Harmonic damper

A harmonic damper is a device fitted to the free (accessory drive) end of the crankshaft of an internal combustion engine to counter torsional and resonance vibrations from the crankshaft. This device must be interference fit to the crankshaft in order to operate in an effective manner. An interference fit ensures the device moves in perfect step with crankshaft. It is essential on engines with long crankshafts (such as straight-8 engines) and V8 engines with cross plane cranks. Harmonics and torsional vibrations can greatly reduce crankshaft life, or cause instantaneous failure if the crankshaft runs at or through an amplified resonance. Dampers are designed with a specific weight (mass) which is dependent on the damping material/method used and the freedom it affords the mass move allowing to reduce mechanical Q factor, or damp, crankshaft resonances. A harmonic balancer (sometimes crankshaft damper, torsional damper, or vibration damper) is the same thing as a harmonic damper except that the balancer includes a counterweight to externally balance the rotating assembly. The harmonic balancer often serves as a pulley for the accessory drive belts turning the alternator, water pump and other crankshaft driven devices. The harmonic balancer must be interference fit to the crankshaft in order to operate effectively. An interference fit ensures the device moves in perfect step with crankshaft.

The need for a damper will depend on the age of the engine design, its manufacture, strength of components, usable powerband, rev range most importantly the quality of the engines tune. The engines tune especially in computer controlled applications can have a dramatic effect on durability, the aggressiveness of the tune puts the engine at risk of detonation which can be catastrophic to all rotating assembly components. Modern (roughly 1988+) DOHC, SOHC flat 4, I4, flat 6, I6, V6, flat 8, and flat-plane V8 have no need for this device. With or without the presence of a damper, a crankshaft will act as a torsional spring to some extent. Impulses applied to the crankshaft by the connecting rods will "wind" this spring, which will respond (as a spring–mass system) by unwinding and re-winding in the opposite direction. This crankshaft winding will usually be damped out naturally. However, at certain crankshaft rotational speeds, such winding can overlap with the crankshaft's natural resonant frequency, thereby increasing the frequency's amplitude and possibly leading to crankshaft damage.

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