Page 315 - Intro Predictive Maintenance

306 An Introduction to Predictive Maintenance

Figure 14–16 Normal gear set profile is symmetrical.

14.2.4 Gears
All gear sets create a frequency component referred to as gear mesh. The fundamen-
tal gear-mesh frequency is equal to the number of gear teeth times the running speed
of the shaft. In addition, all gear sets create a series of sidebands or modulations that
are visible on both sides of the primary gear-mesh frequency.

Normal Profile
In a normal gear set, each of the sidebands is spaced by exactly the 1¥ running speed
of the input shaft, and the entire gear mesh is symmetrical as seen in Figure 14–16.
In addition, the sidebands always occur in pairs, one below and one above the gear-
mesh frequency, and the amplitude of each pair is identical.

If we split the gear-mesh profile for a normal gear by drawing a vertical line through
the actual mesh (i.e., number of teeth times the input shaft speed), the two halves
would be identical. Therefore, any deviation from a symmetrical profile indicates a
gear problem; however, care must be exercised to ensure that the problem is internal
to the gears and not induced by outside influences.

External misalignment, abnormal induced loads, and a variety of other outside influ-
ences destroy the symmetry of a gear-mesh profile. For example, a single-reduction
gearbox used to transmit power to a mold-oscillator system on a continuous caster
drives two eccentric cams. The eccentric rotation of these two cams is transmitted
directly into the gearbox, creating the appearance of eccentric meshing of the gears;
however, this abnormal induced load actually destroys the spacing and amplitude of
the gear-mesh profile.