Abstract:
A power transfer unit is provided with a torque limiting coupling that limits the amount of torque that can be transferred to the driveline components when torque peaks occur while torque is transferred to the non-slipping wheels. The torque limiting coupling includes an engagement member disposed between an input from a driving member and an output to a driven member. The engagement member is located radially between and provides a frictional engagement between the input and the output. The engagement member is capable of slipping under heavy torque loads in order to protect the driveline components.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/786,263, filed on Mar. 27, 2006. The disclosure(s) of the above application(s) is (are) incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The present disclosure relates to power transfer units for use in motor vehicles and, more particularly, to an improved torque limiting clutch assembly for use in such power transfer units. 
       BACKGROUND 
       [0003]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0004]    In view of increased consumer popularity in four-wheel drive vehicles, a plethora of power transfer systems are currently being utilized in vehicular driveline applications for selectively directing power (i.e., drive torque) from the powertrain to all four wheels of the vehicle. In many power transfer systems, a power transfer unit, such as a transfer case, is incorporated into the driveline and is operable for delivering drive torque from the powertrain to both the front and rear wheels. Many conventional transfer cases are equipped with a mode shift mechanism that can be selectively actuated to shift between a two-wheel drive mode and a four-wheel drive mode. 
         [0005]    It is also known to use “on-demand” power transfer systems for automatically biasing power between the front and rear wheels, without any input or action on the part of the vehicle operator, when traction is lost at either the front or rear wheels. Modernly, it is known to incorporate the “on-demand” feature into a transfer case by replacing the mechanically-actuated mode shift mechanism with a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the clutch assembly is typically maintained in a non-actuated condition such that drive torque is only delivered to the rear wheels. However, when the sensors detect a low traction condition, the clutch assembly is automatically actuated to deliver torque “on-demand” to the front wheels. Moreover, the amount of drive torque transferred through the clutch assembly to the non-slipping wheels can be varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. This on-demand clutch control system can also be used in full-time transfer cases to automatically bias the torque ratio across an interaxle differential. 
         [0006]    Notwithstanding significant sales of four-wheel drive and all-wheel drive vehicles, much emphasis is directed to improving vehicle performance and fuel efficiency while at the same time reducing weight. In conflict with this emphasis is the need to engineer the components of power transfer units to meet all torque requirements anticipated for the vehicle application. Specifically, the components must be sized to survive during torque peak conditions despite the fact that such peak conditions rarely occur during typical use of the motor vehicle. Thus, a need exists to limit the maximum torque transferred by a power transfer unit so as to permit the components to be smaller in size and weight. 
       SUMMARY 
       [0007]    A power transfer unit for use in motor vehicles is provided with a torque limiting coupling that limits the drive torque transferred to the driveline when torque peaks occur. The power transfer unit includes an input member driven by the powertrain, an output member driving the driveline, and a torque limiting coupling disposed between the input member and the output member. The torque limiting coupling establishes a spring-biased drive connection between the input member and the output member. 
         [0008]    Thus, it is an object of the present invention to provide a shaft and sprocket assembly having a torque limiting coupling. 
         [0009]    It is a further object of the present invention to provide a shaft and sprocket assembly equipped with a torque limiting coupling and which is well-suited for use in a transfer case or other vehicular drivetrain devices. 
         [0010]    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 
         [0011]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0012]      FIG. 1  is a schematic view of a four-wheel drive vehicle equipped with a power transfer unit having an actively-controlled torque transfer clutch and a control system according to the present disclosure; 
           [0013]      FIG. 2  is a schematic diagram of the power transfer unit of  FIG. 1  defining a full-time two-speed transfer case having a chain drive assembly equipped with a torque limiting coupling according to the present invention; 
           [0014]      FIG. 3  is a sectional view of a torque limiting coupling embodying the device schematically shown in  FIG. 2 ; and 
           [0015]      FIG. 4  is a schematic diagram of an alternative installation of the torque limiting chain drive assembly within the full-time transfer case of  FIG. 2   
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0017]    Referring now to the drawings, a four-wheel drive vehicle  10  is schematically shown to include a front driveline  12  and a rear driveline  14  both drivable from a powertrain. The powertrain is shown to include an engine  16  and a transmission  18  which may be of either the manual or automatic type. In the particular embodiment shown, vehicle  10  further includes a power transfer unit, hereinafter referred to as transfer case  20 , that is operable for transmitting drive torque from engine  16  and transmission  18  to front driveline  12  and rear driveline  14 . Front driveline  12  includes a pair of front wheels  22  connected at opposite ends of a front axle assembly  24  having a front differential  26  that is coupled to one end of a front drive shaft  28 , the opposite end of which is coupled to a front output shaft  30  of transfer case  20 . Similarly, rear driveline  14  includes a pair of rear wheels  32  connected at opposite ends of a rear axle assembly  34  having a rear differential  36  coupled to one end of a rear drive shaft  38 , the opposite end of which is interconnected to a rear output shaft  40  of transfer case  20 . 
         [0018]    As will be further detailed, transfer case  20  is equipped with a two-speed range unit  42 , an interaxle differential  44 , a clutch assembly  46  and a power-operated actuation mechanism  48  that is operable to control coordinated shifting of range unit  42  and adaptive engagement of clutch assembly  46 . In addition, a control system  50  is provided for controlling power-operated actuation mechanism  48 . Control system  50  includes sensors  52  for detecting operational characteristics of motor vehicle  10 , a mode selector  54  for permitting the vehicle operator to select one of the available drive modes, and an electronic control unit  56  operable to generate control signals in response to input signals from sensors  52  and mode signals from mode selector  54 . The control signals are sent to an electric motor assembly  58  ( FIG. 2 ) associated with actuation mechanism  48 . 
         [0019]    With particular reference to  FIG. 2 , transfer case  20  is schematically shown to include an input shaft  60  adapted to be driven by the output of transmission  18 . Range unit  42  includes a planetary gearset having a sun gear  62  driven by input shaft  60 , a ring gear  64  fixed to a stationary housing  66  and planet gears  68  rotatably supported by a planet carrier  70  and which are meshed with both sun gear  62  and ring gear  64 . Range unit  42  further includes a synchronized dog clutch assembly  72  having a clutch hub  74  journalled on input shaft  60 , a first clutch plate  76  fixed for rotation with input shaft  60  and a second clutch plate  78  fixed for rotation with planet carrier  70 . Synchronized dog clutch assembly  72  further includes a first synchronizer  80  disposed between clutch hub  74  and first clutch plate  76 , a second synchronizer  82  disposed between clutch hub  74  and second clutch plate  78  and a shift collar  84  splined for rotation with and bi-directional axial sliding movement on clutch hub  74 . 
         [0020]    Shift collar  84  is shown in its neutral (N) position where it is disengaged from both first clutch plate  76  and second clutch plate  78 . Shift collar  84  is moveable from its N position to a high-range (H) position whereat shift collar  84  is coupled to first clutch plate  76  and is driven at a direct speed ratio relative to input shaft  60 . In contrast, shift collar  84  can be moved from its N position to a low-range (L) position whereat shift collar  84  is coupled to second clutch plate  78  and is driven by planet carrier  70  at a reduced speed ratio relative to input shaft  60 . First synchronizer  80  functions to establish speed synchronization between shift collar  84  and input shaft  60  during movement of shift collar  84  toward its H position. Likewise, second synchronizer  82  functions to establish speed synchronization between shift collar  84  and planet carrier  70  during movement of shift collar  84  toward its L position. 
         [0021]    Interaxle differential  44  includes an input member driven by shift collar  84 , a first output member driving rear output shaft  40 , and a second output member operably arranged to drive front output shaft  30 . In particular, differential  44  includes an annulus gear  90  fixed for rotation with shift collar  84 , a sun gear  92  fixed to a quill shaft  94  rotatably supported on rear output shaft  40 , and a pinion carrier  96  fixed to rear output shaft  40  which rotatably supports meshed pairs of first pinion gears  98  and second pinion gears  100 . In addition, first pinion gears  98  are meshed with annulus gear  90  and second pinion gears  100  are meshed with sun gear  92 . As such, driven rotation of annulus gear  90  (at either of the direct or reduced speed ratios) causes drive torque to be transmitted to rear output shaft  40  via pinion carrier  96  and to quill shaft  94  via sun gear  92 . Drive torque is transferred from quill shaft  94  to front output shaft  30  via a chain drive assembly which includes a drive sprocket  102  fixed to quill shaft  94 , a driven sprocket  104  supported on front output shaft  30 , and a drive chain  106  meshed with sprockets  102  and  104 . As will be detailed, the chain drive assembly incorporates a torque limiting coupling  168  between driven sprocket  104  and front output shaft  30 . Based on the particular configuration of interaxle differential  44 , a specific torque distribution ratio is established (i.e., 50/50, 64/36) between rear output shaft  40  and front output shaft  30 . However, the magnitude of the torque transfer from driven sprocket  104  to front output shaft  30  can be limited by a torque limiting device  168  as discussed below. 
         [0022]    With continued reference to  FIG. 2 , clutch assembly  46  is shown to be a multi-plate friction clutch comprised of a clutch drum  108  fixed to quill shaft  94 , a clutch pack  109  having outer clutch rings  110  splined for rotation with clutch drum  108  which are interleaved with inner clutch rings  112  splined to rear output shaft  40 , and an apply plate  114  for applying an axially-directed clutch engagement force on clutch pack  109 . 
         [0023]    Power-operated actuation mechanism  48  is operable to cause movement of shift collar  84  between its three distinct positions as well as to generate the clutch engagement force exerted on clutch pack  109  of clutch assembly  46 . In its most basic sense, actuation mechanism  48  includes motor assembly  58 , a driveshaft  120  rotatively driven by the output of motor assembly  58 , a range actuator assembly  122 , and a clutch actuator assembly  124 . Motor assembly  58  is preferably an electric gearmotor equipped with an encoder capable of accurately sensing the rotated position of driveshaft  120 . Range actuator assembly  122  includes a range cam  126  fixed for rotation with driveshaft  120 . Cam  126  is cylindrical and includes a high-range circumferential groove  128 , a low-range circumferential groove  130  and a spiral intermediate groove  132  connecting circumferential grooves  128  and  130 . Range actuator assembly  122  further includes a range fork  134  having a follower segment  136  shown retained in spiral groove  132  and a fork segment  138  retained in an annular groove formed on shift collar  84 . 
         [0024]    As will be appreciated, rotation of range cam  126  results in axial movement of shift collar  84  due to retention of follower segment  136  in spiral groove  132 . Specifically, rotation of driveshaft  120  in a first direction causes concurrent rotation of range cam  126  which, in turn, causes follower segment  136  to move within spiral groove  132  until shift collar  84  is located in its H position. At this position, follower segment  136  enters high-range dwell groove  128  which permits continued rotation of drive shaft  120  in the first direction while shift collar  84  is retained in its H position with the high-range drive connection established between input shaft  60  and annulus gear  90 . Thereafter, rotation of driveshaft  120  and range cam  126  in the opposite second direction causes follower segment  136  to exit high-range dwell groove  128  and re-enter intermediate spiral groove  132  for causing shift collar  84  to begin moving from the H position toward its L position. Upon continued rotation of range cam  126  in the second direction, follower segment  136  exits spiral groove  132  and enters low-range dwell groove  130  for locating shift collar  84  in its L position and establishing the low-range drive connection between planet carrier  70  and annulus gear  90 . 
         [0025]    Clutch actuator assembly  124  is also driven by motor assembly  58  and includes a ball-ramp unit  140  and a gear assembly  142 . Ball-ramp unit  140  includes a first ball-ramp plate  144 , a second ball-ramp plate  146 , and a plurality of balls  148  disposed in ramped grooves  150  and  152  formed in corresponding face surfaces of plates  144  and  146 . First ball-ramp plate  144  is non-rotatably secured to housing  66  and is supported for bi-directional axial movement. Specifically, first ball-ramp plate  144  is shown to coaxially surround rear output shaft  40  and is arranged to move axially for exerting an axially-directed clutch engagement force on apply plate  114  for frictionally engaging clutch pack  109 . A thrust bearing is shown located between apply plate  114  and first ball-ramp plate  144  for permitting relative rotation therebetween. Second ball-ramp plate  146  also coaxially surrounds rear output shaft  40  and is supported for limited rotation relative to first ball-ramp plate  144 . Second ball-ramp plate  146  is axially restrained relative to rear output shaft  40  via a backing plate  153 . A thrust bearing is shown located between backing plate  153  and second ball-ramp plate  146 . As such, relative rotation between ball-ramp plates  144  and  146  causes balls  148  to travel along ramped grooves  150  and  152  which, in turn, acts to control the axial position of second ball-ramp plate  146  relative to clutch pack  109 , thereby controlling the magnitude of the clutch engagement force exerted thereon. 
         [0026]    Gear assembly  142  includes a first gear  154  fixed for rotation with driveshaft  120 , a second gear  156  fixed to second ball-ramp plate  146 , and a third gear  158  rotatably supported on an idlershaft  160  and which is meshed with both first gear  154  and second gear  156 . Preferably, second gear  156  is an arcuate gear segment formed integrally with, or rigidly secured to, an outer face surface of second ball-ramp plate  146 . The profile of ramped grooves  150  and  152  and the gear ratio established by gear assembly  142  between drive shaft  120  and second ball-ramp plate  146  are designed to permit bi-directional rotation of drive shaft  120  through a range of travel sufficient to permit shift collar  84  to move between its H and L positions without any significant clutch engagement force being transmitted by ball-ramp unit  140  to clutch assembly  46 . However, additional bi-directional rotation of drive shaft  120 , as accommodate by dwell grooves  128  and  130  in range cam  126 , is designed to cause axial movement of second ball-ramp plate  146  between an “adapt-ready” position and a “locked” position. In the adapt-ready position, a minimum clutch engagement force is exerted on clutch pack  109  such that clutch assembly  46  is considered to be non-actuated. Preferably, this clutch engagement force applies a preload on clutch pack  109  to eliminate driveline clunk and permit instantaneous clutch actuation. Conversely, in the locked position, a maximum clutch engagement force is exerted on clutch pack  109  and clutch assembly  46  is considered to be fully engaged. Thus, by varying the axial position of second ball-ramp plate  146  between its adapt-ready and locked position, the torque bias across differential  44  can be continuously modulated to provide automatic clutch control of clutch assembly  46  in a range between its released and fully engaged conditions. 
         [0027]    Control system  50  is provided to control the rotated position of drive shaft  120  in response to the mode signal delivered to ECU  56  by mode selector  54  and the sensor input signals sent by sensors  52 . While sensors  52  can provide numerous indicators (i.e., shaft speeds, vehicle speed, acceleration/throttle position, brake status, etc.), it is contemplated that clutch assembly  46  is controlled, at a minimum, in response the magnitude of interaxle slip (ΔRPM) between output shafts  40  and  30 . Mode selector  54  permits selection of one an Automatic Full-Time four-wheel high-range (Auto-4WH) drive mode, a Neutral mode, and a Locked four-wheel low-range (Lock-4WL) drive mode. In the Auto-4WH mode, shift collar  84  is located in its H position and the torque biasing generated by clutch assembly  46  is continuously modulated based on value of the sensor signals. In the Lock-4WL mode, shift collar  84  is in its L position and clutch assembly  46  is fully engaged. In the Neutral mode, shift collar  84  is in its N position and clutch assembly  46  is released. Obviously, other available drive modes can be provided if desired. For example, a Locked four-wheel high-range (LOCK-4WH) drive mode can be established by locating shift collar  84  in its H position and fully engaging clutch assembly  46 . 
         [0028]    While actuation mechanism  48  has been disclosed in association with a full-time transfer case, it will be understood that differential  44  could be eliminated such that clutch assembly  46  would function to modulate the drive torque transferred from rear output shaft  40  to front output shaft  30  for establishing an on-demand four-wheel drive mode. It is also understood that the transfer case could be single-speed power transfer unit with elimination of two-speed range unit  42 . 
         [0029]    With additional reference to  FIG. 3 , the chain drive system is shown to include torque limiting coupling  168  disposed between front output shaft  30  and driven sprocket  104 . As such, the combination of sprocket  104 , shaft  30  and torque limiting coupling  168  define a torque-limited shaft and sprocket assembly. Torque limiting coupling  168  is shown to include a pair of cone clutches  170  and  172 , a pair of belleville springs  174  and  176 , a plurality of thrust washers  178 ,  180  and  182 , and a pair of C-rings  184  and  186 . Preferably, cone clutches  170  and  172  are annular ring-shaped components that are completely contained within the width of driven sprocket  104  during normal operation. Cone clutches  170  and  172  have corresponding inner surfaces  188  and  190  and outer surfaces  192  and  194  that are tapered relative to an axis of rotation  196  of front output shaft  30 . Cone clutches  170  and  172  are oriented such that inner surfaces  188  and  190  and outer surfaces  192  and  194  extend generally opposite one another at a taper or ramp angle of approximately seven degrees relative to axis of rotation  196 , although other suitable angles can be utilized. 
         [0030]    Tapered inner surfaces  188  and  190  are adapted to frictionally engage similarly tapered portions of an outer surface  198  on front output shaft  30 . Specifically, outer shaft surface  198  includes first and second conically-tapered portions  200  and  202  which extend generally parallel to inner cone surfaces  188  and  190 , respectively. First and second tapered portions  200  and  202  extend generally opposite one another and are disposed at an angle of approximately seven degrees relative to axis of rotation  196 , although other suitable angles can be utilized. While tapered portions  200  and  202  have been shown formed integrally as part of front output shaft  30 , it is contemplated that conically-tapered hubs could be secured to shaft  30 . 
         [0031]    Outer surfaces  192  and  194  of cone clutches  170  and  172  are adapted to frictionally engage a similarly tapered inner surface  204  of driven sprocket  104 . Specifically, inner sprocket surface  204  includes first and second conically-tapered portions  206  and  208  which extend generally parallel to outer surfaces  192  and  194 , respectively. First and second tapered portions  206  and  208  extend generally opposite one another and are disposed at an angle of approximately seven degrees relative to axis of rotation  196 , although other suitable angles can be utilized. 
         [0032]    A first end wall  210  of cone clutch  172  abuts thrust washer  182 , which, in turn, abuts C-ring  186 . C-ring  186  is axially fixed to front output shaft  30 , thereby preventing axial travel of cone clutch  172  in a first direction toward thrust washer  182 . Cone clutch  172  is generally prevented from axial travel in a second direction generally opposite the first direction through its frictional engagement with front output shaft  30  and driven sprocket  104 . 
         [0033]    A first end wall  212  of cone clutch  170  abuts thrust washer  180 . Thrust washer  180  is biased against first end wall  212  by belleville springs  174  and  176 . Belleville springs  174  and  176  engage thrust washer  178  which abuts C-ring  184 . C-ring  184  is axially fixed to front output shaft  30 , thereby limiting axial travel of cone clutch  170  in the second axial direction mentioned above. Axial travel of cone clutch  170  in the first axial direction mentioned above is limited by its frictional engagement with front output shaft  30  and driven sprocket  104 . Belleville springs  174  and  176  apply a predetermined spring load to cone clutch  170  in the first axial direction. The spring load is determined based on the cone geometry and friction coefficient as well as the desired torque transfer limit. The belleville springs are merely representative of a suitable spring biasing mechanism that can be used to maintain a desired spring load on cone clutches  170  and  172 . 
         [0034]    In the present example, front output shaft  30  and driven sprocket  104  are each made of steel, resulting in similar coefficients of thermal expansion. Cone clutches  170  and  172  are preferably made from bronze. As noted above, cone clutches  170  and  172  are preferably contained within the width of driven sprocket  104 . This arrangement avoids grooving of the bronze friction surfaces. 
         [0035]    In operation, drive torque is transferred by the chain drive assembly from driven sprocket  104  to front output shaft  30  due to frictional engagement of cone clutches  170  and  172  with driven sprocket  104  and front output shaft  30 . As torque transfer is increased above a predetermined limit, the frictional coupling of cone clutches  170  and  172  with driven sprocket  104  and front output shaft  30  due to the bias loading of springs  174  and  176  is overcome. Accordingly, as the peak torque exceeds the predetermined limit, cone clutches  170  and  172  permit slip between driven sprocket  104  and front output shaft  30 , thereby limiting the maximum amount of drive torque transferred to front output shaft  30 . As noted above, this predetermined torque level or capacity can be any desired amount and can be adjusted based upon the parameters noted above. 
         [0036]    To provide adequate lubrication to the interface surfaces between the steel and bronze components, a lube hole  220  in housing  66  permits flow of lubricant to a central lubrication channel  222  formed in front output shaft  30 . Ports  224  and  226  provide a flow path for lubricant from channel  222  to the interface between shaft surface  200  and cone surface  188  and the interface between shaft surface  202  and cone surface  190 . Lubricant is also permitted to flow to the interface between outer cone surface  192  and sprocket inner cone surface  206  as well as the interface between outer cone surface  194  and sprocket inner cone surface  208  via a central radial passage  228 . It is contemplated that cone surfaces  188  and  192  on first cone clutch  170  and cone surfaces  190  and  194  on second cone clutch  172  may be grooved and/or have lube slots formed therein to assist in routing the lubricant across the corresponding shaft and sprocket surfaces. Finally, a friction material may be bonded to the cone surfaces of cone clutches  170  and  172  and/or the mating conical surfaces on front output shaft  30  or driven sprocket  104 . 
         [0037]    Torque limiting coupling  168  has been described as being located between driven sprocket  104  and front output shaft  30 . However, it is understood that torque limiting coupling  168  can be located at any other location where control of torque transfer is desired. To this end,  FIG. 4  illustrates torque limiting coupling  168  operably disposed between quill shaft  94  and drive sprocket  102  with driven sprocket  104  now fixed for rotation with front output shaft  30 . Other potential driveline locations for torque limiting coupling  168  may include the connection interface between one or both propshafts  28  and  38  and their inputs to corresponding differentials  26  and  36 . 
         [0038]    The above reference embodiments clearly set forth the novel and unobvious features, structure and/or function of the present disclosure. However, one skilled in the art will appreciate that equivalent elements and/or arrangements may be used which will be covered by the scope of the following claims.