Height correction valve Fig 1072a b c and d

Automatic height correction is achieved by varying the volume of incompressible fluid between the sphered diaphragm and the piston. Increased vehicle weight lowers the body, thus causing the suspension arms to deflect and at the same time rotate the anti-roll bar. The angular rotation of the anti-roll bar is a measure of the suspension's vertical deflection relative to the vehicle's normal static height. This movement is relayed to the height correction valve via a torsional control rod clamped to the anti-roll bar at one end and to a control rod lever which is attached to the height correction valve at the other end.

To avoid continuous height correction every time a pair of wheels roll over a hump or dip in the road, a delayed response is introduced to the

Fig. 10.70 General layout of the hydropneumatic automatic height correction suspension

height correction valves so that the spring unit cylinder is not being charged on bump or discharged on rebound. Height correction will therefore be achieved only after a small time pause during which time the suspension will have had time to adjust to a change in the loads imposed on the spring units. Once the spring unit cylinder has been fully recharged, or discharged to bring the suspension height back to the standard setting, the height correction valve is made to respond immediately by either moving from inlet charging to neutral cut-off or from exhaust discharge to neutral cut-off position.

Charging the spring unit (spool valve movement from neutral cut-off to inlet open) (Fig. 10.72(a)) An increase in car load causes the lower transverse arm to pivot and the anti-roll bar to rotate. At the same time the control rod twists and tries to tilt the control rod lever, thereby transmitting an axial load to the height correction spool valve. The effect of shifting the spool valve to the left hand side is to move it from the cut-off position to the inlet open position. An increased amount of fluid is now forced between the piston and diaphragm causing the vehicle to rise until the anti-roll bar, which is rotating in the opposite direction, pulls the spool valve back to the neutral cut-off position. The return to the cut-off position is rapid because the spool valve does not offer any resistance in this direction, and the vehicle height will have been brought back to its normal position. To slow down the movement from cut-off to inlet charge positions, the disc valve in the right hand diaphragm chamber is closed. Therefore, the only way the fluid can be transferred from the right to

(a I Front view

Fig. 10.71 (a-c) Detailed arrangement of hydropneumatic spring unit using a transverse double arm linkage suspension

(a I Front view

Fig. 10.71 (a-c) Detailed arrangement of hydropneumatic spring unit using a transverse double arm linkage suspension the left hand chamber is through the restricted passage with the result that the spool valve shift movement is very sluggish.

Discharge the spring unit (spool valve movement from neutral cut-off to exhaust open) (Fig. 10.72(b)) Decreasing the car load has the reverse effect to increasing the load. This time the spool valve moves from the neutral cut-off position to the exhaust open position. The excess fluid between the piston and diaphragm is now expelled to the reservoir tank and the suspension spring unit contracts until the body to ground height has been corrected, at which point the spool valve again will be in the neutral cut-off position. Similarly the discharge process is also slowed down so that the valve does not respond to small changes in dynamic loads caused by suspension vibration as the wheels travel over the road surface irregularities.

Spool valve movement from inlet charge to neutral cut-off (Fig. 10.72(c)) Once the spring unit cylinder has been fully recharged with fluid, the anti-roll bar will have rotated sufficiently to make the spool valve alter its direction of slide towards the neutral cut-off position. This return movement of the spool valve to the cut-off position is rapidly speeded up because the left hand disc valve is in the open position so that when the spool valve first starts to change its direction of slide, fluid in the unrestricted passage will force the right hand valve off its seat. As a result, fluid movement from the left hand to the right hand diaphragm chamber takes place through both the restricted and unrestricted passages, speeding up the fluid transfer and accordingly the spool valve movement to the neutral cutoff position. As soon as the spool valve reaches its cut-off position the disc valve in the left hand diaphragm chamber re-seats. This action stops the spool valve overshooting its cut-off position and therefore avoids the valve going through a second recharge and discharge cycle of correction.

Spool valve movement from exhaust discharge to neutral cut-off (Fig. 10.72(d)) A rapid closing of the exhaust valve takes place once the fluid in an over-charged cylinder has been permitted to escape back to the reservoir thus restoring the suspension

Fig. 10.72 (a-d) Height correction valve action

to its standard height. The ability for the spool valve to respond quickly and close off the exhaust valve is due to the right hand disc valve being open. Thus fluid in the unrestricted passage is permitted to push open the right hand disc valve, this allows fluid to readily move through both the restricted and unrestricted passages from the right to left hand diaphragm chamber. Immediately the tor-sional wind-up of the control rod due to the anti-roll bar rotation causes the spool valve to shift to the neutral cut-off position.

Manual height correction A manual control lever is provided inside the car, the lever being connected by actuating rods to the front and rear height correction units. Its purpose is to override the normal operation of the spool valve and to allow the driver to select five different positions:

Normal — this is the standard operating position

High or low — two extreme positions Two positions — intermediate between normal and high

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