Torsional vibration

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Torsional vibration is angular vibration of an object—commonly a shaft along its axis of rotation. Torsional vibration is often a concern in power transmission systems using rotating shafts or couplings where it can cause failures if not controlled.

In ideal power transmission systems using rotating parts the torques applied or reacted are "smooth" leading to constant speeds. In reality this is not the case. The torques generated may not be smooth (e.g., internal combustion engines) or the component being driven may not react to the torque smoothly (e.g., reciprocating compressors). Also, the components transmitting the torque can generate non-smooth or alternating torques (e.g., worn gears, misaligned shafts). Because the components in power transmission systems are not infinitely stiff these alternating torques cause vibration along the axis of rotation.

Crankshaft torsional vibration

Torsional vibration is a concern in the crankshafts of internal combustion engines because of several factors.

• Alternating torques are generated by the slider-crank mechanism of the crankshaft, connecting rod, and piston.
  • The motion of the piston mass and connecting rod mass generate alternating torques often referred to as "inertia" torques
  • The cylinder pressure due to combustion is not constant through the combustion cycle.
  • The slider-crank mechanism does not output a smooth torque even if the pressure is constant (e.g., at top dead centre there is no torque generated)
• Engines with several cylinders can have very flexible crankshafts due to their long length.
• There is inherently little damping in a crankshaft to reduce the vibration

If torsional vibration is not controlled in a crankshaft it can cause failure of the crankshaft or any accessories that are being driven by the crankshaft (typically at the front of the engine, the inertia of the flywheel normally reduces the motion at the rear of the engine).

This potentially damaging vibration is often controlled by a torsional damper that is located at the front nose of the crankshaft (in automobiles it is often integrated into the front pulley). There are two main types of torsional dampers.

• Viscous dampers consist of an inertia ring in a viscous fluid. The torsional vibration of the crankshaft forces the fluid through narrow passages that dissipates the vibration as heat. The viscous torsional damper is analogous to the hydraulic shock absorber in a car's suspension.
• Tuned absorber type of "dampers" often referred to as a harmonic dampers or harmonic balancers (even though it technically does not dampen or balance the crankshaft). This damper uses a spring element (often rubber in automobile engines) and an inertia ring that is typically tuned to the first torsional natural frequency of the crankshaft. This type of damper reduces the vibration at specific engine speeds when an excitation torque excites the first natural frequency of the crankshaft, but not at other speeds. This type of damper is analogous to the tuned mass dampers used in skyscrapers to reduce the building motion during an earthquake.

Measuring torsional vibration on physical systems

The most common way to measure torsional vibration is the approach of using equidistant pulses over one shaft revolution. Dedicated shaft encoders as well as gear tooth pickup transducers (induction, hall-effect, variable reluctance, etc.) can generate these pulses. The resulting encoder pulse train is converted into either a digital rpm reading or a voltage proportional to the rpm.

The use of a dual-beam laser is another technique that is used to measure torsional vibrations. The operation of the dual-beam laser is based on the difference in reflection frequency of two perfectly aligned beams pointing at different points on a shaft. Despite its specific advantages, this method yields a limited frequency range, requires line-of-sight from the part to the laser, and represents multiple lasers in case several measurement points need to be measured in parallel.