Page 325 - Intro Predictive Maintenance

316 An Introduction to Predictive Maintenance

Figure 14–29 Scrubber roll set.

race defects. The low end of the roll has higher broadband vibration energy, and
dominant frequencies include roll speed (1¥) and multiple harmonics (i.e., the same
as mechanical looseness).

Paired Rolls. Rolls that are designed to work in pairs (e.g., damming or scrubber rolls)
also must be perpendicular to the pass line. In addition, they must be parallel to each
other. Figure 14–29 illustrates a paired set of scrubber rolls. The strip is captured
between the two rolls, and the counter-rotating brush roll cleans the strip surface.

Because of the designs of both the damming and scrubber roll sets, it is difficult to
keep the rolls parallel. Most of these roll sets use a single pivot point to fix one end
of the roll and a pneumatic cylinder to set the opposite end.

Other designs use two cylinders, one attached to each end of the roll. In these designs,
the two cylinders are not mechanically linked and, therefore, the rolls do not main-
tain their parallel relationship. The result of nonparallel operation of these paired rolls
is evident in roll life.

For example, the scrubber/backup roll set should provide extended service life;
however, in actual practice, the brush rolls have a service life of only a few weeks.
After this short time in use, the brush rolls will have a conical shape, much like a
bottle brush (see Figure 14–30). This wear pattern is visual confirmation that the brush
roll and its mating rubber-coated backup roll are not parallel.

Vibration profiles can be used to determine if the roll pairs are parallel and, in this
instance, the rules for parallel misalignment apply. If the rolls are misaligned, the
vibration signatures exhibit a pronounced fundamental (1¥) and second harmonic (2¥)
of roll speed.

Multiple Pairs of Rolls. Because the strip transmits the vibration profile associated
with roll misalignment, it is difficult to isolate misalignment for a continuous-process
line by evaluating one single or two paired rolls. The only way to isolate such mis-