Page 316 - Intro Predictive Maintenance

Failure-Mode Analysis 307

Figure 14–17 Sidebands are paired and equal.

Defective Gear Profiles
If the gear set develops problems, the amplitude of the gear-mesh frequency increases
and the symmetry of the sidebands changes. The pattern illustrated in Figure 14–18
is typical of a defective gear set, where overall energy is the broadband, or total,
energy. Note the asymmetrical relationship of the sidebands.

Excessive Wear. Figure 14–19 is the vibration profile of a worn gear set. Note that
the spacing between the sidebands is erratic and is no longer evenly spaced by the
input shaft speed frequency. The sidebands for a worn gear set tend to occur between
the input and output speeds and are not evenly spaced.

Cracked or Broken Teeth. Figure 14–20 illustrates the profile of a gear set with a
broken tooth. As the gear rotates, the space left by the chipped or broken tooth
increases the mechanical clearance between the pinion and bullgear. The result is a
low-amplitude sideband to the left of the actual gear-mesh frequency. When the next
(i.e., undamaged) teeth mesh, the added clearance results in a higher-energy impact.
The sideband to the right of the mesh frequency has much higher amplitude. As a
result, the paired sidebands have nonsymmetrical amplitude, which is caused by the
disproportional clearance and impact energy.