Balance quality for impellers and rotors, explained for buyers
A balance grade such as G6.3 on a drawing is a speed limit for the rotor's centre of mass, set by ISO 21940-11. This guide explains what the G-grades mean, how to turn a grade into a permissible residual unbalance in g·mm, which grades ISO 14694 sets for fans, and what to put in an RFQ so the balancing is quoted right the first time.
A lower G-number means a tighter balance.
Every rotor has some unbalance left after balancing. The balance grade says how much is acceptable. ISO 21940-11 defines the grade G as the permitted residual specific unbalance multiplied by the maximum service angular velocity, in mm/s. G6.3 allows the centre of mass to move at 6.3 mm/s at service speed; G2.5 allows 2.5 mm/s. The grades step by a factor of 2.5 (G1, G2.5, G6.3, G16, G40 and so on), and a lower number is a tighter requirement. ISO 21940-11:2016 replaced ISO 1940-1, so older drawings that cite ISO 1940-1 use the same G-grades.
How to turn a grade into a permissible residual unbalance.
Two inputs decide the number: the rotor mass m in kg and the maximum service speed n in rpm. The permissible residual specific unbalance is eper = 9,549 × G / n, in g·mm/kg (numerically the same as µm of centre-of-mass offset). Multiply by the rotor mass to get the permissible residual unbalance Uper = eper × m, in g·mm. ISO 21940-11 then gives rules for allocating Uper to the tolerance planes, usually the bearing planes; for a rotor with two planes and its centre of mass midway between the bearings, each plane gets roughly half.
| Balance grade | eper (g·mm/kg) | Uper for the whole rotor (g·mm) |
|---|---|---|
| G16 | 152.8 | 229,176 |
| G6.3 | 60.2 | 90,238 |
| G2.5 | 23.9 | 35,809 |
| G1 | 9.5 | 14,324 |
The same rotor at G6.3 may keep about 90 kg·mm of unbalance, roughly 45 g at a 1,000 mm radius on each of two planes. Going from G6.3 to G2.5 cuts the allowance by 60%, which usually means more balancing runs and, for a welded part, tighter control of the geometry before balancing starts.
Typical grades by machine type.
ISO 21940-11 lists guidance grades for common rigid rotors. They are a starting point when the equipment specification gives none. The grade on your drawing, or in the OEM's specification, always takes precedence.
ISO 21940-11 applies to rotors that behave rigidly at service speed. Long, slender rotors that run near or above a bending critical speed are flexible rotors and fall under ISO 21940-12, which needs a different balancing procedure. If your rotor is flexible, say so in the RFQ.
Fans: ISO 14694 balance categories.
For industrial fans, ISO 14694 groups applications into fan application categories and links each category to a balance quality category from BV-1 to BV-5, with a G-grade for each. The same standard also sets vibration limits for the fan in the factory and on site.
If you buy a fan impeller as a spare part, give the BV category or the G-grade of the original fan. The category of the complete fan decides the grade, not the size of the impeller.
Balance the part you will actually run.
An impeller balanced on its own, on a balancing arbor, is not yet a balanced rotor. Fit clearances, runout of the shaft seat and the key all add unbalance when the impeller is mounted. For tight grades, specify whether you want component balancing, assembled-rotor balancing or both. Keys matter too: ISO 21940-32 sets the key convention, and the half-key convention is the usual default. Say which one applies, or the balance can be correct on the machine and wrong in the pump.
For welded impellers, the sequence matters as much as the balancing machine. Stress relieving (post-weld heat treatment) before final machining reduces the risk of the part distorting after the last cut, and final machining before balancing removes the mass errors that welding leaves behind. If the part is coated after balancing, agree in advance whether a check balance is needed.
What to put in a balancing RFQ.
These eight items let a supplier quote the balancing without a round of questions:
How we handle balancing.
First In Service dynamically balances impellers, rotors and fan components up to Ø 4,000 mm and 4,000 kg in the same order as the machining, so one supplier is responsible for both. Impellers up to Ø 4,000 mm are welded to EN ISO 3834-2, stress-relieved as a routine step, finish-machined on the FERMAT WRF 130 floor-type boring mill or another machine in the park, and then balanced. Parts up to Ø 2,700 mm can also be turned on the SC27 vertical lathe. Grinding and polishing are done by certified partners within the same order. Send the grade from your drawing or specification with the enquiry, and we will quote to it.
Primary sources: ISO 21940-11:2016 · ISO 21940-32:2012 · ISO 14694:2003
Need an impeller or rotor balanced to a set grade?
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