Reference / Balance
Rotor Balance Grades for Electric Motors
A G number on a drawing is not a tolerance until you know the rotor mass and the speed. This works it out and puts the answer in grams.
Written by the sourcing desk that reads your RFQLast reviewed How we write these
Work out the permissible unbalance
ISO 1940-1 defines the permissible specific unbalance as the G value times 9549, divided by the maximum service speed. Multiply by rotor mass and you have the limit in gram millimetres. Runs in your browser, nothing is sent anywhere.
Why the last line matters
The unbalance limit in gram millimetres is hard to picture. Divided by the correction radius it becomes a mass, and a mass you can compare against things that actually happen on a shop floor.
A burr left on one side of a keyway, a drilled hole that broke out further than intended, a chip of swarf welded into a corner, an off centre bore left by a part that moved in the chuck. On a fast, light rotor the whole budget can be a fraction of a gram, which is smaller than most people expect and smaller than a lot of ordinary machining variation.
That is the link between balancing and machining. Nobody balances a part during machining, but the machining decides how much correction is left to do, and whether the correction is even possible.
Two planes or one
A rotor that is short relative to its diameter can often be corrected in a single plane. A long rotor, which is most motor rotors, needs two. The split shown is the simple even division. Where the correction planes sit far apart relative to the bearing span, the allowance per plane changes, and that calculation belongs with whoever runs the balancing machine.
The grades, and where motors sit
The grouping below follows ISO 1940-1. Two of these six rows cover almost every electric motor rotor.
| Grade | e × ω (mm/s) | Typical application |
|---|---|---|
| G0.4 | 0.4 | Precision grinding spindles and gyroscope rotors. Rare in a motor programme. |
| G1 | 1 | Small armatures with special requirements, precision drives. |
| G2.5 | 2.5 | Turbines and turbo compressors, machine tool drives, small electric armatures, and medium to large armatures with special requirements. |
| G6.3 | 6.3 | Fans, pump impellers, flywheels, general machinery, and electrical armatures from about 80 mm shaft height without special requirements. |
| G16 | 16 | Drive shafts with special requirements, parts of crushing and agricultural machinery. |
| G40 | 40 | Car wheels and wheel rims. Not a motor rotor grade. |
The grade number is the product of the specific unbalance and the angular velocity, in millimetres per second. A lower number is a tighter requirement. Doubling the speed halves the permissible unbalance for the same grade.
What the machine shop actually controls
Balancing happens after assembly, so it is tempting to treat it as somebody else's problem at the drawing stage. Four things on a machining drawing decide how well it goes.
Concentricity between the journals and the stack seat
If the surface the rotor stack sits on does not run true to the bearing journals, the stack itself is mounted off axis. That is a mass eccentricity built into the part, and it is the largest single contributor a shop can cause. It is also cheap to avoid if the journals and the seat are cut in the same setup.
A true datum to balance against
The balancing machine has to support the rotor on something. Normally that is the bearing journals, which means the journals must exist and must be finished before balancing. If the drawing has them ground after balancing, the balance is measured against a surface that is about to change.
Material left for correction
If correction is by removal, there has to be metal to remove somewhere that does not matter structurally. If it is by added weights, there has to be a feature to hold them. Neither appears by accident. Put it on the drawing or a shop will machine the part to the outline and leave you nowhere to go.
Keyways and cross holes
Any feature that removes mass from one side is an unbalance the correction has to absorb. A keyway is usually accounted for. A cross drilled oil hole or an off centre flat sometimes is not, and on a light high speed rotor it can be a meaningful share of the whole budget.
Runout and balance are not the same specification
This is worth stating plainly because the two get swapped on drawings regularly.
Runout is geometry. It asks how much a surface moves as the part turns about a stated datum axis, and it is checked with an indicator or a CMM on a stationary part. It is a machining characteristic and it belongs on the machining drawing with its datums named.
Balance is mass distribution. It asks where the centre of mass sits relative to the axis of rotation, and it is measured on a spinning machine after assembly. It is an assembly characteristic.
A rotor can pass one and fail the other in either direction. Specify both, on the right drawings, each with its own limit. A machining supplier that is asked to guarantee a balance grade on a bare shaft is being asked for something it cannot control, and the sensible ones will say so.
Questions about balancing
Which balance grade does an electric motor rotor need?
ISO 1940-1 puts electrical armatures from about 80 mm shaft height in G6.3 when there are no special requirements, and in G2.5 when there are. Small armatures in mass production where vibration matters also sit at G2.5. Above that, G1 and G0.4 belong to precision spindles rather than to motors. Pick the grade from the application, not from a habit.
Is balancing the same as runout?
No, and specifying one does not give you the other. A balancing machine measures how mass is distributed around the axis of rotation. A runout check measures how a surface moves relative to a stated datum axis. A shaft can be balanced to G2.5 and still have a bearing journal that runs out, and a shaft with perfect runout can be out of balance because of a keyway or an off centre bore. Call out both, separately.
Who does the balancing, the machine shop or the motor builder?
Usually the motor builder, because the rotor is only balanced once the stack, the fan and anything else on the shaft is assembled. Balancing a bare shaft rarely helps. What the machine shop controls is the geometry that makes balancing possible: concentric journals, a bore that runs true to the outside diameter, and enough material left where correction weights or balance cuts go.
Does a tighter balance grade cost more to machine?
Indirectly. The grade itself is set at assembly, so it is not a machining operation. What it does is tighten the geometry the shop has to hold upstream, and it may require a balancing feature on the part, such as a rim that can be drilled or a ring that can be turned. Say on the drawing if material has to be left for correction, because a shop will otherwise machine it away.
Standards and references
- ISO 1940-1, Mechanical vibration. Balance quality requirements for rotors in a constant (rigid) state. Part 1: Specification and verification of balance tolerances. Source of the G grades and of the permissible unbalance formula used above.
- ISO 21940-11, Mechanical vibration. Rotor balancing. Part 11: Procedures and tolerances for rotors with rigid behaviour. The current revision that supersedes ISO 1940-1. The grade numbering is unchanged.
- ISO 1101, Geometrical product specifications (GPS). Geometrical tolerancing. Runout and total runout callouts, which are the machining side of this page.
Figures on this page are typical ranges drawn from the standards above and from ordinary shop practice. They are not a specification for your part. The shop that quotes you confirms what it can hold on your geometry.