Page 92 - An Introduction To Predictive Maintenance

82 An Introduction to Predictive Maintenance

Special attention should be given to the thrust bearings, which are used in conjunc-
tion with helical gears. Because helical gears generate a relatively strong axial force,
each gear shaft must have a thrust bearing located on the backside of the gear to absorb
the thrust load. Therefore, all helical gear sets should be monitored for shaft run-out.

The thrust, or positioning, bearing of a herringbone or double-helical gear has little
or no normal axial loading; however, a coupling lockup can cause severe damage to
the thrust bearing. Double-helical gears usually have only one thrust bearing, typi-
cally on the bullgear. Therefore, the thrust-bearing rotor should be monitored with at
least one axial data-measurement point.

The gear mesh should be in a plane opposing the preload, creating the primary data-
measurement point on each shaft. A secondary data-measurement point should be
located at 90 degrees to the primary point.

Gear-Mesh Frequencies
Each gear set generates a unique profile of frequency components that should be mon-
itored. The fundamental gear-mesh frequency is equal to the number of teeth in the
pinion or drive gear multiplied by the rotational shaft speed. In addition, each gear set
generates a series of modulations, or sidebands, that surround the fundamental gear-
mesh frequency. In a normal gear set, these modulations are spaced at the same
frequency as the rotational shaft speed and appear on both sides of the fundamental
gear mesh.

A narrowband window should be established to monitor the fundamental gear-mesh
profile. The lower and upper limits of the narrowband should include the modulations
generated by the gear set. The number of sidebands will vary depending on the reso-
lution used to acquire data. In most cases, the narrowband limits should be about 10
percent above and below the fundamental gear-mesh frequency.

A second narrowband window should be established to monitor the second har-
monic (2X) of gear mesh. Gear misalignment and abnormal meshing of gear sets
result in multiple harmonics of the fundamental gear-mesh profile. This second
window provides the ability to detect potential alignment or wear problems in the
gear set.

Running Speeds
A narrowband window should be established to monitor each of the running speeds
generated by the gear sets within the gearbox. The actual number of running speeds
varies depending on the number of gear sets. For example, a single-reduction gearbox
has two speeds: input and output. A double-reduction gearbox has three speeds: input,
intermediate, and output. Intermediate and output speeds are determined by calcula-
tions based on input speed and the ratio of each gear set. Figure 5–3 illustrates a typical
double-reduction gearbox.