Torsional vibration is a topic that is not always well understood. This type of vibration is typically the angular vibration of a
component such as a shaft along its axis of rotation. However there are other types of angular vibration such as the torsional
vibration failure of the Tacoma-Narrows Bridge in Washington State. The bridge, a long narrow structure, was excited
torsionally by the effects of flutter. This is a phenomenon of concern in aircraft design whereby significant interactions between
the ambient wind conditions and the structure occur. The torsional moments that were generated served to overstress the
supporting cables and consequently ruptured the roadway.
POWER TRANSMISSION SYSTEMS
In power transmission systems the generated torques and/or driven components may not react to these torques in a smooth
manner. Components such as elastic drive belts, worn gears, and misaligned shafts can generate non-linear torques.
Furthermore, the transmission components such as the shafting may itself twist and vibrate. Typical sources of non-smooth
torques are those generated by internal combustion engines, reciprocating compressors, and universal joints if the shafts are
not parallel. This is caused by the non-linear nature of the mechanisms.
CONTROLLING TORSIONAL VIBRATION
The torsional failure of the Tacoma-Narrows Bridge was caused by insufficient torsional stiffness and low damping. Both
factors contributed to the failure by allowing excessive motions with little ability to absorb and dissipate the energy.
In power transmission systems such as automobile engines, there is little inherent damping to reduce the vibration level.
Therefore the vibration is controlled by the use of torsional dampers located at the front of the crankshaft. Another type of
device is the tuned mass damper that limits vibration at specific torsional natural frequencies. This type of device is similar
to that used in buildings to limit its motion during high wind conditions and earthquakes.