Page 182 - Root Cause Failure Analysis

170 Root Cause Failure Analysis

Figure 13-8 Result of misalignment (Le., nonpamllel operation).

Nonparallel operation of paired rolls reduces their life. For example, the scrubber/
backup roll set should provide extended service life. However, the brush rolls some-
times have a life of only a few weeks. The brush roll wear profile shown in
Figure 13-8 is clear evidence of nonparallel operation. After a very short service life,
the brush rolls become conical in shape, much like a bottle brush. This wear pattern is
visual conformation that the brush roll and its mating rubber-coated backup roll are
not parallel.

Poor alignment has another negative affect on rolls. It changes the load zone so that
one of the rolls must absorb more of the load than the other. In conveying applica-
tions, this may not seriously affect roll life or cause catastrophic failure. However, in
applications where the roll must support extreme loads or impart large forces, like
comgators or rolling mills, this shift in the load zone can result in catastrophic roll
failure.

OPERATING METHODS
Abnormal induced loads is the most serious operator-controlled variable that affects
roll performance. Operating methods should ensure that proper tension, product
tracking, and torsional loads are maintained at all times.

In particular, operators should look for torsional-load variations caused by speed and
load changes. In most cases, roll damage occurs when they are subjected to momen-
tary radical changes in torsional load. These transients normally occur during startup,
process-speed changes, and shutdown. Operating procedures should be developed and
followed to minimize such transients.

Abnormal loading caused by improper tension or tracking of the product, belt, or
other media carried by the rolls also will reduce the performance and useful life of
process rolls. The load induced by the carried media should be equally and uniformly
distributed across the entire roll face. If the load is concentrated off-center, it will
cause premature wear and possible failure of the bearing, roll neck, and shaft. Operat-
ing procedures should provide positive methods that monitor and correct abnormal
tension or tracking.