8 - Rotor Balancing

Abstract

Mechanical imbalance is one of the most common causes of machinery vibration and is present to some degree on nearly all machines that have rotating parts or rotors. This chapter provides the information needed to understand and solve the majority of balancing problems by using a vibration/balance analyzer—a portable device that detects the level of imbalance, and misalignment in a rotating part based on the measurement of vibration signals. Two major sources of vibration caused by mechanical imbalance in equipment with rotating parts or rotors are assembly errors and incorrect key length guesses during balancing. Even when parts are precision balanced to extremely close tolerances, vibration caused by mechanical imbalance can be much greater than necessary because of assembly errors. Potential errors include relative placement of each part's center of rotation, location of the shaft relative to the bore, and cocked rotors. Further, with a keyed-shaft rotor, the balancing process can introduce machine vibration if the assumed key length is different from the length of the one used during operation. Such an imbalance usually results in a mediocre or “good” running machine as opposed to a very smooth running machine. To prevent an imbalance from occurring, the balance operator should not be allowed to guess the key length. It is strongly suggested that the actual key length be recorded on a tag that is attached to the rotor to be balanced. The tag should be attached in such a way that another device cannot be attached until the balance operator removes the tag.