For correct operation of the bearing it is essential that, after fitting, a clearance exists between the shaft and the bore of the bearing, to provide for lubrication and dimensional movement due to changes in temperature and moisture content. When a bearing is press fitted into the housing, there will be a reduction in bore size approximately equal to the interference between shell and housing.
Countrose bearing sizes refer to the nominal size of the shaft. The actual internal size of the bearing includes a minimum clearance, calculated by Countrose, based on shaft and housing sizes machined to ISO286-2 tolerances. These sizes must be accurate if the minimum clearance is to be maintained after fitting.
The clearances designed into our standard range of bearings are based upon the shaft being manufactured to ISO g6 tolerances. Plain bearings are supplied machined on the outside diameter generally to ISO p6 tolerances, to suit a housing which is within ISO H7 tolerances. This assumes that the bearing will be press fitted, as the reduction in bore diameter will be small enough to be accommodated within the designed clearances. Incorrect housing diameter will lead either to a loose bearing or the insertion force being too great, resulting in damage to the bearing and excessive bore closure.
Where deviations from the above shaft tolerances are necessary, the displacement should be towards a smaller shaft so that the minimum clearance is not reduced.
It is important to ensure that the housings, bearings and shaft are all correctly aligned along the same axis. Any misalignment between the bearing bore and the shaft will effectively reduce the running clearance available to the bearing and may cause problems in operation, such as vibration or excessive wear.
Generally bearings are pressed into their housings with an interference fit. Alternatively, a light push fit can be used and the bearing retained by a radial grub screw, partially penetrating the shell. In this type of assembly care must be taken that the screw does not distort the shell or interfere with the lining.
Flanged bearings are similarly assembled with a light push fit to control concentricity, and retained by bolts or screws in the flange.
If it becomes necessary to establish the clearance prior to installation the bearing can be placed freely on the shaft with a dial test indicator at its TDC (Top Dead Centre) position. Lifting the bearing to its highest position relative to the shaft will indicate the clearance. To measure the clearance after interference fitting, a dial indicator may be used in a similar way, to measure free movement of the shaft.