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u.Learn.Vibration Basic (Category 1&2)

u.Learn.Vibration Basic (Category 1&2)

Basic Motion Including Multiple Frequencies Topic: Basic Motion (A)Vibration can include response to a broad variety of sources. The simplest sources will include

unbalance (1xRPM) and misalignment (1xRPM, possibly 2x, 3x, etc). Real vibration signals are rarely

composed of only one frequency, but normally have multiple frequencies present with different

amplitudes. A very simple time waveform is shown below for a machine with primarily unbalance. This

plot shows mostly 1xRPM with some harmonic content observed on the waveform. The source of this

measurement was a proximity probe, where the harmonic content (minor 2xRPM) was likely electrical

noise (i.e. runout) on the shaft surface opposed to real vibration.

It should be apparent from the time waveform above that the frequency content of the FFT should be

primarily a single peak in the FFT, since the waveform is very nearly a sine wave with consistent periodic

shape. A “pure” sine wave would look like the plot below, where there is no harmonic content.  

For a very “clean” waveform as above, the FFT would have a single frequency at 1xRPM.  

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KelmEngineering, LLC

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Suite 202

Friendswood, TX 77546

(281) 993-3717

(979)319-4165 FAX

u.Learn.Vibration Basic (Category 1&2)

In cases where multiple frequencies exist, some waveforms should be easily identifiable because they

occur often. One common case a situation where two frequencies exist that are exactly a factor of 2

apart. This can happen with misalignment (1xRPM and 2xRPM) or with a loose bearing (½ x RPM and

1xRPM). The example below is for a loose bearing with a strong ½ x as well as strong 1xRPM.

The distinguishable feature in the waveform is obviously more than one frequency (not a pure sine

wave), with a distinct repeating shape. In this case, it takes two shaft rotations (each black dot on the

waveform indicates where the tachometer event occurred) to complete one cycle. Since it took twoshaft rotations to complete one full “cycle”, the two frequencies have to be ½ speed and 1xRPM.  

The pulsing high/low waveform is the telltale sign of two distinct frequencies. You would use the

tachometer pulses to determine if the signal contains 1x and 2x or ½ x and 1x. If the full “cycle” would

have occurred in one tachometer pulse, it would contain 1x and 2x instead of the ½ x and 1xRPM.

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Suite 202

Friendswood, TX 77546

(281) 993-3717

(979)319-4165 FAX

u.Learn.Vibration Basic (Category 1&2)

It should also be noted that for the case of two frequencies where the frequencies are exactly a factor of

2:1, the waveform shown in figure 1 would be constant over time. If the two frequencies were not

EXACT 2:1 multiples (like with oil whirl), the waveform shape would be continuously changing over time.

Again, the key with the 2:1 shape is a constant waveform shape with the high/low look as detailed

below.

You could also look at the period between two adjacent “high” peaks to see that the spacing is 0.024

seconds (2494 CPM) where the tach pulse spacing is 0.012 seconds (4972 CPM). You should also note

that the machine RPM is indicated at the top of the plot as 4972 RPM.

High High High High High

Low Low Low Low

Tachometer Pulses

1 full

vibration

cycle = 2 shaft

rotations

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Suite 202

Friendswood, TX 77546

(281) 993-3717

(979)319-4165 FAX

u.Learn.Vibration Basic (Category 1&2)

Possible test question:

Figure 1 - Time Waveform

In the time waveform in figure 1, what frequencies are present?

A. 

1xRPM

B. 

1xRPM and 2xRPM

C. 

Motor and fan shaft speeds

D. 

½ xRPM and 1xRPM

Correct answer is D