Measuring Radial Internal Clearance

Measuring Radial Clearance

Clearance Standards

The ABMA and ISO have established standards for five (5) defined classifications of radial internal clearance. The nomenclature for these classifications (in ascending order of size from the smallest clearance to the largest) are C2, normal (no designation), C3, C4, and C5. Typically, bearings stocked by distributors are C3 clearance. Clearance must be specified when ordering to obtain the proper bearing for the application. The illustration at the top of this page demonstrates how internal clearance is measured. The example shows a radial ball bearing, so the radial internal clearance is measured. The bearing is grasped at one point on the inner ring and at another point on the outer ring, directly opposite (see large arrows). The bearing is held together to assure radial contact between the inner raceway, balls, and outer raceway. This allows measurement of the bearing’s internal clearance at a point on the opposite side of the bearing--180°-- from where the points of contact are being made. The small gap between the top ball and the raceway represents the bearing’s radial internal clearance. The C2 through C5 classifications are further defined within strict mini- mum and maximum ranges, according to the bore size of the bearing. An actual example of this range as it applies to each internal clearance classification for a SRDG ball bearing with a bore size of over 10mm through 18mm is demonstrated in the table below.

Bore Diameter	Over (mm)	Incl (mm)
	10	18
Clearance	Min (µ)	Max (µ)
C2	0	9
Normal	3	18
C3	11	25
C4	18	33
C5	25	45

Note: All NTN internal clearance specifications to ISO standards are stated in microns, an abbreviation for "micrometers." (One micron is one- millionth of a meter, or, 25,400 microns equal one (1) inch.)

Normally, a bearing is pressed onto a shaft with a tight fit (press fit). This slightly stretches the inner ring and effectively removes some of the unmounted internal radial clearance. The resulting remaining radial internal clearance is the mounted internal clearance. The mounted internal clearance may also be reduced in the bearing as it is pressed into a housing. As a general rule, you will not see an application where both the shaft and housing will call for a pressed fit. This could squeeze the bearing and cause a very quick failure. Under normal operating conditions heat is usually generated, causing the shaft to expand and eliminating additional internal clearance.