Jim Bryan
EASA Technical Support Specialist

Much has been said and much work performed to produce the “perfect” bearing fit. For any single bearing, there is an inner fit to the shaft and an outer fit to the housing. It is required that one of the two fits be able to slide in order to assemble the machine. If the bearing-to-shaft fit (journal) is tight, then the bearing-to-housing (bore) must be loose. Of course tight and loose are relative terms and the quest for the perfect fit must define these terms.

A tight fit, also known as an inter­ference fit, is usually recommended for a motor bearing journal. The range for radial ball bearing journal fits is from j5 to m5, and the housing fit is H6 (see Table 1). These are the “standard” fits and may be different depending on the machine design­ers understanding of the application. Table 1 is derived from Table 2-13 of ANSI/EASA AR100 Recommended Prac­tice for the Repair of Rotating Electrical Apparatus. It shows the relationship of bearing size to fit tol­erances. Generally, as the bearing gets larger, the tolerance widens. The key to this chart is that the journal fit is always interference and the bore fit is always line-to-line to loose. See AR100 for additional radial ball and roller bearing sizes.

Categories: Bearings

Tags: Bearings

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