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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 11, Gear Vibration

Section 3, Effect of a Beat on Phase

Beats are easily recognized by their periodic pulsations in amplitude. Sometimes it is less known how beats also affect a vibration's phase. If phase is indicated via a strobelight on a reference mark, then the reference mark will appear to be oscillating back and forth. If while measuring vibration on one machine, a small interfering beat causing vibration is added from another machine, the phase oscillation will be slight -- usually only a few degrees or so. The larger the interference vibration from the other machine, the greater the beat amplitude and the wider the angle of oscillating phase.

A fairly large beat can cause an angular oscillation of the reference mark of anywhere from a few degrees to almost 180°. However, sometimes the phase does not oscillate back and forth but instead rotates completely around at a rate equal to the beat frequency. The explanation is as follows:

If the vibration pickup is on machine Number 1, but the vibration from machine Number 2 is greater in amplitude than that supplied by machine Number 1, then the phase mark will appear to be rotating at a speed equal to the beat frequency. Obviously, such a machine cannot be balanced or properly use phase for analysis unless machine Number 2 is shut down, or unless the instrument that is used is capable of synchronous time averaging to eliminate the effect of all but machine Number 1. A little confusing to follow, but don't give up!

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