Selecting the right shaft and housing tolerance
A
NSK UK product story
Edited by the Engineeringtalk editorial team
Mar 6, 2002
According to NSK, the first step towards achieving long bearing life is selecting a bearing of the proper type and size.
The first step towards achieving long bearing life is selecting a bearing of the proper type and size.
But that's not enough, because even a properly selected bearing will fail prematurely if the shaft and housing fits are incorrect.
Too much or too little interference between the mating parts can cause problems - or early failure.
This becomes even more critical when bearings are replaced.
A failed bearing can damage the shaft and housing, causing them to be out of tolerance.
A loose fit between the shaft and bearing inner ring (or the housing and the outer ring) can lead to relative movement, or "creep" between these parts.
Creep wears out the mating surfaces, increasing the clearance between them.
Eventually, the process can generate abnormal heat, vibration and possible contamination from accumulated wear particles.
At the other end of the spectrum, excessive interference causes other problems that can decrease service life.
Two key concerns are fracturing the inner ring and reducing the bearing's internal clearance.
Too much interference builds high stress, which can sometimes fracture inner rings.
Also, an interference fit can decrease the internal clearance of a bearing due to growth of the inner ring or shrinkage in the outer ring.
When the interference is too great, internal clearance becomes negative, resulting in excessive heat build-up and premature bearing failure.
Clearly, proper fit selection has a narrow margin for error.
As a rule of thumb, the rotating part should have the interference fit.
To specify the correct fit, the main factors that influence fit recommendations should be understood.
The first of these concerns the operating conditions: which ring rotates, the inner or outer one? Is the load stationary? These factors determine which ring should have the interference fit.
There are three possible combinations: tight (or interference) fit on the inner ring - a common approach; tight fit on the outer ring - also common; or tight fit on both rings - rarely seen.
The second consideration is the load, which reduces the interference of the inner ring.
Thus, heavier loads require more interference.
Thirdly, consider the housing and shaft materials.
Look at the composition of the housing and shaft materials.
Varying material strengths and thermal expansion rates make the proper fit different for each material.
For example, aluminium expands more than steel.
Thus, an interference fit on an outer ring with an aluminium housing requires more interference than a steel housing would require.
In addition, special considerations should be given to thin wall housings, hollow shafts, split housing and high vibration when making fit recommendations.
Split housings should be avoided when using a tight fit on an outer ring - the combination often leads to an oval-shaped housing and bearing ring.
Applications with heavy vibration may need tight fits on both inner and outer rings.
Using these guidelines, the shaft and housing should be inspected to verify that they meet the manufacturer's original specifications.
This will ensure that the replacement bearing achieves maximum life.
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Contact NSK UK
Tel +44 1636 605123
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