Page 105 - An Introduction To Predictive Maintenance

Machine-Train Monitoring Parameters 95

quencies; however, rolls are highly prone to loads induced by the process. In most
cases, rolls carry some form of product or a mechanism that, in turn, carries a product.
For example, a simple conveyor has rolls that carry a belt, which carries product from
one location to another. The primary monitoring parameters for process rolls include
bearings, load distribution, and misalignment.

Bearings
Both nonuniform loading and roll misalignment change the bearing load zones. In
general, either of these failure modes results in an increase in outer-race loading. This
is caused by the failure mode forcing the full load onto one quadrant of the bearing’s
outer race. Therefore, the ball-pass outer-race frequency should be monitored closely
on all process rolls. Any increase in this unique frequency is a prime indication of a
load, tension, or misaligned roll problem.

Load Distribution

By design, process rolls should be uniformly loaded across their entire bearing span
(see Figure 5–12). Improper tracking and/or tension of the belt, or product carried by
the rolls, will change the loading characteristics.

The loads induced by the belt increase the pressure on the loaded bearing and decrease
the pressure on the unloaded bearing. An evaluation of process rolls should include a
cross-comparison of the overall vibration levels and the vibration signature of each
roll’s inboard and outboard bearing.

Misalignment
Misalignment of process rolls is a common problem. On a continuous process line,
most rolls are mounted in several levels. The distance between the rolls and the change
in elevation make it extremely difficult to maintain proper alignment. In a vibration
analysis, roll misalignment generates a signature similar to classical parallel mis-
alignment. It generates dominant frequencies at the fundamental (1X) and second (2X)
harmonic of running speed.

5.3.5 Pumps
A wide variety of pumps is used by industry, which can be grouped into two types:
centrifugal and positive displacement. Pumps are highly susceptible to process-
induced or installation-induced loads. Some pump designs are more likely to have
axial- or thrust-induced load problems. Induced loads created by hydraulic forces also
are a serious problem in most pump applications. Recommended monitoring for each
type of pump is essentially the same, regardless of specific design or manufacturer;
however, process variables such as flow, pressure, load, and so on must be taken into
account.