Practical Approach to Precision Balancing

2- Basic Understanding of Various Dynamic Unbalance Forces in Rotors

Unbalance as the Resultant of Many Unbalance Forces in a Rotor:

Within a rotor, there are usually many sources of unbalance and in several planes. For example, in one plane the unbalance may be due to a gas hole in the casting. In another plane, the unbalance may be the vectorial resultant of unequal distribution of cast material and off-center machining of the bore. In another plane, unbalance may be due to parts of unequal weights that were attached to the casting, .

The balancing machine or balancing process doesn't separate each and every one but instead deals with the vectorial summation of all these forces as they act in the chosen measurement planes (correction planes).

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."

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."

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