Motorbearing fits are essential to longer bearing life and motor run life. Seems like a sensible idea, one that most any competent shop could achieve. But you might be surprised to learn that more than 40% of motor failures are due to bearing related problems, all of which can be minimized with good repair procedures. The causes of bearing failures range from contamination to poor fitting procedures to improper handling of the bearings and the lubrication. So how does bearing fitting go wrong? Let's look at a few problems. First, shops often use inside micrometers to measure the bore. These leave room for error, lots of room. To test this idea, have several texts take a bore measurement using a micrometer. You'll see quite a range of readings. The tolerance is required of motor bearings need to be very precise. For example, in a standard 6201 bearing, which is common to many motors, the fit tolerance is 3 tenths of 1,000th of an inch. That's less than a third the width of a human hair. A better method is to use dial bore gauges, which give far more precise measurements. There's simply too much room for error with micrometers. Another area of potential failure is the shop procedure itself. Bearing fits must be inspected prior to quoting a repair. Done properly, the motor will be disassembled and cleaned before taking bearing fit measurements. A reliable fit intolerance source should be consulted and multiple measurements taken before tendering a quote. Good shop practice is to take 8 to 12 measurements at different depths for each bore. Moving the bore gauge about 60 degrees with each measurement will ensure consistency. Inconsistent measurements could indicate eccentricity that would need to be corrected for maximum reliability and run life. There are some standard guidelines any good repair shop should follow to achieve better bearing fits. Consult the reliable fits reference, like those from SKF. Use dial bore gauges, not micrometers. Take multiple measurements to determine if the bore is oval or egg-shaped and establish a calibration program for the measurement equipment through a reliable third party. If you have questions about this or any other topic, contact us at ecoonline.com.