A balance specification that gives only a maximum allowable unbalance is incomplete. Unbalance is always expressed relative to a plane, and moving that plane changes what the number means. Two parts can meet the same numerical tolerance and impose very different loads on their bearings.
Drawn from Specifying Unbalance and the Location of Tolerance Planes by Gary K. Grim and Jake Schlaegel. The original paper is available as a PDF.
What a Complete Specification Contains
A two-plane unbalance specification needs two things:
1. The maximum allowable residual unbalance, in a weight-times-length unit — oz·in, g·in, g·mm, g·cm or kg·m.
2. The location of the planes that figure applies to.
The second is routinely left off drawings, and it is the one that determines whether the specification actually protects the machine the part goes into.
Why Tolerance Planes Belong at the Bearings
The forces produced by residual unbalance are transmitted to the rest of the assembly through the bearings. If the tolerance planes are defined at the bearing planes, then the specified limit corresponds directly to the force each bearing will see — and static and couple unbalance are represented on equal terms.
Move the tolerance planes inboard and that correspondence breaks. The same numerical limit now permits a larger couple, because a couple specified across a shorter arm implies larger forces once resolved out at the bearings.
What That Looks Like in Numbers
Take a rotor turning at 1,800 rev/min carrying 1.0 oz·in of unbalance in each of two planes.
With the two unbalances at the same angular position — pure static unbalance — the result is about 5.75 pounds of force per bearing.
With the two unbalances 180° opposed — pure couple unbalance — the bearing forces depend entirely on where those planes sit. Specified at the bearing planes, the force is again about 5.75 pounds per bearing. Specified at inner planes one third of the way in from each bearing, the same numerical tolerance produces only about 1.92 pounds.
Read that the other way round and the problem becomes clear: a tolerance written at inner planes allows roughly three times the couple unbalance, for the same number on the drawing.
| Condition | Tolerance plane location | Force per bearing |
|---|---|---|
| Both unbalances at the same angle (static) | Bearing planes | ~5.75 lb |
| Both unbalances 180° opposed (couple) | Bearing planes | ~5.75 lb |
| Both unbalances 180° opposed (couple) | Inner planes, 1/3 in from each bearing | ~1.92 lb |
Practical Guidance
Put the tolerance planes at the bearing planes wherever the part geometry allows it. Where it does not — because there is nowhere to correct at the bearings — specify the tolerance at the bearing planes anyway and let the correction planes be somewhere else. Measurement and correction do not have to happen in the same place, and a good balancing machine resolves between them.
Where a drawing already specifies inner planes, the figure can be translated to an equivalent bearing-plane value provided the plane locations are known. It is worth doing that translation before assuming two specifications are comparable.
For how grade and service speed set the magnitude in the first place, see balance tolerance and ISO 21940-11, or work out a specific figure with the ISO balance tolerance calculator, or jump straight to the calculator itself.
Setting the tolerance is only half the job — the other half is showing a balancing process holds it reliably in production. Our CpK calculator works out process capability directly from your balance tolerance, mean outgoing reading and process sigma.
Frequently Asked Questions
Why does the location of the tolerance plane matter?
Because unbalance is a weight times a distance. Changing the plane changes the distance, so the same numerical tolerance permits a different amount of actual couple — and therefore a different force at the bearings.
Where should tolerance planes be specified?
At the bearing planes, because that is where unbalance forces are transmitted into the rest of the assembly. This makes static and couple unbalance equally represented by the same limit.
Do the correction planes have to be the same as the tolerance planes?
No. Measurement, tolerance and correction planes can all differ. A balancing machine resolves measured unbalance into whichever planes are specified, provided the geometry is entered correctly.
What units should a balance tolerance use?
Any weight-times-length pairing: oz·in, g·in, g·mm, g·cm or kg·m. What matters is that the drawing, the specification and the machine all use the same one.
Source
Adapted from Specifying Unbalance and the Location of Tolerance Planes by Gary K. Grim and Jake Schlaegel, Balance Technology Inc. Download the original paper (PDF).
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