SYNOPSIS
In the present scenario of automobile industry manufacturers are trying to produce comfortable and safe vehicles which the consumers are looking for. A shock absorber is a damping element of the vehicle suspension, and its performance directly affects the comfortability, dynamic load of the wheel and dynamic stroke of the suspension. The conventional type of shock absorbers has got the main drawback that it causes foaming of the fluid at high speeds of operation. This results in a decrease of the damping forces and a loss of spring control. The gas filled shock absorbers are designed to reduce foaming of the oil and provide a smooth ride for a long period.
INTRODUCTION
For a smooth and comfortable ride the disturbing forces should be eliminated or reduced considerably by using some devices. Shock absorbers are such devices which isolate the vibrations by absorbing some disturbing energy themselves. Of the many types telescopic shocks are widely used which has got the draw back that the flow of oil in the cylinder can cause foam of oil and air to form. These limit the optimum throughout of the flow in the valves. Gas shocks represent an advance over traditional shocks. Nitrogen filled gas shock absorbers are the results of years of extensive research and development with top flight shock design engineers. They are designed for both lowered and stock vehicles to provide shock absorbers that would out perform anything on the market today. Nitro shock absorbers are high quality, nitrogen filled shocks designed and gas charged specifically for each vehicle application. The addition of nitrogen under pressure limits the foaming effect and increases efficiency.
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 mean time, 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. A shock absorber is basically a hydraulic damping mechanism for controlling spring vibrations. It controls spring movements in both directions: when the spring is compressed and when it is extended, the amount of resistance needed in each direction is determined by the type of vehicle, the type of suspension, the location of the shock absorber in the suspension system and the position in which it is mounted. Shock absorbers are a critical product that determines an automobile’s character not only by improving ride quality but also by functioning to control the attitude and stability of the automobile body.
PRINCIPLE OF OPERATION
The damping mechanism of a shock absorber is viscous damping. Viscosity is the property of a fluid by virtue of which it offers resistance to the motion of one layer over the adjacent on. The main components of a viscous damper are cylinder, piston and viscous fluid. There is a clearance between the cylinder walls and the piston. More the clearance more will be the velocity of the piston in the viscous fluid and it will offer less value of viscous damping coefficient. The basic system is shown below. The damping force is opposite to the direction of velocity.
[pic] I-CLEARNCE, II-PISTON, III-VISCOUS FLUID The damping resistance depends on the pressure difference on the both sides of the piston in the viscous medium. The figure shown below shows the example of free vibrations with viscous damping. [pic]
The equation of motion for the system can be written as mx + cx +kx = 0
Energy dissipation in viscous damping : For a vibratory body some amount of energy is dissipated because of damping. This energy dissipation can be per cycle. Rate of change of work W is called energy. For a viscously damped system the force F is expressed as
F= cx = cdx/dt, where x = dx/dt
Work done W = Fx = (cdx/dt) x
The rate of change of work per cycle
i.e. Energy dissipated
Let us assume the simple harmonic motion of the type x = Asinωt (dx/dt) ² = ω²A²cos²ωt
The equation for
This shows that the energy dissipation per cycle is proportional to the square of the amplitude of motion.
The total energy of a vibrating system can be either maximum of its potential or kinetic energy. The maximum kinetic energy of the system can be written as E = (KE) max = 1/2mx²max = 1/2mω²A²
SHOK ABSORBER ACTION
Shock absorbers develop control or resistance by forcing fluid through restricted passages. A cross-sectional view of a typical shock absorber is shown below. Its main components and working is also given below.
[pic]
The inside parts of a shock absorber
The upper mounting is attached to a piston rod. The piston rod is attached to a piston and rebound valve assembly. A rebound chamber is located above the piston and a compression chamber below the piston. These chambers are full of hydraulic fluid. A compression intake valve is positioned in the bottom of the cylinder and connected hydraulically to a reserve chamber also full of hydraulic fluid. The lower mounting is attached to the cylinder tube in which the piston operates.
During compression, the movement of the shock absorber causes the piston to move downward with respect to the cylinder tube, transferring fluid from the compression chamber to the rebound chamber. This is accomplished by fluid moving through the outer piston hole and unseating the piston intake valve. During rebound, the pressure in the compression chamber falls below that of the reserve chamber. As a result, the compression valve will unseat and allow fluid to flow from the reserve chamber into the compression chamber. At the same time, fluid in the rebound chamber will be transferred into the compression chamber through the inner piston holes and the rebound valve.
|[pic] |[pic] |
|Spring |Schematic Diagram of the Interior of a Shock Absorber |
FORMS OF SUSPENSIONS AND TYPES OF SHOCK ABSORBERS
Various types of shock absorbers are available in the market. Out of that the widely used types and their characteristics are given below.
| Type | Product | Characteristics |
| |[pic] | |
| |Double-tube | |
| | |The outer part of the double tube is used as a |
| | |gas chamber, which is filled with low- pressure |
| | |nitrogen gas. This type can provide stable |
| | |damping force. |
| | | |
| | | |
| | | |
|Double-wishbone | | |
|(Multilink) | | |
| |[pic] | |
| |Single-tube | |
| | |Separation between oil and nitrogen gas by a free|
| | |piston provides stable damping force, as well as |
| | |high performance. |
| | | |
| | | |
| | | |
| | | |
| | | |
| |[pic] | |
| |Double-tube | |
| | |This type consists of double tubes that comprise |
|Strut | |part of the support structure of the suspension. |
| | |Filled with low-pressure nitrogen gas, it |
| | |provides stable damping force. |
| |[pic] | |
| |Inverted type | |
| | |Structurally, this is a single-tube type placed |
| | |upside down. Its large-diameter pipe provides |
| | |sufficient rigidity to bear the heavy load from |
| | |the car body, characteristic of a strut. |
| |[pic] | |
| |With a steering arm | |
| | |When connected to the power steering system at a |
| | |point higher than normal, this type allows the |
| | |cabin space to be expanded and the maneuvering |
| | |stability improved. |
| |[pic] | |
| |Unit damper | |
| | |Because the spring is mounted separately, this |
|Type with separately mounted spring | |type features a simple structure comprised of a |
|(rigid axle, etc.) | |damping mechanism |
WHY GAS FILLED SHOCK ABSORBERS?
The rapid movement of the fluid between the chambers during the rebound and compression strokes can cause foaming of the fluid. Foaming is the mixing of free air and the shock fluid. When foaming occurs, the shock develops a lag because the piston is moving through an air pocket that offers up resistance. The foaming results in a decrease of the damping forces and a loss of spring control. During the movement of the piston rod, the fluid id forced through the valuing of the piston. When the piston rod is moving quickly, the shock absorbers oil cannot get through the valuing fast enough, which causes pressure increases in front of the piston and pressure decreases behind the piston. The result is foaming and a loss of shock absorber control. The need for a gas filled shock absorber arises here.
GAS FILLED SHOCK ABSORBER
The gas filed shock absorbers is designed to reduce the foaming of the oil. It uses a piston and oil chamber similar to other shock absorbers. The difference is that instead of a double tube with a reserve chamber, a dividing piston separates the oil chamber from the gas chamber. The oil chamber contains a special hydraulic oil and the gas chamber contains nitrogen at 25 times atmospheric pressure. The schematic diagram showing the inside parts of a gas filled shock absorber is shown below.
[pic] The inside parts of a gas-filled shock absorber.
When the piston rod is moved into the shock absorber, oil is displaced as in double tube principle. This oil displacement causes the dividing piston to press in the gas chamber, thus reducing it in size. With the return of the piston rod the gas pressure returns the dividing piston to its starting position.
Whenever the oil column is held at a static pressure of approximately 25 times atmospheric pressure, the pressure decreases behind, the working piston cannot be high enough for the gas to exit from the oil column. Consequently, the gas filled shock absorber operates without foaming.
TYPES OF GAS FILLED SHOCK ABSORBERS • Twin– tube with low pressure gas. • Single- tube with high pressure gas.
LOW PRESSURE TWIN- TUBE SHOCKS Twin- tube gas technology design retains the classical twin-tube while adding at the top of the reserve tube nitrogen under relatively low pressure 2.5- 5 bars instead of 25- 30 bars used in high pressure shock absorbers. This pressure is sufficient to radically improve the efficiency of the shock absorbers.
HIGH PRESSURE SINGLE- TUBE SHOCKS Gas shock absorbers operate in the same principle of movement of the piston in an oil filled tube but they contain at one end a small quantity of nitrogen under high pressure (25 bars). The gas is prevented from mixing with the oil by a floating piston. When the piston rod passes into the body and displaces oil, the oil compresses the nitrogen even further. The volume of gas changes playing the role as an equalization tube. The permanent pressure exerted on the oil by the gas guarantees an instantaneous response and the quieter piston valve operation. At the same time this constant pressure eliminates cavitations and foaming which could momentarily degrade the effectiveness of the shock absorber.
WORKING
TWIN– TUBE SHOCK ABSORBERS :
The main components are: • Outer tube, also called reservoir tube • Inner tube, also called cylinder • Piston connected to a piston rod • Bottom valve, also called foot valve • Upper and lower attachment
How does it work?
Bump Stroke: When the piston rod is pushed in oil flows without resistance from below the piston through the orifices and the non-return valve to the enlarged volume above the piston. Simultaneously, a quantity of oil is displaced by the volume of the rod entering the cylinder. This volume of oil is forced to flow through the bottom valve into the reservoir tube (filled with air (1 bar) or nitrogen gas (4-8 bar)). The resistance, encountered by the oil passing through the footvalve, generates the bump damping. [pic]
Rebound Stroke: When the piston rod is pulled out, the oil above the piston is pressurized and forced to flow through the piston. The resistance, encountered by the oil on passing through the piston, generates the rebound damping. Simultaneously, some oil flows back, without resistance, from the reservoir tube through the footvalve to the lower part of the cylinder to compensate for the volume of the piston rod emerging from the cylinder.
[pic]
MONO- TUBE SHOCK ABSORBERS :
The main components are: • Pressure cylinder, also called housing • Piston rod connected to a piston rod • Floating piston, also called separating piston • Piston rod guide • Upper and lower attachment
How does it work?
Bump Stroke: Unlike the bi-tube damper, the mono-tube has no reservoir tube. Still, a possibility is needed to store the oil that is displaced by the rod when entering the cylinder. This is achieved by making the oil capacity of the cylinder adaptable. Therefore the cylinder is not completely filled with oil; the lower part contains (nitrogen) gas under 20-30 bar. Gas and oil are separated by the floating piston. When the piston rod is pushed in, the floating piston is also forced down the displacement of the piston rod, thus slightly increasing pressure in both gas and oil section. Also, the oil below the piston is forced to flow through the piston. The resistance encountered in this manner generates the bump damping. [pic]
Rebound Stroke: When the piston rod is pulled out, the oil between piston and guide is forced to flow through the piston. The resistance encountered in this manner generates the rebound damping. At the same time, part of the piston rod will emerge from the cylinder and the free (floating) piston will move upwards.
ADVANTAGES OF NITRO SHOCKS
Instantaneous response : • Because the high pressure eliminates aeration (foaming), action is always is immediate. • The low mass of gas and the single tube further improves response time.
Better fade resistance : • Since there is no outer tube, cooling is much better which gives a drastic reduction in fade. Thus more consistent handling and control.
Better durability : • Single-tube construction also allows for a larger internal working area, reducing stress and fatigue for better durability. • De Carbon’s monodisc valving system features a single moving part that drastically reduces inertia and friction, to improve durability and performance. • Better cooling of the mono tube design results in lower operating temperatures and thus longer life.
No need for re-adjustment: • The viscosity of hydraulic fluid changes as temperature changes. This may because of climate, season (summer/winter) or heavy duty (motorway cruising). The high pressure gas compensates immediately and automatically for changes in viscosity.
TIPS BEFORE MOUNTING
A stiff suspension does not necessarily mean good handling. Often the contrary. If still a stiff suspension is needed it should come from the springs. The function of the shock absorber is to dampen oscillations of the spring by converting energy to heat. Do not use shock absorbers to obtain a stiff suspension. Shock absorbers and springs each have their own function. Respect those functions. Do not use new shocks to compensate for old and tired springs. The shocks will soon fail when the springs are bad. Worn shocks do not only reduce safety and handling, they also increase the risk of having a broken spring as the spring is allowed to oscillate.
When to buy shocks?
Shock absorbers last a long time, but they tend to degrade slowly throughout their life. So when is it time to replace them? In some cases, a seal will rupture. A shock covered in oil is a good indication that it has failed. The age-old test of bouncing on a fender is really only a rough guide as to whether the vehicle needs new shocks. Usually the slow degradation in shock absorber's performance won't be noticed until it affects handling fairly dramatically. Depending on how rough the roads are, modern shocks can last 80-100,000 miles, but remember that a shock with 60,000 miles on it won't perform as well as a new one.
Which ones are right? Choosing which shocks to buy largely depends upon what kind of vehicle and the kind of driving. As with most automotive components, it is important the specific vehicle, since mismatched shocks can drastically affect handling and could even be dangerous. The best advice will probably come from a mechanic who is familiar with the vehicle.
CONCLUSION
In the current scenario of automobile industry the need for vehicles which provides smooth and comfort ride is growing. Nitro shock absorbers are designed to be ultimate in performance and comfort. In a country like ours whose roads are not up to world standards the need for automotive components like nitro shocks are necessary. It goes without saying that if the right choice is made the improvements in vehicles ride and handling can be shocking.
REFERENCES
1. “In For A Shock”, S.B.L Beohar; Mechanical Systems and Signal Processing. 2. Automotive Encyclopedia; Tobolt, Johnson. 3. Auto Mechanical Fundamentals; Stockel. 4. http://belltech.net/ 5. http://monroeshocks.com
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