Page 321 - Intro Predictive Maintenance

312 An Introduction to Predictive Maintenance

winds, much like a spring, as the torsional power increases. Normally, this torque and
the resultant twist of the spindle are maintained until the torsional load is reduced. At
that point, the spindle unwinds, releasing the stored energy that was generated by the
initial transient.

Repeated twisting of the spindle’s tube or the solid shaft used in jackshafts results in
a reduction in the flexible drive’s stiffness. When this occurs, the drive loses some of
its ability to absorb torsional transients. As a result, the driven unit may be damaged.

Unfortunately, the limits of single-channel, frequency-domain data acquisition prevent
accurate measurement of this failure mode. Most of the abnormal vibration that results
from fatigue occurs in the relatively brief time interval associated with startup, when
radical speed changes occur, or during shutdown of the machine-train. As a result, this
type of data acquisition and analysis cannot adequately capture these transients;
however, the loss of stiffness caused by fatigue increases the apparent mechanical
looseness observed in the steady-state, frequency-domain vibration signature. In most
cases, this is similar to the mechanical looseness.

14.2.6 Process Rolls
Process rolls commonly encounter problems or fail because of being subjected to
induced (variable) loads and from misalignment.

Induced (Variable) Loads
Process rolls are subjected to variable loads that are induced by strip tension, track-
ing, and other process variables. In most cases, these loads are directional. They not
only influence the vibration profile but also determine the location and orientation of
data acquisition.

Strip Tension or Wrap. Figure 14–24 illustrates the wrap of the strip as it passes over
a series of rolls in a continuous-process line. The orientation and contact area of this
wrap determines the load zone on each roll.

In this example, the strip wrap is limited to one-quarter of the roll circumference.
The load zone, or vector, on the two top rolls is on a 45-degree angle to the pass
line. Therefore, the best location for the primary radial measurement is at 45 degrees
opposite to the load vector. The secondary radial measurement should be 90 de-
grees to the primary. On the top-left roll, the secondary measurement point should be
to the top left of the bearing cap; on the top-right roll, it should be at the top-right
position.

The wrap on the bottom roll encompasses one-half of the roll circumferences. As a
result, the load vector is directly upward, or 90 degrees, to the pass line. The best loca-
tion for the primary radial-measurement point is in the vertical-downward position.
The secondary radial measurement should be taken at 90 degrees to the primary.