When performing a steady-state torsional analysis, the first step taken by EDI is to create a mass-elastic model from the drawings and information provided. Then the torsional natural frequencies and mode shapes of the system are calculated with an Eigenvector-Eigenvalue Method. The relative torsional oscillation at each mass station is plotted to create mode shape plots.
Possible excitations usually occur with frequencies at multiples of running speed. An interference or Campbell diagram is plotted to show the speeds where these excitations coincide with the torsional natural frequencies. The separation margin between these resonant speeds and the machines’ operating speed range is then evaluated.
After the torsional natural frequencies are calculated, forced vibration response calculations are performed to calculate alternating shear stresses in the shafts and vibratory torques in the couplings. These levels are plotted vs. speed for all excitation orders as well as the combined levels as shown in the example below. They are then compared to the shaft material endurance limits and coupling allowable torque values to determine if the system design is satisfactory.
In cases where the design needs to be improved, a parametric analysis is performed to identify elements which may be adjusted to shift the torsional responses and make the system acceptable. For example, the coupling selection and torsional stiffness could be optimized to detune resonances away from the running speed. Note that changes in coupling weight can also affect the lateral critical speeds. Therefore, a lateral analysis may be performed if the coupling is significantly different than originally specified.
