4 Tolerances and fits Recommended fits
Fits for aluminium housings The coefficient of expansion for aluminium is more than two times greater than for cast iron or steel. Therefore, for motors with alu minium housings in particular, steps should be taken to support the outer ring and pre vent it from rotating in its seat. To do this, start by selecting a tighter tolerance for the housing, e.g. use a J7 instead of an H7. A more effective way to prevent the outer ring from rotating is to cut an O-ring groove into the bearing seat and install a rubber O-ring. When designed correctly, the O-ring will apply enough pressure so that the bearing outer ring will be held in place and unable to spin in the housing bore († fig. 4, chapter 2 on page 42).
Influence of temperature difference when selecting the housing fit Electric motors and generators generate heat in the rotor and stator coils and are often equipped with an internal fan for cooling pur poses. The forced airflow dissipates unwanted heat from the housing (motor-end shield) and can create a temperature difference between the housing and the bearing outer ring. This can cause a problem with the non-locating bearing if it needs to move axially in its seat to accommodate thermal elongation of the shaft. To correct the problem, switch to a looser housing fit, i.e. from H7 to G6, or place the axially free bearing in a position where there is hot air flow against the end shield.
Table 4 Geometrical tolerances for bearing seats on shafts and in housings A t3 A-B
A
B
t4 A-B
B
dA
dB
DB
DA
t4 A-B t3 A-B Surface Permissible deviations Characteristic Symbol for Bearings of tolerance class1) geometrical tolerance Normal P6 characteristic zone
P5
Cylindrical seat Total radial runout
t3
IT5/2
IT4/2
IT3/2 IT2/2
t4
IT5
IT4
IT3 IT2
Flat abutment Total axial runout Explanation
For normal demands
1)
70
For special demands with respect to running accuracy or even support
For bearings with a tolerance class higher than Normal (tolerance class P4 etc.), refer to Super-precision bearings († skf.com/super-precision).