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, the standard recommends limiting the split 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 \(0.7 \cdot U_{\mathrm{per}}\).

\(0.3 \cdot U_{\mathrm{per}} \le U_{\mathrm{per}A},\, U_{\mathrm{per}B} \le 0.7 \cdot U_{\mathrm{per}}\)

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.