Page 305 - Intro Predictive Maintenance

296 An Introduction to Predictive Maintenance

Figure 14–8 Product modulation for a speed-increaser gearbox.

This type of coupling effect is common in single-reduction/increase gearboxes or other
machine-train components where multiple running or rotational speeds are relatively
close together or even integer multiples of one another. It is more destructive than
other forms of coupling in that it coincides with real vibration components and tends
to amplify any defects within the machine-train.

Product
With product modulation, the two speeds couple in a multiplicative manner to create
a set of artificial frequency components (see Figure 14–8). In the previous example,
product modulations occur at 50rpm (i.e., 10rpm ¥ 5rpm) and 500rpm (i.e., 50rpm
¥ 10 teeth).

Beware that this type of coupling may often go undetected in a normal vibration analy-
sis. Because the ghost frequencies are relatively high compared to the expected real
frequencies, they are often outside the monitored frequency range used for data acqui-
sition and analysis.

14.1.6 Process Instability
Normally associated with bladed or vaned machinery such as fans and pumps, process
instability creates an unbalanced condition within the machine. In most cases, it
excites the fundamental (1¥) and blade-pass/vane-pass frequency components. Unlike