ArticleUnderstanding Amplitude and Phase in Rotating Machinery
by Edgar J. Gunter, Ph.D.
The measurement of phase is essential in the single- or multi-plane balancing of rotating machinery. The rate of change of phase is important, as it may indicate a critical speed, and from the rate of change of phase one may be able to deduce the amplification factor or log decrement of a particular mode. Having a phase mark is particularly critical on complex machinery such as gas turbines and space shuttle oxygen and hydrogen pumps. Without a phase signal, it is impossible to determine the component of total vibration that is synchronous, external, or sub-synchronous vibrations. This article is about the use of a timing mark to track amplitude and phase, determine critical speeds and amplification factors, and perform balancing.
The understanding of amplitude-phase relationships is extremely important for all users of turbo-machinery, as well as the manufacturers. One of the greatest problems in the diagnosis and correction of field problems with rotating machinery is the lack of adequate vibration instrumentation on the machinery to observe the vibrations, the lack of adequate balancing planes, and most important, the lack of a phase reference signal. The field installation of vibration instrumentation and a phase indication probe are often very difficult and time consuming to achieve. A great deal of time, effort, and efficiency would be achieved if manufacturers had adequately designed the machinery with the idea of sufficient instrumentation, balance planes, and phase reference signal for machine diagnosis and balancing.
For example, without an adequate reference signal it is difficult to separate synchronous from nonsynchronous motion on a turbo-rotor and makes the problem of field balancing extremely difficult. Some notable examples that will be discussed are the correction of the failure problem encountered with the space shuttle main engine oxygen pump. The oxygen pump vibration failure is an example in the extreme of what happens without the application of a suitable reference signal. The vibration of interest in the oxygen pump was initially drowned out by the high intensity, high frequency vibrations generated by the rocket engine. It was only after a reference signal was generated laboriously from the four-lobed speed pickup that the vibration could be suitably analyzed and the pump design corrected.
Once the timing mark was secure, synchronous tracking filters showed that the oxygen pump was operating at a second critical speed. The pump seals then could be redesigned to suppress this critical speed in order for the pump to operate safely through the entire speed range. Another notable example was the attempt to analyze the vibration characteristics of a typical commercial small jet aircraft engine. Although years of vibration data had been taken on this engine, it proved to be worthless since there was no corresponding phase reference mark to determine the nature of the critical speeds and the amount of damping in the modes. New data had to be generated, which included a timing reference mark, in order to ascertain the problems. Many field problems in vibration diagnostics and field balancing could be more easily resolved if adequate provisions had been initially made to provide adequate phase measurements.”
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