Abstract:
A vehicle drive train includes a first power disconnection device and a first driveline for transferring torque to a first set of wheels. A second driveline for transferring torque to a second set of wheels includes a differential gearset having an output coupled to a second power disconnection device. A hypoid gearset is positioned within the second driveline in a power path between the first and second power disconnection devices. The second power disconnection device includes a clutch having a first set of clutch plates fixed for rotation with the differential gearset output. The clutch further includes a second set of clutch plates fixed for rotation with a shaft adapted to transfer torque to one of the wheels of the second set of wheels. A valve limits a flow of coolant to the clutch when the second power disconnection device operates in a disconnected mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/232,882, filed on Aug. 11, 2009. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to a driveline for a motor vehicle having a system for disconnecting a hypoid ring gear from rotating at driveline speed. In particular, a power transfer device such as a power take-off unit or a transfer case includes a coupling for ceasing the transfer of torque from a power source to the hypoid ring gear of a secondary driveline while another disconnect selectively interrupts the flow of power from a vehicle wheel to the hypoid ring gear on the secondary driveline. 
     BACKGROUND 
     Typical power take-off units transfer power from a transaxle in receipt of torque from a vehicle power source. The power take-off unit transfers power to a propeller shaft through a gear arrangement that typically includes a hypoid cross-axis gearset. Other gear arrangements such as parallel axis gears may be provided within the power take-off unit to provide additional torque reduction. 
     Power take-off units have traditionally been connected to the transaxle output differential. Accordingly, at least some of the components of the power take-off unit rotate at the transaxle differential output speed. Power losses occur through the hypoid gear churning through a lubricating fluid. Efficiency losses due to bearing preload and gear mesh conditions are also incurred while the components of the power take-off unit are rotated. 
     Similar energy losses occur when other driveline components are rotated. For example, many rear driven axles include hypoid gearsets having a ring gear at least partially immersed in a lubricating fluid. In at least some full-time all-wheel drive configurations, the rear drive axle hypoid gearset continuously rotates during all modes of operation and transmits a certain level of torque. In other applications, the rear axle hypoid gearset still rotates but without the transmission of torque whenever the vehicle is moving. In other configurations, a transfer case selectively transfers power to a front drive axle equipped with a front drive axle hypoid gearset. Regardless of the particular configuration, churning and parasitic losses convert energy that could have been transferred to the wheels into heat energy that is not beneficially captured by the vehicle. As such, an opportunity may exist to provide a more energy efficient vehicle driveline. 
     SUMMARY 
     A vehicle drive train includes a first driveline being adapted to transfer torque to a first set of wheels and includes a first power disconnection device. A second driveline is adapted to transfer torque to a second set of wheels and includes a differential gearset having an output coupled to a second power disconnection device. A hypoid gearset is positioned within the second driveline in a power path between the first and second power disconnection devices. The second power disconnection device includes an active multi-plate clutch having a first set of clutch plates fixed for rotation with the differential gearset output. The clutch further includes a second set of clutch plates fixed for rotation with an output shaft adapted to transfer torque to one of the wheels of the second set of wheels. A valve is operable to limit a flow of coolant to the multi-plate clutch when the second power disconnection device operates in the disconnected mode. 
     In another form, a vehicle drive train includes a first driveline adapted to transfer torque from a power source to a first set of wheels and includes a power take-off unit. A second driveline includes a hypoid gearset in receipt of torque from the first driveline. The power take-off unit includes a first power disconnection device selectively ceasing the transfer of torque to the hypoid gearset. The second driveline transfers torque to a second set of wheels and includes a second power disconnection device selectively interrupting a transfer of torque from the second set of wheels to the hypoid gearset. The second power disconnection device includes a multi-plate clutch controlled by a ball ramp actuator selectively providing a first rate of axial apply plate travel per degree of rotation and a second lesser rate of axial apply plate travel per degree of rotation. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a schematic of an exemplary vehicle equipped with a vehicle drive train of the present disclosure; 
         FIG. 2  is a fragmentary cross-sectional view of a rear drive axle including a disconnect coupling; 
         FIG. 3  is a fragmentary cross-sectional view of a ball ramp actuation mechanism; 
         FIG. 4  is a fragmentary sectional view of another portion of the ball ramp mechanism; 
         FIG. 5  is a partial fragmentary cross-sectional view of a rear drive axle having a clutch lubrication flow valve; and 
         FIG. 6  is a fragmentary cross-sectional view of the axle and the clutch lubrication flow valve having a flow reducer in a position to restrict fluid flow. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
     In general, the present disclosure relates to a coupling and hypoid disconnect system for a driveline of a motor vehicle. A power take-off unit may be equipped with an active coupling or a dog clutch/synchronizer to disconnect the power source from a portion of the driveline and to reconnect through synchronization of said driveline. Additionally, another active coupling may be provided to disconnect a portion of the driveline from the vehicle wheels. The hypoid gearing of the vehicle driveline may be separated from the driving source of power to reduce churning losses and other mechanical inefficiencies. 
     With particular reference to  FIG. 1  of the drawings, a drive train  10  of a four-wheel drive vehicle is shown. Drive train  10  includes a front driveline  12  and a rear driveline  14  both drivable from a source of power, such as an engine  16  through a transmission  18  which may be of either the manual or automatic type. In the particular embodiment shown, drive train  10  is a four-wheel system incorporating a power transmission device  20  for transmitting drive torque from engine  16  and transmission  18  to front driveline  12  and rear driveline  14 . Power transmission device  20  is shown as a power take-off unit. 
     Front driveline  12  is shown to include a pair of front wheels  24  individually driven by a first axle shaft  26  and a second axle shaft  28 . Front driveline  12  also includes a reduction speed gearset  30  and a differential assembly  32 . Power transmission device  20  includes a clutch  34  and a right-angled drive assembly  36 . Clutch  34  may be configured as a dog clutch, a synchronized clutch, a roller clutch, a multi-plate clutch, or another torque transferring disconnection mechanism. If speed synchronization may be accomplished between the rotating members to be connected, a simple dog clutch may suffice. However, under certain conditions, the reconnection of a previously disconnected driveline may become more challenging due to rotational speed differences across the power disconnection device. For example, front wheel slip may occur that will result in the front driveline speed being greater than the rotational speed of rear driveline components being driven by the rear wheels. In this case, a speed differential will be realized across the power disconnection device making it difficult or impossible for a dog clutch to be actuated from a non-torque transferring mode to a torque transferring mode. Accordingly, a roller clutch or synchronizer may be implemented at any of the locations depicted as a dog clutch or similar power disconnection device. By implementing the roller clutch or synchronizer, a controller may initiate reconnection and torque transfer once a specified range of speed difference between the two members being connected is met. This control arrangement may result in improved system performance including a reduction in the time required to operate the vehicle in one of the drive modes. 
     Rear driveline  14  includes a propeller shaft  38  connected at a first end to right-angled drive assembly  36  and at an opposite end to a rear axle assembly  40 . Rear driveline  14  also includes a pair of rear wheels  42  individually driven by a first rear axle shaft  44  and a second rear axle shaft  46 . Rear axle assembly  40  also includes a hypoid ring and pinion gearset  48  driving a differential assembly  50 . A disconnect coupling  52  may selectively drivingly connect and disconnect second rear axle shaft  46  from ring and pinion gearset  48  and differential assembly  50 . 
       FIG. 2-4  depict portions of rear axle assembly  40 . A housing  60  rotatably supports a pinion shaft  62  of ring and pinion gearset  48  via bearings  64 ,  66 . A pinion gear  68  is integrally formed with pinion shaft  62 . Ring and pinion gearset  48  also includes a ring gear  70  in meshed engagement with pinion gear  68  and fixed for rotation with a carrier  72 . Carrier  72  is rotatably supported within housing  60  by bearings  74 . Differential assembly  50  includes a pair of pinion gears  76  supported on a cross pin  78  fixed to carrier  72 . First and second side gears  80 ,  82  are in meshed engagement with pinion gears  76 . Second side gear  82  is fixed for rotation with a stub shaft  84 . Bearing  74  rotatably supports stub shaft  84  within housing  60 . Seals  86  engage stub shaft  84  and separate a cavity  88  containing disconnect coupling  52  from a cavity  89  containing differential assembly  50 . 
     Disconnect coupling  52  includes a drum  90  fixed for rotation with stub shaft  84 . A driven spindle  94  is rotatably supported within a removable portion  96  of housing  60  by bearings  98 . A hub  100  is fixed for rotation with driven spindle  94  via a splined connection  102 . Disconnect coupling  52  also includes a plurality of outer friction plates  104  fixed for rotation with and axially moveable relative to drum  90  as well as a plurality of inner friction plates  106  fixed for rotation with and being axially moveable relative to hub  100 . Outer friction plates  104  are interleaved with inner friction plates  106 . 
     A clutch actuator  110  is operable to selectively apply a force to an actuator plate  112  for compressing outer clutch plates  104  and inner clutch plates  106  to transfer torque between stub shaft  84  and driven spindle  94 . A spring  113  is positioned to engage hub  100  and actuator plate  112  to urge actuator plate  112  away from clutch plates  104 ,  106 . Actuator  110  includes an electric motor  114  driving a ball ramp mechanism  115  via a worm gear  116  and sector gear  117 . Ball ramp mechanism  115  includes a first cam plate  118  spaced apart from a second cam plate  120 . First cam plate  118  includes a plurality of tapered grooves  122 . Second cam plate  120  includes a corresponding pair of tapered grooves  124  that are circumferentially spaced apart from one another and positioned to oppose first grooves  122 . Balls  126  are positioned within pairs of tapered grooves  122 ,  124 . Relative rotation between first cam plate  118  and second cam plate  120  causes second cam plate  120  to translate and axially move actuator plate  112 . 
     As shown in  FIG. 4 , first tapered grooves  122  include a relatively steep ramp angle portion  128  adjacent to a relatively shallow ramp angle portion  130 . Second grooves  124  also include corresponding steep and shallow ramp angle portions  132  and  134 , respectively. To reduce frictional losses across disconnect coupling  52  when the coupling is operated in an open or disconnected mode, it may be advantageous to space outer friction plates  104  from inner friction plates  106  a maximum distance from one another. The shape and depth of first grooves  122  and second grooves  124  acting with spring  113  may accomplish this task. However, a relatively large distance needs to be traversed when torque transfer across disconnect coupling  52  is desired. The steep ramp angle portions  128 ,  132  function to accomplish this goal by axially translating second cam plate  120  a relatively large amount based on a relatively small amount of relative rotation between first cam plate  118  and second cam plate  120 . Once most of the clearance between outer clutch plates  104 , inner clutch plates  106  and actuator plate  112  has been removed, balls  126  act on the relatively shallow ramp angle portions  130 ,  134  to apply an amplified force and control the torque generated by disconnect coupling  52 . 
     Clutch actuator  110  may alternatively include a hydraulic motor, or some other source of energy to cause relative rotation between first cam plate  118  and second cam plate  120 . Furthermore, it should be appreciated that ball ramp mechanism  115  may be replaced by a hydraulic actuation system with similar behavior. In a first step, a piston in the hydraulic system travels quickly with a small available force. In a second step, the piston travels slowly, but with a high possible actuation force. An exemplary system is described within U.S. Patent Application Publication No. 2009/038908 which is hereby incorporated by reference. 
     During vehicle operation, it may be advantageous to reduce the churning losses associated with driving ring and pinion gearset  48  and right-angled drive assembly  36 . A controller  140  is in communication with a variety of vehicle sensors  142  providing data indicative of parameters such as vehicle speed, four-wheel drive mode, wheel slip, vehicle acceleration and the like. At the appropriate time, controller  140  outputs a signal to control clutch  34  and place it in a deactuated mode where torque is not transferred from engine  16  to rear driveline  14 . Controller  140  also signals clutch actuator  110  associated with disconnect coupling  52  such that energy associated with rotating rear wheels  42  will not be transferred to ring and pinion gearset  48  or differential assembly  50 . Accordingly, the hypoid gearsets do not rotate at the rotational output speed of differential assembly  32 , nor do they rotate at the rotational speed of rear wheels  42 . The hypoid gearsets are disconnected from all sources of power and are not driven at all. 
     It is contemplated that any one or more of the previously described clutches including interleaved inner and outer clutch plates may be either a wet clutch or a dry clutch. Wet clutches are lubricated and cooled with a fluid that may be pumped or sloshed across the friction surfaces of the inner and outer clutch plates. The wet clutches provide excellent torque transfer characteristics and operate in a sealed environment containing the lubricant. A pump (not shown) may provide pressurized fluid to cool the wet clutch. Alternatively, the fluid acting on the clutch plates may be the same fluid used to lubricate members of the gear train including the ring and pinion gears. 
     When a wet plate clutch is used as a disconnect device and active all wheel drive coupling, viscous drag torque losses are associated with the plates of the wet clutch shearing through the fluid in contact with the plates. To reduce the drag losses within the wet clutch, the inner and outer plates may be axially spaced apart from one another a relatively large distance, as previously discussed. To further reduce the fluid shearing losses, actuator  110  may include a valve  150  associated with a clutch lubrication pickup tube  152 . Lubrication pickup tube  152  is stationary within housing  60  and may be fixed to first cam plate  118 . Valve  150  functions to control lubricant flow in the vicinity of outer clutch plates  104  and inner clutch plates  106 . When disconnect coupling  52  is in a torque transferring mode, a substantial flow of lubricant is allowed. When disconnect coupling  52  is in the open or disconnected mode, valve  150  functions to restrict or discontinue the flow of lubricant to the friction plates  104 ,  106 . With the lubricant flow restricted or stopped, fluid previously positioned between outer clutch plates  104  and inner clutch plates  106  will drain such that the shearing losses will be further reduced. More particularly, and as shown in  FIGS. 5 and 6 , it is contemplated that valve  150  includes a flow reducer  154  fixed to second cam plate  120 . Flow reducer  154  is shown rotated out of a flow restricting position in  FIG. 5 .  FIG. 6  depicts flow reducer  154  blocking at least a portion of pickup tube  152 . The angular orientation of second cam plate  120  determines the position of flow reducer  154 . 
     By positioning actuator  110  within housing  60  as previously discussed, the forces generated by disconnect coupling  52  and its associated actuator  110  are retained and reacted in housing portion  96  thus minimizing any losses across support bearings  74  or  98 , thereby improving system control and accuracy. Furthermore, the actuation forces related to operating disconnect coupling  52  are not influenced by forces generated by ring and pinion gearset  48  or differential assembly  50 , thus improving control accuracy and reducing drag losses. 
     It should be appreciated that the concepts previously discussed regarding the operation and location of multiple disconnects in relation to a transverse oriented engine and transmission as depicted in  FIG. 1  may also be applied to a longitudinal engine arrangement. While a number of vehicle drivelines have been previously described, it should be appreciated that the particular configurations discussed are merely exemplary. As such, it is contemplated that other combinations of the components shown in the Figures may be arranged with one another to construct a drive train not explicitly shown but within the scope of the present disclosure.