Page 137 - Intro Predictive Maintenance
P. 137
128 An Introduction to Predictive Maintenance
t ω
Figure 7–9 Two harmonic motions with a phase angle
between them.
Figure 7–10 Nonharmonic periodic motion.
two vibrations. Because of f, the two vibrations do not attain their maximum dis-
f
placements at the same time. One is seconds behind the other. Note that these two
w
motions have the same frequency, w. A phase angle has meaning only for two motions
of the same frequency.
Nonharmonic Motion
In most machinery, there are numerous sources of vibrations; therefore, most time-
domain vibration profiles are nonharmonic (represented by the solid line in Figure
7–10). Although all harmonic motions are periodic, not every periodic motion is har-
monic. In Figure 7–10, the dashed lines represent harmonic motions. Figure 7–10 is
the superposition of two sine waves having different frequencies. These curves are
represented by the following equations:
