Basic Boundary Dimensions

Basic Boundary Dimensions

The ABMA and ISO assign bearing dimensional specifications for basic boundary dimensions, sometimes referred to as the bearing’s envelope dimensions. These dimensions are established for all types and sizes of bearings and are normally listed in every bearing manufacturer’s catalog as the outside diameter, inside diameter and width.

Note: Most basic boundary dimensions are measured in millimeters (mm), or a fraction thereof. One millimeter = 1/1,000 (.001) of a meter, or 25.4 mm = one (1) inch.

The Outside Diameter (D)

The outside diameter, (O.D.), is the straight line measurement from the outside edge of the outer ring across the entire distance of the bearing to the opposite, outside edge of the outer ring. This dimension is noted as D in NTN catalogs. The outside diameter normally fits into a housing.

The Inside Diameter (d)

The inside diameter, (I.D.), sometimes referred to as the bearing’s bore, is the straight line measurement from the inside edge of the inner ring across the hole in the center of the bearing to the opposite inside edge of the inner ring. This dimension is noted as a d in NTN catalogs. The shaft normally fits into the bore of the bearing.

The Width (B)

The width is the straight line measurement from one side of the outer ring across the outside surface to the opposite side of the outer ring. This measurement may also be taken from one side of the inner ring, across its surface, to the opposite side. In most cases, the width of the inner and outer rings will be identical; however, it does not have to be the same. This NTN catalog designation is B.

The Chamfer (r)

The chamfer refers to the corner (edge) formed when the outside surface of the outer ring intersects the edge surface (or face) of the outer ring. It also refers to the corner (edge) formed, when the inside surface of the inner ring intersects the edge surface (or face) of the inner ring. This measurement is noted in NTN’s catalog with an r.