A torsiograph is an instrument that roataes with the shaft and is used to measure angular velocity (deg/sec) or displacement (degrees).
An HBM torsiograph operates on the seismometer principle, with a mass retained by springs whose relative motion compared to the stator is converted into an electrical signal by inductive proximity detectors. The frequency range is approximately 3 – 1,000 Hz. The device must be mounted on a free end of the shaft, preferably near an anti-node (point of maximum torsional oscillation) for best results.
While the instrument is easy to install, it is sensitive to lateral vibration and will require that the shaft end be true and drilled and tapped such that the torsiograph is centered on the shaft. Also, downtime of the system will be required for installation. Note that the amount of oscillation may not be an indicator of shaft stresses. For example, high oscillation can occure in a system with a soft coupling, but the stresses may be low. If only torsional oscillations are measured, the torsional analysis should be normalized to evaluate the stresses throughout the system.