Tolerancing of all mating parts for super precision bearing installation is vitally important to a long successful life of the application. Having said this, some systems can handle more of an open tolerance span than others. In a perfect world, a resultant tolerance span fit of 10 microns or less would be great! However, it is very much a reality that this cannot happen for many applications, and actually does not need to happen for many applications. Know that much of the below is geared towards precision angular contact bearings, but it very much applies on a general basis to precision radial ball bearings as well. The below tables outline a good, standard tolerancing strategy. Some key aspects of bearing tolerancing are:
- General Strategy
- 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 ID of the bearing to the shaft, and a slight gap on the OD of the bearing to the housing. The tables below mostly outline this. The tables are created 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 is not part of the intentional design. In many circumstances the inner race press needs to be higher than the below tables. BUT know that too much press fit can decrease a precision angular contact bearing contact angle to a detrimental level. Precision nut force, pre-load force, and mounting shoulders also prevent raceway rotation. Please consult GMN USA Engineering for tolerance strategy support.
Effects of Application RPM
Application RPM will tie in with the overall tolerance strategy within a specific application. The main effect of RPM on tolerancing is inner race lift off resulting from centrifugal forces. As stated above, 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 tables are a great starting point to adjust the DIN EN ISO 1101 numbers in the above tables. The first table shows a good suggested target press fit for a given RPM for the S series GMN angular contact bearings. 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 SM and KH series bearings utilized at high speeds.
Effects of Application Temperature
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 utilized 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.
As an example, a S6005 CTA A7 UL GMN 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. There are many applications where this level of engineering is required. Please consult GMN USA Engineering for tolerancing support.
Potential Application Effects
Always keep in mind that there may be other application parameters that can effect 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).
If you feel that there are extenuating or special circumstances within your application, do not hesitate to call GMN USA Engineering for application specific tolerancing advice.