THE RELATIONSHIP BETWEEN BEARING FIT AND LOAD TYPE

When choosing a fit, the first consideration should be the type of load that the collar bears. There are usually three types of loads that the ferrule can bear:

1 Fixed load

The composite radial load acting on the ring is borne by the local area of the ring raceway and transmitted to the relative area of the shaft or bearing seat. This load is called a fixed load. The characteristic of a fixed load is that the synthesized radial load vector is relatively stationary with the collar. Both the collar and the composite radial load do not rotate or rotate at the same speed, which are fixed loads. Loose fitting can be used for ferrules that bear fixed loads.

2 Rotating load

The composite radial load acting on the ferrule rotates in the circumferential direction of the raceway and is sequentially borne by various parts. This load is called rotational load.

The characteristic of rotating load is the composite radial load vector rotating relative to the ferrule. There are three types of rotational loads:

(1) The load direction remains fixed and the collar rotates;

(2) The load vector rotates and the collar remains stationary;

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

3. Swing load and indefinite load

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

The swinging load and the indefinite load should be treated equally with the rotating load in the fit, as a loose fit can cause damage to the mating surface.

Transition fit or interference fit should be selected for the collar and shaft or seat hole that rotate relative to the load direction. The size of the interference is based on the principle that when the bearing operates under load, its collar does not produce "creep" phenomenon on the mating surface of the shaft or seat hole. When the load is very light or the ferrule occasionally rotates at low speed under heavy loads, a transition fit can be selected. At this time, the surface of the shaft should have higher hardness and smaller surface roughness.