When selecting the fit, the type of load bearing of the ring should be considered first. Generally, there are three types of ring load:

1 Fixed load

The resultant radial load acting on the ring is borne by a local area of the ring raceway and transmitted to a relative area of the shaft or housing. This load is called a fixed load. The characteristic of the fixed load is that the resultant radial load vector is relatively static with the ring. Both ring and composite radial loads do not rotate or rotate at the same speed are fixed loads. Looser fit can be chosen for the ring bearing fixed load.

2 Rotating load

The resultant radial load acting on the raceway rotates along the circumference of the raceway and is successively borne by various parts. This load is called the rotational load.

The characteristic of the rotating load is to synthesize the radial load vector relative to the ring rotation. There are three cases of rotating load:

(1) The load direction is fixed and the ring rotates;

(2) The load vector rotates and the ring is stationary;

(3) The load vector and the ring rotate at different speeds.

Oscillating load and indefinite load

Sometimes the direction and size of the load cannot be determined accurately. For example, in high-speed rotating machinery, in addition to the load bearing the rotor weight is fixed in the direction, there is also the rotating load caused by the unbalanced mass. If the rotating load is much larger than the fixed load, then the resultant load is still the rotating load. If the rotary load is much smaller than the fixed load, the resultant load is the oscillating load. The magnitude and direction of the rotating or swinging load are constantly changing. But in the variable working state, some rings may be subjected to a rotating load, a fixed load, and a swinging load. This load is called an indefinite load.

Swing load and variable load should be treated equally with rotary load, too loose fit will lead to fit surface damage.

The transition fit or interference fit should be selected for the ring and shaft or seat hole rotating in the direction of load. The interference size is based on the principle that when the bearing is working under load, no "creep" phenomenon occurs on the matching surface of its ring on the shaft or in the seat hole. When the load is very light, or when the ring rotates at low speed occasionally under heavy load, the transition fit can be selected. In this case, the shaft surface should have a higher hardness and a smaller surface roughness.