Page 380 - Intro Predictive Maintenance

A Total-Plant Predictive Maintenance Program 371

In dynamic imbalance, the two imbalances do not have to be equal in magnitude
to each other, nor do they have to have any particular angular reference to each
other. For example, they could be 0 (in-phase), 10, 80, or 180 degrees from each
other.

Although the definition of dynamic imbalance covers all two-plane situations, an
understanding of the components of dynamic imbalance is needed so that its causes
can be understood. An understanding of the components also makes it easier to under-
stand why certain types of balancing do not always work with many older balancing
machines for overhung rotors and very narrow rotors. The primary components of
dynamic imbalance include number of points of imbalance, amount of imbalance,
phase relationships, and rotor speed.

Points of Imbalance. The first consideration of dynamic balancing is the number of
imbalance points on the rotor because there can be more than one point of imbalance
within a rotor assembly. This is especially true in rotor assemblies with more than one
rotating element, such as a three-rotor fan or multistage pump.

Amount of imbalance. The amplitude of each point of imbalance must be known to
resolve dynamic balance problems. Most dynamic balancing machines or in situ bal-
ancing instruments are able to isolate and define the specific amount of imbalance at
each point on the rotor.

Phase relationship. The phase relationship of each point of imbalance is the third
factor that must be known. Balancing instruments isolate each point of imbalance and
determine their phase relationship. Plotting each point of imbalance on a polar plot
does this. In simple terms, a polar plot is a circular display of the shaft end. Each point
of imbalance is located on the polar plot as a specific radial, ranging from 0 to 360
degrees.

Rotor speed. Rotor speed is the final factor that must be considered. Most rotating
elements are balanced at their normal running speed or over their normal speed range.
As a result, they may be out of balance at some speeds that are not included in
the balancing solution. For example, the wheels and tires on your car are dynamically
balanced for speeds ranging from 0 to the maximum expected speed (i.e., 80 miles
per hour). At speeds above 80 miles per hour, they may be out of balance.

Coupled Imbalance. Couple imbalance is caused by two equal noncolinear imbalance
forces that oppose each other angularly (i.e., 180 degrees apart). Assume that a rotor
with pure couple imbalance is placed on frictionless rollers. Because the imbalance
weights or forces are 180 degrees apart and equal, the rotor is statically balanced;
however, a pure couple imbalance occurs if this same rotor is revolved at an appre-
ciable speed.

Each weight causes a centrifugal force, which results in a rocking motion or rotor
wobble. This condition can be simulated by placing a pencil on a table, then at one