Page 301 - Intro Predictive Maintenance

292 An Introduction to Predictive Maintenance

Figure 14–4 Horizontal looseness creates first and second harmonics.

neutral. The double change in direction as the leg first deflects to one side then the
other generates a frequency at two times (2¥) the shaft’s rotating speed.

Other
Many other forms of mechanical looseness (besides vertical and horizontal movement
of machine legs) are typical for manufacturing and process machinery. Most forms of
pure mechanical looseness result in an increase in the vibration amplitude at the fun-
damental (1¥) shaft speed. In addition, looseness generates one or more harmonics
(i.e., 2¥, 3¥, 4¥, or combinations of harmonics and half-harmonics); however, not all
looseness generates this classic profile. For example, excessive bearing and gear clear-
ances do not generate multiple harmonics. In these cases, the vibration profile con-
tains unique frequencies that indicate looseness, but the profile varies depending on
the nature and severity of the problem.

With sleeve or Babbitt bearings, looseness is displayed as an increase in subharmonic
frequencies (i.e., less than the actual shaft speed, such as 0.5¥). Rolling-element bear-
ings display elevated frequencies at one or more of their rotational frequencies. Exces-
sive gear clearance increases the amplitude at the gear-mesh frequency and its
sidebands.

Other forms of mechanical looseness increase the noise floor across the entire band-
width of the vibration signature. Although the signature does not contain a distinct