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How to use it
Permissible unbalance. Enter the rotor mass, the highest service speed and the required balance quality grade. You get the permissible residual unbalance in g·mm and the specific unbalance in g·mm/kg, which corresponds to the permissible eccentricity of the centre of gravity in micrometres. With the distance between bearings and the position of the centre of gravity the value is split over the two planes; with the correction radius it becomes a mass in grams.
Grade achieved. Start from the residual unbalance read on the balancing machine and check which G grade the rotor meets at its service speed.
Formulas used
Typical balance quality grades
ISO 21940-11, which replaced ISO 1940-1, suggests a grade for each type of machine. These are starting values: the drawing or the customer's specification takes precedence.
- G 40: car wheels and rims, crankshafts of fast engines on elastic mounts.
- G 16: drive shafts, parts of agricultural and crushing machinery.
- G 6.3: fans, flywheels, pump impellers, parts of machine tools and process machinery, rotors of standard motors.
- G 2.5: turbines, machine tool spindles, turbochargers, rotors with special requirements.
- G 1: grinding machine drives, small precision rotors.
- G 0.4: spindles and wheels of precision grinding machines, gyroscopes.
What remains to be checked
The calculation applies to rigid rotors, that is, rotors running well below the first bending critical speed. Flexible rotors, overhung rotors, rotors with the centre of gravity outside the bearings and assemblies balanced part by part need a dedicated split and often balancing on more planes. Tooling and key errors matter too: with tight tolerances they can exceed the permissible unbalance. For choosing the grade, defining the planes and the note on the drawing there is the service balance quality grade to ISO 21940.
FAQ
What is the difference between ISO 1940 and ISO 21940?
ISO 21940-11 replaced ISO 1940-1 in 2016. The balance quality grades G and the formula are the same: what changes is the numbering of the standard and some guidance on the split between planes and on the verification of the residual unbalance.
What does G 6.3 mean?
It means that the product of the residual eccentricity of the centre of gravity and the angular speed does not exceed 6.3 mm/s. For the same grade, a faster rotor must have a smaller residual eccentricity.
Which speed should I enter?
The highest service speed of the rotor, not that of the balancing machine. A rigid rotor can be balanced at low speed, but the tolerance is calculated on the speed at which it will run.
Why is the unbalance split over two planes?
A long rotor has a static unbalance and a couple unbalance: correcting both requires two planes. The total tolerance is split between the planes in inverse proportion to the distance from the centre of gravity, with a minimum of 30% and a maximum of 70% per plane.
Is a single plane enough?
For disc-shaped rotors, narrow compared with the diameter and with small axial run-out, usually yes: the whole tolerance is assigned to a single plane. This is the case for many pulleys, flywheels and narrow impellers.