Why does the spacing between balancing planes matter?

The short answer

Plane spacing changes the relationship between a pair of eccentric mass moments and their resulting couple. A balancing report should therefore identify correction-plane positions. Comparing the grams listed in two reports without their radii, angles and axial separation can misrepresent a wind drivetrain’s residual condition.

Separate a force pair from a net force

Equal and opposite unbalance vectors in two separated planes can have zero net static unbalance while retaining a couple. Increasing the distance between those planes increases the couple produced by the same vectors. Thus a rotor can have its centre of mass on-axis yet still require a two-plane assessment. A single overall eccentricity number cannot describe the full distribution along the shaft.

Retain the dimensions in the result

In a simplified hypothetical example, opposing unbalance magnitudes of 0.01 kg m separated by 0.4 m produce an unbalance couple magnitude of 0.004 kg m squared. This is a mass-distribution quantity, not an operating torque. The corresponding rotating mechanical moment also depends on speed squared. Reporting only “10 grams correction” omits the correction radius and cannot reconstruct either unbalance magnitude or couple.

Request comparable plane definitions

For a generator or rotor assembly, request axial plane coordinates, correction radii, angular reference and the residual result under the specified method. Clarify whether bearing measurement planes and physical correction planes are different. Schenck equipment documentation distinguishes these planes in balancing practice. No universal plane separation or MARSTON residual criterion is provided here. Use a qualified balancing assessment for the exact assembly rather than transfer a correction mass or acceptance value from another rotor geometry. The axial reference should remain fixed when a report changes correction planes. Equivalent representations can use different correction masses, so a larger reported mass does not by itself mean a worse balanced rotor.