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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 2, Unbalance Units

For simplicity in Fig. 1, first visualize a narrow, disc-shaped rotor that is perfectly balanced. Then mentally assume that in Fig. 2, the net result of all the unbalances in that plane are resolved to an equivalent unbalance weight acting at a distance from the rotor's centerline. Unbalance units are expressed as the product of the unbalance weight times the radius at which it is acting. The weight always precedes the distance. For English units, unbalance is expressed in ounce-inches which we now abbreviate as "oz•in" (no capital letters, no periods, the - is replaced by a dot placed midway between the bottom and top. The dot is the modern expression for the x symbol meaning "times" or "multiplied by." Metric units are in gram-millimeters (g•mm).

Although unbalance in English units is normally specified "oz•in," unbalance weight measured in ounces is too crude and cumbersome for weighing as there are 28.35 grams per ounce. Gram units allow more sensitive weight scales and easier numbers with which to work. Therefore, most balancing machine operators in North America translate the specifications into a mixture of English and metric units or "g•mm." (Eventually, all countries will use metric units.)

Engineers and technicians are usually knowledgeable about the expression "center of gravity." There are certain ways in which the location for a rotor's center of gravity determines how
unbalance forces affect rotor unbalance. Therefore, a very short review of some of the basic concepts will be given so that all will be using the concepts in the same manner.

The expression "center of gravity" is usually referred to as the "CG." On drawings, its position and symbol is indicated by: . For a rotor with uniform weight distribution, the CG would be equidistant from each end. Another way to visualize the CG is to imagine that the rotor is balanced on a fulcrum or knife edge. The rotor will remain level only if the fulcrum is in the plane of the CG.

"Correction" planes are those planes on a rotor where counterbalancing weights are added or removed in order to balance the rotor. The same planes are often called "measuring planes."

There is a standard, official definition for dynamic unbalance. However, even balancing engineers sometimes have difficulty understanding it. Therefore, Update has developed its own easy to understand, practical definition that is just as accurate. Update's definition: "Dynamic unbalance is any combination of unbalances resolved to at least two correction planes." Review the diagram showing how all possible combinations of unbalance, whether they be static, couple or anything else, create "dynamic unbalance." (The vectors shown in the unbalance diagrams represent the unbalance units after they have been converted to centrifugal forces at operating speed.)


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