Tips for the right bearing clearance class

You’ve spent a lot of time choosing the right tolerances (shaft, housing and bearing tolerances) for the installation of a roller bearing, so everything will go well during operation, right?

Of course not. That’s because the running properties are dependent to a large extent on the operating clearance and not on the choice of fit. Many other factors also affect the operating clearance: the bearing clearance class (CN, C2, C3...), the material of the shaft and housing as well as the heat development in the roller bearing during operation.

But why?

The German industry standard DIN 620-2 defines the possible bearing clearance classes for roller bearings. This is the radial clearance of the inner ring in an uninstalled state when the outer ring is being retained. When a roller bearing is installed on a shaft with a tight fit or a transition fit, the inner ring of the bearing is widened and the shaft is compressed. This results in a reduction in the bearing clearance in the installed state. During operation, the shaft usually heats up more than the roller bearing because the heat accumulates in the shaft and first has to be dissipated via the roller bearing and housing. At the same time, the housing cools down more due to its larger surface. This leads to a further reduction in the bearing clearance.

The material combinations of a shaft made of aluminum, a housing made of steel or vice versa also lead to a reduction in the operating clearance due to the different thermal expansion coefficients. Many aluminum alloys in particular have larger thermal expansion coefficients such that the shaft expands more than the roller bearing during heat development. Design engineers have to take this into account. The aluminum housing is an even more critical aspect because the housing may expand so much faster that the bearing no longer fits correctly and starts to slip. Special roller bearings with rubber rings in the outer ring reliably prevent this.

There may also be a considerable increase in the overall bearing friction depending on the choice of lubricant and seal. Operation under a heavy load or at high speeds may cause significant heat development that may result in a loss in the desired operating clearance.

Characteristics of critical operating conditions

A reduction in the residual bearing clearance manifests itself as an increase in noise generation and operating temperatures. The high operating temperatures in turn lead to a further loss in the operating clearance. This leads to a vicious circle that results in a premature bearing failure or, at worst, to a blockage of the roller bearing.

It’s the same at sub-zero temperatures, just in the opposite direction: the components contract. So what can you do as a user in order to quickly find the right dimensions.

1. Send us the technical questionnaire for the attention of our application engineering department.

2. Refresh your knowledge with a training course in our roller bearing academy.

3. Is there a concrete damage pattern for an existing application and you think it might be a design error or product defect? Then request a damage analysis.