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.
4.3 Air Spring Bags (Fig. 4.3 (a) (b))
Air
spring bags may be of the two or three convoluted bellows (Fig. 4.3 (a)) or
rolling lobe (diaphragm) type (Fig. 4.3 (b)), each having distinct
characteristics. In general, the bellows
air spring (Fig. 4.3 (a)) 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. 4.3 (b)) 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 it
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 bags normally consists 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 minimise the loss of air.
In
the case of the bellow type springs, the air bags (Fig. 4.3 (a)) 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. 4.3 (b)) 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.
4.4 Anti-roll Rubber Blocks (Fig. 4.4)
A conventional anti-roll
bar can be incorporated between the trailing arms to increase the body roll
stiffness of the suspension or alternatively built-in anti-roll rubber blocks
can be adopted. During equal bump or
rebound travel of each wheel the trailing arms swing about their front
pivots. However, when the vehicle is
cornering, roll causes one arm to rise and the other to fall relative to the
chassis frame. Articulation will occur
at the rear end of the training arm where it is pivoted to the lower spring
base and axle member. Under these
conditions, the trailing arm assembly adjacent to the outer wheel puts the
rubber blocks into compression, whereas in the other trailing arm, a tensile
load is applied to the bolt beneath the rubber block. As a result, the total roll stiffness will be
increased. The stiffness of these rubber
blocks can be varied by adjusting the initial rubber compressive perload.
4.5 Air
Filter
It
is incorporated between auxiliary reservoir and levelling valve. Sintered bronze filters are fitted in the
ports to protect the internal mechanism from damage by foreign matter which may
be carried back from the interior of the spring.
4.6 Pressure
Regulator
As
the level of air pressure demanded by the brakes is greater than that of the
suspension system, a pressure regulator valve is incorporated between the
unloader valve and suspension reservoir valve, its function is to reduce the
delivery pressure for suspension to approximately 5.5 bar.
4.7 Drier
Any
moisture in the air in air-spring will corrode the interior of the spring. Hence, drier is fitted in suspension air
circuit. It contains silica gel which
absorbs moisture from air. Exhaust air
from air spring is used as a re-generative agent to remove moisture from silica
gel and make it fresh.
5. AIR SPRING CHARACTERISTICS
The
bounce frequency of a spring decreases as the sprung weight increases and
increases as this weight is reduced.
This factor plays an important part in the quality of ride which can be
obtained on a heavy goods or passenger vehicle where there could be a fully
laden to unladen weight ratio of up to 5 : 1.
An
inherent disadvantage of leaf, coil and solid rubber springs is that the bounce
frequency of vibration increases considerably as the sprung spring mass is
reduced (Fig. 5.1). Therefore, if a heavy
goods vehicles is designed to give the best ride frequency, say 60 cycles per
minute fully laden then as this load is removed, the suspension’s bounce
frequency could rise to something like 300 cycles per minute when steel or
solid rubber springs are used, which would produce a very harsh, uncomfortable
ride. Air springs, on the other hand,
can operate over a very narrow bounce frequency range with considerable changes
in vehicle laden weight, say 60 – 110 cycles per minute for a rolling lobe air
spring (Fig. 5.1). Consequently the
quality of ride with air springs is maintained over a wide range of operating
conditions.
Steel
springs provide a direct rise in vertical deflection as the spring mass
increases, that is, they have a constant spring rate (stiffness) whereas air
springs have a rising spring stiffness with increasing load due to their
effective working area enlarging as the spring deflects (Fig. 5.2). This stiffening characteristic matches far
better the increased resistance necessary to oppose the spring deflection as it
approaches the maximum bump position.
To
support and maintain the spring mass at constant spring height, the internal
spring air pressure must be increased directly with any rise in laden
weight. These characteristics are shown
in Fig. 5.3 for three different set optimum spring heights.
The
spring vibrating frequency will be changed by varying the total volume of air
in both extra tank and spring bag (Fig. 5.4).
The extra air tank capacity, if installed, is chosen to provide the
optimum ride frequency for the vehicle when operating between the unladen and
fully laden conditions.
6. VOLVO OPTIMISED AIR SUSPENSION (VOAS) SYSTEM
The
Volvo optimised air suspension (VOAS) system is a modified version of the
previous air suspension on rear axles of Volvo GM heavy duty trucks (Figure
6.1). A number of modifications in the
VOAS system provide improved ride, reduced unspring weight, increased
durability, and better alignment ability.
The VOAS system is available on all Volvo GM on-highway tractors. The operation of the VOAS system is similar
to the operation of other air suspension systems explained previously.
The
spring brackets in the VOAS system are manufactured from ductile iron, and
these brackets have closed sections and cutouts to reduce the weight of each
bracket by 4 lb (2 kg) (Fig. 6.2).
As
in previous models, a polyethylene wear pad provides a very smooth surface for
the end of the spring to contact (Fig. 6.3).
This type of wear pad eliminates noise caused by contact between the
spring and metal wear pad in other suspension systems. The wear pad in the VOAS
50 % is lighter compared with the wear pad in previous suspension systems. A single fastener retains the wear pad to the
spring bracket legs.
The
Z-spring is redesigned with a larger clamping surface at the axle seat and
improved alignment with other suspension components (Fig. 6.4). The Z-spring is also redesigned to provide
more clearance between this spring and the lower shock absorber mounting
bracket.
The
rear of the radius spring is mounted between the axle seat and the
Z-spring. The front end of each radius
spring contains a bushing that is bolted to openings in the lower end of the
spring bracket. In the VOAS system each
radius spring has an improved bushing for increased durability. Each radius spring has a locating pin for
accurate positioned of the radius spring, Z-spring, and axle seat. This design provides improved axle alignment.
The
crossbeam in the VOAS system is manufactured from thinner, high-strength steel
with cutouts. Each crossbeam is 12 lb (5
kg) lighter than previous models.
7. ELECTRONICALLY CONTROLLED AIR SUSPENSION SYSTEMS
Electronically
controlled air suspension systems are not widely used as standard equipment at
present. This type of suspension system
is in the experimental and developmental stage.
However, with the ever-expanding use of electronics in the trucking
industry, electronically controlled air suspension systems may be standard
equipment on new tractors in the near future.
Because electronically controlled air suspension systems are not
standard equipment at present, the discussion is brief.
An
electronic control unit (ECU) in the suspension system received voltage input
signals from various sensors including two rear axle and one front axle height
sensor, air pressure sensor, accelerator position sensor, and steering angle
sensor. The steering angle sensor is
mounted on the steering column shaft and senses the amount and speed of
steering wheel rotation. Other ECU
inputs may include the brake light switch, ignition switch, vehicle speed, and
engine speed. Solenoids mounted in a
solenoid valve block are operated electronically by the ECU (Fig. 7.1). Air pressure is supplied from the air brake
system to these solenoids. If the left rear
axle height sensor indicates the left rear frame height is less than specified,
the ECU energises the appropriate solenoid.
This action opens the solenoid, and supplies air pressure to the left
rear air spring to restore the frame height to specifications. When the left rear height sensor signal to
the ECU indicates the left rear frame height is within specifications, the ECU
de-energises the solenoid. Under this
condition the solenoid closes and maintains the air pressure in the solenoid.
When
a height sensor signal indicates to the ECU that the frame height is above
specifications, the ECU energises the appropriate solenoid so it is in the vent
mode. Under this condition air pressure
is released from the air spring to restore the frame height to specifications. The ECU can switch the air suspension very
quickly to a firm or hard mode by rotating valves in the shock absorbers
(dampers). During the soft shock
absorber mode the shock absorber valves are positioned so they offer less
restriction to the flow of oil. If the
ECU enters the firm mode and rotates the shock absorber valves, these valves
move to a position that provides more restriction to oil movement. This firm mode may be entered by the ECU and
shock absorbers during fast avoidance manoeuvres, fast cornering, hard braking,
rapid traction changes, continued road irregularities, cross winds, and high
static load. The capability of the
electronically controlled suspension system to switch quickly to the firm mode
provides improved vehicle stability and safety.
8. ADVANTAGES OF AIR SUSPENSION SYSTEM
1.
The
improved ride quality provided by an air suspension system reduces driver
fatigue.
2.
An
air suspension system provides improved tire-to-road contact, and this improves
braking capability.
3.
An
air suspension system provides low frequency and viscous damping of the
suspension, and this action reduces wheel hop while braking.
4.
The
improved roll resistance of an air suspension system provides better vehicle
stability.
5.
An
air suspension provides pneumatic equalising between the forward and rearmost
rear axles for improved weight distribution between these axles. This helps to reduce overloading of one axle
and one set of tires.
6.
Because
the vehicle attitude is also constant, changes in headlamp alignment due to
varying loads are avoided.
7.
The
spring rate varies much less between the laden and unladen conditions, as
compared with that of conventional steel springs. This reduces the dynamic loading.
8.
The
improved standard of ride comfort and noise reduction attained with air springs
reduces both driver and passenger fatigue.
9. COMPARISON
OF AIR SUSPENSION SYSTEM AND CONVENTIONAL SUSPENSION SYSTEM
Sr. No.
|
Parameter
|
Air Suspension
|
Conventional Suspension
|
1.
|
Load Carrying Capacity
|
Higher
|
Lower
|
2.
|
Ability to absorb shocks and
vibrations
|
Better
|
Good
|
3.
|
Car height control
|
Better
|
No
|
4.
|
Cost
|
Costly
|
Cheaper
|
5.
|
Maintenance
|
More
|
Less
|
6.
|
Passenger Comfort
|
Excellent
|
Not satisfactory
|
7.
|
Electronic control
|
Possible
|
Not possible
|
8.
|
Life
|
More
|
less
|
9.
|
Noise level
|
Less
|
Higher
|
10. CONCLUSION
The
air springs in air suspension system take the place of the leaf springs in a
conventional suspension system. An air
suspension system eliminates the interleaf friction encountered in a
leaf-spring suspension system. Compared
with a leaf-spring suspension system, an air suspension system minimises road
shock transferred from the suspension to the truck frame, driver, and cargo. An air suspension adjusts automatically to
different load conditions, and provide a softer suspension system with light
loads and a firmer suspension system with heavier loads. An air suspension system provides a constant
frame height under various load conditions.
Compared with a leaf-spring suspension system, an air suspension system
provides a considerable unsprung weight saving.
From
all above points it is clear that air suspension system makes significant
overall improvement to vehicle ride and comfort. It is clear from the seminar that air
suspension system provides a better alternative for conventional suspension
system.
