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:

Uper = 9549 · G · m/n

where:
Uper – 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.