Air Suspension system:part 1

1.       INTRODUCTION

            The automobile chassis is mounted on the axles, not direct but through some form of springs.  This is done to isolate the vehicle body from the road shocks which may be in the form of bounce, pitch, roll or sway.  These tendencies give rise to an uncomfortable ride and also cause additional stress in the automobile frame and body.  All the parts which perform the function of isolating the automobile from the road shocks are collectively called a suspension system.  It includes the springing device used and various mountings for the same.

 

1.1       Objective of Suspension System

                        The following are the objectives of suspension system,

·                     To safeguard the occupants against road shocks and provides riding comfort.

·                     To isolate the structure of vehicle from shock loading and vibration due to irregularities of road surface without affecting its stability.

·                     To minimise the rolling and pitching tendency.

·                     To keep vehicle body in perfect level while travelling over rough roads.

·                     To support the weight of the vehicle.

 

1.2       Basic Suspension movements

·                     Bouncing – The complete body movement in the vertical direction on rising up and down of vehicle body known as ‘Bouncing’.

·                     Pitching – The rotating action produced in a vehicle about a transverse axis through it parallel to ground known as ‘Pitching’.  It is due to out of phase movement of front suspension with respect to rear suspension causing rotating effect in the vehicle.

·                     Rolling – The centre of gravity of the vehicles is considerably above the ground.  Due to this reason, while taking the turns, the centrifugal force acts outwards, on the C.G. of vehicle, while the road resistance acts inward, at the wheels.  This gives rise to a couple turning the vehicle about a longitudinal axis.  This is called rolling. (Ref. Fig. 1.2))

 

2.       WHY AIR SUSPENSION SYSTEM?

                        Passenger’s comfort mainly depends upon ability of suspension system to absorb socks and vibrations.  Natural frequency or vibration of mass (m) can be written as :

            Fn        = (½ p) x 

Where,             K         = Effective stiffness,

                        m         = Effective mass of vehicle

Putting,           m         = F/g,   we get

                        F_n      =                                                              ---- (1)

where,             F          = dynamic force acting on vehicle due to shocks and vibrations

                                                For passenger comfort F_n should be constant

            From equation (1) we can write,

                        F_n a (K/F)       i.e. (F_n)² (K/F)

            To keep F constant K must vary with F, i.e.

            Spring rate of system must directly vary with dynamic force.

                        None of the conventional system has an ability to change ‘K’ with ‘F’ so as to keep ‘Fn’ constant.

                        However, by controlling air flow in and out of the air-spring.  F_n can be kept constant (approximately) giving better passenger comfort.

3.       LAYOUT OF AIR SUSPENSION

                        A rigid six wheel truck equipped with pairs of air springs per axle is shown in Fig. 3.1.  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.

                        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 central levelling valve at the front (Fig. 3.2) 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 pair 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 minimises 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.

4.       MAIN COMPONENTS OF AIR SUSPENSION SYSTEM

4.1.      Levelling Valve (Fig. 4.1(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 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. 3.2).  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 dam spindle.  This caused both leaf springs to deflect outwards and at the same time 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 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.

a)         Vehicle Being Loaded (Fig. 4.1 (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 spring weight receive or lose air.  It is therefore the normal operating position for the levelling valve and springs.

b)         Vehicle Being Unloaded (Fig. 4.1 (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 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 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.

4.2       Isolating Valve (Fig. 4.2 (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. 

                        When the air springs are being charged, compressed air enters the inlet part of the valve from 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.  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 preventing the highly pressurised 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.