Abstract:
A method and system for bypassing a control valve that would otherwise disengage differential and/or inter-axle locking means. The bypass is achieved by a valve that can be actuated by a vehicle condition change.

Description:
FIELD 
     The present disclosure relates to differential locking systems on vehicles, and specifically to such systems where incorporating an automatic lock disengagement mechanism. 
     BACKGROUND 
     It is well known in the art of motor vehicle design to provide a differential to enable wheels at opposed ends of an axle to rotate at different speeds, for example to avoid undue tire wear. It is also well known to provide certain vehicles with a locking differential which selectively forces the wheels to rotate at the same speed no matter what the difference in traction, thereby providing a tractive advantage in some circumstances. 
     Also, inter-axle differentials have been developed for use on vehicles with multiple axles, whereby the differential can be locked and power is transmitted equally to all axles. The locking differential system locks the wheels on an axle, while the inter-axle system locks the multiple axles together thereby forcing the drivetrain to transmit power to all axles equally for maximum traction. In the case of certain vehicles designed for pulling heavy loads, such as a road/rail power unit when in rail operation mode, it is important that the differential and inter-axle locks remain engaged during operation. 
     Despite the advantages of the selective locking system, it has been determined that in certain circumstances it may be desirable to disengage the locks and later re-engage them, and such disengagement means have become a factory standard addition. For example, in some trucks the inter-axle lock may be designed to automatically disengage in response to a condition such as a low-traction event in which the anti-lock braking system (ABS) initiates, which allows for more effective braking. The lock then re-engages automatically after cessation of the low-traction event. 
     However, the automatic nature of the lock disengagement is problematic in other contexts. The locks are designed primarily to maximize traction, and an operator hauling a heavy load may therefore wish to have the locks engaged at all times during hauling even when faced with intermittent low-traction events. In the case of snow plows ascending an icy slope, disengagement of the inter-axle lock can reduce adhesion and terminate the ascent, and similar situations have been noted with logging trucks pulling heavy loads on washboard road surfaces. As a further example, it is critical in a road/rail vehicle in rail transport operation mode that traction not be lost when pulling railcars, but it is common to experience traction loss or slippage on a rail that could result in ABS initiation and lock disengagement, and subsequent automatic re-engagement under load can damage the differentials and axles. 
     What is needed, therefore, is a system and method for selectively bypassing the factory differential lock disengagement means. 
     BRIEF SUMMARY 
     The present disclosure therefore seeks to provide a method and system for selectively bypassing the differential lock disengagement means. 
     According to a first aspect, then, there is provided a method for selectively bypassing a disengagement system for differential locking means in a vehicle having a selectively lockable differential, the differential locking means disengageable by means of a control valve in communication with the differential locking means, the method comprising the steps of: 
     a. providing a bypass valve moveable between first and second positions; 
     b. positioning the bypass valve between the control valve and the differential locking means; 
     c. setting the bypass valve in the first position, thereby allowing unimpeded communication between the control valve and the differential locking means; and 
     d. selectively actuating the bypass valve to move the bypass valve to the second position, thereby blocking communication between the control valve and the differential locking means and preventing disengagement of the differential locking means. 
     In exemplary embodiments of the first aspect, the disengagement system disengages the differential locking means in response to a low-traction event. The differential locking means are preferably fluid-powered and the control valve is a solenoid valve capable of controlling fluid feed to the differential locking means. The vehicle is most preferably provided with a pneumatic system capable of use with the differential locking means. The communication between the control valve and the differential locking means is preferably fluid communication, with the bypass valve a pneumatic valve configured to control passage therethrough of a working gas. The step of setting the bypass valve in the first position is preferably achieved by biasing the bypass valve in the first position. The step of selectively actuating the bypass valve to move the bypass valve to the second position is preferably achieved by introduction of working gas pressure to an actuator of the bypass valve, which introduction of working gas pressure preferably occurs in response to a vehicle condition change; where the vehicle is a road/rail vehicle, the vehicle condition change is preferably inflation of air bags during conversion to a rail mode of vehicle operation. 
     According to a second aspect, there is provided a bypass system for use with a disengagement system for differential locking means in a vehicle having a selectively lockable differential, the differential locking means powered by a power fluid selectively allowed by disengagement control means, the bypass system comprising: 
     valve means for receiving a power fluid alternatively from the disengagement control means in a first position and a power fluid source in a second position; 
     biasing means for biasing the valve means in the first position; 
     actuation means for switching the valve means from the first position to the second position; and 
     power fluid transfer means for supplying the disengagement control means and the valve means; 
     such that in the first position, the valve means allows unimpeded power fluid flow between the disengagement control means and the differential locking means; and 
     in the second position, the valve means blocks power fluid flow between the disengagement control means and the differential locking means and thereby prevents disengagement of the differential locking means by the disengagement control means while allowing power fluid flow directly from the power fluid source to the differential locking means. 
     In exemplary embodiments of the second aspect, the disengagement system disengages the differential locking means in response to a low-traction event, the disengagement control means comprise a solenoid valve capable of controlling power fluid feed to the differential locking means, and the power fluid is a pressurized gas. The actuation means preferably move the valve means to the second position by introduction of power fluid to an actuator of the valve means, which introduction of power fluid occurs in response to a vehicle condition change; where the vehicle is a road/rail vehicle the vehicle condition change is inflation of air bags during conversion to a rail mode of vehicle operation. 
     A detailed description of an exemplary embodiment of the present invention is given in the following. It is to be understood, however, that the invention is not to be construed as being limited to this embodiment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the accompanying drawings, which illustrate an exemplary embodiment: 
         FIG. 1  is a simplified schematic view of a bypass system in the un-actuated position; and 
         FIG. 2  is a simplified schematic view of the bypass system of  FIG. 1  in the actuated position. 
     
    
    
     An exemplary embodiment of the method and system of the present disclosure will now be described with reference to the accompanying drawings. 
     DETAILED DESCRIPTION 
     A pneumatic control system is described in the following, but it will be clear to those skilled in the art that the bypass method and system of the present disclosure could be applied with any other suitable system including a hydraulic control system. Only those parts or components of the vehicle systems that are necessary for an understanding of the present disclosure will be described herein, as those skilled in the art will fully understand the broader mechanical and operational context of the bypass system of the present disclosure and its application in particular situations. 
     In prior art systems, a solenoid valve is inserted in the air feed line between the air source and the differential lock and inter-axle lock (the differential lock and inter-axle lock collectively referred to herein as the “differential lock” or “lock”). The solenoid therefore acts as a gate to alternatively allow or restrict air flow to the lock depending on the solenoid design. As explained above, such solenoids are designed to respond to ABS initiation (low-traction events) to block air flow to the locks, thereby disengaging the locks, and then subsequently allow air flow back to the locks once the trigger event has ceased. 
     Turning now to  FIGS. 1 and 2 , a bypass system  10  is illustrated. The method and system will be described with reference to these Figures. 
     In the bypass system  10 , the factory standard control valve  12  (a solenoid) is in place between the air source  16  and the output line  44  to the differential locks. The control valve  12  is fed by a feed line  18  from the air source  16  and comprises an inlet  20  and an outlet  22 , the outlet  22  feeding air to an output line  28 . This portion of the illustrated embodiment is similar to the prior art design, and the control valve  12  is wired to receive signals from the ABS in a conventional manner that will not be described further herein. The bypass system  10  also comprises a bypass valve  14 , which in the illustrated embodiment is an air piloted three-port air valve. The bypass valve  14  is operated remotely by pneumatic signals provided by pressurized gas, as will be explained below. 
     The bypass valve  14  comprises upper and lower blocks  24 ,  26 . The upper block  24  comprises a closed port  34  and an open port  36 , while the lower block  26  comprises a closed port  30  and an open port  32 . The bypass valve  14  further comprises an actuator  38  that is controlled by means of an air bag air pressure source  40  and pressurized air supply line  42 , the actuator  38  of conventional design. 
     The bypass valve  14  is biased by means of a spring  46  into a first position, which is illustrated in  FIG. 1 , and there is no counteracting pressure  40  through supply line  42  to signal the actuator  38  to switch the bypass valve  14  to the second position. In this first position, pressurized air is supplied to the differential locks through the control valve  12 . Pressurized air is provided by the air source  16  and is forced through the feed line  18 . As the lower port  30  of the lower block  26  is closed, the pressurized air must flow to the control valve  12  through the inlet  20 . At this point, the control valve  12  will either allow the pressurized air to flow through the outlet  22 , output line  28  and open port  32  of the lower block  26  to the output line  44  to the locks (in which case the differential is locked) or will block the flow of pressurized air to the locks (in which case the differential is unlocked). In the exemplary embodiment, the control valve  12  is configured to receive a signal from the vehicle ABS, such that the control valve  12  blocks air flow in response to initiation of an ABS event and subsequently allows air flow in response to a signal indicating cessation of the ABS event. In the first position, then, the control valve  12  determines on an automatic basis whether the locks will be engaged or disengaged. 
     In a road/rail vehicle, this first position would normally be preferred when the vehicle is in the road transport mode of operation. In the rail transport mode of operation, however, this would be problematic, as described above. The bypass valve  14  is accordingly capable of shifting to a second position as described below. 
     The bypass valve  14  can be shifted into the second position, as illustrated in  FIG. 2 . When it is desired to operate a road/rail vehicle in a rail transport mode of operation, for example, air bags are inflated to lower the rail gear relative to the vehicle body and push the vehicle body upwardly, such that the rubber tires are elevated and the rail wheels can engage the rails. Inflating the air bags  40  sends pressurized air through the supply line  42  to the actuator  38  of the bypass valve  14 , thereby countering the force of the spring  46  and switching the bypass valve  14  to the second position. For other vehicles, other means of signalling the actuator  38  would be appropriate and within the knowledge of those skilled in the art. 
     In the second position, the upper block  24  is now engaged. Pressurized air is provided by the air source  16  and is forced through the feed line  18 , but the lower port  36  is open and pressurized air can therefore flow directly through the bypass valve  14  to the output line  44  for the differential locks. The upper port  34  is closed, with the result that pressurized air fed through the feed line  18  and inlet  20  to the control valve  12  can pass through the outlet  22  into the output line  28  but is blocked from passing through the bypass valve  14 . Therefore, in the second position, the effect of the control valve  12  is negated such that it does not impact pressurized air supply to the locks, while a direct open supply of pressurized air to the locks is supplied through the open port  36 . Pressurized air is accordingly constantly supplied to the locks during this bypass phase, such that the locks remain engaged even in the event of a low-traction event triggering the control valve  12  flow restriction. 
     When the bypass valve  14  is switched back to the first position, the control valve  12  once again can automatically allow or restrict pressurized air supply to the locks, as shown in  FIG. 1 . In the case of a road/rail vehicle, for example, this would occur when the vehicle was converted from a rail transport mode of operation to a road transport mode of operation by deflation of the air bags and release of the pressure on the actuator  38 . 
     The foregoing is considered as illustrative only of the principles of the invention. The scope of the claims should not be limited by the exemplary embodiment set forth in the foregoing, but should be given the broadest interpretation consistent with the specification as a whole.