THE RIGHT FIT HOW SHAFT AND HOUSING TOLERANCE IMPACT BEARING LIFE

Published: 01st August 2009
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THE RIGHT FIT
HOW SHAFT AND HOUSING TOLERANCE IMPACT BEARING LIFE
In every industry and every application, reaching maximum bearing life is of utmost importance. When bearings reach their full potential service life, maintenance and replacement costs are reduced, and machinery operates at peak efficiency. There are many factors that reduce the life of a bearing, particularly contamination or vibration, but issues like tolerances and fit can also play and important role.

SELECTION AND SIZE
Selecting the proper bearing type and size is essential, but not enough, because even a properly selected bearing will fail prematurely if it isn't installed correctly. If shaft and housing fits are incorrect, any bearing is likely to prematurely fail, whether as a result of too much or too little interference. The issue of fit becomes even more critical when bearings are being replaced, because a failed bearing can damage both the shaft and housing, causing them to be out of tolerance.
LOOSE FITS
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.
TIGHT SQUEEZE
At the other end of the spectrum, excessive interference causes further 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 higher stress, which can sometimes fracture inner rings. Also, an interference fit can decrease the internal clearance of a bearing due to expansion of the inner ring or shrinkage of the outer ring. When the interference is too great, internal clearance becomes negative, resulting in excessive heat buildup and premature bearing failure.
FIT FOR LIFE!
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, understanding the main factors that influence fit recommendations is important:
1. Operating Conditions:
Which rotates, the inner or the outer ring? Is the load stationary? These factors indicate which ring should have the interference fit. There are three possible combinations:
o Tight (or interference) fit on the inner ring - a common approach
o Tight fit on the outer ring - also common
o Tight fit on both rings - rarely seen
See Table 1 for fit recommendations (loose or tight) based on typical operating conditions. (Please click here for larger image.)
2. Load: Load decreases the interference of the inner ring. Thus, heavier loads require more interference.
3. 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, aluminum expands more than steel, therefore, an interference fit on an outer ring with an aluminum housing requires more interference than a steel housing.
Bearings can meet their expected service life under the right conditions; finding the right tolerance and fit for your bearing/application combination will help reduce replacement and maintenance costs and increase productivity.
Visit www.thinknsk.com to learn more about ways to maximize uptime in a wide range of industry applications.


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