Diagnosing Damaged Bearings
Nov 26, 2017
It's a no-brainer to diagnose why a bearing failed if its seals are gone and the balls are rattling around inside the races like a pinball machine. But sometimes bearings fail because of installation mistakes or other preventable problems. A close examination before disassembly can often determine cause of failure and ensure the replacement bearing doesn't meet a similar fate.
Lock collars are often overlooked contributors to bearing failures. Check to see if the lock collar is loose because it was "locked" the wrong direction, if the set screw backed out, or if the eccentric ring on the bearing's inner race broke away from the bearing itself. Broken eccentric rings are usually related to the lock collar being hammered too hard when it was locked to the bearing. The force exerted between the eccentric rings on lock collars and bearings is immense--all it takes is a few firm taps to lock them with hundreds of pounds of torque. If you hammer hard enough when locking a collar so that the little hole where you stick the punch is distorted, you've hammered too hard. There should be just a slight distortion, a raised edge, on the edge of the locking hole when a lock collar is properly tapped into place.
Another cause of a cracked eccentric is a worn shaft. If you're inspecting a high-hour machine that has a lock collar that cracked its bearing, there's a chance somebody replaced that bearing onto a worn shaft. The lock collar portion of the shaft was normal size, but the shaft where the bearing rides has been spun and is now several thousandths smaller than it's supposed to be. When the firm fitting lock collar is tightened, the bearing cocks on the spun portion of the shaft, and the misalignment cracks the lock collar/bearing race junction.
In a perfect world, a worn/spun shaft should be replaced to guarantee proper alignment between lock collar and bearing. In our world, there are options. Use a hammer and center punch to create a series of tiny, raised-edge craters all around the worn portion to increase its diameter. Or, use something like LocTite's bearing mounting compound to fill a gap of a couple thousandths of an inch. If possible reverse the shaft, shim out the bearing mounts or do something so the new bearing and lock collar are positioned on a portion of the shaft that is full-diameter.
Have I made such stop-gap, temporary repairs? Yes. Am I proud of them? Only in that they got the customer going long enough to finish the day, the week, or the harvest. Sometimes you do what you've got to do to make the customer happy.