Torsional vibrations are phenomena that occurs in the angular domain: it consists in rotational speed fluctuations of machine components, leading to shaft break, premature aging, failed acceptance tests and loss of efficiency.
As for linear vibrations it is possible to identify resonances and damping phenomena. Torsional vibrations are not transmitted to the structure. They can be measured through local strain stress measurement using telemetry. Another way is to measure the instantaneous speed variation using non-contact sensors, like optical or magnetic sensors.
Torsional vibrations analysis consists of looking at speed variation, identify torsional resonances, and identify critical torsional frequencies.