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This results in a residual unbalance that is less than 10 percent of the rotor weight. So, it is more conservative.
MIL-STD-167 was developed by the U.S. Navy and was based on a study aimed at limiting machinery vibrations to make it difficult for sonar operators to track naval vessels. The unbalance calculation in this standard for rotors operating above 1,000 rpm is:
U = 4W/N where:
U = maximum allowable residual unbalance in oz-inches.
W = total rotor weight in pounds
N = rotor operating speed in RPM
ISO1940-1 was developed by a consortium of equipment manufacturers and operating companies aimed at optimizing economics and balance quality requirements. In this standard, the balance quality grade differs for different rotor types. The two key assumptions in this standard are:
1. The permissible residual unbalance for geometrically similar rotors varies inversely with the rotor speed.
2. Rotor and bearing stresses will be the same for geometrically similar rotors running at the same speed and same residual unbalance.
Thus, the product of acceptable residual unbalance multiplied by the rotor speed varies depending the rotor type. The basic formula is:
Uper = G*6.015*W/N where:
Uper = the total allowable residual unbalance in oz-inches
G = allowable residual unbalance times rotor-angular velocity in mm/sec.
W = rotor weight in pounds
N = rotor operating speed in RPM
"G" is also called the balance grade, and it varies depending on the rotor type. It ranges from G4000 for certain types of crankshaft/drives to G0.4 for armatures of precision grinders. Most pumps and process fans fall into a balance grade of G6.3, while most gas and steam turbines and turbo-compressors fall into a balance grade of G2.5.
In this standard, Uper is the sum of the residual unbalance in all balance planes. For a two-plane balance, the residual unbalance in each plane is allocated such that it approximates the bearing load. For a two-plane balance:
Uper = Uper1 + Uper2 where:
Uper = the total allowable residual unbalance
Uper1 = maximum permissible residual unbalance in plane 1
Uper2 = maximum permissible residual unbalance in plane 2
