To determine the shaft and housing fits required for a particular application, one must consider such variables as load, ambient temperature, the type of bearing and the type of shaft and housing. The fit, or amount of interference that exists between mating components, can be devised into three resultant categories: press fit, transition fit and loose fit.

An improper fit selection may allow the bearing to creep on the shaft or in the housing. Creep is defined as the inner ring slipping on the shaft or the outer ring slipping in the housing. This occurs when the interference fit is not sufficient for the operating conditions. As a result, creep can cause wear, ring fracture and excessive vibration.

On the other hand, an excessive interference fit will eliminate the bearing internal clearance and cause a rise in operating temperature. A shaft interference fit expands the inner ring; similarly, a housing interference fit compresses the outer ring. It will also make the installation of the bearing more difficult and more susceptible to damage during installation.

Typically, an inner ring rotating application will use a tight or interference fit on the shaft, and the housing will use a loose or clearance fit. Likewise, with an outer ring rotating application, the opposite is true.

Bearing fits are selected by reviewing the application details, general guidelines and with experience. When reviewing the application, the following questions should be answered:

	What is the bearing size and type?
	Is the application inner or outer ring rotating?
	What is the load direction and condition?
	What kind of shaft and housing are being used?
i.e.	-Is the shaft solid or hollow?
	-Are the shaft and housing of steel or aluminum construction?

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

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