Couplings are typically modeled with an inertia at each hub connected by the torsional stiffness. The torsional stiffness of couplings can be calculated or obtained directly from the manufacturer. "Certified coupling drawings" from the vendor are recommended to ensure that the torsional analysis is made with the correct coupling properties.
The vendor's coupling stiffness may assume one-third penetration of the shaft into the hub, which means that two thirds of the shaft length in the hub has no relative motion. The adjacent shaft sections are thus simulated up to the edge of the coupling hub.
Some couplings have a constant torsional stiffness independent of speed or load, while other couplings utilize nonlinear elastic elements which add damping to the system as shown below. These couplings with added damping serve to reduce the peak amplitudes at resonance. The elastic elements cause a change of torsional stiffness which varies with load and speed. At each speed, the coupling torsional stiffness must be calculated for the transmitted torque.
