Air Suspension Spring
● A volume of air enclosed either in a cylinder fitted with a piston or in a flexible bellows can be used as spring.
● Under static load the air is compressed to predetermined pressure and subsequently motion of the piston either increase and decreases the pressure and consequently increases or decreases the force acting on piston.
● Air springs are fairly widely employed on vehicle whose laden and unladen weight differ greatly.
Advantages of air suspension
(1) It maintains constant road height from road surface.
(2) The stiffness of system increases with increases in deflection
(3) Improve standard of ride comfort and reduces noise.
● The common type of air suspension springs used are explain below,
Type of air suspension springs
(a) Bellow type air spring
(b) Piston type air spring
(a) Bellow type air spring
● This spring consist of rubber bellow which is generally circular in cross section. The bellow has two convolutions for proper functioning. The constructional details are shown in the simplified sketch. The type of spring is a good replacement of coil spring.
(b) Piston type air spring
● This type spring consists of a metal air container in the form of an inverted drum. The drum is fixed to the frame and the sliding piston is attached to the lower wishbone. For making the system leak proof a seal is provided by a flexible diaphragm. The diaphragm is secured at its outer circumference to the lip of the drum and its centre to the piston.
Air Suspension System
● Now-a-days pneumatic suspension systems are employed in some tourist vehicles to improve the riding comfort of the passenger
● A line diagram of the air suspension system with air. It consists of four air bags.
● The elastic element consists of housing, diaphragm with suitable air inlet and piston group. The housing is linked with the frame (sprung mass). The housing contains the piston group which is linked with unsprung mass (rear and front axle)
● The housing and piston group are connected by a metallic diaphragm for better sealing and to reduce friction between the members of clastic elements.
● Each air bag is filled with compressed air. The sketch of air bag.
● The component of air suspension system are air filter, compressor, air reservoir, relief valve, pressure regulator, solenoid valve, levelling valve and "T check valve.
● An air compressor is mounted on the vehicle frame supplies high pressure air into the reservoir. The compressor sucks the air from atmosphere through air filter.
● The pressure in the reservoir is maintained at about 20 kg/cm.sq
● The compressed air in the air bags support the weight of vehicle whenever vehicle come across the bump of the room surface, the air in the air bag compressed and absorbs the shocks.
● Air is admitted into the four air bags through two circuits. In one circuit a pressure reduced to 12 kg/cm by pressure regulator.
● This pressure is kept in air bag through levelling valve if the pressure in one of the bag is low, the levelling arm move and air is admitted into low pressure air bag through inlet valve.
● The outer circuit has air pressure of 20 kgf/cm. This circuit pressure is used for additional loading of vehicle.
● This can be achieved by special regulator, which keeps the same distance between wheel and frame under various load. This circuit maintain the vehicle level constant in loaded or unloaded condition
● For this situation, the air at 20 kgf/cm pressure is admitted into the levelling valve through solenoid valve.
● Also the air is quickly released by levelling valve whenever load is decreased from corresponding air bag. This lower the air bag and hence the vehicle to the proper level.
Advantages
1. Constant frequency of vibration
2. Highly smooth run due to low rigidity.
3. Provide constant body height under all static load taken by elastic element.
4. Avoid change in head lamp alignment due to varying load.
5. Longer service life
Disadvantages
1. Complicated in design.
2. System is more costly
3. Risk of breakdown.
4. Freezing of moisture in air in cold weather.
5. Greater maintenance required.
Shock Absorber
● For providing comfortable ride to the passengers and driver as well as reduce additional stresses to vehicle frame. It necessary to provide a proper device in the suspension system for damping out the effect of road irregularities vehicle must be roll or bounce or sway during turning on good road condition.
● The springing device must be a compromise between flexibility and stiffness. If it is rigid, it will not absorb the shock efficiently and if it is more flexible it will continue to bounce or vibrate even after the bump has passed
● In leaf spring damping is provided due to friction but because of uncertainty in lubrication, friction between blade or leaves varies its damping characteristics due to this reason an additional damper (shock absorber) must be provided.
● The shock absorber or damper is a device which absorb shock energy by damping and dissipate into heat. So it serve to control amplitude and frequency of bouncing It cannot support weight and has zero resilience
● There are many types of shock absorber operating on friction, on compressed air and hydraulically
● The hydraulic shock absorber is the only type in common use at present The hydraulic shock absorber contain fluid that forces through restricting orifices as the shock absorber is operated spring flexure.
Principle of Hydraulic Shock Absorber
● When a fluid forces through restricting or small orifices it develop the resistance to the movement of the fluid through the restricting orifices by absorbing shock energy of damping and control amplitude and frequency of bouncing and dissipate into heat.
● It cannot support weight and has zero resilience. This effect quickly damped and spring oscillation damping is proportional to square of the speed.
Telescopic Shock Absorber :
Construction:
● It consist of a cylinder to which head is welded.
● The head is screwed into the top end of the outer tube.
● To the bottom end of the outer tube is welded with a pressed steel cap and eye or ring. This eye is connected to wheel axle.
● The piston slides inside the cylinder. This piston is secured to the piston rod which at its upper end has an eye welded to it. With this eye, the piston rod is attached to the frame of the vehicle the outside part of the piston rod is protected by a dust shield (cover) which is welded to the fixing eye.
● A gland prevents oil leakage from where the piston rod passes through the head
● The gland consist of a piston rod, oil seal, oil seal gasket, seal retainer, oil seal spring and oil seal cup.
● Any fluid scraped off by the gland packing passes down a drain hole to the reservoir space between the cylinder and outer tube
● The piston has two concentric rings of holes drilled through it.
● The outer ring of hole is covered at the top by a disc valve which is held down by a star shape die spring
● The inner ring hole is covered by dise valve from bottom by the coil spring.
● There is a foot valve assembly at the bottom of the cylinder. The foot valve assembly is similar to that piston assembly except that the lower disc valve which covers the inner ring of holes is held up by a dise valve spring instead of coil spring
● Both ends of the cylinder are completely filled by a mixture of 60% transformer oil and 40% turbine oil and the space between the tube and cylinder.
Working:
● When a vehicle come across the bump, the bottom eye is moved upwards, then the fluid below the piston muk displaced to the top side of the piston.
● The fluid will now pass through the outer ring of hole in the piston by lifting the top disc against the disc spring. But the volume above the piston is less due to piston rod.
● As such, fluid from the bottom of the piston will also get displaced through inner ring of holes in the foot valve and enter the reservoir space between the cylinder and outer tube.
● So the fluid level in the reservoir space will rise
● The pressure set up in the system will depend upon the size of the passage open by valve in the piston and foot valve.
● This will depend on the square of the speed at which the cylinder is moved upward.
● When the cylinder moves downward, fluid will be displaced from the upper end of the cylinder to lower end through the inner ring of hole in the piston by opening the lower disc valve against coil spring.
● Because of the volume of the piston rod that leaves the cylinder, the fluid will be drawn into the lower end of the cylinder from the reservoir space through the outer ring of hole in the foot valve. This passing of fluid through opening provide damping.
Gas Filled Shock Absorbers :
● Gas filled shock absorbers work along the same principle as a conventional hydraulic shock absorber but also have a gas cushion that be compressed, therefore creating space for the compressed oil.
● The gas filled shock-ups can be easily differentiated by 'Canister' which holds the nitrogen gas inside and attached to the conventional spring loaded shock absorber.
● The canister is used to be filled with pressurized Nitrogen gas, which further act as reservoir for both oil and Nitrogen gas. The role of the Nitrogen gas is to keep the damping oil pressurized so that the gas molecules present in the oil remain bonded with the oil molecules and must not get loosen to form bubbles/foam and hence prevent the Cavitation .
● A floating piston separates oil and gas, thus preventing them from mixing. If the oil is displaced when the spring is compressed, the piston compresses the gas cushion. When the spring then extends, the gas pushes the oil back, similar to a spring. The gas pressure is very high and ensures that even the smallest movements are damped
(a) Single cylinder or mono tube shock absorber
● In a mono-tube shock absorber, the shocks components are contained within one tube. A mono-tube system conta shell case which works as a cylinder. Within that cylinder you would find a piston valve, oil and gas. The construct of a mono-tube does also include some differences compared to the twin-tube design.
● A mono-tube design utilizes a free piston which separates the oil chamber from the gas chamber within the shell cas Air is not able to penetrate the oil or gas since they are completely separated. As temperature increases, oil is able release heat with less effort.
● It has zero restrictions on installation angles and Less chance of cavitation.
Disadvantage
● More expensive and high pressured gas creates a higher amount of stress on scals which can cause more friction
(b) Twin-Tube
● In a twin-tube shock absorber there are two cylinders. In one cylinder is placed inside the shell case and the second cylinder contains the piston valve which moves up and down within this second cylinder known as the "inner cylinder With a twin-tube design, there is no piston or barrier between the oil chamber and gas chamber.
● Advantage of this is Unlike the inverted design, friction can be stopped. External damage to the shell case does not effect the shock absorber. Low gas pressure reduces stress on scalp and also keeps friction to a minimum.
Disadvantage of this Oil capacity is reduced in comparison to the mono-tube. Low Pressure - Generally very Seft.
