Let us now look into the most important aspect of bearing selection. This is a very important criterion which each designer should look into in detail.
10. Accuracy of Running: This is another important criterion stating the purpose for which the shaft is running. It is ultimately the bearing that controls the running accuracy of the shaft in the following terms
i. The run-out expected in the shaft during continuous running
ii. The axial play allowable in the shaft
Applications like the spindle in a CNC machine need to rotate very accurately with least run out, axial play, etc., in order to produce a component with least errors and consistency. These kinds of applications require bearings with good accuracy in order handle all the above requirements.
The main terminology that caters to this is “Radial Internal Clearance". The accuracy of running is fulfilled by the availability of bearing with various levels of “Radial Internal Clearance".
First of all let us know what “Radial Internal Clearance" means. As you know the bearing is made up of three main constituents of inner race, outer race and the rolling element. By manufacturing and construction, a clearance is required between these three, so that the rolling element rolls with minimum friction between the inner race and outer race. Moreover this clearance is reduced when the bearing is fitted on a shaft by its inner race and on the housing by its outer race, due to the expansion of the inner race and compression of the outer race.
The bearings are available in 4 categories of clearance. The values of the clearances are specified separately for each type of bearing. They are
· Normal Clearance or CN – This is the normal level of clearance in any rolling element bearing.
· C2 – The least clearance available in a rolling element bearing. This clearance type is basically most suited for applications having frequent alternating loads with oscillation motion.
· C3 – The internal clearance is higher than CN
· C4 – The internal clearance is higher than C3
· C5 – The internal clearance is higher than C4
C3, C4 and C5 clearance bearings are used in cases where the bearings are fitted on to the shaft or the housing with interference fit. These are also used in cases where the bearings tend to work in elevated temperatures like above 80 Deg C. The effect of the heat tends to expand the elements of the bearings causing a reduction in the internal clearance. A reduced radial internal clearance or the non presence of radial internal clearance can lead to seizure of the bearing, leading to failure.
Click here to see the Radial Internal Clearance diagrammatically.