The two primary factors to determine permissible unbalance (also called the balancing tolerance) are the mass of the rotating part and the maximum operational speed.

Calculating a balancing tolerance based on these parameters is realtively simple. However, these calculated tolerances are for the journal planes, and must be transposed into the correction planes.

Based on experimental data, the potential for damage is proportional to the Balancing Quality Grade. Larger G numbers cause more structural stress.

The EasyBalance software has a built-in balancing quality calculator for ISO, MIL, API and special standards, like 4W/n and such.

Enter rotor mass, service speed and desired quality grade, and click CALCULATE. The correct unbalance tolerance will be calculated, and automatically transposed from journal planes into correction planes.

ISO 1940 is obsolete and has been replaced with ISO 21940-11, edition 2016-11-15. The EasyBalance software Tolerance Calculator has been updated to this new ISO standard.

Balancing Quality Grade G number	Vibration velocity in mm/s	Rotor types General examples
G 4000	4000	Crankshaft drives for large, slow marine diesel engines (piston speed below 9 m/s), inherently unbalanced
G 1600	1600	Crankshaft drives for large, slow marine diesel engines (piston speed below 9 m/s), inherently balanced
G 630	630	Crankshaft drives, inherently unbalanced, elastically mounted
G 250	250	Crankshaft drives, inherently unbalanced, rigidly mounted
G 100	100	Crankshaft drives of large Diesel engines Complete reciprocating engines for cars, trucks and locomotives
G 40	40	Crankshaft drives for engines of trucks and locomotives Cars: wheels, wheel rims, wheel sets, drive shafts Crankshaft drives, inherently balanced, elastically mounted
G 16	16	Parts of crushing machinery Agricultural machinery Crankshaft drives, inherently balanced, rigidly mounted Crushing machines Drive shafts (cardan shafts, propeller shafts)
G 6.3	6.3	Fly-wheels Fans Aircraft gas turbine rotors Electrical armatures Process plant machinery Pump impellers 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
G 2.5	2.5	Machine-tool drives Computer drives Turbo compressors Small electric armatures 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 Turbine-driven pumps
G 1	1	Grinding machine drives Audio and Video drives Textile bobbins Automotive turbochargers
G 0.4	0.4	Gyroscopes Disk-drives Spindles and drives of high-precision applications

NOTE 1 Typically, completely assembled rotors are classified here. Depending on the particular application, the next higher or lower grade may be used instead. For components, see Clause 9 of ISO 21940-11 .

NOTE 2 All items are rotating if not otherwise mentioned (reciprocating) or self-evident (e.g. crankshaft drives).

NOTE 3 For some additional information on the chosen balance quality grade, see Figure 2 which contains generally used areas (service speed and balance quality grade G) based on common experience.

NOTE 4 For some machines, specific International Standards stating unbalance tolerances exist.

NOTE 5 The selection of a balance quality grade G for a machine type requires due consideration of the expected duty of the rotor when installed in situ which typically reduces the grade to a lower level if lower vibration magnitudes are required in service.

NOTE 6 The shaft height of a machine without feet, or a machine with raised feet, or any vertical machine, is to be taken as the shaft height of a machine in the same basic frame, but of the horizontal shaft foot-mounting type. When the frame is unknown, half of the machine diameter should be used.

ISO balancing grades, what are they?