Motor Bearing Fits
Proper assessment of motor bearing fits requires sound references and precision methods. Every motor repair requires an assessment of bearing fit, and fast-paced motor repair shops face the added challenge of repeated accuracy. This post will help you understand what your shop should be doing at a minimum to assess bearing fits.
Why are motor bearing fits so important?
Accurate bearing fit is critical to bearing life and the run life of the motor. Over 40% of motor failures are bearing related.
The most prominent causes of bearing failure include:
Contamination
Fitting and mount
Handling
Fatigue
Improper lubrication
Getting the most out of a newly repaired motor can be a real challenge, because the fit tolerance required is often beyond the reach of many motor repair shops. "Inside" micrometers are often used, however, they leave too much room for error.
Get together with three machinists and have them measure the same bore using inside micrometers. They will typically record very different figures. It takes a much more precision method to assess motor bearing fits. A standard 6201 bearing fit is .4726 maximum and .4723 minimum. That's a tolerance of .0003, or 3/10 of one-thousandth of an inch. That's less than a third of the width of a human hair!
How better shop practice can lead to longer bearing life
The recommended fit and tolerance must be sourced, which may, at time, require securing OEM data.
To inspect a motor to these tolerances, surfaces must be completely disassembled and cleaned.
There are 32 - 38 machine (motor) fits to inspect, on average.
Embracing bearing fit tolerance makes an impact on progressive shop culture. There are implications to equipment, calibration, procedure, and turn-around time. Given that fits must be inspected prior to quoting a repair, it also makes you wonder about "free inspections" offered by some motor shops.
What to look for to improve the accuracy of bearing fits:
Have a trustworthy fits reference on hand. SKF is a great partner for this kind of information and they produce a wealth of material on bearing fits.
Use dial bore gauges. A three-pronged bore gauge is self centering. There is a spring loaded button between the two adjacent prongs that drives the gauge.
Calibrated set gauge blocks are used to measure the deflection, providing a repeatable and precision measurement of the bore.
Take multiple measurements (9-12) across the face of the bore, rotating approximately 60 degrees between each measurement. This will help identify and oval or egg shape.
Maintain a calibration program of all precision measurement instruments by a qualified third party.
Motor Bearing Fit Video:
It is a well accepted fact that accurate bearing fit is critical to bearing life and run life of the motor. With more than 40% of motor failures related to bearings, this best practice cannot be emphasized enough, especially since the variables of bearing fits are easily controlled in the motor shop.
This video looks at the four most prominent causes of bearing failure. Do you know what they are?
Precise bearing fits are also a function of how you check fit tolerances - the tools used and the procedure. Dial bore gauges are preferred over inside micrometers and for very good reasons, as the video demonstrates. With tolerances less than the width of a human hair, precise bearing fits are serious business if you want to optimize the life of the bearings and the motor.
