# Preload in mounted bearing sets

After mounting, sets of universally matchable bearings and matched bearing sets can have a heavier preload than the pre-set preload, predetermined during manufacture. The increase in preload depends mainly on the actual tolerances for the shaft and housing seats and whether these result in an interference fit with the bearing rings.

An increase in preload can also be caused by deviations from the geometrical form of associated components, such as cylindricity, perpendicularity or concentricity of the bearing seats.

During operation, an additional increase in preload can also be caused by:

• the centrifugal force caused by the rotational speed of the shaft, for constant position arrangements
• a temperature difference between the inner ring, outer ring, and balls
• different coefficient of thermal expansion for the shaft and housing materials compared to bearing steel

If the bearings are mounted with zero interference on a steel shaft and in a thick-walled steel or cast iron housing, preload can be determined with sufficient accuracy from

where

 Gm = preload in the mounted bearing set [N] GA,B,C,D = pre-set preload in the bearing set, prior to mounting [N] (→ product table) f = bearing factor dependent on the bearing series and size (→ product table) f1 = correction factor dependent on the contact angle (→ product table) f2 = correction factor dependent on the preload class (→ product table) fHC = correction factor for hybrid bearings (→ product table)

Considerably tighter fits may be necessary, for example, for very high speed spindles, where centrifugal forces can loosen the inner ring fit on its shaft seat. These bearing arrangements must be carefully evaluated. In these cases, contact the SKF application engineering service.
For preload in a mounted bearing set (Gm) of S70 .. W series bearings, contact the SKF applications engineering service.
Calculation example
What is the preload in a matched bearing set 71924 CD/P4ADBC after mounting?

The pre-set preload for the set of two bearings in the 719 CD series, prior to mounting, preload class C, size 24 is GC = 1 160 N (→ product table).

With the bearing factor f = 1,26 and correction factors f1 = 1 and f2 = 1,09 (→ product table), the preload of the mounted bearing set is

= 1,26 x 1 x 1,09 x 1 160 ≈ 1 590 N