3.3.11.1 Calculation Methods For Approximate Torsional Critical Speed. 3.3.11.1.1 Torsional Critical Speed Curves - Torsional critical speed curves for 1500 rpm and 1800 rpm units are shown in Figures 17 thru 34. These curves are based on the use of Solar’s standard flexible dry-disc, shear type, coupling. Applications requiring a different coupling may affect the validity of these curves. Solar will notify the supplier if any changes to the rotor are required after completing a torsional analysis of the entire drive train. Generator mass shall include the total mass of the rotating components, not just the mass between the bearings.