By Jim Bryan
EASA Technical Support Specialist (retired)

Much has been said and done to produce the "perfect" fit for rolling element bearings in motors and other rotating equipment. Assembly of these machines requires that either the inner fit to the shaft (journal) or the outer fit to the housing (bore) is able to slide; so if one fit is tight, the other must be loose. While "tight" and "loose" are relative terms that must be defined in the quest for the perfect fit, any fit that's too loose or too tight can lead to early bearing failure and costly downtime.

A tight (interference) fit is usually recommended for motor bearing journals. Standard fits for radial ball bearing journals range from j5 to m5; the standard housing fit is H6. These are the "standard" fits and may be different depending on the machine designer's understanding of the application.

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