Patent Publication Number: US-7895913-B2

Title: Spiral cam clutch actuation system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/626,510 filed on Jan. 24, 2007, now U.S. Pat. No. 7,650,808 which application claims the benefit of U.S. Provisional Application Ser. No. 60/765,489 filed Feb. 3, 2006. The entire disclosures of each of the above applications are incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates generally to power transfer systems for controlling the distribution of drive torque between the front and rear drivelines of a four-wheel drive vehicle. More particularly, the present invention is directed to a power transmission device for use in motor vehicle driveline applications having a power-operated clutch actuator that is operable for controlling actuation of a multi-plate friction clutch assembly. 
     BACKGROUND 
     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 transfer case is incorporated into the driveline and is operable in a four-wheel drive mode 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 part-time four-wheel drive mode. In addition, many transfer cases also include a range shift mechanism which can be selectively actuated by the vehicle operator to engage a reduction gearset for shifting between four-wheel high-range and low-range drive modes. 
     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 friction clutch assembly and a power-operated clutch actuator that is interactively associated with an electronic control system and a sensor arrangement. During normal road conditions, the friction clutch assembly is typically maintained in a released condition such that drive torque is only delivered to the rear wheels. However, when the sensors detect a low traction condition, the clutch actuator is actuated for engaging the friction clutch assembly to deliver drive torque “on-demand” to the front wheels. Typically, the amount of drive torque transferred through the friction clutch assembly to the non-slipping wheels is varied as a function of specific vehicle dynamics, as detected by the sensor arrangement. This on-demand clutch control system is also used in “full-time” transfer cases to automatically bias the torque ratio across an interaxle differential. 
     In some two-speed transfer cases, the range shift mechanism and the clutch assembly are independently controlled by separate power-operated actuators. For example, U.S. Pat. No. 5,407,024 discloses a two-speed range shift mechanism actuated by an electric motor and a friction clutch assembly actuated by an electromagnetic ballramp unit. In an effort to reduce cost and complexity, some transfer cases are equipped with a single power-operated actuator that is operable to coordinate actuation of both the range shift mechanism and the clutch assembly. In particular, U.S. Pat. Nos. 5,363,938 and 5,655,986 each illustrate a transfer case equipped with a motor-driven sector having cam surfaces adapted to coordinate actuation of the range shift mechanism and the clutch assembly for establishing a plurality of distinct two-wheel and four-wheel drive modes. 
     While transfer cases equipped with such coordinated actuation systems have been commercially successful, a need exists to develop alternative clutch actuation systems which further reduce the cost and complexity of two-speed actively-controlled transfer cases. 
     SUMMARY 
     Accordingly, it is an objective of the present invention to provide a transfer case equipped with a two-speed range unit, a mode clutch assembly and a power-operated actuation mechanism for controlling coordinated actuation of the range unit and the mode clutch assembly. 
     It is another objective of the present invention that the transfer case be interactively associated with a control system for controlling operation of the power-operated actuation mechanism to establish various four-wheel high-range and low-range drive modes. 
     It is another objective of the present invention to locate the mode clutch assembly across an interaxle differential to provide automatic torque biasing and slip limiting features in a full-time four-wheel drive mode. 
     It is another objective of the present invention to locate the mode clutch assembly between the front and rear output shafts of the transfer case to provide automatic torque transfer in an on-demand four-wheel drive mode. 
     Another objective of the present invention is to provide a synchronized range unit for permitting on-the-move shifting between the high-range and low-range drive modes. 
     It is another objective of the present invention to provide a power-operated actuation mechanism having a range actuator assembly operable to control actuation of the two-speed range unit, a mode actuator assembly operable to control actuation of the mode clutch assembly and a motor-driven geartrain operable to control actuation of the range and mode actuator assemblies. 
     It is another objective of the present invention to provide the mode actuator assembly with a roller ramp unit having a face cam with cam surfaces and a control gear with rollers engaging the cam surfaces. 
     It is another objective of the present invention to mount the rollers on pins to permit radial travel of the rollers within spiral or other non-constant radius cam surfaces formed on the face cam. 
     According to these and other objectives of the present invention, a transfer case is provided with a two-speed range unit, a mode clutch assembly, a power-operated actuation mechanism and a control system. The range unit includes a planetary gearset driven by an input shaft and a range clutch for releasably coupling one of the input shaft and an output component of the planetary gearset to a first output shaft. The mode clutch assembly is a multi-plate friction clutch operably disposed between the first output shaft and a second output shaft. The power-operated actuation mechanism includes an electric motor, a geartrain driven by the motor, a range actuator assembly and a mode actuator assembly. The range actuator assembly includes a driveshaft driven by the geartrain, a range cam rotatively driven by the driveshaft and a shift collar associated with the range clutch. Rotation of the range cam results in transitional movement of the shift collar between high-range (H), neutral (N) and low-range (L) positions. The mode actuator assembly is a roller ramp unit having a face cam with cam surfaces and a control gear with rollers engaging the cam surfaces. The control gear is rotatively driven by the geartrain for initially causing concurrent rotation of the face cam. This initial rotary non-translational movement of the face cam permits sufficient rotation of the driveshaft to move the shift collar between its three range position while the friction clutch is maintained in a disengaged state. An anti-rotation mechanism limits rotation of the face cam upon continued rotation of the control gear such that engagement of the rollers on the cam surfaces causes translational non-rotary movement of the face cam. Such translational movement of the face cam functions to control the magnitude of a clutch engagement force applied to the friction clutch. The control system is adapted to control the magnitude and direction of rotary motion of the driveshaft and the control gear through controlled energization of the electric motor. 
     The power-operated actuation mechanism of the present invention is arranged to permit sufficient bi-directional rotation of the geartrain to cause movement of the shift collar between its H and L positions without causing the roller ramp unit to engage the multi-plate friction clutch. However, once the shift collar is positively located in either of its H or L positions, continued rotation of the geartrain causes actuation of the roller ramp unit for generating and applying the clutch engagement force on the multi-plate friction clutch. 
     Additionally, a power transmission device for a motor vehicle includes a clutch for transferring torque between a first shaft and a second shaft. A clutch actuation system includes a drive member and a cam member in cooperation with one another. The drive member is driven by an electric motor and includes first and second rollers rotatably mounted thereon. The cam member includes first and second circumferentially extending channels, each having a continually reducing radius. The first and second channels are separate from and overlap one another. The first roller is positioned within the first channel to engage the drive member and the cam member. The second roller is positioned within the second channel to engage the drive member and the cam member such that relative rotation between the drive member and the cam member translates the cam member along an axis of relative rotation to vary a magnitude of force applied to the clutch. 
     Furthermore, the present disclosure describes a power transmission device for use in a motor vehicle having a powertrain and a driveline. A clutch selectively transmits drive torque between an input shaft adapted to be driven by the powertrain and an output shaft adapted to drive the driveline. An electric motor rotates a driveshaft. A clutch operator includes a first member rotatably driven by the driveshaft, a second member axially moveable between first and second positions for controlling the magnitude of a clutch engagement force applied to the clutch, and a cam mechanism for converting rotary movement of the first member into axial movement of the second member. The cam mechanism includes a roller rotatably mounted to the first member and a spiral channel formed in the second member within which the roller is disposed. The channel includes a cam surface engaged by the roller and configured to cause radial movement of the roller and axial movement of the second member between its first and second positions in response to rotation of the first member relative to the second member. 
    
    
     
       DRAWINGS 
       Further objects, features and advantages of the present invention will become apparent from analysis of the following written specification including the appended claims, and the accompanying drawings in which: 
         FIG. 1  is a schematic view of a four-wheel drive vehicle equipped with a transfer case and a control system according to the present invention; 
         FIG. 2  is a sectional view of a two-speed full-time transfer case constructed in accordance with one preferred embodiment of the present invention; 
         FIGS. 3 through 5  are enlarged partial views of  FIG. 2  showing the two-speed range unit, interaxle differential, mode clutch assembly and power-operated actuation mechanism associated with the two-speed full-time transfer case in greater detail; 
         FIG. 6  is a side view of a control gear associated with a roller ramp unit; 
         FIG. 7  illustrates various components associated with the power-operated actuation mechanism; 
         FIG. 8  is a side view of a face cam associated with the roller ramp unit; 
         FIG. 9  is a partial sectional view taken along line A-A of  FIG. 8  showing recessed channel-type cam surfaces formed in the face cam; 
         FIG. 10  is similar to  FIG. 9  except that it depicts raised flange-type cam surfaces formed on the face cam; 
         FIGS. 11A through 11G  are views of the components associated with the power-operated actuation mechanism in different positions for establishing the various available drive modes; 
         FIG. 12  is a sectional view of a two-speed on-demand transfer case according to an alternative preferred embodiment of the present invention; 
         FIG. 13  is a sectional view of a single-speed on-demand transfer case according to yet another preferred embodiment of the present invention; and 
         FIG. 14  illustrates a modified face cam associated with the roller ramp unit shown in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, a four-wheel drive vehicle  10  is schematically shown to include a front driveline  12 , a rear driveline  14  and a powertrain for generating and selectively delivering rotary power (i.e., drive torque) to the drivelines. 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 transfer case  20  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 driveshaft  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 driveshaft  38 , the opposite end of which is interconnected to a rear output shaft  40  of transfer case  20 . 
     As will be further detailed, transfer case  20  is equipped with a two-speed range unit  42 , an interaxle differential  44 , a mode 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 mode clutch assembly  46 . In addition, a control system  50  is provided for controlling actuation of actuation mechanism  48 . Control system  50  includes vehicle sensors  52  for detecting real time operational characteristics of motor vehicle  10 , a mode select mechanism  54  for permitting the vehicle operator to select one of the available drive modes, and an electronic controller unit (ECU)  56  that is operable to generate electric control signals in response to input signals from sensors  52  and mode signals from mode select mechanism  54 . The control signals are sent to an electric motor assembly  58  associated with actuation mechanism  48 . 
     With particular reference to  FIGS. 2 and 3 , transfer case  20  is shown to include an input shaft  60  adapted to be driven by the output shaft of transmission  18 . Range unit  42  includes a planetary gearset having a sun gear  62  fixed (i.e., splined) for rotation with input shaft  60 , a ring gear  64  non-rotatably fixed to a portion of a housing  66  and a set of planet gears  68  rotatably supported from a planet carrier  70 . Each planet gear  68  is 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  journaled 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 axial sliding movement on clutch hub  74 . As will be detailed, shift collar  84  is arranged to selectively drive an input member of interaxle differential  44 . 
     Shift collar  84  is shown in its central neutral (N) position where it is disengaged from both first clutch plate  76  and second clutch plate  78 . With shift collar  84  in its N position, transfer case  20  is in a Neutral non-driven mode with input shaft  60  uncoupled from driven connection with the input of interaxle differential  44 , whereby no drive torque is transmitted to either of the output shafts. 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 . Accordingly, location of shift collar  84  in its H range position functions to establish a high-range drive connection between input shaft  60  and the input to interaxle differential  44 . 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 . Such movement of shift collar  84  to its L range position functions to establish a low-range drive connection between input shaft  60  and the input to interaxle differential  44 . 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. 
     It is contemplated that transfer case  20  could be equipped without synchronizers  80  and  82  if a non-synchronized range shift system is desired. Likewise, the planetary gearset and range shift arrangement shown are intended to merely be representative of one type of two-speed range unit available for use in transfer cases. To this end, any two-speed reduction unit having a shift member moveable to establish first and second ratio drive connections is considered to be within the scope of this invention. 
     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, interaxle differential  44  includes an annulus gear  90  fixed for rotation and axial sliding movement with shift collar  84 , a sun gear  92  fixed to a quill shaft  94  that is rotatably supported on rear output shaft  40 , and a pinion carrier assembly  96  that is fixed (i.e., splined) for rotation with rear output shaft  40 . Pinion carrier assembly  96  includes a first carrier ring  96 A fixed (i.e., splined) for rotation with rear output shaft  40 , a second carrier ring  96 B, and pins rotatably supporting meshed pairs of first pinion gears  98  and second pinion gears  100  (see  FIG. 2 ) therebetween. In addition, first pinion gears  98  are meshed with annulus gear  90  while 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 assembly  96  and to quill shaft  94  via sun gear  92 . Drive torque is transferred from quill shaft  94  to front output shaft  30  through a transfer assembly  101  which includes a drive sprocket  102  fixed to quill shaft  94 , a driven sprocket  104  fixed to front output shaft  30 , and a drive chain  106  meshed with sprockets  102  and  104 . Based on the particular configuration and sizing of the gears associated with 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 . 
     Referring primarily to  FIG. 4 , mode clutch assembly  46  is shown to include a clutch hub  110  fixed via a spline connection  112  to a tubular end segment of quill shaft  94 , a clutch drum  114  fixed via a spline connection  116  to rear output shaft  40 , and a multi-plate clutch pack  118  operably disposed between hub  110  and drum  114 . Clutch pack  118  includes a set of outer clutch plates that are splined for rotation with and axial movement on an outer cylindrical rim segment  120  of drum  114 . Clutch pack  118  also includes a set of inner clutch plates that are splined for rotation with and axial movement on clutch hub  110 . Clutch assembly  46  further includes a reaction plate  122  that is splined for rotation with outer rim segment  120  of drum  114  and retained thereon via a lock ring  124 , and a pressure plate  126  that is also splined for rotation with outer rim segment  120  of drum  114 . Pressure plate  126  is adapted to move axially for exerting a compressive clutch engagement force on clutch pack  118  in response to resilient pivotal movement of disk levers  128 . Disk levers  128  are shown to be located between pressure plate  126  and a radial plate segment  130  of drum  114 . 
     Pressure plate  126  is axially moveable relative to clutch pack  118  between a first or “released” position and a second or “locked” position. With pressure plate  126  in its released position, a minimum clutch engagement force is exerted on clutch pack  118  such that virtually no drive torque is transferred through clutch assembly  46  so as to establish a differentiated or full-time four-wheel drive mode. In contrast, location of pressure plate  126  in its locked position causes a maximum clutch engagement force to be applied to clutch pack  118  such that front output shaft  30  is, in effect, coupled for common rotation with rear output shaft  40  so as to establish a non-differentiated or locked four-wheel drive mode. Therefore, accurate control of the position of pressure plate  126  between its released and locked position permits adaptive regulation of the torque biasing between rear output shaft  40  and front output shaft  30 , thereby establishing an adaptive all-wheel drive mode. 
     Power-operated actuation mechanism  48  is operable to coordinate movement of shift collar  84  between its three distinct range positions with movement of pressure plate  126  between its released and locked positions. In its most basic form, actuation mechanism  48  includes an electric motor assembly  58 , a reduction geartrain  140  driven by motor assembly  58 , a range actuator assembly  144  and a mode actuator assembly  146 . 
     Reduction geartrain  140  is shown to include a first gearset  150  and a second gearset  152 . First gearset  150  is preferably a bevel gearset having a drive pinion  154  driven by an output shaft of electric motor assembly  58  and which is meshed with a bevel gear  156  so as to provide a first reduction ratio. As seen, bevel gear  156  is rotatably supported by a bearing assembly  160  from housing  66  for rotation about a first rotary axis. The first reduction ratio established by bevel gearset  150  is preferably in the range of 3:1 to 10:1 and, more preferably, is about 6:1. Second gearset  152  is preferably a spur gearset having a first gear  162  rigidly secured to bevel gear  156  for common rotation about the first rotary axis and which is meshed with a second gear  164  so as to provide a second reduction ratio. Second gear  164  is rotatably supported from housing  66  by a bearing assembly  166  for rotation about a second rotary axis. Preferably, the second reduction ratio provided by spur gearset  152  is similar in range to that of bevel gearset  150  with a preferred ratio of about 6:1. A cumulative speed reduction ratio of about 36:1 between the output shaft of electric motor assembly  58  and second gear  164  permits the use of a small, low power electric motor. 
     Referring primarily to  FIG. 5 , range actuator assembly  144  is shown to include a driveshaft  142  and a range cam  172  that is fixed for rotation with driveshaft  142 . As seen, driveshaft  142  has a first end fixed via a spline connection  166  for common rotation with second gear  164  and a second end that is rotatably supported in a socket  168  formed in housing  66 . In addition, an angular position sensor, such as an encoder unit  170 , is provided for accurately detecting the rotated position of second gear  164 . Range cam  172  is cylindrical and includes a groove  173  comprised of a high-range dwell segment  174 , a low-range dwell segment  176  and a helical intermediate shift segment  178  interconnecting dwell segments  174  and  176 . Range actuator assembly  144  further includes a range fork  180  having a tubular sleeve  182  surrounding range cam  172 , a follower pin  184  which extends from range fork sleeve  182  into groove  173 , and a fork segment  186  extending from sleeve  182  into an annular groove  190  formed in shift collar  84 . 
     Rotation of range cam  172  results in controlled axial movement of shift collar  84  due to the movement of follower pin  184  within shift segment  178  of groove  173 . Specifically, when it is desired to shift range unit  42  into its high-range drive mode, electric motor  58  is energized to cause rotation of second gear  164  and driveshaft  142  in a first direction which, in turn, causes concurrent rotation of range cam  172 . Such rotation of range cam  172  causes follower pin  184  to move within intermediate shift segment  178  of groove  173  until shift collar  84  is axially located in its H range position. With shift collar  84  in its H range position, the high-range drive connection is established between input shaft  60  and annulus gear  90 . Continued rotation of driveshaft  142  in the first direction causes follower pin  184  to exit shift segment  178  and enter high-range dwell segment  174  which is configured to maintain shift collar  84  in its H range position. Thereafter, concurrent rotation of second gear  164 , driveshaft  142  and range cam  172  in the opposite or second direction causes follower pin  184  to exit high-range dwell segment  174  and re-enter helical shift segment  178  for causing shift collar  84  to begin moving from its H range position toward its L range position. Upon continued rotation of range cam  172  in the second direction, follower pin  184  exits shift segment  178  and enters low-range dwell segment  176  of groove  173  for axially locating shift collar  84  in its L range position and establishing the low-range drive connection between planet carrier  70  and annulus gear  90 . 
     As best seen from  FIGS. 2 and 4 , mode actuator assembly  146  surrounds rear output shaft  40  and includes a drive member  200 , a cam member  202 , and a thrust member  204 . Drive member, hereinafter referred to as control gear  200 , has a cylindrical inner rim segment  206  rotatably supported by a bearing assembly  208  on an inner sleeve segment  210  of clutch drum  114 , a cylindrical outer rim segment  212 , and a plate-like web segment  214  therebetween. Outer rim segment  212  is shown to have external gear teeth  216  extending entirely around its outer circumference that are in constant meshed engagement with gear teeth  218  on second gear  164 . The relative orientation of geartrain  140  and electric motor  58  to control gear  200  is best shown in  FIG. 7 . According to a preferred construction, the size and number of teeth  218  on second gear  164  are identical to the size and number of teeth  216  on control gear  200  to provide a direct (i.e., 1:1) ratio therebetween. Control gear  200  further includes a pair of diametrically opposed rollers  220 A and  220 B that are retained in channels  222  formed in web segment  214 . In particular, rollers  220 A and  220 B are each shown to be mounted for rotation and sliding movement on a pin  224  which is secured between the inner and outer rim segments of control gear  200 . 
     As best seen from  FIG. 8 , cam member, hereinafter referred to as face cam  202 , is a ring-like structure having a central aperture surrounding inner sleeve segment  210  of drum  114  and an outwardly extending anti-rotation lug  225 . Lug  225  is retained between a pair of diametrically opposed anti-rotation shoulder stops  226 A and  226 B formed on housing  66  so as to permit rotation of face cam  202  through a range of angular travel delimited by anti-rotation stops  226 A and  226 B. In the arrangement shown, the range of rotary movement for face cam  202  is about 180°. Face cam  202  defines a first face surface  228  and a second face surface  230 . Extending inwardly from first face surface  228  are a first channel  232  and a second channel  234 , with each channel having a “spiral” or other non-constant radial path relative to the central rotary axis of face cam  202 . First channel  232  defines a cam surface  236  having a first or high-range ramp segment  236 A and a second or low-range ramp segment  236 B, both of which have an angular length of about 180°. Likewise, second channel  234  defines a cam surface  238  having a first or high-range segment  238 A and a second or low-range segment  238 B, both of which have an angular length of about 180°. 
     Roller  220 A of control gear  200  is retained within first channel  232  and rollingly engages first cam surface  236  while roller  220 B is retained within second channel  234  and rollingly engages second cam surface  238 . As noted, rollers  220 A and  220 B slide on pins  224  which function to accommodate the non-constant radial path defined by channels  232  and  234 . In fact, high-range ramp segments  236 A and  238 A are similarly tapered or otherwise contoured to control axial movement of face cam  202  between a retracted position and an extended position relative to control gear  200  when shift collar  84  is located in its H range position. Likewise, low-range ramp segments  236 A and  236 B are similarly tapered or otherwise contoured to control axial movement of face cam  202  between its retracted and extended positions when shift collar  84  is located in its L range position. As will be detailed, face cam  202  is axially moved between its retracted and extended positions when it is prevented from rotating with control gear  200  due to engagement of its lug  225  with one of anti-rotation stops  226 A and  226 B. 
       FIG. 9  is partial sectional view showing channels  232  and  234  formed in first face  228  of face cam  202 . The depth of channels  232  and  234  will vary due to the tapered profile of cam surfaces  236  and  238 , but the edge surfaces function to maintain rollers  220 A and  220 B therein. As an option,  FIG. 10  illustrates face cam  202  having raised cam surfaces  236 ′ and  238 ′ formed on first face surface  228  in place of channels. To accommodate the non-constant radial path of cam surface  236 ′ and  238 ′, rollers  220 A and  220 B would be ridge or otherwise provided with flanged portions to overhang opposite sides of the cam surfaces. 
     Thrust member  204  includes a hub segment  240  surrounding inner sleeve segment  210  of drum  114 , a plate segment  242  extending radially from hub segment  240  and a plurality of circumferentially-spaced thrust pins  244  that extend axially from plate segment  242 . Each thrust pin  244  has a terminal end which extends through a bore  246  formed in plate segment  130  of drum  114  and which is adapted to engage the free end of disk levers  128 . A thrust bearing assembly  248  is provided between second face surface  232  of face cam  202  and plate segment  242  of thrust member  204 . 
     The biasing force exerted by disk levers  128  on thrust member  204  acts to maintain constant engagement of control gear rollers  220 A and  220 B with respective cam surfaces  236  and  238  on face cam  202 . Accordingly, when face cam  202  is axially located in its retracted position, disk levers  128  are released from engagement with pressure plate  126 , whereby pressure plate  126  is located in its released position and clutch assembly  46  is considered to be in a released or non-engaged state. In contrast, axial movement of face cam  202  from its retracted position toward its extended position causes thrust pins  244  to deflect disk levers  128  which, in turn, causes pressure plate  126  to move axially from its released position toward its locked position. As noted, such movement of pressure plate  126  causes a compressive clutch engagement force to be applied to clutch pack  118  for transferring drive torque through clutch assembly  46 . Since control gear  200  is restrained from moving axially, rotation of control gear  200  relative to face cam  202  causes rollers  220 A and  220 B to ride along cam surface  236  and  238  on face cam  202  which, in turn, results in axial movement of face cam  202 . 
     As noted, power-operated actuation mechanism  48  coordinates axial movement of shift collar  84  with axial movement of face cam  202  to establish a plurality of different four-wheel drive modes. The available drive modes include a full-time four-wheel high-range (4WH) drive mode, an adaptive all-wheel high-range (AWH) drive mode, a locked four-wheel high-range (LOCK-4WH) drive mode, a Neutral mode, a full-time four-wheel low-range (4WL) drive mode, an adaptive all-wheel low-range (AWL) drive mode and a locked four-wheel low-range (LOCK-4WL) drive mode. While it is contemplated that mode select mechanism  54  would most likely limit the available selection to the AWH, LOCK-4WH, N and LOCK-4WL drive modes in a typical vehicle application, the following description of each drive mode is provided. 
     In operation, when mode select mechanism  54  indicates selection of the 4WH drive mode, ECU  56  signals electric motor  58  to rotate geartrain  140 . Specifically, second gear  164  is rotated in a first (i.e., clockwise) direction to a position where: A) concurrent rotation of driveshaft  142  has caused shift collar  84  to move into its H range position; and B) the resulting rotation of control gear  200  in a first (i.e., counter-clockwise) direction has caused concurrent rotation of face cam  202  until its lug  225  engages anti-rotation stop  226 A. As seen from  FIGS. 8 and 11A , rollers  220 A and  220 B on control gear  200  bear against cam surfaces  236  and  238  at their respective low or “detent” points  236 C and  238 C such that face cam  202  is axially located in its retracted position. Furthermore, rollers  220 A and  220 B are both located at a first radial distance “A” from the origin of face cam  202 . As such, pressure plate  126  is located in its released position and clutch assembly  46  is released. With mode clutch assembly  46  released, differential  44  acts as an open differential permitting unrestricted speed differentiation between the two output shafts. 
     When mode select mechanism  54  thereafter indicates selection of the AWH drive mode, ECU  56  energizes electric motor  58  to cause geartrain  140  to continue rotating second gear  164  in its first direction. As indicated, high-range dwell segment  174  of groove  173  in range cam  172  accommodates this additional rotation of driveshaft  142  resulting from such continued rotation of second gear  164  for maintaining shift collar  84  in its H range position. As is evident, continued rotation of second gear  164  in its first direction results in continued rotation of control gear  200  in its first direction. However, such continued rotation of control gear  200  now causes non-rotary axial movement of face cam  202  from its retracted position toward an intermediate or “adapt” position. Specifically, such axial movement of face cam  202  occurs since tab stop  226 A prevents further concurrent rotation of face cam  202  with control gear  200 . Thus, the resultant relative rotation of control gear  200  relative to face cam  202  causes rollers  220 A and  220 B to exit dwell points  236 C and  238 C and travel along complimentary high-range ramp segments  236 A and  238 A of face cam  202  to the position shown in  FIG. 11B . Such movement of rollers  220 A and  220 B results in initial axial movement of face cam  202  from its retracted position to its adapt position. The adapt position is selected to locate pressure plate  126  in a ready position so as to provide a predetermined low level of torque transfer across mode clutch assembly  46  to take-up clearances in clutch pack  118  in preparation for subsequent adaptive control. Thereafter, ECU  56  determines when and how much torque needs to be transmitted across mode clutch assembly  46  to limit excessive interaxle slip between the output shafts based on the current tractive conditions and vehicular operating characteristics detected by sensors  52 . 
     The limits of adaptive torque control in the AWH drive mode are established by controlling bi-directional rotation of control gear  200  through a range of motion operable for axially moving face cam  202  between its adapt and extended positions. Specifically, axial movement of face cam  202  to its extended position results from further rotation of second gear  164  in its first direction until rollers  220 A and  220 B are located at the end of high-range ramp segments  236 A and  238 A, as shown in  FIG. 11C . Bi-directional rotation of control gear  200  within this range of travel is controlled by ECU  56  controlling energization of electric motor  58  based on a pre-selected torque control strategy. Preferably, the length of high-range ramp segments  236 A and  236 B of channels  232  and  234  permits about 180° of rotation for control gear  200 . As will be understood, any control strategy known in the art for adaptively controlling actuation of clutch assembly  46  can be used with the present invention. 
     If mode select mechanism  54  indicates that the vehicle operator has selected the LOCK-4WH drive mode, electric motor  58  is energized to rotate second gear  164  and control gear  200  in their respective first directions until rollers  220 A and  220 B on control gear  200  are located in the positions shown in  FIG. 11C . As such, rollers  220 A and  220 B have rolled up high-range segments  236 A and  236 B of cam surfaces  236  and  238  which, in turn, has caused face cam  202  to move axially to its extended position. As noted, such movement of face cam  202  to its extended position causes pressure plate  126  to move to its locked position for fully engaging mode clutch assembly  46 . As shown in  FIG. 8 , face cam  202  is located in its axially extended position when rollers  220 A and  220 B are located at a second radial distance “B” from the center of face cam  202 . 
     To limit the on-time service requirements of electric motor  58 , a power-off brake  250  can be provided to brake rotation of the motor shaft so as to prevent back-driven rotation of geartrain  140  for maintaining pressure plate  126  in its locked position. In this manner, electric motor  58  can be shut-off during operation of transfer case  20  in its LOCK-4WH drive mode. To reiterate, shift collar  84  is maintained in its H range position because high-range dwell segment  174  of groove  173  in range cam  172  accommodates the additional rotation of driveshaft  142  caused by rotation of second gear  164  in its first direction which also functions to rotate control gear  200  relative to face cam  202 . 
     If the Neutral mode is selected, second gear  164  is rotated in its second (i.e., counter-clockwise) direction for concurrently rotating driveshaft  142 . Such rotation of driveshaft  142  causes follower pin  184  on range fork  180  to ride within shift segment  178  of groove  173  in range cam  172  until shift collar  84  is located in its N position. During such range shifting, mode clutch  46  is maintained in its released state. Specifically, the rotation of second gear  164  in its second direction also causes rotation of control gear  200  in its second (i.e., clockwise) direction from the position shown in  FIG. 11A  to that shown in  FIG. 11D . The continuous engagement of face cam  202  with rollers  220 A and  220 B on control gear  200  due to the biasing of disk levers  128  causes face cam  202  to also rotate in its second direction in concert with control gear  200 . Furthermore, this biasing also causes rollers  220 A and  220 B to be located at their detent points  236 C and  238 C, respectively, thereby maintaining face cam  202  in its retracted axial position. As seen, lug  225  is generally located halfway between stops  226 A and  226 B when the Neutral mode is established. 
       FIG. 11E  illustrates the position of the components associated with transfer case  20  for establishing the 4WL drive mode. In particular, second gear  164  has been rotated in its second direction to a position whereat: A) concurrent rotation of driveshaft  142  has caused shift collar  84  to move into its L range position; and B) the resulting rotation of control gear  200  in its second direction has caused face cam  202  to rotate until its lug  225  now engages anti-rotation stop  226 B. In this position, face cam  202  is in its retracted axial position such that mode clutch assembly  46  is released. 
     When mode select mechanism  54  indicates selection of the AWL drive mode, ECU  56  energizes motor  58  to cause geartrain  140  to continue rotation of second gear  164  in its second direction. Shift collar  84  is maintained in its L range position due to follower pin  184  entering low-range dwell segment  176  of groove  173  in range cam  172  during such continued rotation of driveshaft  142 . Furthermore, engagement of lug  225  with stop  226 B prevents further rotation of face cam  202  while control gear  200  continues to rotate until rollers  220 A and  220 B are located in the positions shown in  FIG. 11F . This relative rotation causes face cam  202  to move axially to its adapt position due to rollers  220 A and  220 B engaging portions of low-range ramp segments  236 B and  238 B of corresponding cam surfaces  236  and  238 . Similar to operation in the AWH drive mode, ECU  56  controls the magnitude of engagement of clutch assembly  46  by controlling movement of the rollers on control gear  200  between the positions shown in  FIGS. 11F and 11G  which, in turn, moves face cam  202  between its adapt position and its locked positions. Such adaptive control is again based on a predetermined control strategy utilizing the signals inputted to ECU  56  from sensors  52 . 
     Referring to  FIG. 11G , the components are shown for establishing the LOCK-4WL mode with shift collar  84  in its L range position and mode clutch assembly  46  fully engaged due to second gear  164  being rotated in its second direction until control gear  200  is rotated to locate the rollers in the positions shown. In this position, rollers  220 A and  220 B are radially located a third radial distance “C” from the origin of face cam  202  on low-range ramp segments  236 B and  238 B such that face cam  202  is located axially in its extended position. Thus, pressure plate  126  is located in its locked position, thereby fully engaging clutch assembly  46 . Again, brake  250  would be engaged to prevent rotation of geartrain  140  and hold second gear  164  in the position defining the LOCK-4WL drive mode while permitting electric motor  58  to be de-energized. 
     According to the present invention, mode actuator assembly  146  and range actuator assembly  144  are interconnected by a common geartrain  140  so as to permit coordinated actuation of both using a single power-operated device, namely electric motor  58 . Mode actuator assembly  146  accommodates actuation of range actuator assembly  144  while mode clutch  46  is maintained in a released state for permitting movement of shift collars  84  between its three distinct range positions. Likewise, range actuation assembly  144  accommodates actuation of mode actuator assembly  146  when shift collar  84  is positively located in one of its H and L range positions to permit adaptive engagement of clutch assembly  46 . To this end, bi-directional rotation of second gear  164  through two distinct ranges of angular travel achieves this coordination feature. Specifically, a first range, identified in  FIG. 7  as angle “X”, controls movement of shift collar  84  while cam member  202  is maintained in its retracted position. A second angular range, identified as angle “Y” controls engagement of clutch assembly  46  while shift collar  84  is maintained in either of its H or L range positions. 
     While actuation mechanism  48  has been disclosed in association with full-time transfer case  20 , it will be understood that interaxle differential  44  could be eliminated such that mode clutch assembly  46  functions to modulate the drive torque transferred from rear output shaft  40  to front output shaft  30  to establish an on-demand four-wheel drive mode. A modified version of transfer case  20  shown in  FIG. 2  is now shown in  FIG. 12  as transfer case  20 A which is operable to define various two-wheel and four-wheel drive modes. Basically, shift collar  84  now includes an annular drive ring  254  that is splined to a drive hub  256  fixed (i.e., splined) to rear output shaft  40  while clutch assembly  46  is arranged to transfer drive torque from rear output shaft  40  to front output shaft  30 . Again, power-operated actuation mechanism  48  is operable to coordinate movement of shift collar  84  and face cam  202  to establish various locked and on-demand four-wheel high-range and low-range drive modes as well as two-wheel drive modes. 
     When on-demand transfer case  20 A of  FIG. 12  is used in association with vehicle  10  of  FIG. 1 , mode select mechanism  54  would permit selection of a variety of available modes including, for example, a two-wheel high-range (2WH) drive mode, an on-demand four-wheel high-range (AUTO-4WH) drive mode, a part-time four-wheel high-range (LOCK-4WH) drive mode, a Neutral mode, and a part-time four-wheel low-range (LOCK-4WH) drive mode. Specifically, in the 2WH drive mode, geartrain  140  would be rotated until face cam  202  and rollers  220 A and  220 B on control gear  200  are located in the positions shown in  FIG. 11A . As such, shift collar  84  would be located in its H range position and clutch assembly  46  would be released such that all drive torque is delivered to rear output shaft  40 . In the AUTO-4WH mode, shift collar  84  would be located in its H range position and engagement of clutch assembly  46  would be continuously varied based on the value of the sensor signals to vary the torque distribution ratio between rear output shaft  40  and front output shaft  30  in a range between 100:0 and 50:50. This mode is established by controlling rotation of geartrain  140  for moving rollers  220 A and  220 B on control gear  200  relative to face cam  202  between the positions shown in  FIGS. 11B and 11C . In the LOCK-4WH position, actuation mechanism  48  rotates geartrain  140  to the position shown in  FIG. 11C , whereby shift collar  84  would still be located in its H range position and clutch assembly  46  would be fully engaged to effectively couple front output shaft  30  to rear output shaft  40 . Selection of the Neutral mode causes actuator mechanism  48  to rotate geartrain  140  for locating face cam  200  and rollers  220 A and  220 B on control gear  200  in the positions shown in  FIG. 11D . Since shift collar  84  is located in its N range position, no drive torque is transferred to rear driveshaft  40 . When the LOCK-4WL mode is selected, ECU  56  controls actuation mechanism  48  to rotate geartrain  140  to the position shown in  FIG. 11G , whereby shift collar  84  is located in its L range position while fully engaging clutch assembly  46  is fully engaged. 
     The arrangement described for power-operated actuation mechanism  48  is an improvement over the prior art in that the torque amplification provided by reduction gearset  140  combined with the force amplification provided by mode actuator assembly  146  and disk levers  128  permit use of a small low-power electric motor and yet provides extremely quick response and precise control over the position of face cam  202 . In addition, since the axially-directed clutch engagement force is inversely proportional to the radial position of the rollers, the design engineer can use the radius as a variable for selectively increasing or decreasing the mechanical advantages. A face cam configured to move the rollers radially inward would function to increase the mechanical advantage for a given face cam taper profile or lead. Conversely, a face cam configured to move the rollers radially outward would function to decrease the mechanical advantage. If a constant mechanical advantage is desired, the lead of the cam surfaces could be varied to compensate for the change in mechanical advantage resulting from changes in the radial position of the rollers. 
     Transfer cases  20  and  20 A were both shown to include two-speed range unit  42  with power-operated actuation mechanism  48  operable to coordinate actuation of range unit  42  with that of mode clutch assembly  46 . However, the advantages provided by spiral or otherwise non-constant radius cam surfaces on face cam  202  in cooperation with radially-moveable rollers  220  are not limited to such applications. Specifically, power-operated actuation mechanism  48  can be modified to only control adaptive engagement of a friction clutch for use in various power transmission devices. To illustrate this feature,  FIG. 13  shows a single-speed transfer case  20 B which is a revised version of transfer case  20 A in that range unit  42  and range actuator assembly  144  have been eliminated with input shaft  60  coupled (i.e., splined) to rear output shaft  40 . Due to the similarity or many components, common reference numerals are used to identify components previously disclosed. 
     Transfer case  20 B is operable to establish a two-wheel drive mode (2WD), a part-time four-wheel drive mode (4WD) and an automatic or on-demand four-wheel drive mode (AWD). Specifically, the 2WD mode is established when face cam  202 ′ is axially located in its retracted position such that pressure plate  126  is located in its released position, thereby releasing engagement of mode clutch assembly  46 . The 4WD mode is established when face cam  202 ′ is located in its extended position for locating pressure plate  126  in its locked position, thereby fully engaging mode clutch assembly  46 . The AWD mode is established by controlling axial movement of face cam  202 ′ between its adapt and extended positions for moving pressure plate  126  between its ready and locked positions thereby adaptively controlling the transfer of torque from rear output shaft  40  to front output shaft  30 . 
     Face cam  202 ′ is shown in  FIG. 14  to be generally similar to face cam  202  of  FIG. 8  except that a first channel  232 ′ and a second channel  234 ′ define corresponding first and second cam surfaces  236 ′ and  238 ′ that are each configured to provide uni-directional clutch control feature. In particular, lug  225 ′ is shown retained between a pair of stops  226 ′ provided for prohibiting rotation of face cam  202 ′ while permitting its axial movement. In accordance with one embodiment, the contour of cam surfaces  236 ′ and  238 ′ are configured to move rollers  220 A and  220 B on control gear  200  radially inwardly to cause axial movement of face cam  202 ′ from its retracted position toward its extended position. As an alternative, cam surface  236 ′ and  238 ′ can be configured to move rollers  220 A and  220 B on control gear  200  radially outward to cause axial movement of face cam  202 ′ from its retracted position toward its extended position. With this arrangement almost 360° of angular travel of rollers  220 A and  220 B within channels  232 ′ and  234 ′ is provided to accommodate precise actuation of mode clutch assembly  46 . 
     The above-referenced embodiments clearly set forth the novel and unobvious features, structure and/or function of the present invention. However, one skilled in the art will appreciate that equivalent elements and/or arrangements made be used which will be covered by the scope of the following claims.