Unbalance - Improper Mating of Surfaces Due to Burrs or Dirt in Bolt Holes

Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 19, Improper Mating of Surfaces Due to Burrs or Dirt in Bolt Holes

The previous section refers to metal that has been displaced by sharp-edge to sharp-edge impact forces and by a portion of a screw hole's thread pulled above the mating surface. In like manner, loose particles of dirt, grease, or burrs can often be left in a bolt hole and of small enough size to not be readily visible. When the bolt is put through the hole, it can push the small particle of loose material such as greasy dirt or loose burrs into a position between the mating surfaces. The very slight displacement causes the attached rotor, such as a flywheel or gear, to be cocked slightly, causing couple unbalance. It is difficult to visualize how much difference this can cause in a rotor's unbalance until it is shown experimentally, using a shim of 4 or 5 mils. The results have surprised even Update's instructors who are already well informed on assembly errors.

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