Page 91 - An Introduction To Predictive Maintenance

Machine-Train Monitoring Parameters 81

an “S” shape between the two joints. This “S” or second-mode shape generates an
elevated frequency at both the fundamental (1X) frequency and the second harmonic
(2X) of turning speed. In extreme cases, the jackshaft deflects further and operates in
the third mode. When this happens, it generates distinct frequencies at the fundamental
(1X), second harmonic (2X), and third harmonic (3X) of turning speed.

As a rule, narrowband windows should be established to monitor at least these three
distinct frequencies (i.e., 1X, 2X, and 3X). In addition, narrowbands should be estab-
lished to monitor the discrete frequencies generated by the couplings or joints used to
connect the jackshaft to the driver and driven unit.

Universal Joints
A variety of universal joints is used to transmit torsional power. In most cases, this
type of intermediate drive is used when some misalignment between the drive and
driven unit is necessary. Because of the misalignment, the universal’s pivot points gen-
erate a unique forcing function that influences both the dynamics and vibration profile
generated by a machine-train.

Figure 5–2 illustrates a typical double-pivot universal joint. This type of joint, which
is similar to those used in automobiles, generates a unique frequency at four times
(4X) the rotational speed of the shaft. Each of the pivot-point bearings generates a
passing frequency each time the shaft completes a revolution.

5.2.3 Gearboxes
Gear sets are used to change speed or rotating direction of the primary driver. The
basic monitoring parameters for all gearboxes include bearings, gear-mesh frequen-
cies, and running speeds.

Bearings
A variety of bearing types is used in gearboxes. Narrowband windows should be estab-
lished to monitor the rotational and defect frequencies generated by the specific type
of bearing used in each application.

Figure 5–2 Typical double-pivot universal joint.