Hydraulic Shock Absorber

What is the hydraulic shock absorber?

The hydraulic shock absorber is a major part of the suspension system of the vehicles or moving machines. It is a device which is made to absorb and reduce the shock impulses. To reduce the rebound and moving load, the hydraulic shock absorber plays a vital part in the suspension system; other devices such as compression spring, dashpot, or rubber snubber also plays a role in reducing the impulses during driving. The mechanism of the hydraulic shock absorber involves the conversion of kinetic energy to thermic energy. There is fluid in the hydraulic shock absorber which produces hydraulic resistance by flowing through the restricted outlets and valve systems.


The anatomy of hydraulic shock absorber

The hydraulic shock absorber comes in with the style of a tube-like device which contains the outer tube, the inner tube, the valve system, the piston rod and the return spring. The outer tube is also called the reservoir tube. The inner tube of the absorber is also called the cylinder. The piston rod is in the very middle of the shock absorber which has the mobility to move upward and downward in the absorber. The valve system is also called the bottom valve or the foot valve. The valve surrounds the piston in the cylinder. And lastly, there are upper and lower attachment on the top and bottom of the hydraulic shock absorber which connects the vehicle and the tire.

The tubular hydraulic shock absorber can be compressed and extended. When the absorber receives the impact from the ground through the tires, it will be compressed. When it is compressed, the piston rod inside the inner tube moves upwards. The hydraulic resistance will then force the piston rock to go back to where it was and therefore the absorber extends. The process that the piston rod goes upward and downward, or the absorber compressed or extended, is called the bump stroke and rebound stroke. By the different inner structure and mechanism the shock absorbers have, they can be categorized into two subtypes. There are bi-tube or twin-tube shock absorbers and there are mono-tube shock absorbers. The hydraulic shock absorbers are often the twin-tube absorbers.


How does the bi-tube hydraulic shock absorber work?

When the shock absorber receives impact, the piston rod inside the inner tube is forced to go upward. The oil inside the absorber flows from below the piston through the cavity around the cylinder. When the piston rod advances inside the cylinder, the oil is forced to flow through the bottom valve into the reservoir tube, the outer tube. The resistance occurs when the oil passes through the bottom valve and the impact is hence damped and reduced. This whole process that the piston is compressed, oil forced to go into the outer tube and the resistance generates is called the bump stroke. The resistance that is generated during the bump stroke is called the bump damping.

When the piston rod is pulled out from the cylinder and the shock absorber is extended, the oil at the bottom is forced to flow through the piston rod. When the oil passes through the piston, the resistance is generated and it is called the rebound damping. When the piston rod is pulled out from the cylinder, the oil that former went into the reservoir tube during the bump stroke now flows back into the inner tube through the bottom valve. This process that the piston rod and oil relocate and generate the rebound damping is called the rebound stroke. The impact that is received by the hydraulic shock absorber is damped and reduced during the bump stroke and rebound stroke.


How does the mono-tube hydraulic shock absorber work?

The mono-tube shock absorber does not have the reservoir tube, the outer tube. Since there is no outer tube for the oil to go when the piston rod is forced into the cylinder, the amount of the oil in the mono-tube hydraulic shock absorber is adjusted. The cylinder of the mono-tube absorber is not filled fully with oil; there is also nitrogen gas inside the cavity of the cylinder. There is a floating piston to separate the gas and the oil inside the cylinder. When the piston rod is pushed inside the cylinder, the floating piston is forced down ward and pressurized both the gas and oil. The resistance is generated while the oil flows through the piston. This is the bumping damping of the mono-tube absorber during the bump stroke.

During the rebound stroke, the piston rod is pulled out from the cylinder and the oil is forced to relocate to where it was. When the oil passes through the piston rod, the resistance is generated and hence the rebound damping. When the piston rod goes back downwardly, the floating piston is forced to go upwards again.


What happens when the hydraulic shock absorber does not work?

The hydraulic shock absorbers are designed to reduce the impact from the ground when a vehicle is moving. If the shock absorber does not function as it is supposed to do, the impact and bounce will become very obvious and the vehicle may dive and squat excessively. The influence is not just that the passengers feel bouncing and uncomfortable and get hurt potentially during the ride. The motion control of the vehicle thus becomes difficult and it is dangerous when the vehicle is on the road.

Also, when the hydraulic shock absorbers fail to absorb the direct impact, the body of the vehicle does instead. Once the shock spreads directly to the moving parts or components of the vehicle, the components will be broken or fail to function; this makes it extremely risky to keep on riding the vehicle and disastrous consequences can be resulted in. It is strongly recommended that one should have the shock absorber checked as soon as possible when he or she is driving the vehicle and feeling that the motion control or the suspension is tricky. Do not drive it in a high speed and avoid sudden turns or stops until the absorbers are fixed or replaced.

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