While a Turbine rotor needs balancing at operating speed, why does the coupling, not need balancing at operating speed?
A turbine rotor is classified as a flexible body, since it is designed to generally operate at speeds higher than its critical speed. Consequently any machine which is to operate at higher than its critical speed, may demonstrate characteristics different from those displayed by it below its critical speed. Hence to avoid out of balance situation, at the operating speed it is necessary to balance such units like turbine rotors at their operating speed. In the case of couplings, as was explained in the earlier question, the design ensures that the critical speed of the coupling would always be far higher than the operating speed of the unit to which it is coupled. Hence it would never pass through its critical speed, thus eliminating the need for balancing it at its operating speed.
