To simulate the torsional response of an entire system, the rotating masses are represented by a series of lumped torsional inertias connected by torsional springs which represent the shaft stiffnesses between the torsional mass inertias.
Torsional Inertias
Each significant polar mass moment of inertia should be considered in the analysis. Typical inertias include motor and generator rotors, turbine wheels, coupling hubs, compressor or pump impellers, gears, and reciprocating crank throws. Each vendor normally provides the torsional mass moment of inertia values for their equipment components. However, these inertia values should be checked prior to performing the computer analysis.
Torsional Stiffness
The torsional stiffnesses between masses (torsional inertias) are normally calculated from dimensioned shaft drawings. The stiffness of a uniform diameter shaft depends on the polar area moment of inertia, shaft length, and material shear modulus. An equivalent shaft stiffness is calculated for shaft sections with diameter changes between masses. The effective spring constant is determined by:
For more complicated geometries such as motor cores and crankshafts, EDI uses Finite Element Analysis for calculating torsional stiffness.
