What are Rotational and Torsional Vibration
Torsional vibrations are angular vibrations of an object, typically a shaft along its axis of rotation. They are mechanical vibrations caused by time-alternating torques which are superimposed on the otherwise steady running speed of a rotating shaft. In automotive engineering torsional vibration is primarily driven by fluctuations in engine power output.
Torsional vibrations are evaluated as the variation of rotational speed within a rotation cycle. RPM variations are typically induced by a rough driving torque or a varying load.
Rotational vibration is simply the dynamic component of the rotational speed. If we measure the rotational speed of the shaft with high precision, we will notice that we get a high deviation of rotational speed in some regions of the run-up. This is caused by the angular vibration crossing the angular natural frequency of the shaft. It is calculated by cutting off the DC component of the rotational speed or the rotation angle
The level of torsional vibration is influenced by a number of parameters, such as material properties and operating conditions such as temperature, load, RPM, etc.