Fan Balance Grade Chart: G16 to G1 Explained (ISO 1940-1)
"Fan balance grade" is a number people quote and few people can put values to. This page gives the actual figures from ISO 1940-1 — what each grade permits in residual unbalance, how that converts to vibration in mm/s, and how to decide which grade a given fan rotor actually needs.
What a balance grade is
ISO 1940-1 does not fix an unbalance in grams. It fixes a balance quality grade, written G, and ties the permissible residual unbalance to the rotor's speed. The higher the speed, the tighter the unbalance must be for the same grade. This is why the same physical part can need re-balancing to a different grade simply because it turns faster on a new duty.
The core relation is:
eper = G · 1000 / ω
where eper is the permissible eccentricity in g·mm/kg (numerically equal to µm of centre-of-gravity offset), G is the balance grade number in mm/s, and ω is the angular velocity in rad/s. Permissible residual unbalance is then Uper = eper · m / 1000, in g·mm, for a rotor of mass m in kg.
The balance grade chart
Permissible eccentricity depends on speed. This table gives the values at three common fan speeds, and the equivalent balance grade a vibration meter would read.
| Grade | Typical application | eper @ 950 rpm | eper @ 1450 rpm | eper @ 2900 rpm |
|---|---|---|---|---|
| G16 | Drive shafts, agricultural machinery | 161 µm | 105 µm | 53 µm |
| G6.3 | General machinery, standard fans, pump impellers | 63 µm | 41 µm | 21 µm |
| G4.0 | Direct-driven fans, jet fans, mine fans | 40 µm | 26 µm | 13 µm |
| G2.5 | Gas and steam turbines, high-speed fans, jet fans on VFD | 25 µm | 16 µm | 8.2 µm |
| G1 | Precision grinding spindles, small high-speed rotors | 10 µm | 6.6 µm | 3.3 µm |
eper = G · 1000 / ω. Values rounded to two significant figures. The mm/s figure quoted in some specifications is the same grade number: G6.3 permits 6.3 mm/s of vibration velocity at the critical frequency, G2.5 permits 2.5 mm/s.
How to convert to residual unbalance in your rotor
The permitted unbalance in g·mm is the eccentricity multiplied by the rotor mass:
Uper [g·mm] = eper [µm] × m [kg] / 1000
Two worked examples at 1450 rpm:
- A 90 kg impeller to G4.0: eper = 26 µm, so Uper = 26 × 90 / 1000 = 2.3 g·mm. That is roughly 0.5 g of mass at a 460 mm radius.
- The same impeller to G6.3: eper = 41 µm, so Uper ≈ 3.7 g·mm.
Expressed that way, the difference between two grades is often only a gram or two of correction — and yet it decides whether the bearing reaches its L10 life.
Which grade does a fan need?
The grade is a contract term, not an opinion. What follows is the practice we work to:
- Belt-driven, low speed, flexible mounting — G6.3 is normally adequate.
- Direct-driven fan, rigid or semi-rigid mounted — G4.0. Belt drives absorb unbalance and direct drives do not; this is where most vibration complaints originate.
- Tunnel jet fans and machine-mounted fans — G4.0 as a minimum, and G2.5 where the fan runs on a speed controller over a wide range or where the mounting is on a slender bracket.
- Fans with a VFD and a long shaft — grade the whole assembled rotor, not the impeller alone, and specify an overspeed test.
The two things that are usually missing from the specification
- Which rotor the grade applies to. Balancing the bare impeller and then fitting it to a shaft with a keyway can move the balance out again. State that the grade applies to the complete rotor as installed — impeller, shaft, key and locking element.
- The correction plane. A single-plane grade is only valid for a rotor that is short relative to its diameter. A wide impeller needs two-plane correction and the drawing should say so.
Balance grade versus vibration limits
It is worth keeping two standards apart. ISO 1940-1 governs the rotor as a manufactured article — the unbalance left after balancing. ISO 10816 governs the vibration measured on the machine in service, and its limits include the foundation, the ductwork and the operating point. A rotor can be correctly balanced to G2.5 and still exceed a site vibration limit if the mounting or the duct is at fault. When a customer reports vibration, we ask for both numbers.
How we balance
SINDFAN balances rotors on our own dynamic balancing machine, in the workshop where the impeller was cast. Cast-aluminium and GRP impellers are balanced after the blades are fixed and the paint or coating has been applied — not before, because a 90 kg rotor gains enough weight in paint to matter. We report the residual unbalance and the grade achieved, with the correction planes stated.
What to tell us when you ask for a quote
- Rotor mass and diameter, and the shaft diameter.
- Operating speed, and whether it is fixed or on a controller.
- The balance grade you need, and on which rotor (bare impeller or complete assembly).
- Whether an overspeed test is required, and at what percentage.
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