BALANCE TOLERANCE CALCULATOR
This calculator gives the permissible residual unbalance for a rigid rotor from its mass, service speed, and balance quality grade, per ISO 1940-1. Enter the rotor mass and speed, pick the grade the drawing calls out, and read the tolerance for each correction plane in the units you work in. It is the same calculation that runs inside ProBal DBS machine software, free to use, with nothing to sign up for.
Results
- Left
- oz-in
- oz @ 3.5 in
- Right
- oz-in
- oz @ 3.5 in
- Static
- oz-in
- oz @ 3.5 in
HOW THIS CALCULATOR WORKS
ISO 1940-1 sets a permissible residual unbalance for a rigid rotor from three things: the balance quality grade G, the rotor mass, and the service speed. The grade is a velocity in mm/s. Dividing it by the rotor's angular velocity gives the permissible specific unbalance, the offset of the centre of mass from the axis of rotation, and multiplying that by the rotor mass gives the permissible residual unbalance as a moment.
e_per = G / 1000 (G in mm/s becomes m/s)
omega = 2 x pi x n / 60 (n is the service speed in rpm; omega in rad/s)
U_per = e_per x m / omega (m in kg; the result in kg-m, converted to your units)
The mass tolerance at a radius is that moment divided by the correction radius you enter. Left and right share the one Left/Right Radius; Static uses its own Static Radius. Every value converts in to kilograms and metres and back out to your display units, so the answer is the same whichever units you enter it in.
One assumption you should know about. This calculator divides the permissible residual unbalance evenly between the two correction planes. That split is exact for a rotor whose centre of gravity sits midway between the planes. A rotor with an offset centre of gravity needs a per-plane allocation based on the bearing distances, which ISO 1940-1 defines and this calculator does not attempt. If that is your rotor, contact ProBal and we will work it through with you.
ISO 1940-1 has been superseded by ISO 21940-11. The balance quality grades and their values are unchanged, so this page reads the same under either number.
BALANCE GRADE REFERENCE
Which grade to specify is the question behind most tolerance calculations. The guidance below is ISO 1940-1 Table 1, carried unchanged into ISO 21940-11: the grade is chosen by rotor type, and the calculator turns it into a tolerance for your mass and speed.
| G0.4 | Spindles, discs, and armatures of precision grinders; gyroscopes. |
|---|---|
| G1 | Tape recorder and phonograph drives; grinding machine drives; small electric armatures with special requirements. |
| G2.5 | Gas and steam turbines, including marine main turbines; rigid turbo-generator rotors; turbo-compressors; machine tool drives; medium and large electric armatures with special requirements; small electric armatures not qualifying for G6.3; turbine-driven pumps; computer memory drums and discs. |
| G6.3 | Parts of process plant machines; marine main turbine gears; centrifuge drums; paper machinery rolls; print rolls; fans; assembled aircraft gas turbine rotors; flywheels; pump impellers; machine tool and general machinery parts; medium and large electric armatures without special requirements; small electric armatures, often mass produced, in vibration-insensitive applications or on vibration-isolating mountings; individual components of engines under special requirements. |
| G16 | Drive shafts (propeller shafts, cardan shafts) with special requirements; parts of crushing machines; parts of agricultural machinery; individual components of engines (gasoline or diesel) for cars, trucks, and locomotives; crankshaft drives of engines with six or more cylinders under special requirements. |
| G40 | Car wheels, wheel rims, wheel sets, and drive shafts; crankshaft drives of elastically mounted fast four-cycle engines (gasoline or diesel) with six or more cylinders; crankshaft drives of engines for cars, trucks, and locomotives. |
| G100 | Crankshaft drives of fast diesel engines with six or more cylinders; complete engines (gasoline or diesel) for cars, trucks, and locomotives. |
| G250 | Crankshaft drives of rigidly mounted fast four-cylinder diesel engines. |
| G630 | Crankshaft drives of rigidly mounted large four-cycle engines; crankshaft drives of elastically mounted marine diesel engines. |
| G1600 | Crankshaft drives of rigidly mounted large two-cycle engines. |
| G4000 | Crankshaft drives of rigidly mounted slow marine diesel engines with an uneven number of cylinders. |
RELATED PAGES
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