Balance Quality Grades under ISO 21940-11: How to Choose the Right Tolerance for Your Equipment

The quality of balancing should be assessed not subjectively ("the vibration has gone down") but against objective, measurable criteria. International standards set out clear requirements for the permissible residual imbalance after balancing.

The key document is ISO 21940-11 (formerly ISO 1940-1:2007), "Mechanical vibration — Rotor balancing — Procedures and tolerances for rotors with rigid behaviour".

Why standards are needed:

  • They turn a subjective judgement into an objective, measurable criterion
  • They serve as the basis for the customer's acceptance of the work
  • They strike the balance between technical necessity and economic sense
  • They protect both the contractor and the customer in the event of a dispute

What a G grade is, in plain language

The balance quality grade (denoted by the letter G) defines the permissible residual imbalance after balancing. The lower the G number, the stricter the balancing accuracy requirement.

Physical meaning: the G grade characterises the permissible vibration velocity in mm/s caused by the residual imbalance. For example, G6.3 means that the permissible vibration velocity due to imbalance must not exceed 6.3 mm/s.

An important principle: each type of equipment has its own recommended balance quality grade, which stays constant regardless of the rotational speed or the mass of the rotor. For example:

  • Crushers → always grade G16
  • Fans and pumps → always G6.3
  • Turbines → always G2.5
  • Spindles → always G1.0 or G0.4
💡 Why it is not worth demanding excessive accuracy: balancing to grade G1.0 when G6.3 is sufficient considerably increases the cost and time of the work without bringing any practical benefit. The standard helps you find a sensible balance between quality and cost.

A table of G balance quality grades for different equipment

G grade Permissible vibration velocity (mm/s) Type of equipment Rotor examples
G4000 4000 Very coarse balancing Rigidly mounted crankshafts of slow-running marine diesels (with an odd number of cylinders)
G16 16 Coarse balancing Crankshafts of large two-stroke engines, crushers, agricultural-machinery shafts
G6.3 6.3 Standard industrial quality Pump rotors, fan impellers, electric-motor armatures, crusher rotors, process-equipment components
G2.5 2.5 Higher quality Gas- and steam-turbine rotors, turbocompressors, machine-tool drives, special-purpose electric-motor armatures
G1.0 1.0 Precision balancing Grinding-machine drives, spindles
G0.4 0.4 Ultra-precision balancing Precision grinding-machine spindles, gyroscopes

← See also the section on balance quality grades in the complete guide

How to calculate the permissible residual imbalance

ISO 21940-11 lets you calculate a specific value for the permissible residual imbalance, which serves as the target figure during balancing.

The calculation is done in two stages:

Stage 1: Determining the permissible specific imbalance (eper)

Formula:

eper = (G × 9549) / n

Where:

  • G — the balance quality grade (for example, 6.3)
  • n — the working rotational speed, rpm
  • eper — the permissible specific imbalance, μm (or g·mm/kg)

Stage 2: Calculating the permissible residual imbalance (Uper)

Formula:

Uper = eper × M

Where:

  • M — the mass of the rotor, kg
  • Uper — the permissible residual imbalance, g·mm
The balancing tolerance calculator in the Balanset-1A software

Fig. 1. The balancing tolerance calculation window in the Balanset-1A software: automatic calculation to ISO 1940-1

Balancing with verification against the standards

We carry out balancing with the tolerance calculated to ISO 21940-11 and issue a certificate of conformity

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Worked examples

Example 1: an industrial fan

Input data:

  • Mass of the rotor (impeller + shaft): M = 150 kg
  • Working speed: n = 1500 rpm
  • Balance quality grade: G = 6.3 (standard for fans)

Calculation:

  1. eper = (6.3 × 9549) / 1500 = 40.1 μm (g·mm/kg)
  2. Uper = 40.1 × 150 = 6015 g·mm

Conclusion: after balancing, the residual imbalance must not exceed 6015 g·mm (or ~6000 g·mm rounded).

Example 2: a 30 kW electric-motor rotor

Input data:

  • Mass of the rotor: M = 25 kg
  • Working speed: n = 3000 rpm
  • Balance quality grade: G = 2.5 (higher quality)

Calculation:

  1. eper = (2.5 × 9549) / 3000 = 7.96 μm
  2. Uper = 7.96 × 25 = 199 g·mm

Conclusion: the motor requires more accurate balancing (grade G2.5 rather than G6.3) because it runs at high speed.

Example 3: a grinding-machine spindle

Input data:

  • Mass of the spindle with its tool: M = 5 kg
  • Working speed: n = 6000 rpm
  • Balance quality grade: G = 1.0 (precision balancing)

Calculation:

  1. eper = (1.0 × 9549) / 6000 = 1.59 μm
  2. Uper = 1.59 × 5 = 7.95 g·mm

Conclusion: for high-speed precision spindles the requirements are very strict — the tolerance is ten times smaller than for fans.

Practical application: if the final balancing report shows that the residual imbalance is within the calculated ISO tolerance, the work is deemed to have been carried out to a high standard. This is an objective, legally meaningful criterion.

The link to equipment vibration

In addition to ISO 21940-11 (the imbalance tolerance), there is the standard ISO 10816-3, which governs the permissible vibration levels of equipment. It divides machines into 4 groups and 2 foundation types (rigid/flexible).

Table: vibration-condition zone boundaries to ISO 10816-3 (mm/s RMS)
Machine group Power (P), kW Zone boundaries (mm/s)
A/B
Good		 B/C
Acceptable		 C/D
Alarm
Group 1
(Large machines)		 P > 300 kW 2.3 4.5 7.1
50 < P ≤ 300 kW 1.8 2.8 4.5
Group 2
(Medium machines)		 15 < P ≤ 300 kW 1.4 2.8 4.5
P ≤ 15 kW 0.71 1.8 2.8

Note: data for machines on rigid foundations.

Decoding the condition zones:

Zone A: Good

The condition of new equipment. No action is needed.

Zone B: Acceptable

Unrestricted operation is permitted. Monitoring is recommended.

Zone C: Temporarily acceptable

The equipment needs diagnostics to find and eliminate the causes of vibration.

Zone D: Unacceptable (Alarm)

The vibration may cause damage. An immediate stop and repair are required.

🚨 Critical vibration levels:

  • Above 7 mm/s is already considered dangerous under ISO — the unit should be stopped for diagnostics to prevent destruction of the bearings and casing
  • Above 10 mm/s can cause fatigue cracks in the casing welds after as little as 100 hours of operation. This is the critical zone!

The two standards complement each other: ISO 21940-11 defines the target balancing quality, while ISO 10816-3 assesses the machine's actual vibration condition.

Conclusion

ISO 21940-11 is not merely a formal requirement but a practical tool for assuring balancing quality. It lets you:

  • Objectively assess the quality of the work carried out
  • Choose an economically justified level of accuracy
  • Protect the interests of both the customer and the contractor
  • Provide documented proof of quality

Modern balancing instruments such as the Balanset-1A have a built-in tolerance calculator to ISO 1940-1 that automatically computes the target values and compares the results achieved against them.

📖 See also: the types of imbalance and how each is corrected

Balancing to ISO standards

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A built-in tolerance calculator to ISO 1940-1

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