ISO 1940-1 Rotor Balance Quality Grade — Calculator & Tolerance Guide
ISO 1940-1 — Mechanical vibration — Balance quality requirements for rotors in a constant (rigid) state — Part 1: Specification and verification of balance tolerances
ISO 1940-1 is the international standard that specifies balance quality grades (G values, in mm/s) for rigid rotors and defines how to calculate the permissible residual unbalance, permissible eccentricity, and correction mass required to reach a given grade. Grades run from G0.4 — for precision gyroscopes and grinding-machine spindles — up to G40 for assembled crankshafts and drive shafts of passenger cars; each grade represents an order-of-magnitude step in the allowable mass-eccentricity product at the rotor's maximum operating speed.
The standard is used at the balancing shop to set the pass/fail tolerance before a rotor is released, and at the design stage to determine how much residual unbalance the bearing and support structure must tolerate. MechanixCalc implements the ISO 1940-1 balance grade panel inside the Rotor Dynamics Calculator: enter the rotor mass, the residual unbalance (mass × radius), and the maximum operating speed, and the tool returns the achieved grade, the permissible eccentricity for every standard grade, and the correction mass needed at your chosen balance-plane radius.
Calculators that implement ISO 1940-1
What ISO 1940-1 covers
- Defines balance quality grades G (mm/s) = e · ω for rigid rotors operating below the first critical speed
- Specifies the permissible residual specific eccentricity e_per = G / ω for each grade at the rotor's maximum continuous speed
- Covers grades G0.4, G1, G2.5, G6.3, G16, and G40, with example applications for each bracket
- Provides the conversion from permissible eccentricity to permissible unbalance U_per = e_per × M and to correction mass m_corr = U_per / r at a given correction radius
- Applies to single-plane (static) and two-plane (dynamic) balancing of rigid rotors; flexible rotors are outside scope
- Supports both SI and imperial working; MechanixCalc implements the SI form (g, mm, kg, rpm)
Governing formulas
G = e · ω [mm/s]where G = balance quality grade (mm/s); e = specific eccentricity = U / M (mm), where U = residual unbalance (g·mm) and M = rotor mass (kg); ω = maximum angular velocity (rad/s) = 2π·N / 60, N in rpm. A rotor meets grade G_target when the computed G ≤ G_target.
e_per = G_target / ω [mm] ; U_per = e_per · M [g·mm] ; m_corr = U_per / r [g]where e_per = permissible specific eccentricity (mm); G_target = chosen grade value (e.g. 2.5 mm/s for G2.5); ω = angular velocity at max operating speed (rad/s); U_per = permissible residual unbalance (g·mm); M = rotor mass (g); r = correction-plane radius from the rotor axis (mm); m_corr = mass to add or remove at radius r to meet the grade
Frequently asked questions
What is ISO 1940-1 used for?
ISO 1940-1 sets the allowable residual unbalance for rigid rotors. It defines balance quality grades (G values in mm/s) — the product of a rotor's specific eccentricity and its maximum angular velocity — and provides the permissible eccentricity and correction mass for each grade. Engineers use it to write a balancing specification before manufacturing and to set the pass/fail limit on the balancing machine at the shop.
Which balance grade applies to my rotor?
ISO 1940-1 Table 1 lists example applications: G0.4 for precision gyroscopes and grinding spindles; G1 for turbocharger rotors and machine-tool drives; G2.5 for gas and steam turbine rotors, centrifugal compressors, and pumps; G6.3 for general industrial fans, centrifugal pumps, and electric motors; G16 for drive shafts and agricultural machinery; G40 for assembled crankshafts. High-speed or precision applications always target the lower grades.
How does ISO 1940-1 differ from API 684?
ISO 1940-1 defines the static balance tolerance — how much residual unbalance a rotor is allowed to carry. API 684 governs the rotordynamic analysis — critical-speed calculations, separation margins, and unbalance response. In practice both are used together: ISO 1940-1 sets the shop tolerance that keeps the running unbalance force within the level assumed in the API 684 response analysis.
Does the calculator handle single-plane and two-plane balancing?
The ISO 1940-1 panel in the Rotor Dynamics Calculator computes the grade and correction mass for a single unbalance input. For two-plane (dynamic) balancing, you repeat the calculation for each correction plane independently using the unbalance distributed to that plane. The standard itself provides the split between planes based on rotor geometry — the calculator currently implements the single-plane permissible eccentricity formula from ISO 1940-1 §5.
Is the ISO 1940-1 calculator free?
You can use it during a free 30-minute preview with no sign-up, and a free 14-day account trial unlocks every calculator with no credit card required. The branded PDF engineering report and saved calculations are part of a paid plan.
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