Shock absorber
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A shock absorber in United States parlance, or damper in British use, is a mechanical device designed to smooth out or damp a sudden shock impulse and dissipate kinetic energy.
Applied to a structure such as a building or bridge it may be part of a seismic retrofit or as part of new, earthquake resistant construction. In this application it allows yet restrains motion and absorbs resonant energy, which can cause excessive motion and eventual structural failure.
In a vehicle, it reduces the effect of traveling over rough ground. Without shock absorbers, the vehicle would have a bouncing ride, as energy is stored in the spring and then released to the vehicle, possibly exceeding the allowed range of suspension movement. Control of excessive suspension movement without shock absorption requires stiffer (higher rate) springs, which would in turn give a harsh ride. Shock absorbers allow the use of soft (lower rate) springs while controlling the rate of suspension movement in response to bumps.
There are several commonly-used approaches to shock absorption:
- Hysteresis of structural material, for example the compression of rubber disks, stretching of rubber bands, bending of steel springs, or twisting of torsion bars.
- Dry friction as used in wheel brakes, but using disks at the pivot of a lever, with friction forced by springs. Used in early automobiles such as the Ford Model T, up through many of the British sports cars of the 1950s and early 1960s.
- Fluid friction, for example the flow of fluid through a narrow orifice (hydraulics), constitute the vast majority of automotive shock absorbets. An advantage of this type is that using special internal valving the absorber may be made relatively soft to compression (allowing a soft response to a bump) and relatively stiff to extension, controlling "jounce", which is the vehicle response to energy stored in the springs; similarly, a series of valves controlled by springs can change the degree of stiffness according to the velocity of the impact or rebound. Some shock absorbers allow tuning of the ride via control of the valve by a manual adjustment provided at the shock absorber. In more expensive vehicles the valves may be adjustable, offering the driver control of the ride at will while the vehicle is operated. The ultimate control is provided by dynamic valve control via computer in response to sensors, giving both a smooth ride and a firm suspension when needed. Many shock absorbers contain compressed nitrogen, to eliminate the tendency for the oil to foam under a lot of rapid movement, which would temporarily eliminate the damping ability of the unit.
- Compression of a gas, for example pneumatic shock absorbers, which act like springs. In aircraft landing gear these may be combined with hydraulic dampening to reduce bounce. Such struts are called "oileao" struts (combining oil and air).
- Magnetic effects.
- Inertial resistance to acceleration
- Composite hydropneumatic devices which combine in a single device spring action, shock absorption, and often also ride-height control, as in some models of the Citroën automobile.
- Conventional shock absorbers combined with composite pneumatic springs with which allow ride hight adjustment or even ride height control, seen in some large trucks and luxury sedans such as certain Lincoln automobiles. Ride height control is especially desirable in highway vehicles intended for occaisional rough road use and as a means of reducing underbody drag by lowering the vehicle when operating on improved high speed roads.
Pneumatic and hydraulic shock absorbers commonly take the form of a cylinder with a sliding piston inside. Spring-based shock absorbers commonly use coil springs or leaf springs, though torsion bars can be used to absorb torsional shocks as well. Vehicle typically employ both springs or torsion bars and hydraulic shock absorbers. In this combination, "shock absorber" (or simply "shocks") is reserved specifically for the hydraulic piston that absorbs and dissipates vibration. In this combination, the springs are not termed "shock absorbers".
Shock absorbers are an important part of automobile suspensions, aircraft landing gear, and the supports for many industrial machines. Large shock absorbers have also been used in architecture and civil engineering to reduce the susceptibility of structures to earthquake damage and resonance.