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harmonic damper
A harmonic damper is a device fitted to the free (accessory drive) end of the crankshaft of an internal combustion engine. It is essential in engines with long crankshafts (such as straight-8 engines) and is present on most engines as it reduces torsional vibrations that tend to peak at certain speeds. Torsional vibrations can greatly reduce crankshaft life, if not cause instantaneous failure, if the crankshaft runs at or through resonance. Because of this, dampers are designed with a specific weight and diameter to reduce mechanical Q factor, or damp, crankshaft resonances. Factory dampers are often re-sized in the same basic engine with any factory modification of crankshaft material, thickness, weight, or throw.
==Need==
Without the presence of a damper a long crankshaft will tend to act as a torsional spring to some extent. Impulses applied to the crankshaft by the connecting rods will tend to "wind" this spring, which will respond (as a spring-mass system) by unwinding and re-winding in the opposite direction. This will usually be damped out naturally, but at certain crankshaft rotational speeds the impulses from the cylinder firing can be in synchronization with the natural resonant frequency. (Some aircraft engines are restricted from continuous operations at specific RPM to avoid the danger of metal fatigue causing a break in the crankshaft.) In an automotive engine there is little control over operation in such specific speed bands and furthermore there may be uncomfortable vibrations as the engine is operated through such speeds, even if not a problem for engine durability.
In many small block V8 engines, such as Ford Windsor and Cleveland, Chevrolet small block 400 and big block 454, and certain Chrysler V8s, a portion of the crankshaft system's overall balancing may be provided by an unbalanced damper.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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