Torsional vibration is the fluctuation of rotational speed and angle around the mean rotation — the rotating machine's equivalent of a vibration signal. It is measured with a pulse sensor (optical tape, gear tooth pickup, or encoder) connected to a pulse input, decoded into the instantaneous angular speed (IAS), and analysed like any other vibration quantity: waveform, spectra, torsional orders, Campbell maps.
Two analyses are provided in the Analyses pool:
| Parameter | Meaning |
|---|---|
| DC Filter (Hz) | Butterworth high-pass that removes the mean rotation ("rigid body" speed trend) so only the fluctuation remains. 0.1–10 Hz typical; make sure your lowest interesting fluctuation frequency stays above the cutoff. Default 3 Hz. |
| Fluctuation Unit | rad/s, deg/s or rpm for the speed fluctuation output. |
| Max Order | Upper edge of the torsional order map; also drives the PPR alarm. |
| Target Orders | Comma-separated list (e.g. 1,2,4.5). Each produces an
amplitude-vs-RPM cut — on a runup this is the torsional resonance curve of that order. |
| Torsional order map | Order × time colormap of the speed fluctuation (coherent demodulation against the measured shaft angle; linear units, no dB). |
| Fluctuation spectrogram | FFT-vs-time colormap of Δω in linear units. |
| Angular acceleration | Adds the α(t) = dΔω/dt curve. |
| Parameter | Meaning |
|---|---|
| Gear Ratio (n2/n1) | Speed ratio between sensor 2 and sensor 1. Same shaft = 1. For a measurement across a gearbox enter the actual ratio; the second angle is referred to the first shaft before subtraction. |
| Zero Offset (deg) | Explicit static twist to subtract (from a prior zero measurement). |
| Auto zero | Subtracts the mean twist over the analysis window. The absolute mounting angle between the two sensors is arbitrary, so this is on by default. |
| DC Filter (Hz, 0 = off) | Optional high-pass on the twist angle. Leave 0 to keep the quasi-static (load-dependent) twist; set 0.1–10 Hz to isolate the dynamic component. |