil the following conditions;

(a) unavoidable sliding should be reduced to a minimum;

(b) the lolling elements must be properly guided in their motion;

(c) all rolling elements should be of exactly the same size;

(d) the rolling elements and their guides or raceways must be ex­tremely hard and be very smoothly polished;

(e) the pressure should be approximately normal to the surface of contact;

(f) in order to avoid distortion the rolling elements must not be overloaded.

Advantages. Properly designed and well manufactured bearings with rolling contact have the following advantages over bearings with sliding contact:

(a) they will maintain a comparatively accurate alignment over long periods of time;

(b) they can carry heavy momentary overloads without failure or seizure;

(c) their power loss caused by friction is very small;

(d) they are particularly adapted to very low speeds since their friction coefficient is independent of speed;

(e) they have a very low starting friction;

(f) their lubrication is simple and requires but little attention.

Classification. Bearings with rolling contact may be divided into two main classes according to the type of rolling elements used:

(a) ball bearings and

(b) roller bearings with cylindrical, conical, spherical, or concave rollers.

Each of the above classes may be subdivided into the following types:

(a) radial bearings;

(b) thrust bearings, and

(c) radial-thrust or angular bearings which can take both radial and axial forces.

Ball Bearings. Each radial ball bearing consists of four elements:

(a) an inner ring or race, grooved on its outer surface;

(b) an outer race grooved on its inner surface;

(c) steel balls and

(d) a ball retainer or cage for spacing the balls so that they do not touch each other, thus reducing wear and noise.

Types. (Fig. 3) (a) a single-row radial bearing usually made with a deep groove;

(b) an angular-type bearing, which can take an axial load in addition to the radial load;

(c) a double-row bearing;