Page 135 - Intro Predictive Maintenance

126 An Introduction to Predictive Maintenance

Displacement X X o time t

T

Figure 7–6 Typical periodic motion.

Figure 7–7 Simple harmonic motion.

X = X 0 sin (w t)

The maximum value of the displacement is X 0, which is also called the amplitude.
The period, T, is usually measured in seconds; its reciprocal is the frequency of the
vibration, f, measured in cycles per second (cps) or Hertz (Hz).

1
f =
T
Another measure of frequency is the circular frequency, w, measured in radians per
second. From Figure 7–8, it is clear that a full cycle of vibration (wt) occurs after 360
degrees or 2p radians (i.e., one full revolution). At this point, the function begins a
new cycle.

w = 2pf

For rotating machinery, the frequency is often expressed in vibrations per minute
(vpm) or