Abstract:
A differential locking axle control system that can cause the axle to automatically lock and unlock at any vehicle speed, up to a predetermined maximum speed, or any wheel spin rate up to a predetermined maximum, when a vehicle is being steered either in a straight line or around a curve while taking traction and global positioning factors into account.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to road and off-road vehicles, particularly to an automatic locking/unlocking differential drive axle of such vehicles. 
       BACKGROUND 
       [0002]    A drive axle of a road or off-road vehicle typically has a differential gear mechanism which enables its right and left drive wheels to rotate at different speeds during certain conditions including steering of the vehicle along a turning radius and occurrence of a difference between traction of one drive wheel to an underlying surface and traction of the other drive wheel to an underlying surface. When a drive axle has a locking differential, it includes a locking/unlocking mechanism. When the locking/unlocking mechanism is unlocked, the differential gear mechanism operates as described. When the locking/unlocking mechanism is unlocked, the right and left axle shafts are locked together to cause the right and left drive wheels to rotate in unison and thereby disable their rotation at different speeds. 
         [0003]    Current locking differential drive axles may be manually activated by a driver of a vehicle, and then either manually or automatically deactivated. When a driver wants to lock the differential, certain contemporaneous events such as wheel spin and steering angle of the vehicle render locking undesirable or even impossible. 
         [0004]    For example, a driver may have to bring a vehicle to a near or complete stop in order to lock the differential, and such stopping may be hazardous in certain circumstances, such as when a vehicle is travelling on a high speed roadway or is operating off-road. 
         [0005]    If the differential is already locked, and then prematurely unlocked, it can be re-locked only in the above manner. 
         [0006]    Consequently, in intermittent low traction conditions, a driver must decide either to tolerate the stop/slow down engage/disengage cycle, to leave the differential unlocked and risk a low traction event, such as getting stuck or being forced to drive more slowly in order to maintain vehicle control, or to leave the differential locked for extended periods of time and risk increased tire wear and reduced driveshaft life due to equivalent left and right wheel speeds, during turning events, causing intermittent wheel slip and grip. 
         [0007]    Two types of differential locking/unlocking systems are sometimes referred to as “mechanical coupler” and “clutch pack” (“limited-slip”). The mechanical coupler cannot be used in a wheel spin event, unless the vehicle is stopped or nearly stopped, and there has not yet been a remedy to prevent that. The clutch pack engages/disengages automatically at almost any speed, but if engaged while the vehicle is in a turn, it has a tendency to cause understeering on low friction road surfaces. 
       SUMMARY OF THE DISCLOSURE 
       [0008]    The differential locking axle of the present disclosure can automatically lock and unlock at any vehicle speed, up to a predetermined maximum speed, or any wheel spin rate up to a predetermined maximum, when a vehicle is being steered either in a straight line or around a curve. That capability enhances vehicle handling by increasing longitudinal and lateral traction. 
         [0009]    That capability is incorporated in an operational strategy of an electronic controller which is associated with an existing drive axle without, or with only little, mechanical modification, provided that the drive axle&#39;s wheels are part of an ABS traction control system. Consequently, there is little or no axle weight increase, avoiding weight penalty likely imposed by “limited slip” or “no-spin” type locking differential drive axles, which may also be more costly and prone to more frequent maintenance. 
         [0010]    The controller allows a driver of a vehicle to use differential locking in the current manner of manual “on” and manual “off” or automatic “off”, and/or or to select automatic locking/unlocking by the controller as a function of vehicle travelling speed and wheel slip. 
         [0011]    One general aspect of this disclosure relates to a road or off-road vehicle comprising a chassis frame, a prime mover supported on the chassis frame, front steering wheels suspended from the chassis frame for steering the vehicle, and a rear drive axle suspended from the chassis frame for propelling the vehicle. 
         [0012]    The rear drive axle comprises a right drive wheel, a left drive wheel, and an input shaft operated by the prime mover to rotate the right drive wheel through a right axle shaft and the left drive wheel through a left axle shaft, a gear mechanism for enabling one drive wheel to rotate at a speed different from that of the other drive wheel during certain conditions which include steering of the vehicle along a turning radius and occurrence of a difference between traction of one drive wheel to an underlying surface and traction of the other drive wheel to an underlying surface, and a locking/unlocking mechanism for selectively locking the right and left axle shafts together to rotate in unison and thereby disable their rotation at different speeds. 
         [0013]    An ABS system can selectively brake each drive wheel independently of the other, and a controller can set any of multiple operating modes for controlling operation of the locking/unlocking mechanism as a function of at least travelling speed of the vehicle and a difference between speed of one drive wheel and speed of the other drive wheel occurring because of a difference in traction of each drive wheel to the respective underlying surface. 
         [0014]    One operating mode is an automatic operating mode for causing the locking/unlocking mechanism to lock the right and left axle shafts together for rotation in unison when both travelling speed of the vehicle becomes greater than a selected speed and a difference between speed of one drive wheel and speed of the other drive wheel becomes greater than a selected limit. 
         [0015]    The foregoing summary, accompanied by further detail of the disclosure, will be presented in the Detailed Description below with reference to the following drawings that are part of this disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a left side elevation view of a truck vehicle. 
           [0017]      FIG. 2  is a top plan view of a chassis of the truck vehicle. 
           [0018]      FIG. 3  is fragmentary view of a portion of a differential drive axle of the truck vehicle showing unlocked (disengaged) condition. 
           [0019]      FIG. 4  is a view like  FIG. 3  showing locked (engaged) condition. 
           [0020]      FIG. 5  is diagram showing a first portion of an operational strategy embodied in a controller associated with the differential drive axle. 
           [0021]      FIG. 6  is diagram showing a second portion of an automatic operational differential lock/unlock strategy embodied in the controller. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]      FIGS. 1 and 2  show an example of a truck vehicle  10  which comprises a chassis  12  having a chassis frame  14  on which are mounted a prime mover, such as a diesel engine,  16  and a cab  18  having an interior compartment for a driver of the truck vehicle. Right and left front steerable wheels  20 ,  22  respectively are suspended from chassis frame  14  on right and left sides for steering truck vehicle  10 . 
         [0023]    Front and rear tandem axles  24 ,  26  are suspended from chassis frame  14 . Front tandem axle  24  is a locking differential drive axle and rear tandem axle  26  is a tag axle. A tag axle can be either to the rear of a drive axle (as shown) or in front of a drive axle, and the locking differential feature can apply to any number of drive axle and tag axle combinations, such as tandems, tridems, etc. 
         [0024]    Prime mover  16  has an output shaft which is coupled through a drivetrain  28  to an input shaft  30  of drive axle  24 . Drive axle  24  has a casing  32  which contains a differential gear mechanism and right and left axle tubes  34 ,  36  which extend laterally from casing  32 . Right axle tube  34  houses a right axle shaft  35  which is coupled to a right drive wheel  38  while being supported for rotation in the axle tube. Left axle tube  36  houses a left axle shaft  37  which is coupled to a left drive wheel  40  while being supported for rotation in the axle tube. Right axle shaft  35  and left axle shaft  37  share a common axis of rotation  39 , and each axle shaft is coupled to the differential gear mechanism within casing  32 . 
         [0025]    When prime mover  16  operates, torque is transmitted via drivetrain  28  to input shaft  30  to operate the differential gear mechanism and rotate right drive wheel  38  through right axle shaft  35  and left drive wheel  40  through left axle shaft  37  to enable drive wheels  38 ,  40  to rotate at different speeds during certain conditions including steering of truck vehicle  10  along a turning radius and occurrence of a difference between traction of one drive wheel to an underlying surface and traction of the other drive wheel to an underlying surface. 
         [0026]    Tag axle  26  is not a drive axle. It has right and left wheels  42 ,  44  which can rotate independently of each other. 
         [0027]    The differential gear mechanism within casing  32  is essentially conventional. It may comprise, by way of example as in  FIGS. 3 and 4 , a ring gear  49  which is supported for rotation in casing  32  about the common axis  39  of right and left axle shafts  35 ,  37 . It may further comprise a left side axle shaft gear  41  which rotates with left axle shaft  37 , a right side axle shaft gear  43  which rotates with right axle shaft  35 , and differential spider gears  54  supported for independent rotation on opposite sides of a carrier  45  to which ring gear  49  is affixed. The two axle shaft side gears  41 ,  43  can rotate via their own respective bearings on carrier  45  about the common axis  39  of right and left axle shafts  35 ,  37 . Rotation of input shaft  30  rotates ring gear  49  including carrier  45 . Rotation of ring gear  49  is imparted to the respective side axle shaft gears  41 ,  43  because spider gears  54  are in mesh with both side axle shaft gears  41 ,  43 . Right and left axle shafts  35 ,  37  will rotate at equal speeds when truck vehicle  10  is being steered in a straight line, but the differential gear mechanism will allow one axle shaft to rotate at a slower speed than the other when the vehicle is being steered to turn in the direction of the slower rotating axle shaft. 
         [0028]      FIGS. 3 and 4  show a locking/unlocking mechanism  46  for selectively locking right and left axle shafts  35 ,  37  together to rotate in unison and thereby disable their rotation at different speeds. In the locking/unlocking mechanism&#39;s unlocked condition shown in  FIG. 3 , each axle shaft  35 ,  37  can rotate independently of the other. In the locked condition of  FIG. 4 , the two axle shafts  35 ,  37  are locked together to rotate in unison. 
         [0029]    A toothed right dog clutch gear  48  is rigidly connected to carrier  45  and ring gear  49 . A toothed left dog clutch gear  52  rotates in unison with left axle shaft  37  via a spline connection  50  which also enables left dog clutch gear  52  to slide along left axle shaft  37 . In the locked condition of  FIG. 4 , toothed left dog clutch gear  52  has been slid along left axle shaft  37  from the unlocked condition of  FIG. 3  to mesh with external teeth on right dog clutch gear  48 , causing left axle shaft  37 , ring gear  49 , and differential carrier  45  to rotate in unison. This effectively locks spider gears  54  against turning and therefore locks right and left axle shafts  35 ,  37  together so that they rotate in unison. 
         [0030]    Locking/unlocking mechanism  46  comprises an actuator  56  for operating a shift fork  58  to shift left dog clutch gear  52  lengthwise along left axle shaft  37  into and out of engagement with right dog clutch gear  48 . 
         [0031]    Actuator  56  can be any of various types, such as one which is actuated by pneumatic pressure to move shift fork  58  from the  FIG. 3  unlocked position to the  FIG. 4  locked position against the force of a compression spring  60 . When pneumatic pressure to actuator  56  is removed, spring  60  forces shift fork  58  back to the  FIG. 3  position. 
         [0032]    All wheels of truck vehicle  10  comprise pneumatic tires through each of which the respective wheel has contact with an underlying surface. Truck vehicle  10  also comprises an ABS traction control system (anti-lock brake system)  84  ( FIG. 6 ) which includes an associated controller for controlling wheel braking on an individual wheel basis. In truck vehicle  10 , the ABS system includes at least right and left drive wheels  38 ,  40 . 
         [0033]      FIGS. 5 and 6  show an operational strategy embodied in a controller associated with locking/unlocking differential drive axle  24 . The controller may be a powertrain or engine controller which controls various aspects of the prime mover and drivetrain.  FIG. 5  shows a first portion  70  of the strategy, and  FIG. 6  a second portion  72 . The strategy can set any of multiple operating modes for controlling operation of locking/unlocking mechanism  46  as a function of at least travelling speed of truck vehicle  10  and a difference between speeds of right and left drive wheels  38  and  40 , which occurs because of a difference in traction of the tire of each drive wheel to the respective underlying surface with which it is in surface-to-surface contact. 
         [0034]    First portion  70  includes a selector switch  74  which is selectively operable by a driver of truck vehicle  10  either to select a first operating mode for the controller to the exclusion of a second operating mode for the controller or to select the second operating mode to the exclusion of the first operating mode. 
         [0035]    The first operating mode, represented by the function “switch off”  76 , is effective to prevent locking/unlocking mechanism  46  from locking right and left axle shafts  35 ,  37  together, and hence drive wheels  38  and  40  also, regardless of truck vehicle speed. 
         [0036]    The second operating mode, represented by the function “switch on”  78 , is effective to enable locking/unlocking mechanism  46  to lock right and left axle shafts  35 ,  37  together, and hence drive wheels  38  and  40  also, for rotation in unison when travelling speed of truck vehicle  10  is less than a first speed (for example 20 MPH as shown), and when travelling speed of the truck vehicle increases to a second speed greater than the first speed (for example 25 MPH as shown), to unlock right and left axle shafts  35 ,  37  from each other and enable drive wheels  38 ,  40  to rotate at different speeds. Actual engagement may be conditioned on factors  80  like wheel spin and road surface friction meeting certain driver determined conditions. When axle shafts  35 ,  37  are locked and vehicle speed is less than the second speed, they may be unlocked simply by turning selector switch  74  off. 
         [0037]    Second portion  72  comprises an automatic mode enable switch  82  which is operable to enable a third operating mode for the controller. The third operating mode causes locking/unlocking mechanism  46  to lock right and left axle shafts  35 ,  37  together for rotation in unison, and hence drive wheels  38  and  40  also, when both travelling speed of truck vehicle  10  becomes greater than a selected speed and a difference between speeds of drive wheels  38 ,  40  becomes greater than a selected limit. 
         [0038]    When the third operating mode is enabled and the conditions just mentioned are satisfied, the controller causes ABS traction control system  84  to decelerate at least one of the drive wheels  38 ,  40  to bring the speeds of both drive wheels to speeds within a selected range of each other before the controller causes locking/unlocking mechanism  46  to lock right and left axle shafts  35 ,  37  together for rotation in unison. 
         [0039]    The controller need not necessarily cause ABS traction control system  84  to decelerate both drive wheels  38 ,  40 . For example, only the one drive wheel whose speed is greater than that of the other drive wheel may be decelerated to bring the speeds of both drive wheels to speeds within the selected range before the controller causes locking/unlocking mechanism  46  to lock right and left axle shafts  35 ,  37  together. 
         [0040]    The controller may also condition locking and unlocking of locking/unlocking mechanism  46  on satisfaction of at least one condition.  FIG. 6  shows that the controller: conditions unlocking of locking/unlocking mechanism  46  on elapse of a selected interval of time during which drive wheels  38 ,  40  have been locked together for rotation in unison; conditions locking and unlocking of locking/unlocking mechanism  46  on the geographic location of the vehicle; conditions locking and unlocking of locking/unlocking mechanism  46  on a characteristic of the surface underlying at least one of drive wheels  38 ,  40 ; causes output torque of prime mover  16  to decrease before causing locking or unlocking of locking/unlocking mechanism  46 ; and causes output torque of prime mover  16  to increase after causing locking or unlocking of locking/unlocking mechanism  46 .