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Torsional vibration occurs due to unbalance in rotating systems, such as misalignment of a rotating shaft or a weak coupling that allows small-unwanted movements along the axis of rotation. Parts are designed to spin with a constant velocity or, sometimes, required to speed up or slow down. The fewer abrupt or random vibrations a rotating part experiences while in operation, the longer its life. Many torsional components are designed with materials that can withstand long-term torsional damage, also known as torsional fatigue. Without adequate testing under vibrational loading, spinning parts might crack through, failing catastrophically, causing peripheral damage — even killing the machine operator.
Rotating rods, usually part of a power train, such as transmission shafts, camshafts, crankshafts, driveshafts, and spindles experience torsional vibrations as they transmit power from some form of generating device. Such rotating shafts are constructed of ductile materials, such as metals that have greater fracture toughness — resistance to cracking. Metallic rotating parts fail through slow cracking from the surface where the greatest torsional stress is experienced and where the cracks are easiest to identify. Cracks can also grow from rotating couplings, from surface flaws inside the fastener holes. Terminal cracks at failure surfaces grow in an approximate plane perpendicular to the length of the rotating shaft and about the central axis.
A simple example of torsional vibration is a road sign in a steady wind. Mountings and brackets that hold the signs up under normal conditions are not designed to resist rotational motion. In a storm, road signs will whip back and forth in the wind under the influence of torsional vibration. Even some very large signs can be ripped from their moorings, becoming shrapnel to the unwary caught out in a hurricane.
Torsional vibrations can occur with specific resonating geometries of the shaft or when rotational speeds are high, increasing above a certain limiting value. At this point, rotation about the shaft’s axis becomes dynamically unstable and damaging vibrations ensue. These random vibrations, at odds with the normal continuous movement of the shaft, open cracks in the metal and are the primary causes of failure of rotating parts.
If part of a thin rotating component, for example, a turbine blade, experiences catastrophic failure from a through crack, it can lead to larger imbalances that might destroy entire power systems. The reason torsional vibration is difficult to account for is that it is complicated to apply periodic torsional loads during testing. Today, shafts are designed with analytical tools to optimize lengths and diameters of shafts in order to minimize torsional vibrations.