Telescopic dampers can be differentiated into mono tube and twin-tube dampers. Figure shows the configuration and principle of the two damper systems.
Both damper types include a piston consisting of throttle elements which travels in a cylinder filled with liquid by overcoming the flow resistance. The mechanical work absorbed is converted into heat and transferred to the environment over the external tube of the damper. If the damper surface is not large enough for heat dissipation, the surface must be enlarged. Otherwise the absorber would overheat, which could cause damage to the rubber parts.
In the conventional Twin Tube Damper, the liquid volume displaced by the immersing piston rod is transferred over the bottom valve into the casing tube which serves as the compensational volume and returns when the piston rod is withdrawn. At rest, the liquid pressure is equal to the ambient pressure.
In order to reduce cavitation at the valves, considerable damping work can be achieved by those oil flows, which are not drawn into an enlarging part of the working chamber.
During the compression of the twin-tube absorber, most of the damping work is done by the oil flow through the bottom valve into the casing tube as result of the intrusion of the piston rod. A second part of the oil flows into the upper part of the working chamber without any resistance.
On the other hand, during the rebound phase of the conventional twin-tube damper, the damping work is done by oil flow, which takes place through the piston valve from the upper part of the working chamber to the lower part. Accordingly, a resistance-free return of oil takes place from the compensating volume as a result of the returning piston rod.
In the Mono Tube Damper, the liquid volume displaced by the piston rod is compensated by the compression of a gas volume included in the damper. This gas volume when located above the surface of the oil, is separated from it by an impact plate, or by a dividing piston when the gas is placed below the oil surface in the damper.
The force exerted on the piston surface of the damper by gas pressure must be larger than the maximum damping force, since otherwise, the gas volume would be compressed by the sudden movement of the piston, while in the part of the damper working chamber facing the gas volume, the pressure would drop to 0 bar. The sudden reversal of the direction of the piston rod movement, would cause a brief breakdown of the damper and cavitation in the area around the piston valves. The gas pressure usually amounts to 30-40 bar. The gas pressure acting on the cross section of the piston rod results in a force which drives the piston rod outwards. This must be taken into consideration in the design of the body suspension.
The advantage of the twin tube damper is its cost and life span. The disadvantages include its tendency towards cavitation, unfavourable heat dissipation, its relatively large diameter and the fact that it can generally be installed only in the vertical position or at low inclinations.
The main disadvantage of the mono tube damper in comparison to the twin tube damper is the fact that the precision required in production makes it more expensive and its life span may be shorter because of the critical piston rod sealing.