A drawing that says "balance to G6.3" gives the balancing operator a target, but the machine does not read in G grades. It reads unbalance in gram-millimetres or grams at a radius. Turning a grade into a tolerance is a simple calculation once you know the rotor's mass and service speed. This article explains how the balance quality grades of ISO 21940-11 work and how to apply them.
The standard
ISO 21940-11 sets out procedures and tolerances for balancing rotors with rigid behaviour. It replaced the long-used ISO 1940-1, and the familiar G grades carried over. A rotor behaves rigidly when it does not bend significantly at its service speed, which covers most fans, pump impellers, motor armatures, pulleys and many machine tool spindles.
What a G grade means
The G number is the product of the permissible specific unbalance (eccentricity of the mass centre) and the angular velocity at service speed, expressed in mm/s. In practice, it represents a vibration-related limit: a G6.3 rotor at its service speed has its mass centre moving at up to 6.3 mm/s. Lower G numbers mean tighter balance.
| Grade | Typical examples given in the standard's guidance |
|---|---|
| G16 | Parts of agricultural machinery, some drive shafts |
| G6.3 | General machinery: fans, pump impellers, flywheels, electric motor rotors of many sizes |
| G2.5 | Gas and steam turbines, turbo compressors, machine tool drives, some precision motors |
| G1 | Grinding machine drives, small precision rotors |
| G0.4 | Precision spindles and gyroscopes |
These are guidance values. The required grade for your rotor should come from the equipment designer, the customer's specification or an applicable product standard.
Calculating permissible residual unbalance
The permissible residual unbalance Uper in g·mm is:
Uper = 9549 × G × m / n
where G is the grade in mm/s, m is the rotor mass in kg and n is the maximum service speed in rpm.
Worked example: a fan impeller of 20 kg running at 1,450 rpm, balanced to G6.3:
Uper = 9549 × 6.3 × 20 / 1450 ≈ 830 g·mm
If the correction radius is 200 mm, that is about 4.1 g of residual unbalance at that radius. This is a simple illustration; your own figures depend on your rotor.
Splitting tolerance between two planes
Many rotors are balanced in two correction planes. The total permissible unbalance must then be divided between the planes. For a rotor whose mass centre lies midway between two planes that are not too close together, each plane is usually allocated half. When the mass centre is closer to one plane, or the bearing and plane positions are unusual, the standard gives methods to allocate the tolerance in proportion to the geometry. Getting this allocation wrong is a common reason why a rotor passes on the machine but vibrates in service.
Single-plane or two-plane balancing
- Single-plane (static) balancing is usually enough for narrow, disc-shaped rotors such as pulleys and some fans, where couple unbalance is small.
- Two-plane (dynamic) balancing is needed for longer rotors, where unbalance in two planes creates a couple that a single correction cannot cancel.
Practical points on the machine
- Balance at a speed the machine and rotor can safely reach; for rigid rotors the result transfers to service speed.
- Use the actual journals or a precise arbor; arbor errors appear as rotor unbalance.
- Index the rotor on the arbor to separate arbor error from rotor unbalance when tolerances are tight.
- Include keys and half keys consistently with the convention in ISO 21940-32 for shaft and fitment key conventions.
- Verify the machine periodically with a proving rotor and test masses.
Choosing a balancing machine
Machine choice follows from rotor weight range, diameter, length, whether rotors have their own journals, and the drive type (belt, end drive or vertical for disc-type parts). Akuracy offers dynamic balancing machines and hard bearing dynamic balancing machines; the balancing machine collection shows belt-drive, end-drive and vertical versions.
Why balancing at lower speed works for rigid rotors
For a rigid rotor, the unbalance is a property of the mass distribution and does not change with speed. The centrifugal force it produces rises with the square of speed, but the correction needed is the same. That is why a rigid rotor can be balanced on a machine running well below its service speed, while the tolerance is still calculated from the service speed.
Recording balancing results
A balancing record should show the rotor identification, mass, service speed, the grade and calculated tolerance per plane, the initial unbalance, the corrections made (mass and angle, or material removed) and the final residual unbalance in each plane. Keeping this record helps to prove compliance to a customer and to spot problems upstream, for example a casting supplier whose parts consistently arrive with high initial unbalance.
Frequently asked questions
Does a lower G grade always mean a better rotor?
It means tighter balance, but over-specifying adds time and cost without benefit. Use the grade appropriate to the machine type and service conditions.
Is ISO 1940-1 still valid?
ISO 1940-1 has been replaced by ISO 21940-11. Many drawings still quote ISO 1940, and the G grades are the same.
Can flexible rotors be balanced this way?
No. Rotors that bend at service speed need methods for flexible rotors, which are covered by other parts of the ISO 21940 series.
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