23-05-2014, 04:09 PM
A Seminar report on Nitro Shock Absorber
Nitro Shock Absorber.doc (Size: 351 KB / Downloads: 167)
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.
A BRIEF HISTORY
In the early 1900's, cars still rode on carriage springs. After all, early drivers had bigger things to worry about than the quality of their ride - like keeping their cars rolling over the rocks and ruts that often passed for roads.
Pioneering vehicle manufacturers were faced early on with the challenges of enhancing driver control and passenger comfort. These early suspension designs found the front wheels attached to the axle using steering spindles and kingpins. This allowed the wheels to pivot while the axle remained stationary. Additionally, the up and down oscillation of the leaf spring was damped by device called a shock absorber.
Compression cycle
During the compression stroke or downward movement, some fluid flows through the piston from chamber B to chamber A and some through the compression valve into the reserve tube. To control the flow, there are three valving stages each in the piston and in the compression valve.
At the piston, oil flows through the oil ports, and at slow piston speeds, the first stage bleeds come into play and restrict the amount of oil flow. This allows a controlled flow of fluid from chamber B to chamber A.
At faster piston speeds, the increase in fluid pressure below the piston in chamber B causes the discs to open up away from the valve seat.
At high speeds, the limit of the second stage discs phases into the third stage orifice restrictions. Compression control, then, is the force that results from a higher pressure present in chamber B, which acts on the bottom of the piston and the piston rod area.
Extension cycle
As the piston and rod move upward toward the top of the pressure tube, the volume of chamber A is reduced and thus is at a higher pressure than chamber B. Because of this higher pressure, fluid flows down through the piston's 3-stage extension valve into chamber B.
However, the piston rod volume has been withdrawn from chamber B greatly increasing its volume. Thus the volume of fluid from chamber A is insufficient to fill chamber B. The pressure in the reserve tube is now greater than that in chamber B, forcing the compression intake valve to unseat. Fluid then flows from the reserve tube into chamber B, keeping the pressure tube full.
Extension control is a force present as a result of the higher pressure in chamber A, acting on the topside of the piston area.
Basic Twin Tube Design
The twin tube design has an inner tube known as the working or pressure tube and an outer tube known as the reserve tube. The outer tube is used to store excess hydraulic fluid.
There are many types of shock absorber mounts used today. Most of these use rubber bushings between the shock absorber and the frame or suspension to reduce transmitted road noise and suspension vibration. The rubber bushings are flexible to allow movement during suspension travel. The upper mount of the shock absorber connects to the vehicle frame.
Notice that the piston rod passes through a rod guide and a seal at the upper end of the pressure tube. The rod guide keeps the rod in line with the pressure tube and allows the piston to move freely inside. The seal keeps the hydraulic oil inside and contamination out.
The base valve located at the bottom of the pressure tube is called a compression valve. It controls fluid movement during the compression cycle.
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.
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.