Page 303 - Intro Predictive Maintenance

294 An Introduction to Predictive Maintenance

14.1.5 Modulations

Modulations are frequency components that appear in a vibration signature but cannot
be attributed to any specific physical cause or forcing function. Although these fre-
quencies are “ghosts” or artificial frequencies, they can result in significant damage
to a machine-train. The presence of ghosts in a vibration signature often leads to mis-
interpretation of the data.

Ghosts are caused when two or more frequency components couple, or merge, to form
another discrete frequency component in the vibration signature. This generally occurs
with multiple-speed machines or a group of single-speed machines.

Note that the presence of modulation, or ghost peaks, is not an absolute indication of
a problem within the machine-train. Couple effects may simply increase the ampli-
tude of the fundamental running speed and do little damage to the machine-train;
however, this increased amplitude will amplify any defects within the machine-train.

Coupling can have an additive effect on the modulation frequencies, as well as being
reflected as a differential or multiplicative effect. These concepts are discussed in the
sections to follow.

Take as an example the case of a 10-tooth pinion gear turning at 10rpm while
driving a 20-tooth bullgear with an output speed of 5rpm. This gear set generates real
frequencies at 5, 10, and 100rpm (i.e., 10 teeth ¥ 10rpm). This same set can also
generate a series of frequencies (i.e., sum and product modulations) at 15rpm (i.e.,
10rpm + 5rpm) and 150rpm (i.e., 15rpm ¥ 10 teeth). In this example, the 10-rpm
input speed coupled with the 5-rpm output speed to create ghost frequencies driven
by this artificial fundamental speed (15rpm).

Sum

This type of modulation, which is described in the previous example, generates a series
of frequencies that include the fundamental shaft speeds, both input and output, and
fundamental gear-mesh profile. The only difference between the real frequencies and
the ghost is their location on the frequency scale. Instead of being at the actual shaft-
speed frequency, the ghost appears at frequencies equal to the sum of the input and
output shaft speeds. Figure 14–6 illustrates this for a speed-increaser gearbox.

Difference
In this case, the resultant ghost, or modulation, frequencies are generated by the dif-
ference between two or more speeds (see Figure 14–7). If we use the same example
as before, the resultant ghost frequencies appear at 5rpm (i.e., 10rpm –5rpm) and
50rpm (i.e., 5rpm ¥ 10 teeth). Note that the 5-rpm couple frequency coincides with
the real output speed of 5rpm. This results in a dramatic increase in the amplitude of
one real running-speed component and the addition of a false gear-mesh peak.