Technical Definitions

CLEARANCE TOLERANCE FIT - Technical Definitions

Although these three terms are very important in the definition and construction of a bearing, they tend to be confusing and used improperly. It is our intention with the following brief explanation, to help you understand each of these technical words and their application to bearings.

CLEARANCE
A space between two moving parts, left to avoid clashing or to permit relatively free motion.

The bearing clearance is the distance by which one bearing ring can be freely displaced in relation to the other one. With axial clearance the bearing is displaced along its axis, with radial clearance vertically to the bearing axis.

Factors that affect radial clearance:

Temperature: input or dissipation through shaft or housing

Tight fits: expansion of inner ring /constriction of outer ring

The clearance is standardized for most bearing types and sizes and is classified in groups designated C1, C2, - (or CN), C3, C4 C1 being the smallest

TOLERANCE
the permissible range of variation in a "dimension" of an object.

Tolerances are laid down for dimensional accuracy and running accuracy.

Dimensional accuracy: which indicates the tolerance limits of boundary dimensions as well as width variations

Running accuracy: that indicates the tolerance limits of outside cylindrical surface runout with side, radial runout, side runout with bore and axial runout.

The tolerances are classified into four classes:

ISO/ABEC	ISO/ABEC	ISO/ABEC	ISO/ABEC
0 = 1	6 = 3	5 = 5	4 = 7

In ISO, the lower the number the greater the precision.

ABEC stands for Annular Bearing Engineers Committee designated by the AFBMA or Anti-Friction Bearing Manufacturing Association.

FIT
The fit of a rolling bearing determines how tightly or loosely the bearing sits on the shaft and in the housing.

The following fits are considered normal:

	Shaft	Housing
Ball Bearings	j5 to k5	H7 to J7
Roller and needle Bearings	k5 to m5	H7 to M7

Example:
6201 d= 12 mm D= 32 mm B= 10 mm

1) Radial Clearance: Normal CN: 3 to 18 microns (a micron = 1/1000 of a millimeter) C3: 11 to 25 microns

2) Tolerance: in microns inner ring outer ring ISO 0=ABEC1 ISO 6=ABEC3 ABEC1 ABEC3

	INNER	RING
	ISO 0=ABEC1	ISO 6=ABEC3
Bore diam. Dev.	0 to -8	0 to -7
Width deviation	0 to -120	0 to -120
	OUTER	RING
	ABEC1	ABEC3
Bore diam. Dev.	0 to -9	0 to -9
Width deviation	0 to -120	0 to -120

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201 Brighton Avenue, Boston, MA 02134
800-225-4587 800-252-1996 (Fax)