Rotor Balance Standard
A designation that defines the limits of residual unbalance for a rotor. Unbalance is defined as the product of the eccentricity (in millimeters) of the center of mass times the operating frequency (in Hertz). Organizations such as the American Petroleum Institute, International Organization for Standardization, and American National Standards Institute all provide repair shops and original equipment manufacturers with balancing guidelines for pump, turbine, and compressor rotors. The various balance standards define balance limits for various classes of rotating machinery. According to API 610, residual unbalance refers to the amount of unbalance remaining in a rotor after balancing. Balance tolerances are established as maximum allowable residual unbalance for each correction plane of the rotor. Balance tolerances can be determined by different methods. API 610 states that the balance tolerance, or maximum allowable residual unbalance per correction plane, equals: U = 4W/N where U is balance tolerance, in units of oz-in. (for each correction plane); W is bearing journal static weight at each end of the rotor, and N is the maximum service speed of the rotor. The 4W/N API standard balance tolerance equates approximately to the ISO Grade of 0.7 (Note: ISO 1940 balance tolerances are widely used in industry). With a modern balancing machine, along with an experienced operator and any proper tooling, these limits may be achievable but not repeatable.