Introduction

It has been said that 85% of the vibration problems in the world can be solved by balancing. This section has been placed at the back of the book, because, it is hoped, the reader now knows better. A great many other possible forcing frequencies have been described. The possibility should be well understood by now that a high level of vibration can be caused, not by balance or any other forcing mechanism, but by the existence of a natural frequency near what would otherwise be an acceptable level of forcing excitation.

The author has met several vibration consultants who have described a number of fans or pumps in a given facility that must be balanced every 3 months or so. These problems have the following characteristics:

• Most of the vibrational energy is at running speed

• There is no reason to suspect a buildup or erosion on the fan or impeller blades

• The possibility that a natural frequency near running speed has not been checked

• The unhappy owner of the machinery in question pays his bills on time

Of course, most consultants are not disreputable, but some have college degrees in Economics, not Engineering. It will be assumed here that the reader has learned enough to check transfer functions and coherence for signs of a resonance problem and has tried synchronous time averaging to verify that the high-amplitude once-per-revolution signal is, indeed, synchronous with the shaft.

If balancing is necessary, accidental destruction of the machine is not impossible; an...