Dynamic shaft and housing calculations measure thermal effects and centrifugal force to determine how much they will impact the fit of the bearing on the shaft and housing unit.
How a Bearing Should Fit on a Shaft and Housing
In most cases of rotating shafts we recommend a press-fit between the shaft and the bearing’s inner ring (ID) and a slip-fit between the housing and the bearing’s outer ring (OD).
The press fit on the shaft will prevent the inner ring of the bearing from sliding and causing fretting damage to the bearing. The housing slip fit is recommended so you can allow the bearing some space for thermal growth.
If the shaft and housing fits become too tight the contact angle on the bearing will decline and ball pinching could occur.
The two main reasons that a bearing’s fit changes are usually because of:
- Thermal effects
- Centrifugal forces
How Thermal Effects Change a Bearing’s Fit
If the rate of thermal expansion is different enough between the shaft, housing, and bearings, the press fits will either increase or decrease, depending on which material has a higher thermal expansion coefficient. It’s recommended to use a shaft and housing that has a thermal expansion coefficient as close as possible to the bearing. This will help minimize the changes in the resultant fits from the application’s running conditions.
Our web-based thermal range calculator analyzes thermal expansion that results during operation. You’ll immediately see the resultant fits of the bearing for the shaft and housing unit.
All you need to enter is:
- Installation Temperature
- Shaft Run Temperature
- Housing Run Temperature
- Bearing Part Number
- Bearing Ball Material (Select either 52100 Bearing Steel or Silicon Nitride)
- Shaft Material *
- Housing Material*
- Bearing CTE (Thermal Expansion Coefficient)**
- Shaft CTE (Thermal Expansion Coefficient)**
- Housing CTE (Thermal Expansion Coefficient)**
- Bearing ID Min and Max (mm or in)* Bearing OD Min and Max (mm or in)*
- Initial Shaft Min and Max (mm or in)
- Initial Housing Min and Max (mm or in)
*Standard selection options are provided in the dropdown
** Tool will auto-populate these values when standard materials are selected for ball, shaft and housing material.
Rotating shafts produce 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.
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 thermal effects and centrifugal forces that 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.