Method and monitoring device for monitoring a torsional rigidity of a rotationally driven system

A method for monitoring a torsional rigidity of a rotationally driven system, comprising:

• determining a first torsional-rigidity-dependent system parameter ensuing in a first region of the system based on a torsional moment acting on the system;
• determining a second torsional-rigidity-dependent system parameter ensuing in a second region of the system based on the torsional moment;
• presupposing that one region out of the first region and the second region has a predetermined torsional rigidity;
• comparing the first torsional-rigidity-dependent system parameter with the second torsional-rigidity-dependent system parameter;
• and
• determining a change to the torsional rigidity of the system in that other region out of the first region and the second region which has not been presupposed as having the predetermined torsional rigidity, based on a comparison result resulting from the comparing.