Patent Publication Number: US-2005121878-A1

Title: Ram-based positioning system

Description:
FIELD OF THE INVENTION  
      The present invention relates to a ram-based positioning system. The invention relates particularly, but not exclusively, to a steering system for an articulated vehicle.  
     BACKGROUND TO THE INVENTION  
      It is known for the steering system of an articulated vehicle to comprise a ram-based positioning system. Typically, the steering system comprises one or more pairs of master/slave hydraulic rams wherein the slave rams are arranged to steer the rear axle of the vehicle. A problem which such systems is that they can often become misaligned as a result of, for example, loss of hydraulic liquid through leakage or through a pressure relief valve during an emergency operation.  
      In a conventional system, alignment has to be restored manually, typically by centralising the steering axle and king pin sensor to a reference point, then opening and closing the system valves in sequence and manually re-priming the system. This is a relatively time consuming and awkward procedure, not least because the driver needs to leave the vehicle unless he has assistance.  
      It would be desirable, therefore, to provide a ram-based positioning system which may readily be re-aligned by, for example, the driver of an articulated vehicle, without the need for assistance or the need to leave the vehicle.  
     SUMMARY OF THE INVENTION  
      Accordingly, a first aspect of the invention provides a positioning system comprising at least one master ram and respective slave ram, each ram being fluid operable and having a respective piston and a respective piston chamber, the or each master ram and respective slave ram being arranged, when in a state of normal alignment, so that actuation of the master piston with respect to the master piston chamber causes a corresponding actuation of the slave piston with respect to the slave piston chamber, wherein the system further includes at least one travel stop and at least one pressure relief valve, the or each travel stop being arranged to restrict the travel of one or more slave piston when the system is misaligned, at least one pressure relief valve being arranged to vent excess pressure in the operating fluid arising as a result of such restriction.  
      By venting the excess fluid pressure in this way, the system is brought back into normal alignment.  
      Preferably, in the state of normal alignment, the or each master and respective slave pistons are actuatable between a respective fully extended state and a respective fully retracted state, wherein the or each travel stop is arranged to prevent one or more slave piston from reaching at least one of said fully extended or fully retracted states when the system is misaligned. Once the slave piston is prevented from moving further, excess fluid pressure arises as the master piston continues to move to its fully extended or fully retracted state.  
      Preferably, each ram comprises a first port and a second port for the inlet and outlet of operating fluid, wherein the ports of the master ram are connected to the ports of the slave ram with a one-to-one correspondence, and wherein a respective pressure relief valve is connected to at least one pair of connected ports. More preferably, a respective pressure relief valve is connected to each pair of connected ports.  
      Preferably, the, or each, pressure relief valve is arranged to direct vented operating fluid to an accumulator.  
      A second aspect of the invention provides a steering system comprising the positioning system of the first aspect of the invention.  
      Preferably, the or each master ram is mountable on a vehicle such that, upon turning of the vehicle, the or each master piston is actuated, and wherein the or each respective slave piston is adapted for steering at least one wheel of the vehicle. More preferably, said at least one wheel is carried by a steerable axle, the or each respective slave piston being adapted to steer said steerable axle.  
      Preferably, the steering system is arranged for steering an articulated vehicle having a tractor unit pivotably coupled to a trailer unit, wherein the or each master piston is actuated upon relative pivoting movement between the tractor unit and the trailer. More preferably, the arrangement is such that, upon extension or retraction of the or each master piston, the respective slave piston is retracted or extended respectively by a corresponding amount.  
      Preferably, the steering system comprises a first master ram and a second master ram, each associated with a respective slave ram, the respective slave pistons being coupled to a steerable axle of the vehicle at locations on opposite sides of the pivoting axis of the steerable axle, the arrangement being such that, upon turning of the vehicle, one of the first and second master pistons is caused to retract, the other being caused to extend, the corresponding actuation of the respective slave pistons causing the steerable axle to pivot about its pivoting axis.  
      Preferably, the or each travel stop is provided on the chassis of a vehicle on which, in use, the steering system in mounted and arranged to restrict the pivoting movement of the steerable axle. Alternatively, the, or each, travel stop is provided on a respective slave ram.  
      A third aspect of the invention provides a vehicle comprising the steering system of the first aspect of the invention. The invention is particularly suited for use with articulated vehicles.  
      Preferably, the rams are hydraulically operable, the preferred hydraulic operating liquid being oil.  
      Other advantageous aspects of the invention will be apparent to those ordinarily skilled in the art upon review of the following description of a specific embodiment of the invention and with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A specific embodiment of the invention is now described by way of example and with reference to the accompanying drawings in which like numerals are used to indicate like parts and in which:  
       FIG. 1  is a plan view of an articulated vehicle including a preferred ram-based steering system embodying one aspect of the invention;  
       FIG. 2  is a schematic view of a preferred ram-based positioning system embodying another aspect of the invention, and being suitable for use in the steering system in  FIG. 1 ;  
       FIG. 3  is a schematic view of a hydraulic circuit for use with the positioning system of  FIG. 2 ;  
       FIG. 4  is a plan view of the articulated vehicle of  FIG. 1 , wherein the steering system is in a partially locked state and the rear axle is properly aligned;  
       FIG. 5  is a plan view of the articulated vehicle wherein the steering system is in a fully locked state and the rear axle is properly aligned;  
       FIG. 6  is a plan view of the articulated wherein the steering mechanism is in a straight ahead state and the rear axle is misaligned;  
       FIG. 7  is a plan view of the articulated vehicle wherein the steering system is in partially locked state and the rear axle is misaligned. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
      Referring now to  FIG. 1  of the drawings there is shown, generally indicated at  10 , an articulated vehicle comprising a tractor unit, or tractor,  12  and a trailer  14 . The trailer  14  comprises a chassis  16  on which there is pivotably mounted a steerable rear axle  18  carrying a set of wheels  20 . Depending on the size of the trailer  14  it may include one or more further axles and wheel sets. In the illustrated embodiment, the trailer  14  includes a further fixed axle/wheel set  15 . The vehicle  10  comprises a steering system which includes a positioning system embodying the invention. In the preferred embodiment, the positioning system comprises a first and a second slave ram  22 ,  24  and a first and second master ram  26 ,  28 , connected to a hydraulic circuit (not shown in  FIG. 1 ) which includes a valve circuit, or valve block  29 . Each ram  22 ,  24 ,  26 ,  28  preferably comprises a double acting ram and comprises a respective piston and piston chamber in normal manner.  
      The rear axle  18  is pivotable with respect to the chassis  16  by the first and second slave rams  22 ,  24 , which have their respective piston chambers mounted on the chassis  16  and their respective. pistons coupled to the rear axle  18 . The first and second master rams  26 ,  28  are also mounted on the chassis  16 . The master rams  26 ,  28  are coupled to a respective slave ram  22 ,  24  so that operation of a master ram  26 ,  28  causes a corresponding operation of the respective slave ram  22 ,  24 .  
      The trailer  14  is pivotably coupled to the tractor  12  by conventional coupling, or linkage mechanism which, in the preferred embodiment, comprises a kingpin mechanism  30  (commonly referred to as a kingpin steering mechanism). The linkage mechanism  30  also forms part of the steering system of the vehicle  10  in that it transmits angular or pivotal movement of the tractor  12  with respect to the trailer  14  to the master rams  26 ,  28 . To this end, the kingpin mechanism  30  includes a crank  32  for converting rotary movement, imparted to the crank  32  during use by the kingpin mechanism  30 , into reciprocal movement of the pistons of the master rams  26 ,  28 . In the preferred embodiment, the crank  32  is substantially “T-shaped” (sometimes referred to as a bell crank, or unitary bell crank) and has one end coupled to the kingpin  34 , the other two ends being connected to a respective piston of the master rams  26 ,  28 .  
      The steering system also includes a respective stop member, or travel stop  36 ,  38 , associated with each slave ram  22 ,  24 . The travel stops  36 ,  38 , are arranged for engagement with the steerable rear axle  18  and/or the pistons of the slave rams  22 ,  24  in certain dispositions of the axle  18 /rams  22 ,  24  as is described in more detail below. In  FIG. 1  the travel stops  36 ,  38  are mounted on the chassis  16  but need not necessarily be so—for example, the travel stops  36 ,  38  may alternatively be incorporated into the slave rams  22 ,  24  themselves.  
      Referring now to  FIG. 2 , there is shown, generally indicated at  40 , the preferred embodiment of the ram-based positioning system. In the preferred embodiment, each ram  22 ,  24 ,  26 ,  28  comprises a hydraulic ram comprising a respective piston  42 ,  44 ,  46 ,  48  and respective piston chamber in the form of cylinders  52 ,  54 ,  56 ,  58 . Each cylinder  52 ,  54 ,  56 ,  58  includes two respective ports A, B, for the input and output of hydraulic operating liquid, typically oil. The arrangement is such that inflow of hydraulic liquid via port A causes the respective pistons  42 ,  44 ,  46 ,  48  to retract and an outflow of hydraulic liquid from the respective port B, while inflow of hydraulic liquid via port B causes the respective pistons  42 ,  44 ,  46 ,  48  to extend and an outflow of hydraulic liquid from the respective port A. Equally, a retraction of the respective pistons  42 ,  44 ,  46 ,  48  causes an outflow from the respective port B, while an extension of the pistons  42 ,  44 ,  46 ,  48  causes an outflow from the respective port A.  
      The master rams  26 ,  28  are hydraulically connected to the respective slave rams  22 ,  24  such that the respective port As and port Bs are connected together in a one-to-one correspondence. Thus, when the piston  46 ,  48  of a master ram  26 ,  28  extends, the piston  42 ,  44  of the respective slave ram  22 ,  24  retracts. Similarly, when the piston  46 ,  48  of a master ram  26 ,  28  retracts, the piston  42 ,  44  of the respective slave ram  22 ,  24  extends.  
      Before use, the system  40  is primed with hydraulic fluid in conventional manner such that the relative states (in terms of piston extension/retraction) of the rams  22 ,  24 ,  26 ,  28  are properly aligned (normal alignment) for the intended use. Thus, for a given extension/retraction of the master ram  26 ,  28 , there is an appropriate corresponding retraction/extension of the respective slave ram  22 ,  24 . It is noted that in  FIG. 2 , all of the rams  22 ,  24 ,  26 ,  28  are shown in a retracted state, which is not a normal operating mode of the system  40  when used to implement the steering system described above. It will also be noted that, although the present embodiment requires that the respective port As of the rams are connected together, as are the respective port Bs, this need not necessarily be the case. For example, in alternative applications, the arrangement may be such that an extension in one (or both) of the master pistons  46 ,  48  causes an extension in the respective slave piston  42 ,  44 .  
      The system  40  may become misaligned, i.e. removed from the normal alignment state, for a variety of reasons including loss of hydraulic liquid through leakage that commonly occurs over a period of time through, for example, cylinder seals (not shown) and valve assemblies (not shown). Hydraulic liquid may also be lost during emergency operations in which hydraulic liquid is shed through relief valves. When misaligned, the relative states of the rams  22 ,  24 ,  26 ,  28  no longer conform to the required relative arrangement and, as a result, the positioning system  40  does not operate properly.  
      The valve block  29  containing a hydraulic valve circuit (described below with reference to  FIG. 3 ) comprises four main hydraulic ports D, E, F, G for the input and output of hydraulic liquid. The rams  22 ,  24 ,  26 ,  28  are connected to the valve block  29  by hydraulic lines (shown dashed in  FIG. 2 ) such that port D of the valve block  29  is hydraulically connected to the respective port A of rams  22 ,  26 ; port E is hydraulically connected to the respective port A of rams  24 ,  28 ; port F is hydraulically connected to the respective port B of rams  22 ,  26 ; and port G is hydraulically connected to the respective port B of rams  24 ,  28 . The hydraulic circuit within the valve block  29  implements a pressure relief system that enables the positioning system  40  to be readily re-aligned, as is described below.  
      Referring now to  FIG. 3 , there is shown the hydraulic circuit, generally indicated at  50 , implemented by valve block  29 . In addition to ports D, E, F and G, the circuit  50  includes a priming port P which is connectable to a conventional priming pump (not shown) in conventional manner. A one-way valve, or check valve  62 , is associated with the priming port P to prevent hydraulic liquid from leaving the circuit  50  by this port P. The circuit  50  further includes an accumulator port ACC which is connectable to a conventional hydraulic accumulator (not shown) in conventional manner. The priming pump and hydraulic accumulator are used to charge, or prime, the valve circuitry  50  and associated hydraulic circuit in conventional manner.  
      The circuit  50  comprises a main hydraulic feed line  64  which is in hydraulic communication with each of ports D, E, F and G via a respective one-way valve  66 , each valve  66  being arranged to allow hydraulic liquid to flow from the main feed line  64  and out through the respective port D, E, F, G, but not in the reverse direction.  
      Each port D, E, F, G is also in hydraulic communication with a respective conventional pressure relief valve  68  which, in the preferred embodiment, are adjustable (or variable) pressure relief valves. Each pressure relief valve  68  is located between the respective port D, E, F, G and the main feed line  64  and is arranged to allow hydraulic liquid to flow from the respective port D, E, F, G to the main feed line  64  only when the pressure of the hydraulic liquid exceeds a threshold value, the threshold value being determined by the setting of the respective variable pressure relief valve  68 . The pressure relief valves  68  do not allow hydraulic liquid to flow in the reverse direction.  
      Circuit  50  further includes two conventional flow control valves  70  which connect, when open, ports D and G and ports F and E, respectively. In normal operation of the circuit  50 , the valves  70  remain closed and are effectively open-circuit (i.e. isolating port D from port G and port E from port F). During the initial setting up of the position system  40 , however, the flow control valves  70  are opened to facilitate initial alignment of the positioning system  40 . In the context of the steering system outlined in  FIG. 1 , this means that the hydraulic liquid is able to flow directly between the rams on either side of the trailer  14 . This allows the pistons  42 ,  44  of the slave rams  22 ,  24  to move independently of the pistons  46 ,  48  of the master rams  26 ,  28  so that the steering system can be properly aligned during the setting up of the hydraulic circuitry.  
      The operation of the positioning system  40  is now described in the context of a vehicle steering system.  FIG. 1  shows the articulated vehicle  10  in a straight ahead state in which the tractor  12  and the trailer  14  are substantially in register with one another i.e. the respective longitudinal axis of the tractor  12  and trailer  14  are substantially coincident. In this state, when the positioning system  40  is in normal alignment (as shown in  FIG. 1 ), the slave rams  22 ,  24  hold the rear axle  18  substantially perpendicularly with respect to the longitudinal axis of the tractor  12 /trailer  14 . Thus, the wheels  20  are substantially parallel with the trailer  14  i.e. ready for straight ahead movement of the vehicle  10 . Assuming that the slave rams  22 ,  24  are positioned substantially ih register with one another on the chassis  16  (as shown in  FIG. 1 ), the slave pistons  42 ,  44  are extended by substantially equal amounts with respect to their cylinder  52 ,  54 . Similarly, assuming that the crank  32  is symmetrically located on the chassis  16  and that the master rams  26 ,  28  are substantially in register (as shown in  FIG. 1 ), then the master pistons  46 ,  48  are extended by substantially equal amounts with respect to their cylinder  56 ,  58 .  FIG. 1  therefore shows the positioning system in normal alignment.  
      In  FIG. 4 , the steering system of the vehicle  10  is in a partially locked state (i.e. not at full turn) such that the tractor  12  is angularly disposed with respect to the trailer  14  (i.e. the respective longitudinal axis of the tractor  12  and trailer  14  are not coincident). The angular displacement of the tractor  12  with respect to the trailer  14  is translated into a corresponding angular displacement of the crank  32  with respect to its  FIG. 1  “straight ahead” position. As the crank  32  rotates (anti-clockwise as viewed in  FIG. 4 ) it causes the piston  46  of master ram  26  to extend and the piston  48  of master ram  28  to retract. This causes corresponding retraction of the slave piston  42  and extension of the slave piston  44 . As a result, the rear axle  18  is pivoted anti-clockwise (as viewed in  FIG. 4 ). Thus, the wheels  20 , which are now angularly disposed with respect to the chassis  16 , are in a position to facilitate the vehicle  10  making a clockwise turn (as viewed in  FIG. 4 ).  FIG. 4  also shows the positioning system in normal alignment.  
       FIG. 5  shows the vehicle  10  with its steering system in a fully locked state and still in normal alignment. The situation is similar to that of  FIG. 4  except the crank  32  is now in its maximum angular disposition with respect to the chassis  16  (for a clockwise turn in normal use). Consequently, the master pistons  46 ,  48  are in extended and retracted states respectively corresponding to this fully locked state, and the slave pistons  42 ,  44  are in their corresponding retracted and extended states respectively (these piston states may be regarded as the fully extended or fully retracted states of the pistons since they define the limit of the pistons travel during normal use, although these states may not correspond to the physical limits of extension/retraction of which the respective pistons are capable). The travel stop  38  is arranged, or positioned, so that in this fully locked state, the rear axle  18  just comes into contact with, or falls just short of, the travel stop  38 . It may be said that the rear axle  18  substantially abuts with the travel stop  38  such that travel of the slave piston  44  is substantially unrestricted by the travel stop  38 . It will be appreciated that the travel stop  38  may alternatively be arranged to lie in the path of the piston  44  itself such that the piston substantially abuts with the travel stop  38  in the fully locked state.  
      The other travel stop  36  is similarly arranged with respect to the other slave piston  42 . Thus, when the steering system of the vehicle  10  is in the opposite fully locked state (for an anti-clockwise turn as viewed in  FIG. 5 —not illustrated), the rear axle  18  (or alternatively the slave piston  42 ) substantially abuts with the travel stop  36  such that travel of the slave piston  42  is substantially unrestricted by the travel stop  36 .  
      The forgoing descriptions referring to  FIGS. 1, 4  and  5  relate to normal operation of the steering system, and therefore of the positioning system  40 , in normal alignment. In such conditions, the valve circuit  50  is inactive. Hydraulic liquid is prevented from entering the circuit  50  via ports D, E, F and G because the hydraulic pressure is not great enough to allow the liquid to pass through the pressure relief valves  68 .  
       FIG. 6  shows the vehicle  10  in the straight ahead state but with the positioning system  40  misaligned. The relative extension/retraction of the slave pistons  42 ,  44  do not correspond correctly with the respective master piston  46 ,  48  extension/retraction in this state. As a result, the wheels  20  are angularly disposed with respect to the chassis  16  when they should be substantially parallel with it. Clearly, the orientation of the wheels  20  is unsuitable for straight ahead travel of the vehicle  10  and requires re-alignment. Re-alignment is readily achieved using the positioning system  40 , and in particular the valve block  29 , as is now described.  
      Starting from the position illustrated in  FIG. 6 , as the vehicle  10  makes a clockwise turn, the slave piston  44  extends and the other slave piston  42  retracts. However, because the slave piston  44  was already further extended than it should have been before the vehicle  10  started to turn (i.e. out of alignment), the rear axle  18  abuts with the travel stop  38  before the steering system of the vehicle  10  has reached the fully locked state (illustrated in  FIG. 7 ). The travel stop  38  thus prevents the slave piston  44  from reaching the extended state that corresponds to the fully locked state shown in  FIG. 5 . However, as the steering system continues towards the fully locked state, the master piston  48  continues to retract as normal. Because the slave piston  44  is prevented by the travel stop  38  from extending further, this results in a build up of hydraulic pressure since master piston  48  is forcing hydraulic liquid out of port B of master ram  28  but port B of slave ram  24  is unable to receive this hydraulic liquid. The build up of hydraulic pressure causes hydraulic liquid to enter the valve block  29  via port G and flow through the associated pressure relief valve  68 . The pressure relief valve  68  is suitably adjusted to allow hydraulic liquid to pass through it at the pressure levels arising in the situation described above. The hydraulic liquid entering circuit  50  via port G is collected in the accumulator.  
      When the steering system, and therefore the positioning system  40 , has reached the fully locked state, the rear axle  18  still abuts against the travel stop  38  but the excess hydraulic pressure has now been dissipated through the pressure relief valve  68  associated with port G. Thus, when the steering system is taken out of the fully locked state (in this case by an anti-clockwise turn of the vehicle  10 ) the slave piston  44  now retracts correctly as the master piston  48  extends. Moreover, the slave piston  44  will extend correctly the next time the master piston  48  retracts. Hence, the positioning system  40  has been re-aligned to normal alignment.  
      Similarly, by putting the steering system, and therefore the positioning system  40 , into the opposite fully locked position (not illustrated) a misalignment caused by an over-extension of the other slave piston  42  can be corrected. In this case, the slave piston  42  abuts against the other travel stop  36  and the excess hydraulic pressure is vented through port F and the associated pressure relief valve  68 .  
      Thus, it will be appreciated that the positioning system  40  allows a misalignment of a steering system of the type described above to be corrected simply by putting the steering system, and therefore the positioning system  40 , into one fully locked state and, if necessary, then the opposite fully locked state.  
      Ports D and E operate in complement to ports G and F respectively in normal manner when the rams are double acting rams. For example, in the  FIG. 2  arrangement, an inflow of oil at port G may be balanced in normal manner by an outflow of oil at port E. In alternative embodiments (not illustrated), the ports D, E, F, G and associated pressure relief valves  68  may operate in alternative configurations. For example, with reference to  FIG. 2 , if one or more travel stop was arranged to prevent, say, piston  44  of slave ram  24  from retracting beyond a given state, then the build up of excess hydraulic pressure when the positioning system was misaligned would be vented mainly by hydraulic liquid passing through the pressure relief valve  68  associated with port E.  
      It will be understood that the slave rams  22 ,  24  need not necessary be arranged to actuate a rear axle—they could be used to operate any steerable axle. Further, in cases where the wheels themselves are steerable, the slave rams may be arranged to actuate the steerable wheels (rather than the axle). Moreover, there need not necessarily be two pairs of master/slave rams—the invention may be applied in positioning systems comprising one or more pairs of master/slave rams. The rams need not necessarily be hydraulic but may alternatively be operatable by any other suitable operating fluid.  
      The positioning system of the invention is not limited to use in a steering system of an articulated, or any other, vehicle. The invention may be employed in any positioning system comprising at least one pair of master/slave rams in order to correct the relative alignment of the master and slave rams. Depending on the requirements of the application, the, or each, travel stop may be arranged to prevent the, or each, respective slave piston from extending and/or retracting more than is necessary when the system is normally aligned.  
      The invention is not limited to the embodiments described herein which may be modified or varied without departing from the scope of the invention.