51 d) Distortion -although a part may be reasonably well balanced following manufacture, there are many influences which may distort or change the shape of a rotor and alter its original balance. Common causes are stress relief and thermal distortion; e) Clearance tolerances -a common source of unbalance is the stack-up of tolerances possible in the assembly of a machine. Tolerances for the different parts accumulate and produce unbalance; f) Corrosion and wear -many rotors, particularly fan,

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... ower, compressor and pump rotors, are subject to corrosion, abrasion or wear, that usually do not occur uniformly, resulting in the appearance of unbalance; g) Deposit build-up -rotors used in material handling may become unbalanced due to the unequal build-up of deposits (dirt, lime, ash, etc.) on the rotor, and the resulting gradual increase in unbalance can quickly become a serious problem; h) Unsymmetrical configurations -many rotors are manufactured in ways that produce dissymmetry, for example rough surfaces on forgings, core shifts in castings, unsymmetrical parts such as crankshafts, etc; i) Hydraulic or aerodynamic unbalance -oil trapped in oil galleries, oil trapped in grinding wheels, and cavitation or turbulence can sometimes produce unbalance forces. All of the above causes of unbalance can exist to some degree in a rotor.