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.
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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.
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.
The damper consists of an inner hub affixed usually by a key and keyway to the outer end of the crankshaft. An outer circular mass is attached to the hub by an integral rubber or other elastomeric section. This crankshaft and damper together become (in its torsional response) a spring-mass-damper system. By selection of the damping section material and the size of the outer mass the damper may be made to resist and thus quiet the specific torsional vibrations. These vibrations will not be at the same frequency as originally since additional mass has been added to the resonant system, so the damper must be tuned for the resultant circumstances. The overall appearance is that of a thick disk.
The damper will be fitted at the front of the engine (opposite the clutch or transmission, just beyond the cover of the timing chain, gears, or belt, and behind the accessory drive pulley (which may carry one or more V-belts or a single serpentine cog-belt. In older vehicles there will usually be an inscribed timing mark, there for purposes of setting the ignition timing.
Even though it is a simple system, a damper must be inspected by manufacturer. Long period racing requires more attention than other activities such as drag racing. Damper must be balanced otherwise it will hurt the engine rather than protect it from unwanted vibrations.