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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 11, Gear Vibration

Section 1, Vibration Due to Beats

Two machines rotating at constant speed often create a vibration that is not constant in amplitude. The vibration increases then decreases -- sometimes to almost nothing -- increases again to a maximum, decreases, etc. -- with the pulsations (beats) repeating cyclically. Two motors mounted on the same structure may each have an acceptable level of vibration, but if both are operating at approximately (but not exactly) the same speed, beats may occur.

Beats are common when, for example, two or more pumps are connected to the same header or pipe, and the rotating speeds of each pump are not exactly the same due to slightly different slip speeds of their drive motors. For a "rule of thumb," beats usually become noticeable when the differences between the two frequencies causing the beats are within 20 percent of each other. The "pulsating vibration" or beat frequency is most often the simple arithmetic difference between the two frequencies. For example, one pump may be running 1750 rpm and the other running 1740 rpm. The beat frequency will be 10 cycles per minute. If the difference in speeds is greater, then the beat frequency will be higher.

As another example, two pulleys when running at nearly (but not exactly) the same speed may produce beats having an amplitude beyond what is acceptable, even though the vibration level of each is within acceptable limits. For instance, a drive pulley and a driven pulley of the same nominal diameter will often produce beats because both will rarely run at exactly the same speed. Their speeds will not be the same due to a slight difference in diameters or belt slippage.

While decreasing the source vibration will lower the intensity of beats, it will not eliminate them entirely. Therefore, when conditions necessarily produce vibrations that are nearly equal in frequency, it is usually best to try to separate the vibrations physically. This can be done by setting one of two motors on a separate foundation or by placing vibration mounts under the frames of both. Although one or the other of these procedures may not completely eliminate the beats, it may reduce their intensity to an acceptable level. Another way of lowering the intensity of beats is to change rotational speeds so that they differ by more than 20 percent.

Beats may also be formed through multiples of the operating speeds of different parts rather than at the operating speeds themselves. For example, a 3600 rpm motor running on the same structure as a 900 rpm fan can result in beats between the 900 cycles per minute frequency and one-fourth of the 3600 rpm frequency.

 

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