Boundary dimensions

Boundary dimensions are the main dimensions of a bearing (fig. 1 and fig. 2). They comprise:
  • the bore diameter (d)
  • the outside diameter (D)
  • the width or height (B, C, T or H)
  • the chamfer dimensions (r)

The boundary dimensions for standard metric bearings are contained in the general plans as specified in ISO (International Organization for Standardization) standards1):

  • ISO 15:2011 for radial rolling bearings, except Y-bearings, some types of needle roller bearings and tapered roller bearings
  • ISO 104:2004 for thrust bearings
  • ISO 355:2007 for tapered roller bearings

ISO general plans

The ISO general plans for boundary dimensions of radial bearings contain a progressive series of standardized outside diameters for every standard bore diameter, arranged in diameter series 7, 8, 9, 0, 1, 2, 3 and 4 (in order of increasing outside diameter). Within each diameter series, different width series have also been established (width series 8, 0, 1, 2, 3, 4, 5 and 6 in order of increasing width).
The height series for thrust bearings (height series 7, 9, 1 and 2 in order of increasing height) corresponds to the width series for radial bearings.
Dimension series are formed by combining the number for the width or height series with the number for the diameter series (fig. 3).
In the ISO general plan for single row metric tapered roller bearings (ISO 355:2007), the boundary dimensions are grouped for certain ranges of the contact angle a, known as the angle series (angle series 2, 3, 4, 5, 6 and 7 in order of increasing angle). Based on the relationship between the bore and outside diameter, and between the total bearing width and the cross-sectional height, diameter and width series have also been established. Here, a dimension series is obtained by combining an angle series with a diameter and a width series (fig. 4). The dimension series consists of a number for the angle series and two letters. The first letter identifies the diameter series; the second identifies the width series.
With very few exceptions, the bearings described on this website comply with the ISO general plans. Experience has shown that the requirements of the vast majority of bearing  applications can be met using bearings with these standardized dimensions. Following ISO standards for the boundary dimensions is a prerequisite for interchangeability of bearings. Specific information about compliance to dimension standards is provided in each product section.

General plans for inch bearings

A large group of bearings with inch dimensions are inch tapered roller bearings. The dimensions of these bearings are in accordance with AFBMA Standard 19 (ANSI B3.19). ANSI/ ABMA Standard 19.2 has replaced this standard, but does not include dimensions.
In addition to the inch tapered roller bearings, some inch ball bearings and cylindrical roller bearings are also available.
  1. 1)For details on how to obtain relevant ISO documentation, refer to ISO - International Organisation for Standardization.
SKF logo