Page 379 - Intro Predictive Maintenance

370 An Introduction to Predictive Maintenance

which generally can be corrected by adding counterweights. There are four types of
imbalance: (1) static, (2) dynamic, (3) couple, and (4) dynamic imbalance combina-
tions of static and couple.

Static. Static imbalance is single-plane imbalance acting through the center of
gravity of the rotor, perpendicular to the shaft axis. This imbalance can also be sepa-
rated into two separate single-plane imbalances, each acting in-phase or at the same
angular relationship to each other (i.e., 0 degrees apart); however, the net effect is as
if one force is acting through the center of gravity. For a uniform straight cylinder,
such as a simple paper machine roll or a multigrooved sheave, the forces of static
imbalance measured at each end of the rotor are equal in magnitude (i.e., the ounce-
inches or gram-centimeters in one plane are equal to the ounce-inches or gram-
centimeters in the other).

In static imbalance, the only force involved is weight. For example, assume that a
rotor is perfectly balanced and, therefore, will not vibrate regardless of the speed of
rotation. Also, assume that this rotor is placed on frictionless rollers or “knife edges.”
If a weight is applied on the rim at the center of gravity line between two ends, the
weighted portion immediately rolls to the 6 o’clock position because of the gravita-
tional force.

When rotation occurs, static imbalance translates into a centrifugal force. As a result,
this type of imbalance is sometimes referred to as force imbalance, and some bal-
ancing machine manufacturers use the word force instead of static on their machines;
however, when the term force imbalance was just starting to be accepted as the proper
term, an American standardization committee on balancing terminology standardized
the term static instead of force. The rationale was that the role of the standardization
committee was not to determine and/or correct right or wrong practices, but simply
to standardize those currently in use by industry. As a result, the term static imbal-
ance is now widely accepted as the international standard and, therefore, is the term
used in this document.

Dynamic. Dynamic imbalance is any imbalance resolved to at least two correction
planes (i.e., planes in which a balancing correction is made by adding or removing
weight). The imbalance in each of these two planes may be the result of many imbal-
ances in many planes, but the final effects can be characterized to only two planes in
almost all situations.

An example of a case where more than two planes are required is flexible rotors (i.e.,
long rotors running at high speeds). High speeds are considered to be revolutions per
minute (rpm) higher than about 80 percent of the rotor’s first critical speed; however,
in more than 95 percent of all common rotors (e.g., pump impellers, armatures, gen-
erators, fans, couplings, pulleys), two-plane dynamic balance is sufficient. Therefore,
flexible rotors are not covered in this book because of the low number in operation
and the fact that balancing operations are almost always performed by specially trained
people at the manufacturer’s plant.