NSK explains about internal clearance in rolling bearings

NSK explains about internal clearance in rolling bearings

An engineer who deals with rolling bearings is likely to be familiar with the knowledge of external dimensions to achieve a correct fit, however, understanding the space inside a bearing and how it affects performance can be just as important. Bearings are designed with a specific internal clearance that measures the total clearance between the rings and the rolling elements. Internal clearance provides:

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  free rotation of rolling elements

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  compensation for thermal expansion

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  optimum load distribution

Choosing the correct internal clearance is important because bearings hold the rotating parts of a machine in proper position across the whole designed range of conditions. The amount of internal clearance can influence noise, vibration, heat build-up and fatigue life. Vibration, interface fits and temperature can also affect internal clearance. To obtain the optimal internal clearance for a specific application, these parameters must be balanced. In some applications, the correct choice of clearance for the bearings is critical for their successful operation (Plate 3, Figures 1 and 2).

Plate 3 NSK04 – understanding internal clearance

Radial and axial internal clearance

Internal clearance can be separated into two categories: radial and axial. The total internal clearance is the amount that one ring can be displaced relative to the other ring, either radially or axially.

The radial clearance is the total clearance between the raceway and the rolling elements measured normal to the bearing axis. The clearance changes with the expansion or contraction of the bearing rings.

The axial clearance is the total amount that one ring can be displaced relative to the other in an axial direction. In ball bearings, as the radial clearance increases, the axial clearance increases as well. The more room between the balls and the rings (radial clearance), the more the elements can shift in relation to each other.

The best specification for the job

Generally, internal clearances are designated from C1 (the tightest) through to C5 (the loosest or largest). The normal clearance is CN, a range sitting between C2 and C3. With a higher clearance there is more tolerance of thermal effects. When noise and vibration must be restricted, lower clearances are necessary. The specific application and operating conditions determine the appropriate internal clearance.

Paper-drying machines that operate under hot conditions usually need C3 and C4 clearances. For more extreme temperature variations C5 can be used. The severe vibration in vibrating screens normally means that C3 and C4 clearances are required. Selection of the correct radial internal clearance group is by calculation - refer to an NSK catalogue or technical handbook for details. Factors to be assessed are:

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  Expansion of the inner ring due to interference fit on shaft.

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  Contraction of the outer ring due to interference fit in the housing.

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  Differential temperature between the inner and outer rings.

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  Differential expansions due to non-ferrous mountings.

CM clearance classes

In addition to ISO clearance, NSK offers CM clearance specifically for electric motors. This class includes a tighter noise specification. (Tighter noise specification versions are available for other radial internal clearance classes, too).

Chart 1 represents radial clearances for NSK single row ball bearings. The CM clearance falls within the range of the CN clearance. CM clearance is specified by some motor manufacturers where closer radial internal clearance helps reduce noise. C3 clearance, which is frequently used in the aftermarket, is greater than the CN and CM clearances.

Figure 1 Chart 1 – internal radial clearance designations

Figure 2 Chart 2 – bearing internal clearance comparison guide

Miniature bearing internal clearance

Bearings with a bore diameter less than 10mm are considered to be miniature. NSK uses the standard ISO clearance designations such as C2, Normal, C3, C4 and C5 for bearings with an inner ring bore diameter greater than or equal to 10mm. However, when the bore diameter is less than 10mm, NSK designates the clearance as follows: MC1, MC2, MC3, MC4, MC5 or MC6 (MC stands for miniature clearance). Based on the tolerance range of these clearance specifications, an MC3 clearance is considered to be roughly equivalent to a normal (CN) bearing clearance designated by ISO. The miniature bearing clearance specification MC5 is then considered to be approximately equivalent to an ISO C3 clearance specification. Chart 2 shows NSK MC (miniature bearing) clearance specifications for a 9mm bore bearing compared to ISO clearance specifications C2 to C4.

For more information contact: NSK UK Ltd, Mere Way, Ruddington, Notts, NG11 6JZ, UK. Tel: +44 115 936 6600; Fax: +44 115 936400.