ISO balancing grades, what are they?

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ISO 1940 is obsolete and has been replaced with ISO 21940-11, edition 2016-11-15.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
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
Machinery, general
Machine tools
Paper machines
Process plant machines
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
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.