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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 5, Unbalance

Section 1, 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, such as clamps and screws. At another plane, the unbalance may be the result of the rotor's bore not being properly centered or true with its longitudinal axis.

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). (For further understanding of how forces are added vectorially, see section on "Basic Vector Understanding.") In like manner, what is called "single plane static unbalance" is the result of several unbalances in many planes, vectorially combined to form a single vectorial resultant force.



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