A rolling contact bearing is usually designated by three or four digits. The meaning of these digits is as follows:

(i) The last two digits indicate the bore diameter of the bearing in mm (bore diameter divided by 5). For example, XX07 indicates a bearing of 35 mm bore diameter.

(ii) The third digit from the right indicates the series of the bearing. The numbers used to indicate the series are as follows:

Extra light series –1, Light series – 2, Medium series – 3, Heavy series – 4 For example, X307 indicates a medium series bearing with a bore diameter of 35 mm.

(iii) The fourth digit and sometimes fifth digit from the right specifies the type of rolling-contact bearing. For example, the digit 6indicates deep groove ball bearings.

Light series bearings permit smallest bearing width and outer diameter for given bore diameter. They have the lowest load carrying capacities. Medium series bearings have 30 to 40 percent higher dynamic load carrying capacities compared with light series bearings of the same bore diameter. However, they occupy more radial and axial space. Heavy series bearings have 20 to 30 percent higher dynamic load carrying capacities compared with medium series bearings of the same bore diameter.

The ISO plan for the dimension series of the bearing is illustrated in fig (a). It consists of two-digit numbers. The first number indicates the width series 8, 0, 1, 2, 3, 4, 5, and 6 in order of increasing width. The second number indicates the diameter series 7, 8, 9, 0, 1, 2, 3, and 4 in order of ascending outer diameter of the bearing.

Figure (b) shows the proportionate dimensions of SKF bearings belonging to different series with a 50 mm bore diameter (Bearing No. 6010, 6210, 6310 and 6410).