The clearance of the bearing determines to a large extent the life of and the noise in the bearing. The clearance is indicated in:
- The radial play: this is the displacement perpendicular to the centerline of the shaft
- The axial play: this is the displacement parallel to the centerline of the shaft
The internal clearance of a bearing, measured without load, is the distance that one ring can move relative to the other ring. There is usually a difference between the radial play of an unmounted and a mounted bearing. This difference is due to the chosen shaft and housing fit tolerances, which can stretch the inner ring and/or compress the outer ring. The initial play before mounting is always greater than the play during operation. When bearings are in service, there are also changes in clearance due to thermal expansion between the inner and outer ring.
Bearing clearance in operating condition
In general, bearings should have practically no play in service condition. This applies in particular to ball and spherical roller bearings, tapered roller bearings and cylindrical roller bearings. This is a small positive clearance of a few microns. The selection of the correct clearance depends on, among other things, the application, the shaft and housing fits, and the temperature in the bearing and the machine.
The radial clearance classes for unmounted bearings are defined in ISO standards and tables. With the most common bearings, such as deep groove ball bearings, spherical roller bearings and cylindrical roller bearings, the play is determined during production.
C2: Clearance smaller than normal
C0: Standard clearance for applications where the normally recommended fits and under normal operating conditions leave a more or less correct clearance
C3: Clearance greater than C0
C4: Clearance greater than C3
C5: clearance greater than C3
Shaft and housing fits
To achieve the optimum service life of a bearing, the selection of the right bearing is of course a prerequisite. If the shaft and housing fit are not properly matched to the bearing, even the correct bearing will fail prematurely.
The ISO tolerances for the shaft and housing together with the tolerances for the bore and outer diameter of a bearing give the fit. The ISO tolerances are defined in a form of tolerance fields relative to the zero line. The position of the tolerance field is indicated by a letter. Capital letters are used for the houses and small letters for the axes.
The choice of a fit is highly dependent on the operating conditions. The right choice is determined by the rotation ratio. A rotation ratio is about the movement of the bearing ring, which is the inner or outer ring, in relation to the direction of the load.
A rotating load exists when the ring is stationary and the load rotates or when the ring rotates and the load is stationary. A stationary load is the situation where both ring and load are stationary or both are rotating at the same speed. Another possibility is an indefinite load if the direction of the load varies due to impact or vibration.
The material of the shaft or housing is also a factor to consider. For example, aluminum expands more than steel when heated. There are also more factors that play a role. Think of a thin-walled housing, a hollow shaft or high vibrations.
The coherence of the entire ISO fitting system in relation to choices, schedules and recommendations for loose, fixed and sliding fits is further elaborated in the catalogs of our manufacturers.