Torsional (Angular) Vibration

Vibration is a dynamic phenomenon observed as an oscillatory motion around an equilibrium position. Vibration is caused by the transfer or storage of energy within structures, resulting from the action of one or more forces. This obviously applies equally well to translational vibrations (in one or several linear degrees-of-freedom) as it does to torsional vibrations (in one or several angular degrees-of-freedom). In the latter case, the forcing function is one or more moments instead of a "linear" force acting on the test structure.

Accurate analysis of torsional vibration plays an increasingly important role when troubleshooting or designing rotating machinery, yet torsional vibrations remain, without modern day laser Doppler-based techniques, notoriously difficult to measure. It is important to be able to measure and analyse torsional vibration accurately because the vibrations in rotating shafts are well-known sources of numerous vibration problems. Typical problems within the automotive industry and the marine engine industry include:

Lack of powertrain smoothness and quietness
The shafts in the drivetrain of passenger cars are excited to torsional vibrations by the inertia and gas forces of the engine, the cardan joints and the gear mesh. Torsional resonance phenomena in the driveshaft, halfshaft, etc., can create wear and fatigue problems along with reduced passenger comfort.

Gear rattle noise
In an automobile, the inherent fluctuations in the revolutions (torsional vibration) of the combustion engine are transmitted to the input shaft of the gearbox through the flywheel and the clutch disk. In the gearbox, these fluctuations produce circumferential vibration in each gear set, and due to the backlash of these gear sets, the surfaces of the gears hit each other, thereby generating rattle noise. In cars fitted with manual gearboxes, rattle noise is a significant contributor to the overall noise level.

Reduced engine performance
Torsional vibration occurring at the crankshaft is one of the major sources of combustion engine vibration leading to increased mechanical shear stresses and higher noise levels radiating from the engine. Furthermore, crankshaft torsional vibrations will typically excite the auxiliary drives via the belt. Resonance phenomena may then occur in the belt drive, or at generators, compressors and steering pumps, creating excessive belt wear or very unpleasant noise and vibration.

Reduced reliability of ship propulsion systems
Shafts in a marine propulsion system are excited to torsional vibrations by the inertia and gas forces of the engine. Again, as in the automotive industry, the common problems associated with this include wear and excessive mechanical shear stresses in the shafts, with the possibility of shaft failures. Furthermore, the majority of larger marine diesel engines are fitted with a torsional damping device in order to prevent the build-up of large vibrational amplitudes and torsional stresses at critical speeds. A decline in the performance of this damper inevitably results in increased engine wear.