14-01-2014, 04:43 PM
SHOCK ABSORBER
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HISTORY
Historically, over the period of time dampers has changed considerably over the years, in
roughly the following periods:
1) Up to 1910 dampers were hardly used at all. In 1913 Rolls Royce actually
discontinued rear dampers on the silver Ghost, illustrating just how different the situation was
in the early years.
2) From 1910 to 1925 mostly dry snubbers were used.
3) From 1925 to 1980 there was a long period of dominance by simple hydraulics,
initially simply constant-force blow off, then through progressive development to more
proportional characteristic, then adjustables, leading to a mature modern product.
4) From 1980 to 1985 there was excitement about the possibilities for active suspension,
which could effectively eliminate the ordinary damper, but little has come of this commercial
in practice so far because of the cost.
5) From 1985 it became increasingly apparent that good deal of the benefit of active
suspension could be obtained much more cheaply by fast auto-adjusting dampers, and the
damper suddenly became an interesting, developing, component again
6) From about 2000, the introduction, on high price vehicle at least, of controllable
magneto rheological dampers.
NEED FOR SHOCK ABSORBERS
Springs alone cannot provide a satisfactorily smooth ride. Therefore an additional
device called a shock absorber is used with each spring. Consider the action of a coil spring.
The spring is under an initial load provided by the weight of the vehicle. This gives the spring
an original amount of compression. When the wheel passes over a bump, the spring becomes
further compressed. After the bump is passed the spring attempts to return to its original
position. However it over rides its original position and expands too much. This behaviour
causes the vehicle frame to be thrown upward. Having expanded too much, the spring
attempts to compress that it will return to its original position; but in compressing it again
overrides. In doing this the wheel may be raised clear of the road and the frame consequently
drops. The result is an oscillating motion of the spring that causes the wheel to rebound or
bounce up and down several times, after a bump is encountered. If, in the meantime, another
bump is encountered, a second series of rebounding will be started. On a bumpy road, and
particularly in rounding a curve, the oscillations might be so serious as to cause the driver to
lose control of the vehicle.
DESCRIPTION
Pneumatic and hydraulic shock absorbers are used in conjunction with cushions
and springs. An automobile shock absorber contains spring-loaded check valves and orifices
to control the flow of oil through an internal piston.
The shock absorber absorbs and dissipates energy. In most dashpots, energy is
converted to heat inside the viscous fluid. In hydraulic cylinders, the hydraulic fluid heats up,
while in air cylinders, the hot air is usually exhausted to the atmosphere. In other types of
dashpots, such as electromagnetic types, the dissipated energy can be stored and used later. In
general terms, shock absorbers help cushion vehicles on uneven roads.
DRY FRICTION TYPE
Dry friction as used in wheel brakes, by using disks (classically made of leather) at
the pivot of a lever, with friction forced by springs. Used in early automobiles such as
the Ford Model T, up through some British cars of the 1940s. Although now considered
obsolete, an advantage of this system is its mechanical simplicity; the degree of damping can
be easily adjusted by tightening or loosening the screw clamping the disks, and it can be
easily rebuilt with simple hand tools. A disadvantage is that the damping force tends not to
increase with the speed of the vertical motion.
PNEUMATIC SHOCK ABSORBER
Compression of a gas, for example Pneumatic Shock Absorbers, which can act like
springs as the air pressure is building to resist the force on it. Once the air pressure reaches
the necessary maximum, air dashpots will act like hydraulic dashpots. In aircraft landing gear
air dashpots may be combined with hydraulic damping to reduce bounce. Such struts are
called Oleo Struts (combining oil and air).
MAGNETIC EFFECT TYPE
Eddy current dampers are dashpots that are constructed out of a large magnet inside
of a non-magnetic, electrically conductive tube. When a conductive material experiences a
time varying magnetic field, addy currents are generated in the conductor. These eddy
currents circulate such that they generate a magnetic field of their own, however the field
generated is of opposite polarity, causing a repulsive force. The time varying magnetic field
needed to produce such currents can be induced either by movement of the conductor in the
field or by changing the strength or position of the source of the magnetic field. In the case of
a dynamic system the conductor is moving relative to the magnetic source, thus generating
eddy currents that will dissipate into heat sue to the resistivity of the conductor. This process
of generation and dissipation of eddy current causes the system to function as a viscous
damper.
FUNCTION OF AUTOMOTIVE SUSPENSION
SHOCK ABSORBER
The shock absorber is the main damping element of an automotive suspension system
and plays an important role in smoothness and safety of automobiles. When the automobile
runs on different roads, the automobile frame (or bearing chassis) and the driving axle (or
wheels) do reciprocating motion, and the function of the automobile suspension is to transmit
vertical reactive force (bearing force), longitudinal reactive force (tractive force and braking
force) and lateral reactive force of road surface against wheels and torques caused by the
reactive force to the automobile frame (or bearing chassis) so as to guarantee normal running
of the automobile.
The working mechanism of the suspension shock absorber is that when the
automobile frame and the driving axle do reciprocating motion, the piston of the shock
absorber also does reciprocating motion in the cylinder, oil in the shell of the shock absorber
repeatedly flows from one cavity to another cavity via some narrow gaps, at this time, the
friction between the hole walls and oil molecules and between the throttling hole and the oil
molecules form the damping force against the vibration, the vibration energy of the
automobile body and the automobile frame is converted into heat energy, and the heat energy
is absorbed by the oil and the shell of the shock absorber and then emitted to the atmosphere.
The damping force of the shock absorber increases or decreases along with the increase or
decrease of the relative speed of the automobile frame and the driving axle(or wheels), and is
related to viscosity and variety of the oil.
CONCLUSION
In the current scenario of automobile industry the need for vehicles which provides
smooth and comfort ride is growing. In a country like ours whose roads are not up to
world standards the need for automotive components like shock absorber are necessary.
It goes without saying that if the right choice is made the improvements in vehicles ride
and handling can be shocking.