SHOCK ABSORBERS/ DAMPERS: WORKING PRINCIPLE, CLASSIFICATION AND FUNCTIONS
Shock absorbers are basically oil pumps. A piston is attached to the end of the piston rod and works against hydraulic fluid in the pressure tube. As the suspension travels up and down, the hydraulic fluid is forced through tiny holes, called orifices, inside the piston. However, these orifices let only a small amount of fluid through the piston. This slows down the piston, which in turn slows down spring and suspension movement.
All modern shock absorbers are velocity sensitive hydraulic damping devices – meaning the faster the suspension moves, the more resistance the shock absorber provides.
Because of this feature, shock absorbers adjust to road conditions. As a result, shock absorbers reduce the rate of:
• Roll or sway
• Brake dive and Acceleration squat
Shock absorbers work on the principle of fluid displacement on both the compression and extension cycle. A typical car or light truck will have more resistance during its extension cycle then its compression cycle. The compression cycle controls the motion of a vehicle’s unsprung weight, while extension controls the heavier sprung weight.
FUNCTIONS OF DAMPER
The main function of the shock absorber is to absorb the shocks and damp them as soon as possible so that a smooth ride can be obtained.
Some other important functions of shock absorber are
It limits vehicle body movement
It stabilizes our ride as discussed above
It stabilizes vehicle tires which are disturbed due to sudden shock, hence it is very important for safety purpose also
It also minimizes tire and body wear of the automobile and hence reduces overall maintenance cost
It may sound a simple job but this is the main thing on which the comfort level of your ride depends.
To understand the shock absorber, it is very important to understand its working.
First of all, we should know that there are generally two types of shock absorbers one is hydraulic and another one is pneumatic. However, working of both the types of shock absorbers is same.
A shock absorber is generally coupled with a spring, which convert sudden shock waves into oscillatory motion. This oscillatory motion gives us instant relief from the shock but, nobody can have his or her whole ride with these oscillations.
Here is the need of shock absorber arises, it is used to damp those oscillations which are made by the springs.
A general shock absorber contains a perforated piston in a hydraulic chamber. The chamber is totally sealed and hence if piston has to make some movement the only way is to let the hydraulic liquid pass through it.
When a shock comes, piston has to move due to shock. When the piston moves than the hydraulic liquid in the shock absorber has to pass through it.
When the liquid is passed through the very tiny perforated holes in the piston the piston has to do some work against it. That work is done on that expense of the energy generated due to the shock and hence soon the shock absorber loses all the shock energy, which results into no oscillation and smooth ride.
SHOCK ABSORBER DESIGN TYPES
There are several shock absorber designs in use today:
1. Twin Tube Designs
• Gas Charged
• PSD (position sensitive damping)
• ASD (Acceleration Sensitive Damping)
A. Twin Tube – Gas Charged Design
The prime function of gas charging is to minimize aeration of the hydraulic fluid. The pressure of the nitrogen gas compresses air bubbles in the hydraulic fluid. This prevents the oil and air from mixing and creating foam. Foam affects performance because it can be compressed – fluid can not. With aeration reduced, the shock is able to react faster and more predictably, allowing for quicker response time and helping keep the tire firmly planted on the road surface.
• Improves handling by reducing roll, sway and dive
• Reduces aeration offering a greater range of control over a wider variety of road conditions as compared to non-gas units
• Reduced fade – shocks can lose damping capability as they heat up during use. Gas charged shocks could cut this loss of performance, called fade
B. Twin Tube – PSD Design
Ride engineers had to compromise between soft valving and firm valving. With soft valving, the fluid flows more easily. The result is a smoother ride, but with poor handling and a lot of roll/sway. When valving is firm, fluid flows less easily. Handling is improved, but the ride can become harsh.
With the advent of gas charging, ride engineers were able to open up the orifice controls of these valves and improve the balance between comfort and control capabilities available in traditional velocity sensitive dampers.
A leap beyond fluid velocity control is an advanced technology that takes into account the position of the valve within the pressure tube. This is called Position Sensitive Damping (PSD).
The key to this innovation is precision tapered grooves in the pressure tube. Every application is individually tuned, tailoring the length, depth, and taper of these grooves to ensure optimal ride comfort and added control. This in essence creates two zones within the pressure tube.
The first zone, the comfort zone, is where normal driving takes place.
The second zone, the control zone, is utilized during demanding driving situations.
• Allows ride engineers to move beyond simple velocity sensitive valving and use the position of the piston to fine tune the ride characteristic.
• Adjusts more rapidly to changing road and weight conditions than standard shock absorbers
• Two shocks into one – comfort and control
C. Twin Tube -ASD Design (Reflex )
A new twist on the comfort/ control compromise is an innovative technology which provides greater control for handling while improving ride comfort called Acceleration Sensitive Damping (ASD).
This technology moves beyond traditional velocity sensitive damping to focus and address impact. This focus on impact is achieved by utilizing a new compression valve design. This compression valve is a mechanical closed loop system, which opens a bypass to fluid flow around the compression valve.
• Control is enhanced without sacrificing driver comfort
• Valve automatically adjusts to changes in the road condition
• Reduces ride harshness
2. Mono-tube design (Standard Types)
These are high-pressure gas shocks with only one tube, the pressure tube. Inside the pressure tube there are two pistons: a dividing piston and a working piston. The working piston and rod are very similar to the twin tube shock design. The difference in actual application is that a mono-tube shock absorber can be mounted upside down or right side up and will work either way. In addition to its mounting flexibility, mono-tube shocks are a significant component, along with the spring, in supporting vehicle weight. Another difference you may notice is that the mono-tube shock absorber does not have a base valve. Instead, all of the control during compression and extension takes place at the piston.
During operation, the dividing piston moves up and down as the piston rod moves in and out of the shock absorber, keeping the pressure tube full all times.
• Can be mounted upside down, reducing the unsprung weight
• May run cooler since the working tube is exposed to the air
• Original equipment many import and performance domestic passenger cars, SUV and light truck applications