Air Suspension system:part 2

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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.