ISO 1940-1 Rotor Balance Grades (Succeeded by ISO 21940-11:2016)
ISO 1940-1 — Mechanical vibration — Balance quality requirements for rotors in a constant (rigid) state — Part 1: Specification and verification of balance tolerances
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ISO 1940-1 (2003, succeeded by ISO 21940-11:2016) is the international standard that specified 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 the most precise rotors through G40 for coarse assemblies such as crankshafts and on to G4000 for the largest slow-speed engine drives; successive grades step by a factor of about 2.5 in the permissible G value (e · ω), applied 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 grade tolerance (single-plane G = e · ω) in a balance grade panel inside the Rotor Dynamics Calculator, using the 2003 Table 1 grade values: 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 the selected target grade, and the correction mass needed at your chosen balance-plane radius.
What ISO 1940-1 covers
- ⚠️ SUPERSEDED: ISO 1940-1:2003 has been replaced by ISO 21940-11:2016 (Mechanical vibration — Rotor balancing — Part 11: Procedures and tolerances for rotors with rigid behaviour). Cite ISO 21940-11 for new work; MechanixCalc uses the G-grade values and e = G/ω relation of ISO 1940-1:2003. 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
- Grade table G0.4 through G4000 — G0.4, G1, G2.5, G6.3, G16, G40, G100, G250, G630, G1600, G4000 — with example applications for each bracket given in the standard's table (see the standard for the application list). The MechanixCalc grade banding covers the full G0.4–G4000 range; the balancing panel's target-grade picker offers G0.4 to G40
- 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 / 1000 [mm/s], e in µmwhere G = balance quality grade (mm/s); e = specific eccentricity = U / M, where U = residual unbalance (g·mm) and M = rotor mass (kg) — note g·mm/kg is µm, so e comes out in MICROMETRES and the /1000 converts to mm; ω = maximum angular velocity (rad/s) = 2π·N / 60, N in rpm. A rotor meets grade G_target when the computed G ≤ G_target. (Equivalently, keep M in grams and e is already in mm — which is the convention the permissible-eccentricity formula below uses.)
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?
The standard's Table 1 (carried into ISO 21940-11) lists example applications for each grade; look up your machine type there. As a rule, precision or high-speed rotors target the lower grades (G0.4, G1, G2.5), general industrial rotors such as fans, pumps and motors sit around G6.3, and coarse assemblies such as drive shafts and crankshafts use G16 to G40.
How does ISO 1940-1 differ from API 684?
ISO 1940-1 (and its successor ISO 21940-11) defines the 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 relation (e_per = G/ω); the plane-split rules are not implemented.
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