LIFT AXLE CONTROL SYSTEM FOR HEAVY TRUCK HAVING MECHANICAL SUSPENSION
FIELD OF INVENTION
[001] The present invention relates to a lift axle control system for lifting and lowering the pusher axle of a heavy trucks having twin front axles and tandem leaf spring suspensions such as non-reactive bell crank and tie rod tandem leaf spring suspension or slipper ended rocker arm tandem leaf spring suspension or bogie inverted leaf spring single point suspension.
BACKGROUND OF INVENTION
[002] Heavy commercial trucks having multi-axle (MAV) are commonly used for carrying heavy load. Figure 1 shows a schematic of the heavy commercials vehicle, having five axles (i.e., MAV 10x2 and 10x4 configurations) operated. Such heavy commercial trucks are used to carry heavy loads. The vehicles have a pusher lift axle (1). The pusher axle (1) is lifted from the road surface in unladen condition and lowered to road surface in laden condition. The pusher lift axle (1) is either self-steer axle or non-steer axle. The pusher lift (1) is generally equipped with an air suspension. The self-steer axle would have single tire (2) at each end. The non-steer axle would have dual tires (3) at each end.
[003] Parallelogram type lift axle is commercially successful in developing countries. The lift axle mechanism has a pair of ride air springs, a pair of lift air springs, four control arms, eight bushings assemblies and an axle. The ride air springs are inflated when the vehicle is in laden condition and the load air springs are deflated and the lift air springs are inflated when the vehicle is un-laden condition. The pusher lift axle (1)
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is mainly used to increase fuel efficiency of the vehicle in unladen condition and extend the tire life.
[004] In developing countries the axles have own leaf spring suspensions in both front and rear axles. In general, the twin front axles (6a and 6b) are equipped with conventional leaf spring suspension. Both the front axles (6a and 6b) are steered by a hydraulic system (therefore, it is called power steer axles). The heavy duty multi-axle vehicles (MAV), having 10x2 configuration has two rear axles (7a and 7b). Usually, the first rear axle (7a) is usually a drive axle (7a) and second rear axle (7b) is a tag axle (or called as dummy axle). The heavy duty multi-axle vehicles (MAV), having 10x4 configuration has also two rear axles and the both the rear axle are drive axle.
[0051 in order to better load distribution between the two rear axles (in both 10x2 and 10x4 configuration vehicles), the rear axles are interconnected by any one of the following conventional tandem rear suspensions: (a) non-reactive bell crank tandem leaf spring suspension (4) which is simply called as non-reactive suspension system (4), (see figure 2b, 2c), or (b) slipper end and balancer beam leaf spring suspension or (c) reactive pivot beam suspension system; so that, if one axle rises relatively to the vehicle frame (5) the other axle will automatically be lowered; thereby the load acting on the vehicle is equally distributed to these axles.
[006] In order to lift the axle in unladen condition and lower the pusher axle (1) in laden condition, a commercial available automatic control system (8) is used (Figure 2a). The automatic lift axle control system (8) is specially designed for the vehicle having tandem rear leaf spring suspension. The lift axle mechanism lifts and lowers the pusher axle depending on the load on the rear axles. The automatic lift axle control system consists of a lift axle control valve (LACV), a pressure limit valve PLV, NRV, a load sensing valve, a pair of lift air springs and a pair of ride air
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springs (also called as load air springs), etc. The lift axle control valve (LACV) is electrically operated direction control valve. The lift axle control valve (LACV), opens and closes their valve ports according to the electrical signal received from the control valve. The lift air springs and ride air springs are attached with the lift axle suspension.
[007] Two methods are commonly used to mount the load sensing valve in the vehicle, according to prior arts (Figure 2b and 2c). Figure 2b shows the schematic of the first method. In this method, the load sensing valve (LSV, 9) is mounted in single axle (i.e., in between the chassis frame and the one of rear axles (7a or 7b), which is either drive axle (7a) or tag (dummy) axle (7b). Even though it is simple, a costly ball joint (not shown) is being used at the axle mounting location. Figure 2c shows the schematic of the second method. In second method, the load sensing valve is mounted in between the chassis frame and a linkage mechanism (10) (that is connected in between the both the rear axles (7a and b), Figure 2b).The linkage mechanism (10) has many parts such as a pair of coil springs (or four coil springs), multiple metallic ball joints, multiple long moveable linkages, multiple short linkages (i.e., multiple fixed linkages), and multiple small fasteners. As it has many parts, many failures are observed in the linkage mechanism (10). In addition, the ball joints are failed (i.e., struck up, not rotates freely) frequently as it is splashed (i.e., hit) by small debris and sand etc., when the vehicle is operated. In addition, the long linkages are susceptible to become bent or broken at the threaded portion (not shown). Therefore, warranty cost of the linkage mechanism (10) is increased. As the linkage mechanism fails frequently, the prior art lift axle control system does not work as efficiently and consistently as required.
[008] The automatic control system monitors the load on the vehicle by metering the distance variation in between the rear axle(s) (7a and 7b) to the chassis frame (5). The
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height of the vehicle is varied according to the load on the load body. Figure 3 a shows the load sensing valve (LSV, 9) mounted in the frame near to the first rear axle (7a) according to the prior art. The leaf springs bend depends on the load acted on the wheels. The load sensing valve (9) regulates the outlet pressure according to the distance variation in between the rear axle(s) and the chassis frame (5). A pre-set pressure is set in the lift axle control valve. If the outlet pressure of the load sensing valve (9) is reached to the pre-determined pressure, an electrical signal is disconnected to the solenoid of the control valve. Therefore, spool of the valve is moved to original position (i.e., changes in the direction of outlet). The lift axle direction control valve inflates the load air springs and deflates the lift air spring. Thereby, the axle is lowered to the road surface. If the outlet pressure of the load sensing valve reached to below the pre-set pressure, the electrical signal is connected to the solenoid of the valve. The solenoid of the valve is energized. Therefore, the spool of the direction control valve is moved upward. That is, the lift air springs are inflated and the ride air springs are deflated. The axle is lifted-off from the road surface and maintained at a desired distance from the road surface.
[009] Figure 3a and 3b illustrate schematic of frame failure observed in the vehicle having conventional lift axle control system and pusher lift axle. When the vehicle is uneven loading condition, a sever bend is observed in the frame where the lift axle location mounted; especially at the ride air springs mounted location. Therefore, vehicle manufacturer (OEM) has to spend a lot of money and manpower due to the warranty cost of the frame structure. Therefore, it leads to a sever expenditure to both OEM and vehicle user.
[010] From on road trials and laboratory experiments, it was observed that the chassis frame (5) was experienced a sever bending load. It was occurred when uneven loading condition in the vehicle. In general, the vehicle is loaded from the rear side of
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the vehicle (Figure 3b). When heavy density materials such as steel coil, machine tools, are placed at the rear side of the vehicle or above the rear axles, the distance between the frame and the axle(s) is reduced. The lift axle control valve (LACV), upon recognizing that the vehicle is in laden condition, supplies compressed air to the ride air springs of lift axle with a maximum pressure. Therefore, the ride air springs of the lift axle try to lift the front side of the vehicle; therefore, the magnitudes of reaction loads (Fl and F2) on both the front axles are significantly reduced. At this time, the ride air springs mounting location of lift axle is acted as a fulcrum point. Simultaneously, weights of the cab (C), engine (E), gear box (G) (concentrated points loads) located at the front of the chassis frame are acted downward at the at the front of the chassis frame (Figure 3b). The weights of the load body and chassis frame (B) and payload (P) are acted on the rear axles and pusher lift axle. The rear axles reaction loads (Rl, R2) and lift axle reaction load (LI) are increased. Therefore, a counterclockwise moment is created due to the above cantilever loads (cab (C), engine (E), gear box (G)) on the frame. The frame experiences a severe bending load due to the above cantilever loads (of cab, engine, gear box). Figure 3c and 3d show schematic illustrations of the frame while loading and after loading, respectively. The frame is subjected to bending at the lift axle location. Therefore, it leads to a high expenditure to both OEM and vehicle user.
[Oil] Tn addition, the brakes consumes a lot of compressed air even when the axle is lifted. As the vehicle is operated on undulated road in developing countries, the brake is applied frequently. It consumes a large volume (i.e., amount) of compressed air. It increases the duty cycle of the compressor. Therefore, fuel consumption is increased.
[012] Accordingly, there is the need to provide a simple automatic lift axle control system which must prevent the failure observed in the chassis frame. It must have a higher durability and at the same should be cost effective. Moreover, it should have
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simple linkage mechanism; so that, the lift axle mechanism works properly. In addition, the axle alignment should be done without losing compressed air from auxiliary air tank so that the vehicle can be moved quickly when the vehicle is restarted. In this regard, the present invention focuses more on the design of a unique lift axle control system for the 10x2 configuration vehicle and 10x4 configuration vehicle having pusher lift axle (1).
OBJECTS OF THE INVENTION
[013] The prime objective of this invention of the present invention is to provide a lift axle control system for lifting and lowering the pusher lift axle of a vehicle having twin front axles and tandem rear suspension such as non-reactive suspension system (4) or slipper end suspension (11) or inverted bogie leaf spring suspension
(12).
[014] Another objective of this invention is to provide automatic lift axle control that should minimize the frame failures while uneven loading the vehicle.
[015] Another objective of this invention is to provide an automatic lift axle control system which should minimize the failure observed in the linkage mechanism connected to the load sensing valve.
[016] Another objective of this invention is to provide an automatic lift axle control system that should lift the pusher lift axle and lower the pusher axle automatically when vehicle is un-laden condition and laden condition, respectively.
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[017] Another objective of this invention is to provide an automatic lift axle control system that should be easily attached to tandem leaf spring suspension which is pivoted to the chassis frame.
[018] Another objective of this invention is to provide an automatic lift axle control system that should be easily attached to first front suspension which is pivoted to the chassis frame.
[019] Another objective of this invention is to provide an automatic lift axle control system which should be easily maintainable.
[020] Another objective of this invention is to provide an automatic lift axle control that should has high reliability.
[021] Another objective of this invention is to provide an linkage mechanism that should has high reliability.
[022] Another objective of this invention is to provide an linkage mechanism that should be simple, low cost and low maintenance.
[023] Yet, another objective of this invention is to provide an linkage mechanism that withstands dynamic loads.
[024] Another objective of this invention is to provide an automatic lift axle control that should consume less amount of compressed air.

[025] Another objective of this invention of the present invention is to provide automatic lift axle control for providing appropriate load sharing control for the auxiliary axles which are not mechanically connected to the drive axle.
[026] Another objective of this invention is to provide automatic lift axle control that should disconnect the lift axle brake when the axle is lifted.
[027] Another objective of this invention is to provide automatic lift axle control that should reliable and low cost and simple provision to perform axle alignment easily.
SUMMARY OF THE INVENTION
[028] According to the present invention, an automatic lift axle control system is provided for lifting pusher axle of heavy commercial vehicle. Especially, the present invention is used to lift the pusher lift axle of 10x2 and or 10x4 vehicle configuration having tandem rear leaf spring suspension. The lift axle mechanism lifts and lowers the auxiliary axle automatically depending on the load on both the front axle and drive axle.
[029] In an embodiment, a pair of load sensing valves are used in the control system to protect the frame form sever bending when uneven loading. The load sensing valves are especially mounted at the first front axle and a rear axle which is either drive axle or tag axle or center of the both the tag and drive axle.
[030] The automatic lift axle control system consists of a primary air tank, a secondary air tank, a NRV, a pressure limiting, a pair of load sensing valves (LSV), a low pressure selecting valve, a electrically operated lift axle control valve (LACV), a brake relay valve, a quick release valve, a exhaust delay valve, a toggle switch, a pair
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of pilot operated direction control valves (DCVI & DCV2) , a pair of linkage mechanisms (namely, first linkage mechanism and second linkage mechanisms). The above parts are integrated with igniting switch (IG), brake valve, brake relay valve and hand brake valve of vehicle.
[031] The primary air tank and secondary air tank are used to provide compressed air to the air springs and brake chambers respectively. The pressure limiting valve (PLV) limits a desire air pressure to the ride air springs. It allows compressed air to the load sensing valve in a controlled mamier. It is fluidly connected in between the outlet port of primary air tank through a non-return valve (NRV) and the inlet port of both the load sensing valves. The inlet port of low selective pressure valve is fluidly connected to the outlet ports of both the LSV (first and second LSV) and the outlet port is fluidly connected to the first inlet port of lift axle control valve (LACV). The first outlet port of LACV is fluidly connected to the ride air springs through a quick release valve. The second inlet port is connected to the primary air tank. The second outlet port is fluidly coimected to the lift air springs through the second direction control valve (DCV2) and exhaust delay valve. The first direction control valve (DCV1) is used to disconnect the brakes of the lift axle when the axle is lifted. It is fluidly conected in between the foot brake valve and the relay valve of the brakes of lift axle. The second pilot operated direction control valve (DCV2) is used to easy of axle alignment carried out in the vehicle. It is fluidly controlled by the hand brake valve. The exhaust delay valve is a flow restriction valve that helps to control the lifting and lowering speed of the axle. The first linkage mechanism (consists of a long link, a pair of rubber joints and a short link) pivotally connected in between the LSVl and the rear axle of the vehicle. The second linkage mechanism, which is similar to the first linkage mechanism, pivotally connected in between the LSVl and the first front axle of the vehicle.
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[032] The vehicle IG switch connected with 24DC battery supplies electrical input to the LACV. A manual lift axle rocker switch (MLS) connected with the vehicle IG switch and the electrical terminal of LACV is provided. The MLS is a rocker switch that activates the main electrical systems for the lift axle control system (LACV). This electrical switch is mounted in dash board of the cabin and is operated directly by vehicle operator to control the lift axles independently.
[033] When the vehicle is in unladen condition, LACV inflates the lift air springs and deflates the ride air springs through the quick release valve. The DCV1 disconnects the brakes of the lift axle; so that the wheels are freely rotated when the axle is lifted. When the vehicle is in laden condition, LACV deflates the lift air springs and inflates the ride air springs through the quick release valve. The DCVI connects the brakes of the lift axle.
[034] When the axle alignment is carried out, the MLS is activated and hand brake is applied. Both the lift air spring and ride air spring are deflated.
BRIEF DESCRIPTION OF THE DRAWINGS:
[035] Figure 1 illustrates the schematic of the multi-axles truck 10x2 configuration having pusher lift axle equipped with air springs in accordance to a prior art: (a) self-steer lift axle having single tire at the each end and (b) non-steer lift axle having dual tires at the each end.
[036] Figure 2 illustrates the schematic of the conventional lift axle control system and single LSV attachment in accordance to a prior art: (a) automatic conventional lift axle control system (b) single LSV fitted with rear dive axle; (c) single LSV fitted with a linkage mechanism connected the first rear axle and second rear axle.
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[037] Figure 3 Illustrates the schematic of the frame failure observed in the multi-axle truck having the conventional lift axle control system with a pusher lift axle.
[038] Figure 4 Illustrates the schematic of the automatic lift axle control system for truck having tandem leaf spring suspension, in accordance to the present invention.
[039] Figure 5 illustrates the first LSV and linkage mechanism fitted with the rear axle and the frame of the vehicle, in accordance with first embodiment of present invention.
[040] Figure 6 illustrates the details of first LSV and linkage mechanism fitted and the rear axle with the frame of the vehicle, in accordance with first embodiment of present invention.
[041] Figure 7 illustrates the second LSV and linkage mechanism fitted with the rear axle and the frame of the vehicle, in accordance with first embodiment of present invention.
[042] Figure 8 illustrates the details of first linkage mechanism LSV fitted with the rear axle with the frame of the vehicle, in accordance with first embodiment of present invention.
[043] Figure 9 illustrates the first embodiment of mounting of load sensing valves in the chassis frame of the vehicle having non-reactive bell crank and tie rod tandem leaf spring suspension or slipper ended rocker arm tandem leaf spring suspension or bogie inverted leaf spring single point suspension, in accordance with the present invention.
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[044] Figure 10 illustrates the second embodiment of mounting of load sensing valves in the chassis frame of the vehicle having non-reactive bell crank and tie rod tandem leaf spring suspension or slipper ended rocker arm tandem leaf spring suspension or bogie inverted leaf spring single point suspension, in accordance with the present invention.
[045] Figure 11 illustrates the third embodiment of mounting of load sensing valves in the chassis frame of the vehicle having non-reactive bell crank and tie rod tandem leaf spring suspension or slipper ended rocker arm tandem leaf spring suspension or bogie inverted leaf spring single point suspension, in accordance with the present invention.
[046] Figure 12 illustrates the fourth embodiment of mounting of load sensing valves in the chassis frame of the vehicle having non-reactive bell crank and tie rod tandem leaf spring suspension or slipper ended rocker arm tandem leaf spring suspension or bogie inverted leaf spring single point suspension, in accordance with the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[047] The present invention relates to the automatic control system which is pneumatically connected to the air springs of pusher lift axle system. The automatic control system inflates and deflates the air springs according to the load on the load body of the vehicle.
[048] Figure 4 shows the schematic of electrical and pneumatic control system (13) for lifting lowering the pusher axle according to the present invention. The control system is used for raising and lowering the axle attached in-front of the conventional
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tandem rear leaf spring suspension in the chassis frame (5). The control system (13) consists of a primary air tank (14), a secondaiy air tank (15), a non-return valve (NRV, 16), a primary load sensing valve (17) and a secondary load sensing valve(18), a pressure limiting valve (PLV, 37), a low pressure selecting valve (19), a lift axle control valve (LACV, 20), a quick release valve (21), a pair of lift air springs (22), a pair of ride air springs (23), a exhaust delay valve (24), a manual lift axle switch (i.e., rocker switch, MLS, 25), a first direction control valve (DCV1, 26), and a second direction control valve (DCV2, 27), a first linkage mechanism (28) and a second linkage mechanism (29). The above parts are integrated with igniting switch (IG, 30), foot brake valve (31) and front brake relay valve (32). The lift air springs (22) and ride air springs (23) are attached with the pusher lift axle suspension (1) which is not the part of present invention. Therefore, the description is not provided hereafter.
[049] The primary air tank (14) is used to store the compressed air that is required for operating the lift axle system especially for supplying air to the ride air springs (23) and lift air springs (22). The secondaiy air tank (15) is used to store the compressed air that is required for brakes (i.e., especially for supplying air to the brake chambers (33) of vehicle. The primaiy air tank (14) and second air tank (15) are fluidly connected through a fluid line (34) and connected to the compressor outlet through a fluid line (35). In order to limit the air pressure of the ride air springs (23), the pressure limiting valve (PLV, 37) is used. The PLV (37) provides a desired air pressure (pre-determined value) to the LACV (20) through the load sensing valves (17,18). The PLV (37) is fluidly connected to the outlet port of primary air tank through a non-return valve (NRV, 16) and through a fluid line (35). The NRV (16) does not allow air from both the ride air springs (23) and lift air springs (22) to the primary air tank (14). In order to sense the load on the vehicle, the first load sensing valve (LSV1, 17) and second load sensing valve (LSV2, 18) are used. These LSV1 and LSV2 are fluidly connected to the outlet port of the primaiy PLV (37) through a
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fluid line (38) and a fluid line (39), respectively. In order to supply a least pressure to the LACV (20), the low pressure selective valve (19) is fluidly connected to the outlet port of both the LSV1 (17) and LSV2 (18). That is, if the outlet pressure of LSV1 (17) is lower than that of LSV2 (18), the low pressure selective valve (19) allows to connect the LSV1 outlet port (40) to the first inlet port of LACV (41) through a fluid line (43). If the outlet pressure of LSV2 (18) is lower than that of LSV1 (17), the low pressure selective valve (19) allows to connect the LSV2 outlet port to the first inlet port of LACV (41) through a fluid line (43). The outlet port (44) of low pressure selective valve (19) is fluidly connected to the first inlet port (41) of lift axle control valve (LACV, 20). The first inlet port (41) is opened to first outlet port (45) without electric power.
[050] The LACV (20) is an electrically operated, normally open type, five port and two ways direction control valve. That is, first inlet port (41) is in normally opened condition with first outlet port (45) without electric power and the second inlet port (46) is closed with the second outlet port (47) and the second outlet port (47) is opened to exhaust port (48) in which a silencer is attached. The solenoid of LACV (20) is electrically connected to the battery of the vehicle through the igniting switch (IG, 30) of the vehicle. When the solenoid is energized, the spool (which is inside of the valve) moves upward. Therefore, the first inlet port (41) is closed with first outlet port (45) and the second inlet port (46) is opened with the second outlet port (47) and the first outlet port (45) is opened to the exhaust port (48).
[051] The first outlet port (45) of the LACV (20) is fluidly connected to the quick release valve (21) through fluid line (49). The quick release valve (21) is connected to the ride air springs (23) through fluid lines (50a, 50b). The outlet port of the primary air tank (14) is fluidly connected to the second inlet port (46) of the LACV through the NRV (16) and a fluid line (51). The second outlet port (47) of LACV (20) is
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fluidly connected to the inlet port (52) of the second pneumatically operated direction control valve (DVC2, 27) through a fluid line (56). The outlet port (53) of the second pneumatically operated direction control valve (DVC2, 27) is connected to inlet port (57) of the exhaust delay valve (24) through a T junction; in which the signal (pilot) port (59) of the first pneumatically operated direction control valve (DVCl, 26) is fluidly connected through a fluid line (63). The exhaust delay valve (24) is a flow restriction valve that helps to control the lowering speed of the axle when the axle is lowered. The outlet port (58) of the exhaust delay valve (24) is fluidly connected to the both the lift air springs (22) through a T joints and a fluid lines (64a and 64b).
[052] The outlet port of the secondary air tank (15) is fluidly connected to the inlet port (65) of the hand brake valve (36) (which already exists in the vehicle) through a fluid line (67) and the signal (pilot) port of the relay valve of the brakes through fluid line (63). The outlet port (66) of hand brake valve (36) is fluidly connected to the signal port (i.e., pilot port, 55) of DVC2 (27) through a fluid line. The exhaust port (54) of DVC2 (27) is opened to atmosphere through a silencer. The first outlet port (45) of LACV (20) is fluidly connected to the signal (pilot port 59) of the DVCl (26) through a fluid line (68) and T joint on the fluid line (49). The outlet port (61) of DVCl (26) is fluidly connected to the signal port (70) of the brake relay valve (32) of the lift axle brakes through a fluid line (69). One of outlet port of brake valve (31) is fluidly connected to the exhaust port (62) of DVCl (26) through fluid line (71). When brake is applied, compressed air is passed from the brake valve (31) to the signal port (70) of brake relay valve (32) through the exhaust port (62) of DVCl (26) through fluid line (71) and fluid line (69).
[053] As the LACV (20) is normally open type, first inlet port and first outlet port are normally open position; so that, compressed air is supplied to the ride air springs (23) without electric power. At this condition, the brake chambers (33) can be activated. If
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brake is applied (when pressing the brake pedal of vehicle), compressed air is supplied to the brake chambers (33). At the same time, the second inlet port (46) and second outlet port (47) of LACV (20) are closed position. The DCV1 (26) is provided to disconnect the brakes when the lift axle is lifted.
[054] The vehicle IG switch (30) is electrically connected with a 24v DC battery and the fist electrical terminals of LACV (20) through electrical wire (72). A manual lift axle switch (MLS, 25) is connected with the vehicle IG switch (30) and the second electrical terminals of LACV (20) through electrical wire (73).The MLS switch (25) is a rocker switch that activates the main electrical systems for the lift axle control system (13). The lift axle switch (MLS, 25) is either attached with the dash board of the cabin near to the operator or with the frame near the lift axle is mounted on the chassis frame (5). It is operated by vehicle operator to control the lift axle manually.
[055] When IG switch (30) is on position, electrical current is passed to the LACV (20). If the vehicle is in unladen condition, compressed air is supplied to the lift air springs (22) with a required pressure. As the same time, compressed air in the ride air springs (23) and the fluid lines (63) are exhausted through the quick release valve (21) (i.e., the ride air springs are deflated through quick release valve). At the same time, compressed air is disconnected to the signal port (59) of the DCV1 (26) as it is fluidly is connected with the ride air springs (23) through the T joint and is the fluid lines (68). Therefore, the outlet port (61) of DCV1 (26) is closed with the signal port (59) of brake relay valve (32). The residual air in the signal fluid line (69) is exhausted to the inlet port (60) of DCVI where a silencer is attached. The air in the brake chambers (33) is also exhausted. Thereby, the brake chambers (33) are disconnected when the axle is lifted from the ground surface.
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[056] When the vehicle is in laden condition, the second outlet port (47) is opened to the exhaust port (48) of the LACV (20) where a silencer is attached. Therefore, compressed air in lift air springs (22) is exhausted through the silencer. The axle is lowered. At the same time, first inlet port (41) of the TACV (20) is opened to the first outlet port (45) of the LACV (20). Compressed air is passed to the ride air springs (23) to have reaction load of vehicle. In addition, the compressed air passes to the pilot line (59) of DCV1 (26). Therefore, exhaust port (62) of DCV1 is opened to the inlet port (60) of the DCV1 (26) where the signal fluid line of the brake relay valve is connected. If brake is applied, compressed air is supplied from brake valve to the signal fluid line of the brake relay valve. The compressed air is passed from secondary air tank (15) to the inlet port of brake relay valve (32) through fluid line (63). Thereby, the brakes are activated when the axle is engaged to the road surface.
[057] The axle alignment is usually carried out in unladen condition and the vehicle is stationary condition. At this time, hand brake (36) is generally applied to hold the vehicle to prevent the vehicle movement. When the hand brake (36) is applied, compressed air is passed to the signal port (55) of the DCV2 (27). The inlet port (52) of DCV2 (27) is closed and the outlet port (54) is opened where silencer attached. Therefore, the lift air springs (22) are deflated. At the same time, the brakes of lift axle are also disconnected. If the axle alignment is carried out in laden condition, the MLS (25) is operated i.e., MLS is kept on position, so that the ride air springs (23) are also deflated.
[058] In the present invention, two load sensing valves (LSVland LSV2) are used which are mounted to the chassis frame (5). Figure 5 illustrates the first LSV (17) attached to the rear axle (7b) of the vehicle through first linkage mechanism (28). It is an automatic pressure control (regulator) valve. The first linkage mechanism (28) according to the present invention is shown Figure 5b & 6. The first linkage
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mechanism (28) consists of a long link (74), a short link (75), a pair of rubber pivot joints (76) and an axle bracket (77). The short link (75) is rigidly secured to the axle bracket (77) which is connected to the rear axle (7b) in conventional means. The long link (74) is pivotally connected to the short link (75) and the lever of the first LSVl (17) using the rubber joints (76). According to the distance between the rear axle (7b) to the chassis frame (5), the angle of lever of first LSV (17) is changed. According to the variation of angle of lever (78), out-let pressure will be varied. For example, if height between the rear axle (7b) to the chassis frame (5) is high, a low pressure will be supplied to the LACV (20) and vice versa.
[059] Figure 7 illustrates the second linkage mechanism according to the present invention. It is attached to the chassis frame (5) and first front axle (6a) of the vehicle through second linkage mechanism (29). The detail of the second linkage mechanism (29) according to the present invention is shown Figure 7b & 8. It is similar to the first linkage mechanism. It is connected in between the leaf spring (79) of front axle (6a) to the chassis frame (5) of the vehicle. It consists of a pair of second rubber joints (80), a second long link (81), second short link (82) and second axle bracket (83). The second rubber joints (80) are attached to both the end of first long link (81). Top end of the second log link (81) is pivotally attached to the lever (84) of the LSV2 (18) by the second rubber joint (80). Other end of the second long link (81) is pivotally jointed to the second short link (82). Other end of the second short link (82) has thread which is securely connected to the second axle bracket (83). The second axle bracket (83) is connected to the leaf spring (79) of the first front axle (6a).
[060] The LSV2 (18) is either connected with the first front axle (6a) (i.e., through first front leaf spring suspension (79)) or the second front axle (6b) (i.e., through second front leaf spring suspension) through second linkage mechanism (29). Similarly, the LSVl (17) is also either connected with the first rear axle (7a) or
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second rear axle (7b) through first linkage mechanism (28). In addition, the present invention is proposed for lifting and lowering the pusher axle (1) of the heavy trucks having twin front axles and tandem leaf spring suspensions such as non-reactive bell crank and tie rod tandem leaf spring suspension (4) or slipper ended rocker arm tandem leaf spring suspension (11) or bogie inverted leaf spring single point suspension (12). The non-reactive bell cranks and tie rod tandem leaf spring suspension (4) and slipper ended rocker arm tandem leaf spring suspension (11) are generally used in the rigid trucks used for the haulage application. The bogie inverted leaf spring single point suspension (12) is used the rigid trucks used for the tipper or dump truck application.
[061] Figure 9 illustrates the first embodiment of mounting the load sensing valves (17,18) in the chassis frame (5) of the vehicle having non-reactive bell crank and tie rod tandem leaf spring suspension (4) or slipper ended rocker arm tandem leaf spring suspension (11) or bogie inverted leaf spring single point suspension (12), in accordance with the present invention. In the first embodiment, the LSV1 (17) is secured to the chassis frame (5) and pivotally connected with the first rear axle (7a) through the first linkage mechanism (28). The LSV2 (18) is secured to the chassis frame (5) and pivotally connected with the first front axle (6a) (i.e., through first front leaf spring suspension) by the second linkage mechanism (29). In this embodiment, in accordance with the present invention, the load on the vehicle is metered using the LSV1 (17) and LSV2 (18). According the outlet pressures of LSV1 (17) and LSV2 (18), the LACV (20) controls the function of pusher lift axle (1) in the vehicle.
[062] Figure 10 illustrates the second embodiment of mounting the load sensing valves (17,18) in the chassis frame (5) of the vehicle having non-reactive bell crank and tie rod tandem leaf spring suspension (4) or slipper ended rocker arm tandem leaf spring suspension (11) or bogie inverted leaf spring single point suspension (12), in
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accordance with the present invention. In the second embodiment, the LSV1 (17) is secured to the chassis frame (5) and pivotally connected with the second rear axle (7b) through the first linkage mechanism (28). The LSV2 (18) is secured to the chassis frame (5) and pivotally connected with the first front axle (6a) (i.e., through first front leaf spring suspension) by the second linkage mechanism (29). In this embodiment, in accordance with the present invention, the load on the vehicle is metered using the LSV1 (17) and LSV2 (18). According the outlet pressures of LSV1 (17) and LSV2 (18), the LACV (20) controls the function of pusher lift axle (1) in the vehicle.
[063] Figure 11 illustrates the third embodiment of mounting the load sensing valves (17,18) in the chassis frame (5) of the vehicle having non-reactive bell crank and tie rod tandem leaf spring suspension (4) or slipper ended rocker arm tandem leaf spring suspension (11) or bogie inverted leaf spring single point suspension (12), in accordance with the present invention. In the third embodiment, the LSV1 (17) is secured to the chassis frame (5) and pivotally connected with the first rear axle (7a) through the first linkage mechanism (28). The LSV2 (18) is secured to the chassis frame (5) and pivotally connected with the second front axle (6b) (i.e., through second front leaf spring suspension) by the second linkage mechanism (29). In this embodiment, in accordance with the present invention, the load on the vehicle is metered using the LSV1 (17) and LSV2 (18). According the outlet pressures of LSV1 (17) and LSV2 (18), the LACV (20) controls the function of pusher lift axle (1) in the vehicle.
[064] Figure 12 illustrates the fourth embodiment of mounting the load sensing valves (17,18) in the chassis frame (5) of the vehicle having non-reactive bell crank and tie rod tandem leaf spring suspension (4) or slipper ended rocker arm tandem leaf spring suspension (11) or bogie inverted leaf spring single point suspension (12), in accordance with the present invention. In the fourth embodiment, the LSV1 (17) is
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secured to the chassis frame (5) and pivotally connected with the second rear axle (7b) through the first linkage mechanism (28). The LSV2 (18) is secured to the chassis frame (5) and pivotally connected with the second front axle (6b) (i.e., through second front leaf spring suspension) by the second linkage mechanism (29). In this embodiment, in accordance with the present invention, the load on the vehicle is metered using the LSV1 (17) and LSV2 (18). According the outlet pressures of LSV1 (17) and LSV2 (18), the LACV (20) controls the function of pusher lift axle (1) in the vehicle.
Operating principle
[065] The operating principle of the lift axle control system (13) according to the present invention is described below. The vital function of the automatic control system (13) is to lift pusher axle (1) in unladen condition and lower the axle in laden condition.
[066] When the vehicle is loaded, the distance between the chassis frame (5) and the rear axle (7a) is low. Similarly, distance between the chassis frame (5) and the front axle beam (6a) is also low. The first inlet port (41) of LACV (20 ) is opened to the first outlet port (45) and the second outlet port (47) is opened to the exhaust port (48). Thereby, any residual air inside of the lift air springs (22) is exhausted to atmosphere. The second inlet port (46) is remaining in closed position. The lift air springs (22) are connected to the atmosphere pressure. When brake is applied in laden condition, the compressed air is passed from brake valve (31) to exhaust port (62) of DCV1 (26). The outlet port (61) of DCVI (26) is opened with the signal port (70) of the brake relay valve (32). Therefore, the compressed air is passed from the signal port (pilot port, 70) of the lift axle brake relay valve (32). Therefore, the compressed air is passed from the second auxiliary tank (15) to the brake chambers (33) through the
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fluid line (63). When brake releases by the operator when operating the vehicle, the residual compressed in the brake chambers (33) is exhausted through the exhaust port (85) of the brake relay valve (33).
[067] In partially laden condition, leaf springs of the suspension are relaxed and the frame height is increased from the road surface. That is, distance between the frame (5) to the axles is increased. The levers of the LSV1 and LSV2 are moved downward. Therefore, outlet pressures of the LSV1 (17) and LSV2 (18) are reduced. The low pressure selecting valve (19) allows the compressed air to the first inlet port (41) of the LACV (20) which has lower magnitude of pressure (i.e., if the outlet pressure of LSV1 (17) is less than that of LSV2 (18), LSV1 (17) outlet pressure is passed to the first inlet port (41) of the LACV (20) and vice-versa). The LACV (20) is set to a predetermined pressure. When the outlet pressure of the low pressure selective valve is reached to the predetermined pressure of LACV (20), a control valve provided inside the LACV (20) (not shown as it is not the part of present invention) disconnect the electrical current to the solenoid. The solenoid of the LACV (20) is discharged (i.e., de-energized or disconnected). When electrically disconnected, spool provided in the LACV (20) is moved downward (not show, as it is internal operation of the valve). Therefore, first inlet port (41) is closed with first outlet port (45) and second inlet port (46) is opened to the second outlet port (47) which is fluidly connected to the lift air springs (22) through exhaust delay valve (24). The compressed air is passed from first primary air tank (14) to the lift air springs (22). Simultaneously, first outlet port (45) of LACV (20) is opened to exhaust port (48). Therefore, compressed air inside in the ride air springs (23) is exhausted through the exhaust port (48) where the silencer is attached. The axle is slowly lifted because of exhaust delay valve (which restricts immediate exhaust) provided at the lift air springs (22) and maintained at a desired height from road surface. In addition, air in the brake chambers (33) is exhausted to the atmosphere through the inlet port (60) of DCV1
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(26). When the axle is lifted, the pneumatic connection is disconnected to the brake champers (33). Therefore, pusher axle (1) wheels are freely rotated. When the pusher axle (1) tires engage any bump, the pusher axle (1) tires freely roll over the hump. Thereby, pusher axle (1) tire scrub is minimized.
[068] In fully unladen condition, leaf springs of the suspension further relax and the frame height further increases from the road surface. That is, distance between the frame (5) to the axle further increases. The levers of both the LSV1 and LSV2 further move downward. Therefore, outlet pressures of both the LSV1 and LSV2 are reduced. The outlet pressure of the low pressure selective valve is continued. This pressure value is less than the predetermined pressure of LACV (20). The control valve provided inside of the LACV (not shown) continuous to disconnect the electrical current to the solenoid. The solenoid of the LACV (20) is continuously de-energized. Spool of the LACV (20) is maintained in up position (not shown) direction. Therefore, first inlet port (41) is continued in closed position and second inlet port (46) is continued to open to the second outlet port (47) which is fluidly connected to the lift air springs (22) through the exhaust delay valve (24). The compressed air is continuously passed from primary air tank (14) to the lift air springs (22). The axle is lifted and continuously maintained at the desired height from the road surface. Simultaneously, first outlet port (41) of LACV (20) is continuously opened to exhaust port (48). Therefore, residual compressed air inside in the ride air springs (22) is exhausted through the exhaust port (48). When the pusher axle (1) is lifted, the pneumatic connection is disconnected to the brake champers (33). Because, the compressed air is discontinued to the pilot line (68) of the first DCV1 (); therefore, the pilot fluid line (69) of the brake relay valve (32) is connected to atmosphere. Therefore, pusher axle (1) wheels are freely rotates. When the tires engage any bump, the pusher axle (1) tires freely roll over the hump. Thereby, pusher axle (1) tire scrub is minimized.
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[069] Upon again loading the vehicle, pay load of the vehicle increases. Therefore, the leaf spring gets bend and reaches the least height from the road surface. As a result, the distance between the axle to the frame is reduced. Upon continuous loading, the outlet pressure reaches above the pre-set pressure of the LACA (20). Immediately, the electrical signal is supplied to solenoid of the LACV (i.e., through an outlet terminal of the pressure switch provided inside of the LACV, not shown). Therefore, the solenoid of the LACV (20) is energized. Therefore, the spool (not shown) is moved downward (to its original position). The first inlet port (41) is again opened to the first outlet port (45) and second outlet port (46) is open to the exhaust port (48). Thereby, compressed air in the lift air springs (22) is exhausted and compressed air is passed from primary air tank (14) to ride air springs (23) through either of LSV1 (17) of LSV2 (18) which has low pressure. The brake chambers (33) also becomes to their function by open the brake outlet line to signal port of brakes relay valve (24). When brake is applied (by operator) compressed air is passed from the brake valve (31), to the brake chambers (33) through brake relay valve (32). The lift air springs (22) are inflated in laden condition.
[070] During reverse operation of the vehicle, the solenoid of the LACV (20) is continuously energized. Spool of the LACV (20) is maintained in up position (not shown) direction. Therefore, first inlet port (41) is continued in closed position and second inlet port (46) is continued to open to the second outlet port (47) which is fluidly connected to the lift air springs (22) through the exhaust delay valve (24). The compressed air is continuously passed from the secondary air tank (15) to the lift air springs (22). The axle is lifted and continuously maintained at the desired height from the road surface. Simultaneously, the first outlet port (45) of LACV (20) is continuously opened to the exhaust port (48). Therefore, compressed air inside in the ride air springs (23) is exhausted through the exhaust port (48). When the pusher axle
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(1) is lifted, the pneumatic connection is disconnected to the brake champers (33). Therefore, pusher axle (1) wheels are freely rotated. When the pusher axle (1) tires engage any bump at the condition of reverse operation of the vehicle, the pusher axle (1) tires freely roll over the hump. Thereby, the pusher axle (1) tires scrub is minimized.
[071] The present invention helps when the pusher axle (1) alignment is carried out. When the axle (1) alignment is carried out, the MLS switch (25) is kept in switch-on position. Therefore, the first inlet port (41) is closed with the first outlet port (45) and the second inlet port (46) is opened to the second outlet port (47) which is fluidly connected to the lift air springs (22) through the exhaust delay valve (24). The compressed air is passed from the first primary air tank (14) to the lift air springs (22). Simultaneously, the first outlet port (41) of LACV (20) is opened to the exhaust port (48). Therefore, the compressed air inside in the ride air springs (22) is exhausted through the exhaust port (48) where the silencer is attached. The pusher axle (1) is lifted and maintained at a desired height from road surface. In this condition, the hand brake (36) is applied. Therefore, the inlet port (52) of the second DCV2 (27) is closed and the outlet port (53) of the second DCV2 (27) is opened to the exhaust port (54). Thereby, the lift air springs (22) are also deflated easily in the present invention. Now, the axle alignment can be carried out in the lift axle suspension (i.e., the position of control arms of the lift axle suspension is adjusted to maintain required position w.r.t frame of the vehicle. Axle alignment procedure is not the part of the present invention. Therefore, it is not described hereafter.
[072] In addition, the present invention minimizes the frame failure when the vehicle is loaded. At the time of loading the vehicle, the vehicle is loading from front of the load body to rear side of the load body or vice-versa. When the vehicle is loaded from rear side of the load body, the distance from the chassis frame (5) to the rear axle (7a)
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is reduced due to load at the rear side of the load body. The lever of the first LSV1 (17) connected at the rear axle move downward. The first LSV1 (17) supply a high pressure to the low pressure selective valve (19). At the same time, distance from the chassis frame (5) to the front axle (6a) is increased or at higher magnitude. Therefore, the lever of the second LSV2 (18) is in upward position. Therefore, the first LSV1 (17) supply a low pressure to the low pressure selective valve (19). The low pressure selective valve (19) connects the first LSV1 (17) to the first inlet port (41) of the LACV (20). In this condition, the pusher axle (1) will not engage to the road surface. That is, the pusher axle (1) is maintained in lifted position. Similarly, when the vehicle is loading from front side of the load body, the low pressure selective valve (19) connects the second LSV2 (18) to the first inlet port (41) of the LACV (20). In this condition also, the axle will not engage to the road surface (i.e., the axle is maintained in lifted position). Therefore, the chassis frame (5) is protected from the severe bending due to overhanging load of either front weight of the vehicle (i.e., cab, engine gearbox etc.,) or rear weight of the vehicle (i.e., rear axle and wheels ). The pusher axle (1) of is only engaged to the road surface when the vehicle is loaded (i.e., load body is loaded) approximately uniform.
[073] According to the invention, the chassis frame (5) protection is provided which is typically loaded when uneven loading of the heavy vehicle. That is, severe chassis frame bending is minimized when the load body is loading either from rear side or front side. The dual LSVs system allows a low pressure to the LACV (20). Therefore, the pusher axle (1) is only engaged to the road surface when the vehicle is loaded (i.e., load body is loaded) approximately uniform.
[074] Another embodiment of the present invention is the disconnection of lift axles brakes when the pusher axle (1) is lifted. The brake chambers (33) are pneumatically disconnected. Therefore, pusher axle (1) wheels rotate freely. When the pusher axle
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tires engage any bump at the condition of reverse operation of the vehicle, the pusher axle (1) tires freely roll over the bump. Thereby, pusher axle tire life is extended. In addition, total air consumption is reduced. Thereby, duty cycle of compressor and fuel consumption is reduced.
[075] Another embodiment of the present invention is the ease in axle alignment process carried out in the lift axle suspension. When the pusher axle (1) alignment is carried out in the vehicle, the lift air springs (22) and the ride air springs (23) are only deflated instead of entire auxiliary tank air is exhausted. The auxiliary tank air is saved. The vehicle is quickly recharged (while restarting the vehicle) the auxiliary air tanks provided in the vehicle, thereby, the vehicle is operated (i.e., moved) quickly from stationary condition.
[076] Another embodiment of the present invention is the design of first linkage mechanism (28). It is used to connect the lever (78) of the first LSV (17) and the rear axle(s) (7a and 7b) of the vehicle. It has non-lubricated and less number of joints. It is simple, low cost, low maintenance and high durable linkage mechanism.
[077] Another embodiment of the present invention is the design of second linkage mechanism. It is used to connect the level of the second LSVs (18) and the first front axle (6a) of the vehicle. The design is similar to the second linkage mechanism (29). The linkage mechanism (29) ensures a long life; thereby, the lift axle control system (13) works properly. It ensure a proper function pusher lift axle (1) in the heavy duty commercial vehicle.
[078] Many modifications and other embodiments of the invention may come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. In
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addition, some changes may be made to these specific embodiments, and such modifications are contemplated by the principle of the present invention. For example, adding one more LSV in the second front axle (6b) ((such as triple LSV control system), or mounting the second LSV2 with the second front axle (6b) of the vehicle or removing, PLV (37), and first DVC1 (26) valve, or replacing the quick release valve by a relay valve (i.e., in place of quick release valve), etc., and using electric operated direction control valves instead of pneumatic operated direction control valves are possible in the system. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims.
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We claim:
1. A lift axle control system (13) for heavy truck having tandem rear leaf spring suspension according to the present invention comprises:
a lift axle control valve (LACV, 20) for directing compressed air to either ride air springs (23) or lift air springs (22) according to operating conditions of the vehicle;
a first direction control valve (DCVI, 26) for disconnecting brakes of pusher lift axle;
a second direction control valve (DCV2, 27) for deflating the lift air springs (22);
a first level sensing valve (LSVl, 17) and a second level sensing valve (LSV2,18) for vaiying air pressure of the ride air springs (23) for better load distribution between the pusher lift axle (1) and rear axles (7a, 7b) according to load on the vehicle;
a non-return valve (NRV, 16) to allow air from air tanks to control valves;
a pressure limiting valve (37) for limiting required pressure to the lift axle control valve (20) and the ride air springs (23);
a quick release valve (21) to exhaust the air from the ride air springs (23) quickly when the axle is lifting the pusher lift axle; and
a low pressure selective valve (19), used to fluidly connect either the first level sensing valve (LSVl, 17) or the second level sensing valve (LSV2, 18), which has lower outlet pressure, with first inlet port (41) of the lift axle control valve (20).
2. The lift axle control system (13) as claimed in claim 1, wherein a first linkage mechanism (28) connects the first level sensing valve (17) with rear axle (7a) of the vehicle.
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3. The lift axle control system (13) as claimed in claim 1, wherein a second linkage mechanism (29) connects the second level sensing valve (18) with the front axle (6a) through front leaf spring suspension of the vehicle.
4. The lift axle control system (13) as claimed in claim 1, wherein said system includes an exhaust delay valve (24) to limit the speed of the air exhausted from the lift air springs (22) when the pusher axle (1) is being lifted.
5. The lift axle control system (13) as claimed in claim 1, wherein a manual operating switch (MLS, 25), is provided to activate or deactivate the lift axle control valve (20).
6. The lift axle control system (13) as claimed in claim 1, wherein the first load sensing valve (17) and the second load sensing valve (18), regulate the pressure required to the ride air springs (23) according to the load experienced in the rear axle (7a) and front axle (6a) respectively, said first and second load sensing valves (17, 18) are mounted on chassis frame (5) above rear axle (7a) and front axle (6a) respectively, pneumatically connected in-between the pressure limiting valve (37) fluidly connected with a primary air tank (14) through non-return valve (16) and said LACV (20) through the low pressure selective valve (19) used for supplying lower air pressure to the LACV (20).
7. The lift axle control system (13) as claimed in claim 6, wherein the first load sensing valve (17) is pivotally connected with either the drive axle (7a) or the tag axle (7b) of the vehicle through a first linkage mechanism (28).
8. The lift axle control system (13) as claimed in claim 6, wherein the second load sensing valve (18) is pivotally connected with either the first front axle (6a) or second front axle (6b) of vehicle through a second linkage mechanism (29).
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9. The lift axle control system (13) as claimed in claim 1, wherein the first direction
control valve (26) is fluidly connected in between brake chambers (33) through brake
relay valve (32) and foot brake valve (31) and connected with the fluid line (49) of
ride air springs (23), and is configured to disconnect the brake chambers (33), when
the pusher axle (1) is lifted.
10. The lift axle control system (13) as claimed in claim 1, wherein the second direction control valve (27) is fluidly connected with lift air springs (22) through fluid line (56) and exhaust delay valve (24), and fluidly connected with outlet port (66) of hand brake valve (36), and is configured to deflate the lift air springs (22) by supplying compressed air from second auxiliary tank (15) to pilot fluid line (67) of second direction control valve (27).
11. The lift axle control system (13) as claimed in claim 2, wherein the first linkage mechanism (28) consists of
an axle bracket (77) rigidly secured with either first rear axle (7a ) or second rear axle (7b);
a short link (75) fixed vertically and secured with axle bracket (77);
a movable long link (74) pivotally connected to the axle bracket (77) and a lever (78) using a pair of rubbers hinge joints(76).
12. The lift axle control system (13) as claimed in claim 3, wherein the second linkage
mechanism (29) consists of
an axle bracket (83), rigidly secured with either first front axle (6a) or second front axle (6b);
a short link (82) fixed vertically and secured with axle bracket (83);
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a movable long link (81) pivotally connected to the axle bracket (83) and a lever (84) using a pair of rubbers hinge joints(80).
13. The lift axle control system (13) as claimed in claim 1, wherein said system (13) is capable of being used for heavy truck having non-reactive bell crank and tie rod tandem rear leaf spring suspension (4), or slipper ended rocker arm tandem rear leaf spring suspension (11), or bogie inverted leaf spring single point rear suspension (12), wherein the first level sensing valve (17) is secured to chassis frame (5) and pivotally attached to either first rear axle (7a) or second rear axle (7b) through the first linkage mechanism (28).
14. The lift axle control system (13) as claimed in claim 1, wherein said system (13)is capable of being used for heavy truck having non-reactive bell crank and tie rod tandem rear leaf spring suspension (4), or slipper ended rocker arm tandem rear leaf spring suspension (11), or bogie inverted leaf spring single point rear suspension (12), wherein the second level sensing valve (18) is secured to the chassis frame (5) and pivotally attached to either the first front axle (6a) or second front axle (6b) through the second linkage mechanism.