How to Split Total Permissible Residual Unbalance Between Two Planes
For a two-support rotor, the total permissible residual unbalance (U_{mathrm{per}}) is distributed between two correction planes based on rotor geometry and the rotor center of mass (CM) position. Below are the core rules from ISO 21940-11, including the 70:30 limitation set out in Clause 7.2.2 of the standard.
Definitions
- (U_{mathrm{per}}): total permissible residual unbalance for the rotor.
- (U_{mathrm{per}A}), (U_{mathrm{per}B}): permissible residual unbalance allocated to planes A and B.
- (L): distance between planes A and B.
- (L_A): distance from CM to plane A.
- (L_B): distance from CM to plane B.
- Geometry relation: (L = L_A + L_B).
1) Symmetric Rotor
If the rotor is symmetric and the CM is approximately centered between the planes, split the allowance equally:
(U_{mathrm{per}A} = U_{mathrm{per}B} = dfrac{U_{mathrm{per}}}{2})
This method fits most standard cases where the rotor is close to symmetric.
2) Asymmetric Rotor (Lever Rule)
If the CM shifts toward one support, distribute the allowance using the lever rule, proportional to the distance from the CM to the opposite plane:
(U_{mathrm{per}A} = U_{mathrm{per}} cdot dfrac{L_B}{L})
(U_{mathrm{per}B} = U_{mathrm{per}} cdot dfrac{L_A}{L})
Here, (L) is the distance between planes, and (L_A) and (L_B) are distances from the CM to planes A and B respectively.
Important Limitation: 70:30 Rule
To avoid extreme accuracy requirements in one plane, ISO 21940-11:2016 (Clause 7.2.2) stipulates for inboard rotors — i.e. with the centre of mass located between the bearing planes — that the split shall be limited to a 70:30 ratio. Even if the CM is very close to one plane/support:
- The smaller share should not be less than (0.3 cdot U_{mathrm{per}}).
- The larger share should not exceed (0.7 cdot U_{mathrm{per}}).
(0.3 cdot U_{mathrm{per}} le U_{mathrm{per}A},, U_{mathrm{per}B} le 0.7 cdot U_{mathrm{per}})
Note: these limits apply to inboard rotors. For outboard (overhung) rotors, Clause 7.2.3 of the standard allows the larger share to reach (1.3 cdot U_{mathrm{per}}), while the smaller share must still be at least (0.3 cdot U_{mathrm{per}}).
Calculator
Enter (U_{mathrm{per}}) and distances. The calculator supports symmetric split and lever-rule split. It also applies the 70:30 limitation when enabled.
