Tolerancing of all mating parts for super precision bearing installation is vitally important to a long successful life of the application. Some systems can handle more of an open tolerance than others.
In a perfect world, a resultant tolerance span fit of 10 microns or less would be great.
Some key aspects of bearing tolerancing are:
- Other Application Effects
General ID/OD Precision Bearing Tolerance Strategy
In general, it is a good idea to have a light press fit on the inner diameter (ID) of the bearing to the shaft and a slight gap on the outer diameter (OD) of the bearing to the housing (see below images).
The images are per DIN EN ISO 1101 standards and are generally accepted within the industry.
However, inner or outer race slipping may cause premature bearing failure if it’s not part of the intentional design. In many circumstances the inner race press needs to be higher than the above tables. Too much press fit can decrease a precision angular contact bearing contact angle to a detrimental level.
Precision nut force, preload force, and mounting shoulders also prevent raceway rotation.
Effects of Application RPM
The main effect of RPM on tolerancing is inner race lift-off resulting from centrifugal forces.
Race rotation relative to the shaft and/or housing can cause premature bearing failure. Because of this, higher RPM applications require a heavier shaft to inner race press fit (assuming the application is shaft rotation).
The below table is a great starting point to adjust the DIN EN ISO 1101 numbers in the above images. The first table shows a good target press fit for a given RPM in the Angular Contact S Series . These numbers could be looked at as the median of the resultant press fit or to offset the DIN EN ISO numbers in the higher press direction. The second table shows correction factors for the first table for both the Angular Contact SM Series and KH Series bearings used at high speeds.
Dynamic application temperature can have a large effect on mating part tolerances to a precision bearing. What really matters is the resultant press or gap fit the bearing sees while being used in the application.
This running fit is the result of installation fits, RPM, application temperature, and sometimes other external application effects such as loads or lubrication.
Example: An S6005 CTA A7 UL Angular contact bearing is installed at room temperature, ~21°C[70°F]. The running temperature of the application is ~31°C [88°F] at the housing. Within this scenario, it would be a reasonable theoretical assumption to use an ID running temperature of ~41°C [106°F], i.e. 10°C above the housing temperature. The bearing will expand by ~6 microns on the OD, and a 7075 AL housing will expand ~11 microns. This can take a reasonable fit for a bearing OD to a fit that will most likely see some rotation.
Potential Application Effects
Always keep in mind that there may be other application parameters that can affect dynamic application fit for the bearing(s). These can include but are not limited to:
- Application loading – both static and dynamic and both radial and axial
- Lubrication – This can be volume of oil flow
- Nature of system – A blower with high volumes of air flow
- Temperature controlled environment vs. exposed to the elements (at least temperature).