Selecting a bearing

10. Fits

When a ball bearing is used, it is not used only by itself. It is always fitted to either a shaft or a housing bore. Fit is the value of tightness between the shaft and bearing bore when the bearing is installed. Fit can also be the tightness between the housing bore and the bearing outside diameter. Fits are classified into clearance fit, intermediate fit, and interference fit.

Fits

Fits help bearings not to have unfavorable slip called "Creep" by firmly securing the inner ring and outer ring on the shaft and in the housing, respectively. Fits also minimize vibration during rotation. When creep happens, abnormal heat, and wear particles can be generated due to slip wear. Abnormal heat hastens the degradation of grease and retainers. Wear particles could migrate inside the bearing, and cause vibration and degradation. It is necessary to choose a proper fit for each application because improper fits can not only degrade the bearing performance, but also they could cause seizure from burning and early failure. In the case of interference fits, the interference causes a change in radial clearance. The change in radial clearance generated by interference can be calculated as below. (By TIMOSHENKO)

Decrease in internal clearance due to fits

Interference fit of the inner ring to the shaft

The sketches drawn in solid lines and dotted lines are the bearing prior to a fit, and the bearing after a fit, respectively. When press fitted with an interference " i ", the inner ring groove diameter d2 increases by an amount δ. This δ is also equal to the decrease in radial clearance.

Press fits of the inner ring to the shaft :

Figure10-1

δ=	2i (d2/d)
	{ (d2/d)2 - 1 } (d2/d)2 + 1 + 1 + Eb (d/d1)2 + 1 - 1 (d2/d)2 - 1 mb Es (d/d1)2 - 1 ms

Interference fit of the outer ring to the housing

The sketches drawn in solid lines and dotted lines are the bearing prior to a fit, and the bearing after a fit, respectively. When it is press fitted with an interference " I ", the outer ring groove diameter D1 decreases by an amount Δ. This amount Δ is also equal to the decrease in radial clearance.

Press fits of the outer ring to the housing :

Figure10-2

Δ=	2I (D/D1)
	{ (D/D1)2 - 1 } (D/D1)2 + 1 - 1 + Eb (D2/D)2 + 1 + 1 (D/D1)2 - 1 mb Eh (D2/D)2 - 1 mh

Securing with glue

When the bearing is fitted to the shaft and housing by glue without interference, it is necessary to select the proper clearance to enhance the effectiveness of the glue. It is recommended to consult with the glue manufacturer because the proper clearance depends on the type of glue. Please be aware that the roundness of the ring raceways could worsen because of the curing stress of the glue.

Referred from JIS B 0401-1

Deviation of holes for common fits

Deviation of shafts for common fits

Referred from JIS B 1566

Fits of inner ring in radial bearings^*1

Fits of outer ring in radial bearings^*3

Definitions

Rotating inner ring load :

the line of action of the load is rotating in relation to the inner ring of the bearing

Stationary inner ring load :

the line of action of the load does not rotate in relation to the inner ring of the bearing

Stationary outer ring load :

the line of action of the load does not rotate in relation to the outer ring of the bearing

Rotating outer ring load :

the line of action of the load is rotating in relation to the outer ring of the bearing

Indeterminate direction load :

the direction of the load cannot be determined.