Where do Distance between track di Ball diameter de Diametric clearance

Fig. 6.5 Internal bearing diametric clearance taper roller bearings do have clearance slackness or tightness under operating conditions but this cannot be measured until the whole bearing assembly has been installed in its housing.

A radial ball bearing working at operating temperature should have little or no diametric clearance, whereas roller radial bearings generally operate more efficiently with a small diametric clearance.

Radial ball and roller bearings have a much larger initial diametric clearance before being fitted than their actual operating clearances.

The difference in the initial and working diametric clearances of a bearing, that is, before and after being fitted, is due to a number of reasons:

1 The compressive interference fit of the outer raceway member when fitted in its housing slightly reduces diameter.

2 The expansion of the inner raceway member when forced over its shift minutely increases its diameter.

The magnitude of the initial contraction or expansion of the outer and inner raceway members will depend upon the following:

a) The rigidity of the housing or shaft; is it a low strength aluminium housing, moderate strength cast iron housing or a high strength steel housing? Is it a solid or hollow shaft; are the inner and outer ring member sections thin, medium or thick? b) The type of housing or shaft fit; is it a light, medium or heavy interference fit?

The diametric clearance reduction when an inner ring is forced over a solid shaft will be a proportion of the measured ring to shaft interference.

The reductions in diametric clearance for a heavy and a thin sectioned inner raceway ring are roughly 50% and 80% respectively. Diametric clearance reductions for hollow shafts will of course be less.

Working bearing clearances are affected by the difference in temperature between the outer and inner raceway rings which arise during operation. Because the inner ring attached to its shaft is not cooled so effectively as the outer ring which is supported in a housing, the inner member expands more than the outer one so that there is a tendency for the diametric clearance to be reduced due to the differential expansion of the two rings.

Another reason for having an initial diametric clearance is it helps to accommodate any inaccuracies in the machining and grinding of the bearing components.

The diametric clearance affects the axial clearance of ball bearings and in so doing influences their capacity for carrying axial loads. The greater the diametric clearance, the greater the angle of ball contact and therefore the greater the capacity for supporting axial thrust (Fig. 6.6).

Bearing internal clearances have been so derived that under operating conditions the existing clearances provide the optimum radial and axial load carrying capacity, speed range, quietness of running and life expectancy. As mentioned previously, the diametric clearance is greatly influenced by the type of fit between the outer ring and its housing and the inner ring and its shaft, be they a slip, push, light press or heavy press interference.

The tightness of the bearing fit will be determined by the extremes of working conditions to which the bearing is subjected. For example, a light duty application will permit the bearing to be held with a relatively loose fit, whereas for heavy conditions an interference fit becomes essential.

To compensate for the various external fits and applications, bearings are manufactured with different diametric clearances which have been standardized by BSI and ISO. Journal bearings are made with a range of diametrical clearances, these clearances being designated by a series of codes shown below in Table 6.1.

Applied radial load

Applied radial load

Diametrical Clearance

Radial reaction load

Radial reaction load

Fig. 6.6 Effects of diametric clearance and axial load on angle of contact

Table 6.1 Journal bearing diametrical clearances

BSI ISO

SKF

Hoffmann

Designation Group

Designation

Designation

— Group 2

C2

O

DC2 Normal group

Normal

OO

DC3 Group 3

CS

OOO

— Group 4

C4

OOOO

Group 2 These bearings have the least diametric clearance. Bearings of this group are suitable when freedom from shake is essential in the assembled bearing. The fitting interference tolerance prevents the initial diametric clearance being eliminated. Very careful attention must be given to the bearing housing and shaft dimensions to prevent the expansion of the inner ring or the contraction of the outer ring causing bearing tightness.

Note The lower the number the smaller is the bearing's diametric clearance. In the new edition of BS 292 these designations are replaced by the ISO groups. For special purposes, bearings with a smaller diametric clearance such as Group 1 and larger Group 5 are available.

The diametrical clearances 0, 00, 000 and 0000 are usually known as one dot, two dot, three dot or four dot fits. These clearances are identified by the appropriate code or number of polished circles on the stamped side of the outer ring.

The applications of the various diametric clearance groups are compared as follows:

Normal group Bearings in this group are suitable when only one raceway ring has made an interference fit and there is no appreciable loss of clearance due to temperature differences. These diametric clearances are normally adopted with radial ball bearings for general engineering applications.

Group 3 Bearings in this group are suitable when both outer and inner raceway rings have made an interference fit or when only one ring has an interference fit but there is likely to be some loss of clearance due to temperature differences. Roller bearings and ball bearings which are subjected to axial thrust tend to use this diametric clearance grade.

Group 4 Bearings in this group are suitable when both outer and inner bearing rings are an interference fit and there is some loss of diametric clearance due to temperature differences.

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