ISO 21940 balance quality G grades

Balance quality grades G have been established that allow to classify the requirements of the level of permissible residual unbalance for the rotor type which guarantees its satisfactory operation. The classes are presented in the table, where the increasing number means a less restrictive standard. Number represents the maximum allowable vibration velocity of a rotating workpiece.

Grade G [mm/s]	Balance rotors examples
4000	Crankshaft drives for large, slow marine diesel engines (piston speed below 9 m/s), inherently unbalanced
1600	Crankshaft drives for large, slow marine diesel engines (piston speed below 9 m/s), inherently balanced
630	Crankshaft drives, inherently unbalanced, elastically mounted
250	Crankshaft drives, inherently unbalanced, rigidly mounted
100	Complete reciprocating engines for cars, trucks and locomotives
40	Cars: wheels, wheel rims, wheel sets, drive shafts Crankshaft drives, inherently balanced, elastically mounted
16	Agricultural machinery Crankshaft drives, inherently balanced, rigidly mounted Crushing machines Drive shafts (cardan shafts, propeller shafts)
6,3	Aircraft gas turbines Centrifuges (separators, decanters) Electric motors and generators (of at least 80 mm shaft height), of maximum rated speeds up to 950 r/min Electric motors of shaft heights smaller than 80 mm Fans Gears Machinery, general Machine tools Paper machines Process plant machines Pumps Turbo chargers Water turbines
2,5	Compressors Computer drives Electric motors and generators (of at least 80 mm shaft height), of maximum rated speeds above 950 r/min Gas turbines and steam turbines Machine-tool drives Textile machines
1	Audio and video drives Grinding machine drives
0,4	Gyroscopes Spindles and drives of high-precision systems

For justified reasons, it is possible to graduate the allowable rotor vibrations speed more precisely. To calculate the unbalance value in [gmm] depending on the grade G, the following formula is used: U_per = 9549 · G · m/n where: U_per – allowable rotor unbalance [gmm] m – rotor weight [kg] n – maximum rotor service speed [r/min] In the case of balancing rotor in two planes, the above value is the total value for the rotor. Due to its geometry, it must be properly divided into planes 1 and 2.