The Importance of Shaft & Housing Calculations

Shaft and housing calculations are important. They determine how much of an impact thermal effects and centrifugal force will have on the fit of the bearing. 

In this article, we’ll discuss how thermal effects and centrifugal forces affect the bearing’s fit and what steps you can take to prevent the fit from changing during operation.

How Should a Bearing Fit on the Shaft & Housing Units?

In most situations, we recommend the bearing to fit like this on a rotating shaft:

• Press-fit between the shaft and the bearing’s inner ring (ID)
• Slip-fit between the housing and the bearing’s outer ring (OD)

A bearing press-fit on the shaft will prevent the inner ring from sliding and causing fretting damage to the bearing. A slip-fit on the housing leaves space for thermal growth to occur, which happens when temperatures rise during operation.  

Material & Thermal Effects  

If the rate of thermal expansion is different enough between the shaft, housing, and bearing, the press fits will either increase or decrease, depending on which material has a higher thermal expansion coefficient. 

If the press-fit becomes too loose the bearings can rotate on the shaft and housing unit and heat up. This will then result in a premature bearing failure.

If the bearing fits too tightly, the contact angle on the bearing will decline and ball pinching could occur. 

We recommend using a shaft and housing material with a thermal expansion coefficient as close as possible to the bearing. This will reduce the chance that a bearing’s fit will change on the shaft and housing unit during operation.

Try our thermal range calculator which analyzes the thermal expansion that could happen between your bearing, shaft, and housing unit. Enter a few key details and you’ll see the resultant fit of your bearing on the shaft and housing unit.

Centrifugal Force & Bearing Fit 

Rotating shafts produce a centrifugal force on the inner ring of the bearing which causes it to expand, reducing the press fit. Sometimes the result can be so drastic that the inner ring can lose the press-fit and actually lift off the shaft. 

Bearing inner ring and outer ring contact pressure measurements will help to determine how much pressure is being put on the shaft and housing unit.  

If the result is a positive number, then there is enough contact for operation. However, a result of 0 means a slipping situation could result during operation. If you need an engineer to perform a contact pressure measurement analysis then contact us for free bearing performance analysis.

Summary of Bearing Fits on a Shaft & Housing 

As you can see, there are lots of factors that can change the ideal fit of a bearing on a shaft and housing unit. Be aware of these when engineering a new application or redesigning an existing application so you can avoid a bearing failure from happening.  

Our no-cost bearing analysis will calculate the thermal effects and centrifugal forces that will result from your application. If you would like more information about a bearing performance analysis, our onsite engineers would love to help! Contact us via our online contact form or give us a call at 800.323.5725.