10101 Hydropneumatic selflevelling spring unit

This constant height spring unit consists of two sections;

1 a pneumatic spring and hydraulic damper system,

2 a hydraulic constant level pump system.

An approximately constant frequency of vibration for the sprung mass, irrespective of load, is obtained by having two gas springs, a main gas spring, in which the gas is contained behind a diaphragm, and a correction gas reservoir spring (Fig. 10.73(a, b and c)). The main spring is controlled by displacing fluid from the upper piston chamber to the spring diaphragm chamber and the correction gas spring is operated by the lower piston chamber discharging fluid into the reservoir gas spring chamber.

The whole spring unit resembles a telescopic damper. The cylindrical housing is attached to the sprung body structure whereas the piston and integral rod are anchored to either the unsprung suspension arm or axle.

The housing unit comprises four coaxial cylinders;

1 the central pump plunger cylinder with the lower conical suction valve and an upper one way pump outlet disc valve mounted on the piston,

2 the piston cylinder which controls the gas springs and damper valves,

3 the inner gas spring and reservoir chamber cylinder,

4 the outer gas spring chamber cylinder which is separated from hydraulic fluid by a flexible diaphragm.

The conical suction valve which is mounted in the base of the plunger's cylinder is controlled by a rod located in the hollow plunger. A radial bleed port or slot position about one third of the way down the plunger controls the height of the spring unit when in service.

The damper's bump and rebound disc valves are mounted in the top of the piston cylinder and an emergency relief valve is positioned inside the hollow pump plunger at the top.

The inner gas spring is compressed by hydraulic fluid pressure generated by the retraction of the space beneath the piston.

The effective spring stiffness (rate) is the sum of the stiffnesses of the two gas springs which are interconnected by communication passages. Therefore the stiffness increase of load against deflection follows a steeper curve than for one spring alone.

Gas spring and damper valve action (Fig. 10.73 (a and b)) There are two inter-related cycles; one is effected by the pressure generated above the piston and the other relates to the pressure developed below the piston.

When, during bump travel (Fig. 10.73(a)), the piston and its rod move upwards, hydraulic fluid passes through the damper bump valve to the outer annular main gas spring chamber and compresses the gas spring. Simultaneously as the load beneath the piston reduces, the inner gas spring and reservoir expand and fluid passes through the transfer port in the wall to fill up the enlarging lower piston chamber cylinder. Thus the deflection of the diaphragm against the gas produces the elastic resilience and the fluid passing through the bump valve slows down the transfer of fluid to the gas spring so that the bump vibration frequency is reduced.

On rebound (Fig. 10.73(b)) fluid is displaced from the outer spring chamber through the damper rebound valve into the upper piston cylinder and at the same time fluid beneath the piston is pushed out of the lower piston chamber into the inner gas spring chamber where it now compresses the inner gas spring.

Likewise fluid which is being displaced from the main gas spring to the upper piston chamber

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Fig. 10.73 (a and b) Exaggerated diagrams illustrating the self-levelling action of a hydropneumatic suspension unit

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Fig. 10.73 (a and b) Exaggerated diagrams illustrating the self-levelling action of a hydropneumatic suspension unit experiences an increased resistance due to the rebound valve passage restriction so that the fluid transfer is achieved over a longer period of time.

Pump self-levelling action (Figs 10.74(a, b and c) and 10.73(a and b)) The movement of the piston within its cylinder also causes the pump plunger to be actuated. During bump travel (Figs 10.73(a) and 10.74(a)) the plunger chamber space is reduced, causing fluid to be compressed and pushed out from below to above the piston via the pump outlet valve. On rebound (Fig. 10.74(c)), the volume beneath the piston is replenished. However, this action only takes place when the piston and rod have moved up in the cylinder beyond the designed operating height.

The conical suction valve, which is mounted in the base of the plunger's cylinder and is controlled

(c) Rebound position

Fig. 10.74(a-c) Self-levelling hydropneumatic suspension by a rod located in the hollow plunger, and also a radial bleed port or slot, positioned about one third of the way down the plunger, control the height of spring unit when in service.

The damper's bump and rebound disc valves are mounted in the top of the piston cylinder and an emergency relief valve is positioned inside the hollow pump plunger at the top.

The inner gas spring is compressed by hydraulic fluid pressure generated by the retraction of the space beneath the piston.

The pumping action is provided by the head of the plunger's small cross-sectional area pushing down onto the fluid in the pump chamber during the bump travel (Fig. 10.74(a)). This compels the fluid to transfer through the pump outlet valve into the large chamber above the piston. The pressure of the fluid above the piston and that acting against the outer gas spring diaphragm is the pressure necessary to support the vehicle's unsprung mass which bears down on the spring unit. During rebound travel (Fig. 10.74(c)), the fluid volume in the pump chamber increases while the volume beneath the piston decreases. Therefore some of the fluid in the chamber underneath the piston will be forced into the inner gas spring chamber against the trapped gas, whilst the remainder of the excess fluid will be transferred from the lower piston chamber through a passage that leads into an annular chamber that surrounds the pump chamber. The pressurized fluid surrounding the pump chamber will then force open the conical suction valve permitting fluid to enter and fill up the pump chamber as it is expanded during rebound (Fig. 10.74(c)). This sequence of events continues until the piston has moved far enough down the fixed pump plunger to expose the bleed port (or slot) in the side above the top of the piston (Figs 10.74(c) and 10.73(b)).

At this point the hollow plunger provides a connecting passage for the fluid so that it can flow freely between the upper piston chamber and the lower plunger chamber. Therefore, as the piston rod contracts on bump, the high pressure fluid in the plunger chamber will be discharged into the upper piston chamber by not only the pump outlet valve but also by the plunger bleed port (slot) (Fig. 10.74(a)). However, on the expansion stroke some of the pressurized fluid in the upper piston chamber can now return to the plunger chamber and thereby prevents the conical suction valve opening against the pressure generated in the lower piston chamber as its volume decreases. The plunger pumping action still continues while the spring unit height contracts, but on extension of the spring unit (Fig. 10.74(c)) the fluid is replenished not from the lower piston chamber as before but from the upper piston chamber so that the height of the spring unit cannot increase the design spring unit length.

When the spring unit is extended past the design height the underside of the piston increases the pressure on the fluid in the reservoir chamber and at the same time permits fluid to bleed past the conical suction valve into the plunger chamber. If the spring unit becomes fully extended, the suction valve is lifted off its seat, enabling the inner spring chamber to be filled with fluid supplied from the lower piston chamber and the plunger chamber.

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