Shaft and Housing Fits

Determining proper shaft and housing fits is critical to ensure long bearing life. The fit, or amount of interference that exists between mating components (such as the shaft and bearing bore), can be devised into three categories: loose (slip), transition, and press (tight). For more information on recommended shaft and housing fits, please download this catalog excerpt.

A loose fit allows for easy installation, but too loose of a fit may allow the bearing ring to slip or creep on the shaft or in the housing. Creep can cause excessive wear or vibration and in the worst cases lead to ring fracture or bearing seizure.

A slight press fit will generally help prevent creep, but an excessive press fit will eliminate the bearing internal clearance and cause a rise in operating temperature that can lead to early failure. Press fits will naturally make installation of the bearing more difficult and susceptible to damage, but with proper care, most problems can be avoided.

The type of fit required varies on a number of factors such as load, operating temperature, bearing type, and shaft or housing material. A good rule of thumb for most applications is to use a press fit on the rotating component.

That is, a shaft or inner ring rotating application like an electric motor will use a press fit on the shaft and a loose fit on the housing. A housing or outer ring rotating application like a wheel hub will employ the reverse: a loose fit on the shaft and a press fit on the housing. This will greatly reduce the chances of creep or a loosened fit during operation.

You will want to consider the following questions before consulting the fit tables:

• What is the bearing size and type? Larger bearings can take greater interference fits than their smaller counterparts, and a ball bearing will require a different fit than a tapered roller bearing.
• Is the application inner or outer ring rotating? Generally a press fit on the rotating component is the best type of fit.
• What is the load direction and condition? A heavier load will require a tighter fit.
• What material is the shaft and housing? Aluminum housing will expand with temperature and usually require a tighter fit than steel or cast iron.

For more information on shaft and housing fit selection, consult your local NTN Technical Support Team.