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.

Construction

The modern 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.

Identification

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.

Maintenance

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.

Invention

Both Frederick Henry Royce and Frederick W. Lanchester have strong claims to the invention of the vibration damper, with the latest research showing Rolls-Royce using a crankshaft slipper (friction) vibration damper on their 1906 30HP models, however Royce had not submitted it for patent. Lanchester had developed a theoretical multi-plate viscous design in 1910. (patent 21,139, 12 September 1910). The brilliance of Fred Lanchester and his many developments was widely hailed. Royce developed a viscous damper in 1912 that was then further developed and carried through to the B60 engine of the 1950s.^[1]

References