Patent Publication Number: US-2003224896-A1

Title: Hydraulic differential lock

Description:
RELATED APPLICATION  
     [0001] The application claims priority to U.S. Provisional Application No. 60/380,799, which was filed on May 15, 2002. 
    
    
     
       TECHNICAL FIELD  
       [0002] This invention relates to a hydraulic differential lock for a drive axle. Specifically, a locking mechanism is externally mounted to eliminate leakage.  
       BACKGROUND OF THE INVENTION  
       [0003] Vehicle drive axles typically include a pair of axle shafts for driving vehicle wheels. The drive axle uses a differential to control input speed and torque to the axle shafts. Under ideal conditions, when the vehicle is driven along a straight path, the wheels will be turning at approximately the same speed and the torque will be equally split between both wheels. When the vehicle negotiates a turn, the outer wheel must travel over a greater distance than the inner wheel. The differential allows the inner wheel to turn at a slower speed than the outer wheel as the vehicle turns.  
       [0004] Power is transmitted from a vehicle drive shaft to a drive pinion that is in constant mesh with a ring gear. The ring gear is bolted to a differential case that turns with the ring gear. A differential spider having four (4) support shafts orientated in the shape of a cross, has four (4) differential pinions. One differential pinion is supported for rotation on each support shaft. Power is transferred from the differential case to side gears that are splined to the axle shafts. The side gears are in constant mesh with the differential pinions. The outer ends of the axle shafts are bolted to the wheel hubs to which the wheels are mounted.  
       [0005] When the vehicle is driven in a straight path the ring gear, differential case, spider, differential pinions, and side gears all rotate as one unit to transfer power to the axle shafts. There is no relative movement between the differential pinions and the side gears. When the vehicle executes a turning maneuver, the differential pinion gears rotate on their respective shafts to speed up the rotation of one axle shaft while slowing the rotation of the other axle shaft.  
       [0006] Often the differential includes a locking or biasing mechanism. When there are poor traction conditions, e.g., slippery or rough surfaced roads, the biasing mechanism allows maximum wheel traction for improved control. If the differential does not have a biasing mechanism and one tire is on ice, the available traction torque on the opposite wheel is same as on the wheel on ice. Thus, the tire just spins on the ice and the vehicle is prohibited from traveling forward. A biasing mechanism allows the axle shafts to rotate at the same speed while transferring most of the available torque to the tire not on the ice. If the tractive effort at this tire is sufficient, the vehicle can be moved off of the ice. When the mechanism is activated, power is transmitted through the differential gearing, and biasing mechanism rather than through the differential gearing only.  
       [0007] One type of differential locking or biasing mechanism is comprised of a wet disc clutch that locks the differential case to the axle shafts, until a predetermined torque level is exceeded. The wet disc clutch includes a plurality of stationary discs interspersed with rotating discs in a fluid chamber. A piston applies a force to the wet disc clutch to compress the rotating and stationary discs together to apply torque between the differential case to be locked to the axle shafts. The terms stationary and rotating applied to the disc are relative to the differential case.  
       [0008] One disadvantage with a typical wet disc clutch system is fluid leakage. The leakage problem results from the pressurized fluid transfer from stationary members to rotating members to actuate the piston. Complicated rotating seal units, sometimes comprising leak-by recapture circuits, must be incorporated into the differential, which take up valuable packaging space and are expensive. The recapture system recovers the leaked fluid and returns it to a pump that is used for applying pressure to actuate the wet disc clutch. Another disadvantage is that the clutch torque capacity is limited by the discs and actuation mechanism that can be physically fit within the differential case.  
       [0009] Thus, it is desirable to have a simplified actuating mechanism for a differential lock that can deliver pressure from a stationary source to a rotating source while eliminating leakage, in addition to overcoming other deficiencies in the prior art as outlined above.  
       SUMMARY OF THE INVENTION  
       [0010] A differential locking assembly for a vehicle drive axle is installed within an axle housing and external to a differential case. The locking assembly is resistive to both axial and torsional forces generated by a lock activation. A connector is used to connect an external fluid supply to the locking mechanism. The connector is fixed to the axle housing.  
       [0011] In one disclosed embodiment, the differential locking assembly includes a differential case and a differential gearing assembly mounted within the differential case and operably coupled to a pair of axle shafts for driving vehicle wheels. A clutch housing is mounted externally relative to the differential case. An actuator is mounted substantially within the clutch housing for selectively locking the clutch housing and differential case for rotation with one of the axle shafts under predetermined locking conditions. The clutch housing is designed to accommodate thrust and torsion forces resulting from activation of the locking mechanism.  
       [0012] Preferably, the locking mechanism includes a first plurality of discs mounted to the clutch housing and a second plurality of discs, interspersed with the first plurality of discs, mounted for rotation with the axle shaft.  
       [0013] In the preferred embodiment, the actuator includes a rotating member or actuation plate mounted with the clutch housing for rotation with the clutch housing. A piston housing is held fixed with the axle housing and includes a body portion extending into the clutch housing and a leg portion extending out radially from the body portion and external to the clutch housing. The connector includes a first portion that is fixed to the axle housing and a second portion that extends into the leg portion of the piston housing. A piston is mounted within a piston chamber formed within the body portion of the piston housing. A fluid path extends from the external fluid supply, through the connector and through the piston housing into the piston chamber. The actuation thrust force is transferred from the stationary piston to the rotating member via a pair of thrust bearings. Preferably, a ring is inserted over the leg portion, rotated approximately 90 degrees, and is attached to one end of the clutch housing. The ring provides a reaction surface for one of the thrust bearings.  
       [0014] The subject invention provides a differential locking mechanism that is easily incorporated into different axle carrier types with minimal component modification. The subject invention offers several advantages over a conventional positive-locking devices involving splines. Once activated, the differential remains locked until the torque exceeds the clutch capacity. This provides a means of automatically protecting the torque-carrying components from overload. Another major advantage is the much better ability to engage on-the-fly. When locking two components at different speeds, spline based devices are subjected to severe wear of the teeth, and can generate significant noise and vibration. Further, the subject invention provides a method and apparatus for transferring an actuation thrust force from a stationary member to a rotating member without the need for rotating seals. These and other features of the present invention can be best understood from the following specifications and drawings, the following of which is a brief description. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:  
     [0016]FIG. 1 is a cross-sectional view of a prior art differential with a locking mechanism;  
     [0017]FIG. 2 is a top cross-sectional view of a differential assembly with a locking mechanism in accordance with the subject invention;  
     [0018]FIG. 3 is a magnified view of section 3 as indicated in FIG. 2; and  
     [0019]FIG. 4 is a schematic view of an axle assembly incorporating the subject invention.  
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT  
     [0020] Referring to the Figures, where like numerals indicate like or corresponding parts throughout the several views, an axle differential with a prior art differential locking mechanism is shown generally at  10  in FIG. 1. The axle differential  10  includes a differential spider  12  having four (4) support shafts (only two (2) are shown) orientated in the shape of a cross, as is known in the art, with each shaft supporting one of four (4) differential pinions  14  (only two (2) are shown). The spider  12  and pinions  14  are mounted within a differential case that has a first half  16  and a second half  18  that are bolted together with a plurality of fasteners  20  (only one is shown). Power is transferred from the differential case to side gears  22  that are splined to axle shafts  24 . The side gears  22  are in constant mesh with the differential pinions  14 . The outer ends of the axle shafts  24  are locked in rotation to the wheel hubs (not shown) to which the wheels are mounted. A known in the art, the spider  12 , pinions  14 , and side gears  22  can make up at least part of a differential gear assembly.  
     [0021] When a vehicle is driven in a straight path the differential case  16 ,  18 , spider  12 , differential pinions  14 , and side gears  22  all rotate as one unit to transfer power to the axle shafts  24 . There is no relative movement between the differential pinions  14  and the side gears  22 . When the vehicle executes a turning maneuver, the differential pinions  14  rotate on their respective shafts to speed up the rotation of one axle shaft  24  while slowing the rotation of the other axle shaft  24 .  
     [0022] The following describes a clutch and actuation mechanism mounted inside the differential case. The differential  10  includes a locking mechanism comprising a wet disc clutch  26  that locks the differential case  16 ,  18  to one of the axle shafts  24 . The wet disc clutch  26  includes a first plurality of discs  28  that are mounted internally to one of the differential case halves  16 ,  18  and, which are interspersed with a second plurality of discs  30  that are mounted internally within the other of the case halves  16 ,  18  for rotation with the axle shafts  24 . The discs  28 ,  30  are mounted within a fluid chamber defined by the case halves  16 ,  18 . A piston  32  applies a force to the wet disc clutch  26  to compress the discs  28 ,  30  together to reduce rotational speed and allow the differential case  16 ,  18  to be locked to the axle shaft  24 .  
     [0023] A hydraulic input  34  supplies fluid to actuate the piston  32  via a fluid path  36 . A sealing assembly  38  is used to provide a sealed environment as fluid flows from the input  34  to the piston  32  via the fluid path  36 . This sealing assembly  38  is subjected to high rotational speeds and high fluid pressures, which leads to challenging seal designs and can result in undesirable leakage.  
     [0024] The subject invention, shown in FIGS. 2 and 3, eliminates fluid leakage by utilizing a differential locking mechanism that is mounted externally from the differential. An axle differential incorporating the subject invention is shown generally at  40  in FIG. 2. The differential  40  operates as described above, however, the differential  40  includes a locking mechanism  42  that has a first portion mounted externally to a differential case half  48  and a second portion that is mounted to an axle housing  44 .  
     [0025] The first portion of the locking mechanism  42  comprises a clutch housing  46  that is externally mounted via teeth, splines, threaded connection, or other similar means to the differential case half  48 . A first plurality of discs  50  are attached to the clutch housing  46  and a second plurality of discs  52  are attached to an axle shaft  54 . The first  50  and second  52  plurality of discs are interspaced with each other, i.e. are mounted in an alternating pattern with each other, as is known in the art. The discs  50 ,  52  are mounted within a chamber defined within the clutch housing  46 . The first discs  50  and clutch housing  46  rotate with the differential case half  48 . The number of discs  50 ,  52  can vary depending upon the application.  
     [0026] A stationary member  56  is installed in one end of the clutch housing  46 . Preferably the stationary member  56  is a piston housing  56 . The piston housing  56  includes a cylindrical body portion  58  and a transversely extending leg  60 . The leg  60  extends in a radial direction away from the axle shaft  54  and toward the axle housing  44 . The leg  60  is preferably orientated at a right angle relative to the body portion  58 .  
     [0027] An adjusting ring  62  is installed within the clutch housing  46  by first being installed in one direction over the leg  60  and then is rotated approximately 90 degrees to be threaded or otherwise attached to the clutch housing  46 . The ring  62  surrounds and is spaced apart from the piston housing  56 .  
     [0028] The second portion of the locking mechanism  42  includes a connector or fitting  64  that is installed through an opening  66  in the axle housing  44  to connect with the piston housing  56 . The fitting  64  defines a fluid input  68  that supplies fluid to an actuating mechanism  70  via a fluid path  72 . The connector  64  includes a first fluid path portion  72   a , that is in fluid communication with a second fluid path portion  72   b  formed within the leg portion  60  of the stationary member  56 , which in turn is in fluid communication with a third fluid path portion  72   c  formed within the body portion  58  of the piston housing  56 .  
     [0029] The actuating mechanism  70  includes an actuation member  74  or actuation plate that is rotatable with the clutch housing  46  and moveable along a linear path in response to movement of a piston  76  to compress the discs  50 ,  52  together to lock the clutch housing  46  (and differential case half  48 ) to the axle shaft  54 . The piston  76  is received within a piston chamber  94  formed within the body portion  58  of the piston housing  56 . The fluid path  72  in the piston housing  56  is in fluid communication with the piston chamber  94 .  
     [0030] A first bearing  78  is installed between the piston  76  and the actuation member  74  and a second bearing  80  is installed between the piston housing  56  and the ring  62 . When a fluid pressure is applied to the piston  76  to move the actuation member  74 , the fitting  64  holds the piston housing  56  stationary by providing resistance to torsional drag from bearings  78  and  80 , as well as secondary axial forces external to the locking mechanism  42 .. Thus, the fitting  64 , by being anchored to the axle housing  44 , supplies fluid to actuate the piston  76  while also providing a stabilizing effect by resisting torsional and axial forces generated during locking.  
     [0031] The fitting  64  includes a conduit or tube  82  that has a first portion  84  fitted through a plug  86  and a second portion  88  that extends into the piston housing  56 . A first seal  90  surrounds the first portion  84  and a second seal  92  surrounds the second portion  88 .  
     [0032] As shown in FIG. 4, the subject axle differential  40  is incorporated into a drive axle assembly  98 . As described above, the axle differential  40  includes a differential gear assembly  100 . An input pinion  102  defines a pinion axis of rotation  104  and is in meshing engagement with a ring gear  106 , which is operably coupled to the differential  40 . The differential gear assembly  100  is operably connected to drive the pair of axle shafts  54  that drive the vehicle wheels  108  about an axle shaft axis of rotation  110  that is typically perpendicular to the pinion axis of rotation  104 .  
     [0033] An external fluid supply  112  is fluidly connected to the connector  64 , which directs fluid to actuate the locking mechanism  42  in the clutch housing  46 . Preferably, mineral oil is used, however, any type of fluid known in the art could be used.  
     [0034] The subject invention provides a hydraulic differential lock that is installed outside of the differential case. The piston  76  squeezes the first and second plurality of discs  50 ,  52 , which applies a torque between the axle shaft  54  and clutch housing  46 , which is coupled to the differential case. The actuation thrust force is transferred from the stationary piston  76  to the rotating discs  50 ,  52  through thrust bearings  78 ,  80 , thus eliminating the need for rotating seals. The clutch housing  46  is capable of taking both thrust and torsion, which saves radial room for the discs  50 ,  52 . Connection to the external hydraulic system is done through the dual-function connector/fitting  64  that transfers fluid and restrains the clutch assembly both axially and rotatively. The subject invention further provides the benefits of being easily adapted to different carriers at low cost, being capable of smoothly engaging on-the-fly, and automatically protecting torque-carrying components from overload.  
     [0035] Although a preferred embodiment of this invention has been disclosed, one of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.