Page 51 - Prosig Catalogue 2005

SOFTWARE PRODUCTS
HOW TO MEASURE NOISE & VIBRATION IN ROTATING MACHINES

How To Measure Noise

& Vibration In Rotating

Machines

In this article we will look at the basic steps behind a simple rotating Training & Support
machinery study. We won’t look in great detail at some of the techniques
involved – we deal with these elsewhere in this handbook and on the
Noise & Vibration Measurement Blog at http://blog.prosig.com. This Figure 2: Frequency spectrum of the entire noise signal
material is suitable for a newcomer to the field who understands the basic data into sections and look at a series of spectra generated from those
concepts of noise & vibration analysis, but has not dealt with rotating segments. This is done with a “Hopping FFT”. This takes a fixed length
machinery before. section of the time history, performs an FFT and then moves along a small
Why do we need to measure noise & vibration in increment and repeats the process. This produces a series of FFT spectra
rotating machines? spread across the whole time period of the test as seen in Figure 3.
The analysis of rotating machinery is central to refinement activities in
automotive and general industry. It also allows engineers to trace faults in
gearboxes, transmission systems and bearings.
Every rotating part in a machine generates vibration, and hence noise, Condition Monitoring
as a result of small imperfections in the balance or smoothness of the
components of the machine. In addition, there are so-called “blade-
passing” phenomena associated with blades of fans and pumps. In every
case, we can relate the frequency of the vibration to the speed of the
rotating machine. For example, a fan with five equally spaced blades will
generate noise at five times the speed of rotation and sometimes at higher
multiples still depending on the number of supports used to hold the fan
in place. If these are close to the blades, then the frequency becomes the
product of the number of blades and the number of supports. Figure 3: A classic waterfall plot (frequency v time)
These vibrations act as a forcing function on the structure of the machine Figure 3 is what most people visualize when they think of a waterfall plot.
or vehicle, where they are mounted. The most severe effects occur when This data can be represented in many different ways. One of the most
the frequency of excitation generated by the rotating part matches one of popular and useful is an intensity or color plot as seen in Figure 4.
the natural frequencies of the structure. These “coincidence” frequencies
are often the target of much design effort to limit the effects, whether Software
they be fatigue, vibration or resulting noise.
With variable speed machines, it is a considerable challenge to reduce
noise and vibration to acceptable levels. The rotating components are
often transmitting very large amounts of power and, unfortunately, even
very small amounts of power, converted to vibration or noise, can produce
undesirable effects.
Analyzing Data From Rotating Machinery
Let’s assume we have captured a noise or vibration signal from some
sort of rotating machine while we accelerate it through it’s entire speed
range. We will use a short noise signal recorded from a 4-cylinder race car
engine. In figure 1 we can see the time history of the signal. Figure 4: An intensity or color plot
The plot shows frequency along the bottom axis and time runs from Hardware
bottom to top. It is clear that the red lines seen on the left part of the
map represent the rising frequencies as the engine accelerates over time.
So these are the effects due to rotation that we are interested in. What
we are still missing, though, is any information about the speed of the
engine.
Analyzing Speed Of Rotation
We need to know the speed of the engine across the time period of our
test. There are several ways to obtain this, but the most accurate is to
capture some sort of tacho signal. Ideally, a tacho signal should be a
square wave or pulse. It can be anything from a once per revolution
pulse measured from some moving part of the engine to several thousand
Figure 1: Time history of noise signal pulses per revolution generated by an encoder. Figures 5 and 6 show our
tacho signal at two different times in our test. System Packages
The simplest way to analyze the frequency content of this data would be Our signal was generating two pulses per revolution. Both figures
to calculate the auto-spectrum. We see the result in Figure 2. show 0.1s of data so it is clear that in Figure 6 the engine was rotating
Because the engine we were testing was changing speed it is almost significantly quicker than in Figure 5. It is plain that our signal is not
impossible to draw any meaningful results from this spectrum. It is clear a clean square wave, but so long as the tacho processing algorithm
we need a different approach. Our first approach might be to segment the is sophisticated and robust enough this is not important. The tacho

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