Page 89 - Intro Predictive Maintenance

80 An Introduction to Predictive Maintenance

coupling should be evaluated to determine the specific mechanical forces and failure
modes they generate. This section discusses flexible couplings, gear couplings, jack-
shafts, and universal joints.

Flexible Couplings
Most flexible couplings use an elastomer or spring-steel device to provide power trans-
mission from the driver to the driven unit. Both coupling types create unique mechan-
ical forces that directly affect the dynamics and vibration profile of the machine-train.

The most obvious force with flexible couplings is endplay or movement in the axial
plane. Both the elastomer and spring-steel devices have memory, which forces the
axial position of both the drive and driven shafts to a neutral position. Because of their
flexibility, these devices cause the shaft to move constantly in the axial plane. This is
exhibited as harmonics of shaft speed. In most cases, the resultant profile is a signa-
ture that contains the fundamental (1X) frequency and second (2X) and third (3X)
harmonics.

Gear Couplings
When properly installed and maintained, gear-type couplings do not generate a unique
forcing function or vibration profile; however, excessive wear, variations in speed or
torque, or overlubrication results in a forcing function.

Excessive wear or speed variation generates a gear-mesh profile that corresponds to
the number of teeth in the gear coupling multiplied by the rotational speed of the
driver. Because these couplings use a mating gear to provide power transmission, vari-
ations in speed or excessive clearance permit excitation of the gear-mesh profile.

Jackshafts
Some machine-trains use an extended or spacer shaft, called a jackshaft, to connect
the driver and a driven unit. This type of shaft may use any combination of flexible
coupling, universal joint, or splined coupling to provide the flexibility required to
make the connection. Typically, this type of intermediate drive is used either to absorb
torsional variations during speed changes or to accommodate misalignment between
the two machine-train components.

Because of the length of these shafts and the flexible couplings or joints used to trans-
mit torsional power, jackshafts tend to flex during normal operation. Flexing results
in a unique vibration profile that defines its operating mode shape.

In relatively low-speed applications, the shaft tends to operate in the first mode or
with a bow between the two joints. This mode of operation generates an elevated
vibration frequency at the fundamental (1X) turning speed of the jackshaft. In higher-
speed applications, or where the flexibility of the jackshaft increases, it deflects into