Abstract:
A drive system includes a differential for transmitting power between a power source and first axles; a clutch including a first member driveably connected to the power source and a second member secured to a power transfer shaft; and a servo including a cylinder connected to a pressure source, a piston displaceable in the cylinder for mutually disconnecting the first and second members and allowing mutual engagement of the first and second members.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation-in-part application of pending U.S. application Ser. No. 12/838,534, filed Jul. 19, 2010, the entire disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to a motor vehicle driveline, which in operation transmits power continually to a first wheel set and selectively to a second wheel set. 
         [0004]    2. Description of the Prior Art 
         [0005]    Power transfer unit (PTU) efficiency is a concern in all wheel drive (AWD) vehicles derived from front wheel drive vehicle platforms, partially due to kinematics dictated by the speed and torque of the transmission output. For example, an AWD powertrain typically reduces fuel economy by about five percent compared to a front wheel drive (FWD) powertrain. 
         [0006]    Driveline disconnects are a well established fuel economy improvement method in rear wheel drive powertains, but are absent from FWD products. A principal difference between FWD-based and RWD-based AWD products is the transfer case used on RWD-based products provides a convenient disconnect point. In FWD-based powertrains, the relatively small package space available for the power transfer unit makes implementation of driveline disconnects on FWD-based products very challenging. 
         [0007]    A need exists in the automotive industry for a front disconnect located ahead of a power takeoff unit in order to optimize fuel efficiency of the powertrain by minimizing in FWD operation the number of rotating components when AWD is not selected. Preferably the package space required for the disconnect permits the PTU drive system to be located within the transaxle case. 
       SUMMARY OF THE INVENTION 
       [0008]    A drive system includes a differential for transmitting power between a power source and first axles; a clutch including a first member driveably connected to the power source and a second member secured to a power transfer shaft; and a servo including a cylinder connected to a pressure source, a piston displaceable in the cylinder for mutually disconnecting the first and second members and allowing mutual engagement of the first and second members. 
         [0009]    The drive system eliminates the need for various PTU disconnect components due to utilizing the existing hydraulic control system, thereby substantially reducing the total system cost and significantly increasing fuel economy. 
         [0010]    The drive system produces an increase in fuel economy by locating the disconnect between the power source and the secondary wheel set ahead of the PTU bevel pinion and mating bevel gear. 
         [0011]    The drive system can be located within the casing of a transaxle, thereby reducing the required package space. 
         [0012]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a cross section showing a drive system that connects a power source continually to a primary wheel set and selectively to a secondary wheel set; 
           [0015]      FIG. 2  is a cross section showing an alternate drive system that connects a power source continually to a primary wheel set and selectively to a secondary wheel set; and 
           [0016]      FIG. 3  is a cross section showing another arrangement of the drive system of  FIG. 2 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]      FIG. 1  illustrates a drive system  10  that continually connects a power source to a primary wheel set, preferably the front wheels of a motor vehicle, and selectively connects the power source to a secondary wheel set, preferably the rear wheels. The power source is a transmission output shaft  12 . Rotating power is transmitted to the wheels of the primary wheel set through a right-hand axle shaft  14  and a left-hand axle shaft  16 . Power is selectively transmitted to the wheels of the secondary wheel set through a power transfer unit (PTU) input shaft  18 . 
         [0018]    A compound planetary differential  20  includes a sun gear  22 , secured through a spline  24  to axle shaft  16 ; a carrier  26 , secured through a spline  28  to axle shaft  14 ; a ring gear  30 , engaged with a pinion  32  formed on the transmission output shaft  12 ; first planet pinions  34  supported on the carrier and meshing with the ring gear  30 ; and second planet pinions  35  supported on the carrier and meshing with the sun gear  22  and the first planet pinions  34 . One side of ring gear  30  is secured to a disc  36  and supported at a bearing  38 ; the other side of ring gear  30  is secured to a disc  40  and supported at a bearing  42 . Disc  40  is formed with an internal spline  43 , which engages an external spline formed on a coupler sleeve  44 . 
         [0019]    Disc  40  forms a cylinder  46 , which contains a piston  48 , actuated by pressurized hydraulic fluid carried to cylinder  46  through a passage  50 . A compression return spring  52  restores piston  48  to the disengaged position shown in the figure. Piston  48  is secured to coupler sleeve  44  such that they move along an axis  53  and rotate about axis  53  as a unit. 
         [0020]    The volume  54  enclosed by piston  48  and spring retainer  56  forms a balance dam containing hydraulic fluid supplied from source of hydraulic lubricant  58  through a lube circuit, which includes passages  60 ,  62 ,  64 ,  66 . 
         [0021]    In operation, fluid from source of line pressure  68  is carried in passage  70  to a valve  72 , which is controlled by a variable force solenoid  74 . Valve  72  opens and closes a connection between line pressure source  68  and passages  76 ,  78 , which carry piston-actuating pressure to cylinder  46  depending on the state of solenoid  74 . When passages  76  and  78  are pressurized, piston  48  and coupler sleeve  44  move leftward, causing frictional contact at the conical surface between a member  80  and a synchronizing ring  82 . Member  80  is rotatably secured by spline  84  to PTU input shaft  18 . As the speed of member  80  is synchronized with the speed of ring gear  30 , the internal spline of coupler sleeve  44  engages the dog teeth on synchronizing ring  82  and the clutch teeth  86  on the radial outer surface of connecting member  80 , thereby driveably connecting ring gear  30  and PTU input shaft  18 . 
         [0022]    When passages  76  and  78  are vented through valve  72  and passage  88 , piston  48  and sleeve  44  move rightward to their disengaged positions, causing connecting member  80  to disengage the ring gear  30 , thereby disconnecting ring gear  30  from PTU input shaft  18 . 
         [0023]    Although the description refers to the speed of connecting member  80  being synchronized with the speed of ring gear  30  using a synchronizer, a connection between the ring gear and PTU input shaft  18  can be completed using a coupler instead of a synchronizer. 
         [0024]    Referring now to  FIG. 2 , the housing  90  of an interwheel differential  92  is driveably connected to the final drive pinion  94  of an automatic transaxle  96 . The differential&#39;s spindle  98 , which rotates with the housing  90  about axis  100 , carries bevel pinions (not shown), which mesh with side bevel gears  102 ,  104 . The left-side bevel gear  102  is secured to a halfshaft  106 , which extends to a driven axle shaft at the left-hand side of the vehicle. Similarly, the right-side bevel gear  104  is secured to a halfshaft  108 , which extends to a driven axle shaft at the right-hand side of the vehicle. 
         [0025]    Housing  90  is formed with an external spline, which meshes with an internal spline formed on a member  110  of dog clutch assembly  112 . The mating splines form a spline connection  114  between housing  90  and clutch member  110 . 
         [0026]    A second member  116  of dog clutch assembly  112  is formed with an external spline, which meshes with an internal spline formed on a PTU shaft  118 , coaxial with axis  100 . The mating splines form a spline connection  120  between PTU shaft  118  and clutch member  116 . PTU shaft  118  transmits torque to a drive shaft (not shown), which extends along a longitudinal axis of the vehicle to a rear differential, through which torque is transmitted to the rear wheels. 
         [0027]    Dog clutch member  110  and dog clutch member  116  are formed with dog clutch teeth  122 ,  124 , which are shown engaged mutually in  FIG. 2 , but can be disengaged when member  116  is moved rightward along axis  100  against the force of a compression spring  126 , which continually biases member  116  leftward toward member  110 . 
         [0028]    The casing  128  of transaxle  96  is formed with a cylinder  130 , containing an axially displaceable piston  132 , and with a passage  134  for carrying hydraulic fluid under pressure to cylinder  130 . Piston  132  is fixed against rotation by an axial spline connection  136  to casing  128 . The spline connection  136  permits axial displacement of piston  132  relative to casing  128 . 
         [0029]    A thrust bearing  138  is fitted in an axial space between dog clutch member  116  and piston  132 . A second thrust bearing  140  is fitted in an axial space between differential housing  90  and transaxle casing  128 . 
         [0030]    In operation, when cylinder  130  is pressurized through passage  134 , piston  132  moves rightward and transmits axial force through bearing  138  to dog clutch member  116 . That force causes member  116  to move rightward against return spring  126 , thereby disengaging dog teeth  122 ,  124  and opening the drive connection between the tranaxle&#39;s final drive pinion  94  and PTU shaft  118 . 
         [0031]    When cylinder  130  is vented through passage  134 , piston  132  moves leftward due to the force of spring  126  on dog clutch member  116 , bearing  138 , and piston  132 . This leftward movement causes member  116  to move leftward, thereby engaging dog teeth  122 ,  124  and closing the drive connection between the tranaxle&#39;s final drive pinion  94  and PTU shaft  118 . In this state the PTU shaft  118  transmits torque to the rear wheels. 
         [0032]      FIG. 3  illustrates an assembly similar to that of  FIG. 2 , wherein housing  90  is formed with an external spline, which meshes with an internal spline formed on a member  150  of dog clutch assembly  152 . The mating splines form a spline connection  114  between housing  90  and clutch member  150 . 
         [0033]    A second member  156  of dog clutch assembly  152  is formed with an external spline, which meshes with an internal spline formed on a PTU shaft  118 , coaxial with axis  100 . The mating splines form a spline connection  120  between PTU shaft  118  and clutch member  156 . PTU shaft  118  transmits torque to a drive shaft (not shown), which extends along a longitudinal axis of the vehicle to a rear differential, through which torque is transmitted to the rear wheels. 
         [0034]    Dog clutch member  150  and dog clutch member  156  are formed with dog clutch teeth  162 ,  164 , which are shown engaged mutually in  FIG. 3 , but can be disengaged when member  156  is moved rightward along axis  100  against the force of compression spring  126 , which continually biases member  156  leftward toward member  150 . The dog teeth  162 ,  164  are located radially outward from thrust bearing  166 . 
         [0035]    The casing  128  of transaxle  96  is formed with a cylinder  130 , containing axially displaceable piston  132 , and with a passage  134  for carrying hydraulic fluid under pressure to cylinder  130 . Piston  132  is fixed against rotation by friction between the piston seals and the cylinder  130 . 
         [0036]    Thrust bearing  166  is fitted in an axial space between a radial extension  168 , which is secured to an end of piston  132 , and a radial member  170 , which is secured to dog clutch member  156 . The second thrust bearing  140  is fitted in an axial space between differential housing  90  and transaxle casing  128 . 
         [0037]    In operation, when cylinder  130  is pressurized through passage  134 , piston  132  moves rightward and transmits axial force through radial extension  168 , bearing  166  and radial member  170  to dog clutch member  156 . That force displaces member  156  rightward against the force of return spring  126 , thereby disengaging dog teeth  162 ,  164  and opening the drive connection between the tranaxle&#39;s final drive pinion  94  and PTU shaft  118 . 
         [0038]    When cylinder  130  is vented through passage  134 , piston  132  moves leftward due to the force of spring  126  applied to dog clutch member  156  and transmitted through radial member  170 , bearing  166 , and radial extension  168  to piston  132 . This force of spring  126  displaces member  156  leftward, thereby engaging dog teeth  162 ,  164  and closing a drive connection between the tranaxle&#39;s final drive pinion  94  and PTU shaft  118 . In this state the PTU shaft  118  transmits torque to the rear wheels. 
         [0039]    Thrust bearing  166  of  FIG. 3  is smaller than its counterpart bearing  138  of  FIG. 2 , thereby requiring a smaller space. The mating dog clutch teeth  162 ,  164  of  FIG. 3  are located at a greater radial distance from axis  100  that the counterpart dog teeth  122 ,  124  of  FIG. 2 , thereby requiring a lower mating force between the clutch teeth  162 ,  164  for a unit of torque transmitted by the clutch assembly  152  as compared to the force between the clutch teeth  122 ,  124  of clutch assembly  112 . 
         [0040]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.