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Relationship Between Bearing Fit And Load Type

Source:  skf bearing  Time:  2021-07-27

1. Basis for choosing cooperation


According to the load acting on the agent's bearing relative to the rotation of the ferrule, there are three types of loads borne by the ferrule: partial load, cyclic load, and swing load. Generally, cyclic load (rotating load) and swing load adopt tight fit; except for special requirements in use for local loads, tight fit is generally not suitable. When the bearing ring bears the dynamic load and is under heavy load, the inner and outer rings should adopt interference fit, but sometimes the outer ring can be slightly loosened, and it should be able to move axially in the housing hole of the bearing housing; When the bearing ring bears the swing load and the load is light, a slightly looser fit than a tight fit can be used.


2. The choice of fit


The bearing and the shaft adopt the basic hole system, and the coordination with the shell adopts the basic shaft system. The matching of the bearing and the shaft is different from the tolerance matching system used in the machine manufacturing industry. The tolerance zone of the bearing inner diameter is mostly below the change. Therefore, under the same matching condition, the matching of the bearing inner diameter and the shaft is closer than the usual matching. . Although the tolerance zone of the bearing outer diameter and the tolerance zone of the base shaft system are below the zero line, their values ​​are different from the general tolerance system. When installing various types of bearings, select the tolerance zone of the shaft and housing hole. Tolerances of different tolerance grades of bearings, shafts and housings, or bearing samples.

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Therefore, when choosing a fit, first consider the type of load bearing the ferrule. Generally, there are three types of load bearing rings:


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 opposite area of the shaft or bearing seat. This load is called a fixed load. The characteristic of the fixed load is that the composite radial load vector is relatively static with the ferrule. Neither the ferrule nor the composite radial load rotates or rotates at the same speed, which is a fixed load. The ferrule that bears the fixed load can choose a looser fit.


2) Rotating load


The synthetic radial load acting on the ferrule rotates in the circumferential direction of the raceway and is borne by various parts in sequence. This load is called a rotating load.


The characteristic of the rotating load is that the resultant radial load vector rotates relative to the ferrule. The rotating load has the following three situations:


a. The load direction is fixed and the ferrule rotates;


b. The load vector rotates and the ferrule is stationary;


c. The load vector and the ferrule rotate at different speeds.


3) Swing load and variable load


Sometimes the direction and magnitude of the load cannot be determined with certainty. For example, in high-speed rotating machinery, in addition to the load of the fixed direction of the rotor weight, there is also the rotating load caused by the unbalanced mass. If this kind of rotating load is greater than the fixed load If the rotating load is much smaller than the fixed load, the resultant load is still a rotating load. If the rotating load is much smaller than the fixed load, the resultant load is a swing load. Regardless of rotating load or swing load, its size and direction are constantly changing. In the variable working state, the load on some ferrules may sometimes be a rotating load, sometimes a fixed load, and sometimes a swing load. This load is called indefinite load.


Swing load and indeterminate load should be treated in the same way as rotating load in terms of fit. Too loose fit will cause damage to the mating surface.


For ferrules and shafts or seat holes that rotate relative to the load direction, transition fit or interference fit should be selected. The interference size is based on the principle that when the bearing is working under load, the ring does not produce "creeping" on the mating surface of the shaft or in the seat hole. When the load is very light or the ferrule occasionally rotates at a low speed under heavy load, a transition fit can be used. At this time, the shaft surface should have a higher hardness and a smaller surface roughness.


For heavy load occasions, the fit should usually be tighter than the light load and normal load occasions. The heavier the load, the greater the fit interference should be.