Machined parts and components all have tolerance standards when manufactured. This standard produces a consistent, uniform product.
Precision bearings have a tolerance on the inner diameter (ID) and the outer diameter (OD), and machined shaft and housings also have tolerance standards on their parts. These tolerances adjust with the temperature.
During operation, temperatures rise causing the bearings, shaft and housing to expand. Being prepared for these changes, when machining a shaft and housing unit, will ensure that you are choosing the correct bearing and fit for your application.
Typical shaft and housing tolerance ranges are around 10µm but establishing the correct tolerances for the shaft and housing is important for achieving optimal performance in your application.
Bearing Tolerance Factors
Some key aspects of bearing tolerancing are:
- Other Application Effects
Basic Bearing ID & OD Tolerance Strategy
Finding the right shaft OD to match the ID of the bearing and the correct housing ID to match the OD of the bearing (at room temperature), produces the resultant fit.
In rotating shaft applications, we recommend achieving a press fit between the shaft and the ID of the bearing and a slip fit between the housing and the OD of the bearing.
Please note: the images use ISO 1101 standards.
Avoid These Bearing Tolerance Problems
When thinking about your bearing tolerance strategy, you want to avoid two common issues, raceway slippage and inner race lift off.
Watch Out for Raceway Slippage
On applications where the shaft is rotating, using a press fit will prevent inner ring rotation, relative to the shaft, that could result in micro-fretting and premature bearing failure.
In many circumstances the inner race press fit needs to be higher than the above tables. Be careful, because too much press fit can decrease the contact angle on an angular contact bearing.
Keep in mind that precision nut force, preload force, and mounting shoulders can also prevent raceway rotation.
Inner Raceway Lift-Off
For high-speed applications, centrifugal force could cause the inner ring to expand and lift off the shaft. This may result in drastic machine malfunctions and a decrease in the life of the bearing. Increasing the press fit would help counteract this scenario.
Race rotation relative to the shaft and/or housing can cause premature bearing failure. Therefore, higher RPM applications require a heavier shaft to inner race press fit (assuming the application is shaft rotation).
The tables below offer a great start in adjusting the ISO 1101 numbers from the above images.
Press Fit for Our S Series
A good press fit target, for a given speed (RPM), in our angular contact S Series is below. Look at these numbers as the median of the resultant press fit or to offset the ISO 1101 numbers in the higher press direction.
Correction Factors for Our SM & KH Series
Running Temperature: Key Factor in Your Bearing Tolerance
Dynamic application temperature can have a large effect on bearing tolerance for mating parts.
The resultant fit (press or gap) the bearing has in operation is what really matters. Because it’s the result of installation, RPM, application temperature and other external application effects such as loads or lubrication.
Example: An S 6005 C TA 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’s a reasonable 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 result in taking a bearing OD from a reasonable fit to a fit that will most likely see some rotation.
Other Key Factors to Consider
Other application factors can affect the bearing fit and tolerance needed. Some of other key factors are:
- Application loading: Static or dynamic and radial or axial
- System Type: A blower with high volumes of air flow
- Temperature controlled environment vs. exposed to nature
What Do You Think About Calculating Bearing Tolerance?
We hope you found this article informational and helpful. We’d love your feedback and thoughts around this topic. Contact us and let us know or give us a call at 800.323.5725.