Torsional vibration in rotating systems can cause damage and failure. Parts designed to spin with constant speed or change speed are susceptible to torsional fatigue. Ductile materials with increased fracture toughness are used to construct rotating shafts, but cracks can still occur from surface defects or rolling joints. Torsional vibrations can cause random vibrations that open cracks in the metal and lead to failure. Shafts are designed with analytical tools to minimize torsional vibration.
Torsional vibration occurs due to imbalances in rotating systems, such as misalignment of a rotating shaft or a weak coupling that allows for small unwanted movements along the axis of rotation. The parts are designed to spin with a constant speed, or sometimes needed to speed up or slow down. The less sudden or random vibrations a rotating part experiences during operation, the longer its service life. Many torsional components are designed with materials that can withstand long-term torsional damage, also known as torsional fatigue. Without proper testing under vibratory load, rotating parts could fail, failing catastrophically, causing peripheral damage, even killing the machine operator.
Rotating rods, usually part of a power train, such as drive shafts, camshafts, crankshafts, drive shafts, and spindles experience torsional vibration as they transmit power from some form of generating device. Such rotating shafts are constructed of ductile materials, such as metals that have increased fracture toughness – resistance to cracking. Rotating metal parts fail due to slow cracking from the surface where the greatest torsional stress occurs and where cracks are easiest to identify. Cracks can also develop from rolling joints, surface defects within the fixing holes. Terminal cracks on 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. The stands and brackets that hold signs under normal conditions are not designed to resist rotational motion. In a storm, road signs move back and forth in the wind under the influence of torsional vibration. Even some very large signs can be ripped from their moorings, becoming splinters to the unwary caught out in a hurricane.
Torsional vibrations can occur with specific resonant shaft geometries or when rotational speeds are high, increasing beyond a certain limit value. At this point, the rotation around the shaft axis becomes dynamically unstable and damaging vibrations are generated. These random vibrations, in contrast to the normal continuous movement of the shaft, open cracks in the metal and are the main causes of failure of rotating parts.
If part of a thin rotating component, such as a turbine blade, suffers catastrophic failure from a through crack, it can lead to larger imbalances that could destroy entire power systems. The reason why it is difficult to account for torsional vibrations is that it is difficult to apply periodic torsional loads during testing. Shafts today are designed with analytical tools to optimize shaft lengths and diameters in order to minimize torsional vibration.
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