1015 Air suspensions for commercial vehicles

A rigid six wheel truck equipped with pairs of air springs per axle is shown in Fig. 10.106. The front suspension has an air spring mounted between the underside of each chassis side-member and the transverse axle beam, and the rear tandem suspension has the air springs mounted between each trailing arm and the underside of the chassis (Figs 10.107 and 10.108).

Air from the engine compressor passes through both the unloader valve and the pressure regulator valve to the reservoir tank. Air is also delivered to the brake system reservoir (not shown). Once the compressed air has reached some pre-determined upper pressure limit, usually between 8 and 8.25 bar, the unloader valve exhausts any further air delivery from the pump directly to the atmosphere, thereby permitting the compressor to 'run light'. Immediately the air supply to the reservoir has dropped to a lower limit of 7.25 bar, the unloader valve will automatically close its exhaust valve so that air is now transferred straight to the reservoir to replenish the air consumed. Because the level of air pressure demanded by the brakes is greater than that for the suspension system, a pressure regulator valve is incorporated between the unloader valve and suspension reservoir valve, its function being to reduce the delivery pressure for the suspension to approximately 5.5 bar.

Air now flows from the suspension reservoir through a filter and junction towards both the front and rear suspensions by way of a single

Fig. 10.107 Tandem trailing arm rolling diaphragm air sprung suspension
Fig. 10.108 Tandem trailing arm bellows spring suspension with rubber anti-roll blocks

central levelling valve at the front (Fig. 10.109) and a pair of levelling valves on each side of the first tandem axle. These levelling valves are bolted to the chassis, but they are actuated by an arm and link rod attached to the axles. It is the levelling valves' function to sense any change in the chassis to axle height and to increase or decrease the air pressure supply passing to the air springs, thereby raising or reducing the chassis height respectively. The air pressure actually reaching the springs may vary from 5.5 bar fully laden down to 2.5 bar when the vehicle is empty.

To improve the quality of ride, extra volume tanks can be installed in conjunction with the air springs to increase the volume of air in the system. This minimizes changes in overall pressure and reduces the spring rate (spring stiffness), thus enabling the air springs to provide their optimum frequency of spring bounce.

An additional feature at the front end of the suspension is an isolating valve which acts both as a junction to split the air delivery to the left and right hand air springs and to permit air to pass immediately to both air springs if there is a demand for more compressed air. This valve also slows down the transfer of air from the outer spring to the inner spring when the body rolls while the vehicle is cornering.

10.15.1 Levelling valve (Figs 10.109 and 10.110(a and b))

A pre-determined time delay before air is allowed to flow to or from the air spring is built into the valve unit. This ensures that the valves are not operated by axle bump or rebound movement as the vehicle rides over rough road surfaces, or by increased loads caused by the roll of the body on prolonged bends or on highly cambered roads.

The valve unit consists of two parts; a hydraulic damper and the air control valve (Fig. 10.110(a and b)). Both the damper and the valves are actuated by the horizontal operating lever attached to the axle via a vertical link rod. The operating lever pivots on a cam spindle mounted in the top of the valve assembly housing. The swing movement of the operating lever is relayed to the actuating arm through a pair of parallel positioned leaf springs fixed rigidly against the top and bottom faces of the flat cam, which forms an integral part of the spindle.

When the operating lever is raised or lowered, the parallel leaf springs attached to the lever casing pivot about the cam spindle. This causes both leaf springs to deflect outwards and at the same time

Fig. 10.109 Air spring suspension front view layout vy, v, '/VV/zV/,-y//.' w.v/ v,y.

Fig. 10.109 Air spring suspension front view layout



orifice valve spring valve valve

Closed open open (bi Air spring discharging

(a) Air spring charging

Fig. 10.110(a and b) Levelling air control valve orifice valve spring valve valve

Closed open open (bi Air spring discharging

(a) Air spring charging

Fig. 10.110(a and b) Levelling air control valve applies a twisting movement to the cam spindle. It therefore tends to tilt the attached actuating arm and accordingly the dashpot piston will move either to the right or left against the fluid resistance. There will be a small time delay before the fluid has had time to escape from the compressed fluid side of the piston to the opposite side via the clearance between the piston and cylinder wall, after which the piston will move over progressively. A delay of 8 to 12 seconds on the adjustment of air pressure has been found suitable, making the levelling valve inoperative under normal road surface driving conditions.

Vehicle being loaded (Fig. 10.110(a)) If the operating lever is swung upward, due to an increase in laden weight, the piston will move to the right, causing the tubular extension of the piston to close the exhaust valve and the exhaust valve stem to push open the inlet valve. Air will then flow past the non-return valve through the centre of the inlet valve to the respective air springs. Delivery of air will continue until the predetermined chassis-to-axle height is reached, at which point the lever arm will have swung down to move the piston to the left sufficiently to close the inlet valve. In this phase, the springs neither receive nor lose air. It is therefore the normal operating position for the levelling valve and springs.

Vehicle being unloaded (Fig.l0.110(b)) If the vehicle is partially unloaded, the chassis will rise relative to the axle, causing the operating arm to swing downward. Consequently, the piston will move to the left so that the exhaust valve will now reach the end of the cylinder. Further piston movement to the left will pull the tubular extension of the piston away from its rubber seat thus opening the exhaust valve. Excessive air will now escape through the centre of the piston to the atmosphere until the correct vehicle height has been established. At this point the operating lever will begin to move the piston in the opposite direction, closing the exhaust valve. This cycle of events will be repeated as the vehicle's laden weight changes. A non-return valve is incorporated on the inlet side to prevent air loss from the spring until under maximum loading or if the air supply from the reservoir should fail.

10.15.2 Isolating valve (Fig. 10.111(a and b)) An isolating valve is necessary when cornering to prevent air being pumped from the spring under compression to that under expansion, which could considerably reduce body roll resistance.

The valve consists of a T-piece pipe air supply junction with a central cylinder and plunger valve (Fig. 10.111(a and b)).

When the air springs are being charged, compressed air enters the inlet part of the valve from

Fig. 10.111 (a and b) Isolator valve

the levelling valve and pushes the shuttle valve towards the end of its stroke against the spring situated between the plunger and cylinder blank end (Fig. 10.111(a)). Air will pass through the centre of the valve and come out radially where the annular groove around the valve aligns with the left and right hand output ports which are connected by pipe to the air springs.

Once the levelling valve has shut off the air supply to the air springs, the shuttle valve springs are free to force the shuttle valve some way back towards the inlet port. In this position the shuttle skirt seals both left and right hand outlet ports (Fig. 10.111(b)) preventing the highly pressurized outer spring from transferring its air charge to the expanded inner spring (which is subjected to much lower pressure under body roll conditions).

The shuttle valve is a loose fit in its cylinder to permit a slow leakage of air from one spring to the other should one spring be inflated more rapidly than the other, due possibly to uneven loading of the vehicle.

10.15.3 Air spring bags (Figs 10.112 and 10.113) Air spring bags may be of the two or three convoluted bellows (Fig. 10.112) or rolling lobe (diaphragm) type (Fig. 10.113), each having distinct characteristics. In general, the bellows air spring

Fig. 10.112 Involute bellow spring

Fig. 10.113 Rolling diaphragm spring

(Fig. 10.112) is a compact flexible air container which may be loaded to relatively high load pressures. Its effective cross-sectional area changes with spring height — reducing with increase in static height and increasing with a reduction in static height. This is due to the squeezing together of the convolutes so that they spread further out. For large changes in static spring height, the three convolute bellows type is necessary, but for moderate suspension deflection the twin convolute bellow is capable of coping with the degree of expansion and contraction demanded.

With the rolling diaphragm or lobe spring (Fig. 10.113) a relatively higher installation space must be allowed at lower static pressures. Progressive spring stiffening can be achieved by tapering the skirt of the base member so that the effective working cross-sectional area of the rolling lobe increases as the spring approaches its maximum bump position.

The normal range of natural spring frequency for a simply supported mass when fully laden and acting in the direct mode is 90-150 cycles per minute (cpm) for the bellows spring and for the rolling lobe type 60-90 cpm. The higher natural frequency for the bellow spring compared to the rolling lobe type is due mainly to the more rigid construction of the convolute spring walls, as opposed to the easily collapsible rolling lobe.

As a precaution against the failure of the supply of air pressure for the springs, a rubber limit stop of the progressive type is assembled inside each air spring, and compression of the rubber begins when about 50 mm bump travel of the suspension occurs.

The springs are made from tough, nylon-reinforced Neoprene rubber for low and normal operating temperature conditions but Butyl rubber is sometimes preferred for high operating temperature environments.

An air spring bag is composed of a flexible cylindrical wall made from reinforced rubber enclosed by rigid metal end-members. The external wall profile of the air spring bag may be plain or bellow shaped. These flexible spring bags normally consist of two or more layers of rubber coated rayon or nylon cord laid in a cross-ply fashion with an outside layer of abrasion-resistant rubber and sometimes an additional internal layer of impermeable rubber to minimize the loss of air.

In the case of the bellow type springs, the air bags (Fig. 10.112) are located by an upper and lower clamp ring which wedges their rubber moulded edges against the clamp plate tapered spigots. The rolling lobe bag (Fig. 10.113) relies only upon the necks of the spring fitting tightly over the tapered and recessed rigid end-members. Both types of spring bags have flat annular upper and lower regions which, when exposed to the compressed air, force the pliable rubber against the end-members, thereby producing a self-sealing action.

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