Fan Balancing Grade Calculator — Permissible Residual Unbalance and Correction Weight
The “permissible residual unbalance” you set on a balancing machine comes from three numbers: balance grade G, maximum operating speed and rotor weight. Enter them with the correction radius to get the permissible residual per plane and the equivalent weight in grams; enter the machine's measured residual to see the grade actually achieved.
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Permissible Residual Unbalance
eper = 1000 × G / ω = 9,549 × G / n (µm = g·mm/kg); Uper = eper × m. Imperial: Uper (oz-in) = 6.015 × G × W (lb) / n.
Results
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
What balance grade G means
ISO 21940-11 (formerly ISO 1940-1) sets the permissible residual unbalance of rigid rotors through a balance quality grade G. The number is the velocity of the rotor's center of mass at operating speed, in mm/s:
G = eper × ω / 1000// eper = permissible specific unbalance in µm (= g·mm/kg); ω in rad/s eper = 9,549 × G / n// n = maximum operating speed, rpm Uper = eper × m// g·mm, m in kg Uper (oz-in) = 6.015 × G × W / n// W = rotor weight in lb Correction weight = Uper / r// r = correction radius // Two-plane balancing of a symmetric rotor: U/2 per plane; for an asymmetric rotor, split by the distance of the center of mass to each plane
At the same grade, the higher the speed, the smaller the permissible eccentricity: G6.3 allows e ≈ 34 µm (1.35 mil) at 1,750 rpm but only about 17 µm at 3,550 rpm.
Fan balance categories (BV) and G grades
For fans, ISO 14694 and AMCA 204 group fans by application and power into balance and vibration categories BV-1 to BV-5, each with a balance quality grade:
Fan category
Balance grade
Typical use (summary)
BV-1
G16
Small residential fans
BV-2
G16
Residential, small commercial and agricultural fans
BV-3
G6.3
Most HVAC, agricultural and general industrial fans
BV-4
G2.5
Large industrial and process fans
BV-5
G1.0
Applications with special vibration requirements
The exact power limits and vibration values are defined in the standards themselves and in the project specification. Where a job has special needs — data centers, clean rooms, fans near sensitive equipment — you can specify a finer grade.
Worked example: EC backward-curved impeller
Given: an 18 in. backward-curved impeller weighing 9.3 lb (4.22 kg), maximum speed 1,750 rpm, balanced to G6.3 in two planes at a correction radius of 8 in. (203 mm).
ω = 2π × 1,750 / 60 = 183.3 rad/s
eper = 1000 × 6.3 / 183.3 = 34.4 µm (1.35 mil)
Uper = 6.015 × 6.3 × 9.3 / 1,750 = 0.201 oz-in (145 g·mm), or 72.5 g·mm per plane
At an 8 in. radius that is no more than 0.36 g per plane
If the machine shows a 1.2 g residual in one plane: 1.2 × 203.2 × 2 = 488 g·mm, equivalent to G21 — out of tolerance; keep adding or removing weight.
Load the presets “EC backward-curved impeller” and “Check: measured residual” to reproduce these numbers.
Frequently Asked Questions
What balance grade is used for fan impellers?
Most HVAC, ventilation and general industrial fan impellers are balanced to G6.3 (fan category BV-3). High-speed centrifugal blowers and large process fans are often balanced to G2.5, and small residential fans may be G16. A project specification or contract takes precedence.
How do I calculate the permissible unbalance for G6.3?
In imperial units, U (oz-in) = 6.015 × 6.3 × rotor weight (lb) ÷ speed (rpm). For a 9.3 lb impeller at 1,750 rpm: 6.015 × 6.3 × 9.3 ÷ 1,750 ≈ 0.20 oz-in. In metric, U (g·mm) = 9,549 × G × mass (kg) ÷ rpm. Divide by the correction radius to get the permissible weight.
Why calculate at maximum speed?
The same eccentricity produces a centrifugal force that grows with the square of speed. EC and VFD-driven fans may run at their maximum speed, so balance for the maximum speed rather than the rated point or an average speed.
Single-plane or two-plane balancing?
Disc-shaped, axially narrow impellers (most axial impellers) only need single-plane static balancing. Wide centrifugal wheels, cross-flow (tangential) impellers and long rotors have couple unbalance and need two-plane dynamic balancing. As a rule of thumb, use two planes when the wheel is wider than about one-fifth of its diameter.
The fan passed balancing — why does it still vibrate on site?
The balance grade covers only the rotating part. Field vibration is also affected by bearing condition, coupling or belt alignment, base stiffness, looseness and structural resonance. Dust build-up, ice or corrosion on the blades can also unbalance an impeller in service.
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