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Bearing clearance

Selecting a clearance class

The clearance values listed in the relevant product chapters are valid for unmounted bearings. To select the best clearance value for an application, the required operating clearance in the bearing (in operation) must be determined first.
Because there are many factors that can influence operating clearance in a bearing, these calculations are best done with the aid of sophisticated computer programs. As a result, SKF recommends using one of the computer programs available through the SKF application engineering service. These programs consider tolerances, fits and component temperatures, to calculate the required initial internal clearance.
The required initial internal clearance of an unmounted bearing can be estimated using

r = rop + Δrfit + Δrtemp

where
r=required internal clearance for the unmounted bearing [mm]
rop=desired operating clearance [mm]
Δrfit 
=clearance reduction caused by the fit [mm]
Δrtemp  
=clearance reduction caused by temperature difference [mm]

Clearance reduction caused by an interference fit

The reduction equals the effective interference fit multiplied by a reduction factor using

Δrfit = Δ1 f1 + Δ2 f2

where

Δrfit =clearance reduction caused by the fit [mm]
f1 =reduction factor for the inner ring
f2=reduction factor for the outer ring
Δ1 
=effective interference between the inner ring and shaft [mm]
Δ2 
=effective interference between the outer ring and housing [mm]
The reduction factors can be obtained from diagram 1 as a function of the ratio of the bearing bore diameter d to the outside diameter D. It is valid for a solid steel shaft and a cast iron or steel housing. For the effective interference fit, the mean value of the smallest and largest values of the probable interference listed in table 1, table 2, table 3, table 4, table 5, table 6, table 7, table 8, table 9, table 10 and table 11 can be used.

Clearance reduction caused by a temperature difference between the bearing rings

When the inner ring temperature is higher than the outer ring temperature, the internal clearance within the bearing is reduced. The internal clearance reduction can be estimated using

Δrtempα dm ΔT

where

Δrtemp =clearance reduction caused by temperature difference [mm]
dm =bearing mean diameter [mm]
= 0,5 (d + D)
α=thermal coefficient of expansion [°C–1]
= 12 x 10–6 for steel
ΔT  
=temperature difference between the shaft and housing [°C]
The temperature difference between components during start-up can be much higher than under steady state conditions (diagram 2) and unwanted preload may result. It is important to avoid unwanted preload during startup, because even short periods of preload can have a negative impact on bearing service life. One way to avoid excessive heat and the resulting preload is to start the application at a low speed and increase the speed incrementally.
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