Patent Publication Number: US-2010107811-A1

Title: Range and Mode Shift System for Two-Speed On-Demand Transfer Case

Description:
FIELD 
     The present disclosure 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 disclosure is directed to 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. 
     BACKGROUND 
     Due to the popularity of four-wheel drive vehicles, a number of power transfer systems are currently being used in vehicular drivetrain 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 drivetrain 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 having a dog-type mode clutch 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 two-speed range shift mechanism having a dog-type range clutch which can be selectively actuated by the vehicle operator for shifting between four-wheel high-range and low-range drive modes. 
     It is also known to use adaptive 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 such a torque “on-demand” feature into a transfer case by replacing the mechanically-actuated mode clutch with a multi-plate clutch assembly and a power-operated clutch actuator that is interactively associated with an electronic control system. During normal road conditions, the clutch assembly is typically maintained in a released condition such that drive torque is only delivered to the rear wheels. However, when sensors detect a low traction condition, the control system actuates the clutch actuator for engaging the clutch assembly to deliver drive torque 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 sensors. Such on-demand clutch control systems can also be used in full-time transfer cases to adaptively bias the torque distribution ratio across an interaxle differential. 
     In some two-speed transfer cases, actuation of 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 clutch assembly actuated by an electromagnetic ball ramp 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 cam having a pair of 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. Examples of other transfer cases equipped with a single power-operated actuator for controlling coordinated engagement of the range shift mechanism and the mode clutch assembly are disclosed in U.S. Pat. Nos. 6,645,109; 6,783,475; 6,802,794; 6,905,436; 6,929,577 and 7,033,300. 
     While conventional transfer cases equipped with coordinated clutch actuation systems have been commercially successful, a need still exists to develop alternative clutch actuation systems which further reduce the cost and complexity of two-speed actively-controlled transfer cases. 
     SUMMARY 
     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 is disclosed. In addition, the transfer case is interactively associated with a control system for controlling operation of the power-operated actuation mechanism to establish a plurality of distinct two-wheel and four-wheel drive modes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further objects, features and advantages of the present disclosure will become apparent from analysis of the following written specification including the appended claims, and the accompanying drawings in which: 
         FIG. 1  is a diagrammatical illustration of a four-wheel drive vehicle equipped with a transfer case and clutch control system according to the present disclosure; 
         FIGS. 2 and 3  are sectional views of a transfer case constructed according to the present disclosure to include a two-speed range unit, an on-demand mode clutch assembly and a power-operated actuation mechanism; 
         FIG. 4  is an enlarged partial view of  FIG. 3  showing various components of the two-speed range unit and the mode clutch assembly; 
         FIG. 5  is an enlarged partial view of a complete power-operated actuation mechanism in greater detail; 
         FIG. 6  is a graph depicting a contour of a mode cam of the present disclosure; 
         FIG. 7  is a sectional side view of another transfer case; 
         FIGS. 8 through 13  are sectional views showing the mode cam rotated to various positions for establishing different drive modes; 
         FIG. 14  is a sectional side view of another transfer case; and 
         FIG. 15  is a plan view of an alternate actuation mechanism. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1  of 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 tractive 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 the powertrain to front driveline  12  and rear driveline  14 . Front driveline  12  includes a pair of front wheels  22  connected via a front axle assembly  24  and a front propshaft  26  to a front output shaft  30  of transfer case  20 . Similarly, rear driveline  14  includes a pair of rear wheels  32  connected via a rear axle assembly  34  and a rear propshaft  36  to a rear output shaft  38  of transfer case  20 . 
     As will be further detailed, transfer case  20  is equipped with a two-speed range unit  40 , a mode clutch assembly  42  and a power-operated actuation mechanism  44  that is operable to control coordinated shifting of range unit  40  and adaptive engagement of mode clutch assembly  42 . In addition, a control system  46  is provided for controlling actuation of actuation mechanism  44 . Control system  46  includes vehicle sensors  48  for detecting real time operational characteristics of motor vehicle  10 , a mode select mechanism  50  for permitting the vehicle operator to select one of the available drive modes and an electronic control unit (ECU)  52  that is operable to generate electric control signals in response to input signals from sensors  48  and mode signals from mode select mechanism  50 . 
       FIGS. 2-6  depict transfer case  20  including an input shaft  54  that is adapted for driven connection to the output shaft of transmission  18 . Input shaft  54  is supported in a housing  56  by a bearing assembly  58  for rotation about a first rotary axis. Rear output shaft  38  is supported between input shaft  54  and housing  56  for rotation about the first rotary axis via a pair of laterally-spaced bearing assemblies  60  and  62 . In addition, front output shaft  30  is supported in housing  56  for rotation about a second rotary axis via a pair of bearing assemblies  64  and  66 . 
     Range unit  40  is shown to generally include a planetary gearset  68  and a dog clutch  70 . Planetary gearset  68  has a sun gear  72  driven by input shaft  54 , a ring gear  74  non-rotatably fixed to housing  56  and a plurality of planet gears  76  rotatably supported from a planet carrier  78 . As seen, planet gears  76  are meshed with both sun gear  72  and ring gear  74 . Planetary gearset  68  functions to drive planet carrier  78  at a reduced speed relative to input shaft  54 . Dog clutch  70  includes a shift collar  80  coupled via a spline connection for rotation with and axial sliding movement on rear output shaft  38 . Shift collar  80  has external clutch teeth  82  adapted to selectively engage either internal clutch teeth  84  formed on input shaft  54  or internal clutch teeth  86  formed on a carrier ring associated with planet carrier  78 . Shift collar  80  is shown located in a high (H) range position such that its clutch teeth  82  are engaged with clutch teeth  84  on input shaft  54 . As such, a direct speed ratio or “high-range” drive connection is established between input shaft  54  and rear output shaft  38 . Shift collar  80  is axially moveable on rear output shaft  38  from its H range position through a central neutral (N) position into a low (L) range position. Location of shift collar  80  in its N position functions to disengage its clutch teeth  82  from both input shaft clutch teeth  84  and carrier clutch teeth  86 , thereby uncoupling rear output shaft  38  from driven connection with input shaft  54 . In contrast, movement of shift collar  80  into its L range position causes its clutch teeth  82  to engage clutch teeth  86  on planet carrier  78 , thereby establishing a reduced speed ratio or “low-range” drive connection between input shaft  54  and rear output shaft  38 . 
     It will be appreciated that planetary gearset  68  and non-synchronized dog clutch  70  function to provide transfer case  20  with a two-speed (i.e., high-range and low-range) feature. However, the non-synchronized range shift unit disclosed could be easily replaced with a synchronized range shift system to permit “on-the-move” range shifting between the high-range and low-range drive modes without the need to stop the motor vehicle. Furthermore, any two-speed reduction unit having a shift member axially moveable to establish first and second drive connections between input shaft  54  and rear output shaft  38  is considered to be within the scope of this invention. 
     Referring primarily to  FIG. 4 , mode clutch assembly  42  is shown to include a clutch hub  90  fixed via a spline connection  92  for rotation with rear output shaft  38 , a clutch drum  94  and a multi-plate clutch pack  96  operably disposed between hub  90  and drum  94 . As seen, clutch pack  96  includes a set of inner clutch plates splined to a cylindrical rim segment  98  of clutch hub  90  and which are alternately interleaved with a set of outer clutch plates splined to a cylindrical rim segment  100  of drum  94 . Clutch pack  96  is retained for limited sliding movement between a reaction plate segment  102  of clutch hub  90  and a pressure plate  104 . Pressure plate  104  has a face surface  106  adapted to engage and apply a compressive clutch engagement force on clutch pack  96 . Pressure plate  104  is splined to rim segment  98  for common rotation with clutch hub  90  and is further supported for sliding movement on a tubular sleeve segment  108  of clutch hub  90 . A return spring  110  is provided between hub  90  and pressure plate  104  for normally biasing pressure plate  104  away from engagement with clutch pack  96 . 
     Upon engagement of mode clutch assembly  42 , drive torque is transmitted from rear output shaft  38  through clutch pack  96  and a transfer assembly  112  to front output shaft  30 . Transfer assembly  112  includes a first sprocket  114  rotatably supported by bearing assemblies  116  on rear output shaft  38 , a second sprocket  118  fixed via a spline connection  120  to front output shaft  30  and a power chain  122  encircling sprockets  114  and  118 . Clutch drum  94  is fixed for rotation with first sprocket  114  such that drive torque transferred through clutch pack  96  is transmitted through transfer assembly  112  to front output shaft  30 . 
     Pressure plate  104  is axially moveable relative to clutch pack  96  between a first or “released” position and a second or “locked” position. With pressure plate  104  in its released position, a minimum clutch engagement force is exerted on clutch pack  96  such that virtually no drive torque is transferred through mode clutch assembly  42  so as to establish a two-wheel drive mode. Return spring  110  is arranged to normally urge pressure plate  104  toward its released position. In contrast, location of pressure plate  104  in its locked position causes a maximum clutch engagement force to be applied to clutch pack  96  such that front output shaft  30  is, in effect, coupled for common rotation with rear output shaft  38  so as to establish a locked or “part-time” four-wheel drive mode. Therefore, accurate control of the position of pressure plate  104  between its released and locked positions permits adaptive regulation of the torque transfer between rear output shaft  38  and front output shaft  30 , thereby permitting establishment of an adaptive or “on-demand” four-wheel drive mode. 
     Power-operated actuation mechanism  44  is operable to coordinate movement of shift collar  80  between its three distinct range positions with movement of pressure plate  104  between its released and locked positions. In its most basic form, actuation mechanism  44  includes an electric motor  126 , a cam plate  128  driven by electric motor  126 , a range actuator assembly  130  and a mode actuator assembly  132 . A reduction geartrain  134  provides a drive connection between an output spindle of electric motor  126  and a driven shaft  136 . Reduction geartrain  134  may include a planetary gearset positioned within a common housing of electric motor  126 . A worm  138  is fixed to driven shaft  136  and positioned in driving engagement with a worm gear  140  fixed to a transfer shaft  142 . Cam plate  128  is also fixes for rotation with transfer shaft  142 . It should be appreciated that worm gear  140  may alternatively be formed on an outer diameter of cam plate  128 . As such, the need for a separate worm gear  140  would be alleviated. Actuation of electric motor  126  causes worm  138  to rotate worm gear  140  and cam plate  128  about an axis extending perpendicular to an axis of rotation of rear output shaft  38 . The cumulative reduction ratio provided by geartrain  134  and the worm gear set permits the use of a smaller, low power electric motor. An angular position sensor or encoder  150  is mounted to cam plate  128  for providing ECU  52  with an input signal indicative of the angular position of cam plate  128 . Depending on the speed and torque requirements of actuation mechanism  44 , reduction geartrain  134  may not be required. In this instance, only worm gear  140  and worm  138  provide torque multiplication from electric motor  126 . 
     Range actuator assembly  130  is operable to convert bi-directional rotary motion of cam plate  128  into bi-directional translational movement of shift collar  80  between its three distinct range positions. Referring primarily to  FIG. 2 , range actuator assembly  130  is shown to generally include a range shuttle  154 , a range fork  156  and a spring-biasing unit  158 . Range shuttle  154  is a tubular member having an inner diameter surface  160  journalled for sliding movement on a range shaft  161 . An elongated shift slot  162  is formed on one face of cam plate  128  and receives a follower pin  164  that is fixed to range shuttle  154 . Shift slot  162  includes a high-range dwell segment  166 , a neutral segment  167 , a low-range dwell segment  168 , a first shift segment  170  interconnecting high-range dwell segment  166  and neutral segment  167 , and a second shift segment  169  interconnecting low-range dwell segment  168  and neutral segment  167 . Range fork  156  includes a sleeve segment  172  supported for sliding movement on range shaft  161  and a fork segment  174  which extends from sleeve segment  172  into an annular groove  176  formed in shift collar  80 . Sleeve segment  172  defines an interior chamber  178  within which spring-biasing unit  158  is located. Spring-biasing unit  158  is operably disposed between range shuttle  154  and sleeve segment  172  of range fork  156 . Spring-biasing unit  158  functions to urge range fork  156  to move axially in response to axial movement of range shuttle  154  while its spring compliance accommodates tooth “block” conditions that can occur between shift collar clutch teeth  82  and input shaft clutch teeth  84  or carrier clutch teeth  86 . As such, spring-biasing unit  158  assures that range fork  156  will complete axial movement of shift collar  80  into its H and L range positions upon elimination of any such tooth block condition. 
     Range actuator assembly  130  is arranged such that axial movement of range shuttle  154  results from movement of follower pin  164  within shift segment  170  of shift slot  162  in response to rotation of cam plate  128 . As noted, such movement of range shuttle  154  causes range fork  156  to move shift collar  80  between its three distinct range positions H, N and L. Specifically, when it is desired to shift range unit  40  into its high-range drive mode, electric motor  126  rotates driven shaft  136  in a first direction which, in turn, causes concurrent rotation of cam plate  128  due to the worm  138  and worm gear  140  interface. Such rotation causes follower pin  164  to move within shift segment  170  of shift slot  162  for axially moving range shuttle  154  and range fork  156  until shift collar  80  is located in its H range position. With shift collar  80  in its H range position, the high-range drive connection is established between input shaft  54  and rear output shaft  38 . Continued rotation of cam plate  128  in the first direction causes follower pin  164  to exit shift segment  170  of shift slot  162  and enter high-range dwell segment  166  for preventing further axial movement of range shuttle  154 , thereby maintaining shift collar  80  in its H range position. The length of high-range dwell segment  166  of shift slot  162  is selected to permit sufficient additional rotation of cam plate  128  in the first rotary direction to accommodate actuation of mode clutch assembly  42  by mode actuator assembly  132 . 
     With shift collar  80  in its H range position, subsequent rotation of cam plate  128  in the opposite or second direction causes follower pin  164  to exit high-range dwell segment  166  and re-enter shift segment  170  of shift slot  162  for causing range shuttle  154  to begin moving shift collar  80  from its H range position toward its N range position. Upon continued rotation of cam plate  128  in the second direction, follower pin  164  exits shift segment  170  of shift slot  162  and enters neutral segment  167 . Follower pin  164  subsequently enters second shift segment  169  to locate shift collar  80  in its L range position, whereby the low-range drive connection between planet carrier  78  and rear output shaft  38  is established. Continued cam plate  128  rotation causes follower pin  164  to enter low-range dwell segment  168  to maintain shift collar  80  in the L range position. The length of low-range dwell segment  168  of shift slot  162  is selected to permit additional rotation of cam plate  128  in the second rotary direction to accommodate actuation of mode clutch assembly  42 . 
     Mode actuator assembly  132  is operable to convert bi-directional rotary motion of cam plate  128  into bi-directional translational movement of pressure plate  104  between its released and locked positions so as to permit adaptive regulation of the drive torque transferred through mode clutch assembly  42  to front output shaft  30 . In general, mode actuator assembly  132  includes a ballramp unit  182  acting in cooperation with a mode cam portion  184  of cam plate  128 . Mode cam portion  184  is formed on the opposite of cam plate  128  as shift slot  162 . Ballramp unit  182  is supported on rear output shaft  38  between a collar  186  and pressure plate  104 . A lock ring  187  axially locates collar  186  in rear output shaft  38 . Ballramp unit  182  includes a first cam member  188 , a second cam member  190  and balls  192  disposed in aligned sets of tapered grooves  194  and  196  formed in corresponding face surfaces of cam members  188  and  190 . In particular, grooves  194  are formed in a first face surface  198  on a cam ring segment  200  of first cam member  188 . As seen, a thrust bearing assembly  202  is disposed between collar  186  and a second face surface  204  of cam ring segment  200 . First cam member  188  further includes a tubular sleeve segment  206  and an elongated lever segment  208 . Sleeve segment  206  is supported on rear output shaft  38  via a bearing assembly  210 . Lever segment  208  has a roller  212  mounted at its terminal end. Roller  212  engages mode cam portion  184  along a contoured cam surface  214  of cam plate  128  and is able to rotate relative to lever segment  208  and mode cam portion  184 . 
     Second cam member  190  of ballramp unit  182  has its grooves  196  formed in a first face surface  220  of a cam ring segment  222  that is shown to generally surround portions of sleeve segment  206  of first cam member  188 . A thrust bearing assembly  224  and thrust ring  226  are disposed between a second face surface  228  of cam ring segment  222  and a face surface  230  of pressure plate  104 . Second cam member  190  further includes an elongated lever segment  232  having its terminal end restricted from rotation. 
     As will be detailed, the contour of cam surface  214  on mode cam portion  184  functions to control angular movement of first cam member  188  relative to second cam member  190  in response to rotation of cam plate  128 . Such relative angular movement between cam members  188  and  190  causes balls  192  to travel along grooves  194  and  196  which, in turn, causes axial movement of second cam member  190 . Such axial movement of second cam member  190  functions to cause corresponding axial movement of pressure plate  104  between its released and locked positions, thereby controlling the magnitude of the clutch engagement force applied to clutch pack  96 . 
     Due to engagement of roller  212  with cam surface  214  on mode cam portion  184 , first cam member  188  is angularly moveable relative to second cam member  190  between a first or “retracted” position and a second or “extended” position in response to rotation of cam plate  128 . With first cam member  188  in its retracted position, return spring  110  biases pressure plate  104  to its released position which, in turn, urges balls  192  to be located in deep end portions of aligned grooves  194  and  196 . Such movement of first cam member  188  to its angularly retracted position relative to second cam member  190  also functions to locate second cam member  190  in an axially retracted position relative to clutch pack  96 . While not shown, a biasing unit can be provided between lever segments  208  and  232  to assist return spring  110  in normally urging first cam member  188  toward its retracted position. In contrast, angular movement of first cam member  188  to its extended position causes balls  192  to be located in shallow end portions of aligned grooves  194  and  196  which causes movement of second cam member  190  to an axially extended position relative to clutch pack  96 . Such axial movement of second cam member  190  causes pressure plate  104  to be moved to its locked position in opposition to the biasing exerted thereon by return spring  110 . Accordingly, control of angular movement of first cam member  188  between its retracted and extended positions functions to cause concurrent movement of pressure plate  104  between its released and locked positions. 
     As previously noted, cam plate  128  includes cam surface  214  on one side and shift slot  162  on the opposite side. Cam plate  128  is configured to coordinate movement of shift collar  80  and pressure plate  104  in response to energization of electric motor  126  and resultant rotation of cam plate  128  for establishing a plurality of different drive modes. According to one possible control arrangement, mode selector  50  could permit the vehicle operator to select from a number of different two-wheel and four-wheel drive modes including, for example, a two-wheel high-range drive mode, an on-demand four-wheel high-range drive mode, a part-time four-wheel high-range drive mode, a neutral mode and a part-time four-wheel low-range drive mode. Specifically, control system  46  functions to control the rotated position of cam plate  128  in response to the mode signal delivered to ECU  52  by mode selector  50  and the sensor input signals sent by sensors  48  to ECU  52 . 
       FIG. 6  illustrates the contour of cam surface  214  as a line graph. The cam surface includes various sectors corresponding to LOCK-H, ADAPT-H, 2H, NEUTRAL, ADAPT-L AND LOCK-L positions. Cam plate  128  may be rotated to any number of these positions including the “2H” position required to establish the two-wheel high-range drive mode. As understood, the two-wheel high-range drive mode is established when shift collar  80  is located in its H range position and pressure plate  104  is located in its released position relative to clutch pack  96 . As such, input shaft  54  drives rear output shaft  38  at a direct speed ratio while mode clutch assembly  42  is released such that all drive torque is delivered to rear driveline  14 . Roller  212  is shown engaging a detent portion of a first cam segment  214 A of cam surface  214  on mode cam portion  184  which functions to locate second cam member  190  in its retracted position when cam plate  128  is in the 2H position. 
     If the on-demand four-wheel high-range drive mode is thereafter selected, electric motor  126  is energized to initially rotate cam plate  128  in a first direction from its 2H position to the “ADAPT-H” position. In this rotated position of cam plate  128 , follower pin  164  is located within high-range dwell segment  166  of shift slot  162  in cam plate  128  such that shift collar  80  is maintained in its H range position for maintaining the direct drive connection between input shaft  54  and rear output shaft  38 . However, such rotation of cam plate  128  to its ADAPT-H position causes concurrent rotation of mode cam portion  184  to the position shown which, in turn, causes roller  212  to engage a first portion of a second cam segment  214 B of mode cam surface  214 . Such movement of roller  212  from first cam segment  214 A to second cam segment  214 B causes first cam member  188  to move angularly relative to second cam member  190  and move second cam member  190  from its retracted position to an intermediate or “ready” position. With second cam member  190  rotated to its ready position, ballramp unit  182  causes pressure plate  104  to move axially from its released position into an “adapt” position that is operable to apply a predetermined “preload” clutch engagement force on clutch pack  96 . The adapt position of pressure plate  104  provides a low level of torque transfer across mode clutch assembly  42  required to take-up clearances in clutch pack  96  in preparation for adaptive control. Thereafter, ECU  52  determines when and how much drive torque needs to be transmitted across mode clutch assembly  42  to limit driveline slip and improve traction based on the current tractive conditions and operating characteristics detected by sensors  48 . As an alternative, the adapt position for pressure plate  104  can be selected to partially engage mode clutch assembly  42  for establishing a desired front/rear torque distribution ratio (i.e., 10/90, 25/75, 40/60, etc.) between front output shaft  30  and rear output shaft  38 . 
     The limits of adaptive control in the on-demand four-wheel high-range drive mode are established by controlling bi-directional rotation of cam plate  128  between its ADAPT-H and its “LOCK-H” position shown in  FIG. 6 . With cam plate  128  in its LOCK-H position, second segment  214 B of mode cam surface  214  causes second cam member  190  to move to its extended position, thereby causing pressure plate  104  to move to its locked position for fully engaging mode clutch assembly  42 . This range of angular travel of cam plate  306  causes follower pin  164  to travel within high-range dwell segment  166  of shift slot  162  so as to maintain shift collar  80  in its H range position. However, such rotation of cam plate  128  results in roller  212  riding along second segment  214 B of cam surface  214  which, in turn, controls movement of second cam member  190  between its ready position and its extended position. Bi-directional rotation of cam plate  128  within this range of travel is controlled by ECU  52  actuating electric motor  126  based on a pre-selected torque control strategy. As will be understood, any control strategy known in the art for adaptively controlling torque transfer across mode clutch assembly  42  can be utilized with the present invention. 
     If the vehicle operator selects the part-time four-wheel high-range drive mode, electric motor  126  is energized to rotate cam plate  128  in the first direction to its LOCK-H position. As such, shift collar  80  is maintained in its H range position and mode cam portion  184  causes second cam member  190  to move to its extended position which, in turn, moves pressure plate  104  to its locked position for fully engaging mode clutch assembly  42 . To limit the on-time service requirements of electric motor  126 , a power-off brake  245  associated with electric motor  126  can be engaged to brake rotation of the motor output so as to prevent back-driving of cam plate  128  for holding pressure plate  104  in its locked position. In this manner, electric motor  126  can be shut-off after the part-time four-wheel high-range drive mode has been established. 
     If the Neutral mode is selected, electric motor  126  is energized to rotate cam plate  128  in a second direction to the neutral position. Such rotation of cam plate  128  causes follower pin  164  to exit high-range dwell segment  166  and ride within shift segment  170  of shift slot  162  until shift collar  80  is located in its N position. Concurrently, rotation of mode cam portion  184  causes roller  212  to engage a portion of first segment  214 A of cam surface  214  that is configured to move second cam member  190  to a position displaced from its retracted position. Such movement of second cam member  190  results in limited axial movement of pressure plate  104  from its released position toward clutch pack  96 . Preferably, such movement of pressure plate  104  does not result in any drive torque being transferred through mode clutch assembly  42  to front driveline  12 . Continued rotation of cam plate  128  in the second direction occurs when the part-time four-wheel low-range drive mode is selected. At an intermediate “ADAPT-L” position of cam plate  128 , follower pin  164  enters low-range dwell segment  168  of shift slot  162  for locating shift collar  80  in its L range position. Mode cam portion  184  has likewise been rotated for locating roller  212  at the interface between first segment  214 A of cam surface  214  and a third segment  214 C thereof. The contour of third segment  214 C is configured such that first cam member  188  is rotated to move second cam member  190  to its ready position. As previously noted, movement of second cam member  190  to its ready position causes pressure plate  104  to move axially to its adapt position. However, selection of the part-time four-wheel low-range drive mode causes continued rotation of cam plate  128  to its LOCK-L position. Low-range dwell segment  168  in shift slot  162  maintains shift collar  80  in its L range position while third segment  214 C of mode cam surface  214  causes roller  212  to move second cam member  190  to its extended position, thereby moving pressure plate  104  to its locked position for fully engaging mode clutch assembly  42 . Again, power-off brake  245  can be actuated to maintain cam plate  128  in its LOCK-L position. 
     Based on the preferred arrangement disclosed for actuation mechanism  44 , cam plate  128  is rotatable through a first range of angular travel to accommodate range shifting of shift collar  80  as well as second and third ranges of angular travel to accommodate engagement of mode clutch assembly  42 . In particular, the first range of angular travel for cam plate  128  is established between its ADAPT-H and ADAPT-L positions. The second range of travel for cam plate  128  is defined between its ADAPT-H and LOCK-H positions to permit adaptive control of mode clutch assembly  42  with shift collar  80  in the H range position. Likewise, the third range of cam plate travel is defined between its ADAPT-L and LOCK-L positions to permit actuation of mode clutch assembly  42  while shift collar  80  is in its L range position. 
       FIG. 7  illustrates another transfer case  300  equipped with a two-speed range unit, a mode clutch assembly and power-operated actuation mechanism operable to control coordinated shifting of the range unit and adaptive engagement of the mode clutch assembly. Transfer case  300  is substantially similar to transfer case  20  except that a different power-operated actuation mechanism  302  is implemented. Accordingly, like elements will retain their previously introduced reference numerals. Power-operated actuation mechanism  302  includes an electric motor  304 , a cam plate  306  rotatably driven by electric motor  304 , range actuator assembly  130  and mode actuator assembly  132 . An output spindle of electric motor  304  is drivingly coupled to reduction geartrain  134 . The output of geartrain  134  drives a shaft  308 . Driven shaft  308  is affixed to cam plate  306  such that cam plate  306  rotates about the same axis of rotation as driven shaft  308 . An elongated shift slot  310  is formed on one face of cam plate  306  and receives follower pin  164  that is fixed to range shuttle  154 . Shift slot  162  is shaped as previously described in reference to transfer case  20 . However, it should be appreciated that within transfer case  300 , follower pin  164  extends along an axis substantially parallel to the axis about which motor  304  rotates while follower pin  164  of transfer case  20  extends along an axis perpendicular to the rotation of motor  304 . 
     Actuation mechanism  302  is also operable to control mode actuator assembly  132 . A mode cam  312  is coupled to or integrally formed with cam plate  306 . A mode follower  314  is rotatably fixed to the terminal end of first cam member  188 . Mode follower  314  rollingly engages a cam surface  316  formed on an outer peripheral edge of mode cam  312 . As will be detailed, the contour of cam surface  316  on mode cam  312  functions to control angular movement of first cam member  188  relative to second cam member  190  in response to rotation of cam plate  306 . 
       FIG. 8  illustrates cam plate  306  rotated to a “2H” position required to establish the two-wheel high-range drive mode. As understood, the two-wheel high-range drive mode is established when shift collar  80  is located in its H range position and pressure plate  104  is located in its released position relative to clutch pack  96 . As such, input shaft  54  drives rear output shaft  38  at a direct speed ratio while mode clutch assembly  42  is released such that all drive torque is delivered to rear driveline  14 . Mode follower  314  is shown engaging a detent portion of a first cam segment  316 A of cam surface  316  on mode cam  312  which functions to locate second cam member  190  in its retracted position. 
     If the on-demand four-wheel high-range drive mode is thereafter selected, electric motor  304  is energized to initially rotate cam plate  306  in a first direction from its 2H position to the “ADAPT-H” position shown in  FIG. 9 . In this rotated position of cam plate  306 , follower pin  164  is located within high-range dwell segment  166  of shift slot  162  in cam plate  306  such that shift collar  80  is maintained in its H range position for maintaining the direct drive connection between input shaft  54  and rear output shaft  38 . However, such rotation of cam plate  306  to its ADAPT-H position causes concurrent rotation of mode cam  312  to the position shown which, in turn, causes mode follower  314  to engage a first end portion of a second cam segment  316 B of mode cam surface  316 . Such movement of mode follower  314  from first cam segment  316 A to second cam segment  316 B causes first cam member  188  to move angularly relative to second cam member  190  and move second cam member  190  from its retracted position to an intermediate or “ready” position. With second cam member  190  rotated to its ready position, ballramp unit  182  causes pressure plate  104  to move axially from its released position into an “adapt” position that is operable to apply a predetermined “preload” clutch engagement force on clutch pack  96 . The adapt position of pressure plate  104  provides a low level of torque transfer across mode clutch assembly  42  required to take-up clearances in clutch pack  96  in preparation for adaptive control. Thereafter, ECU  52  determines when and how much drive torque needs to be transmitted across mode clutch assembly  42  to limit driveline slip and improve traction based on the current tractive conditions and operating characteristics detected by sensors  48 . As an alternative, the adapt position for pressure plate  104  can be selected to partially engage mode clutch assembly  42  for establishing a desired front/rear torque distribution ratio (i.e., 10/90, 25/75, 40/60, etc.) between front output shaft  30  and rear output shaft  38 . 
     The limits of adaptive control in the on-demand four-wheel high-range drive mode are established by controlling bi-directional rotation of cam plate  306  between its ADAPT-H position of  FIG. 9  and its “LOCK-H” position shown in  FIG. 10 . With cam plate  306  in its LOCK-H position, second segment  316 B of mode cam surface  316  causes second cam member  190  to move to its extended position, thereby causing pressure plate  104  to move to its locked position for fully engaging mode clutch assembly  42 . This range of angular travel of cam plate  306  causes follower pin  164  to travel within high-range dwell segment  166  of shift slot  162  so as to maintain shift collar  80  in its H range position. However, such rotation of cam plate  306  results in mode follower  314  riding along second segment  316 B of cam surface  316  which, in turn, controls movement of second cam member  190  between its ready position and its extended position. Bi-directional rotation of cam plate  306  within this range of travel is controlled by ECU  52  actuating electric motor  304  based on a pre-selected torque control strategy. As will be understood, any control strategy known in the art for adaptively controlling torque transfer across mode clutch assembly  42  can be utilized with the present invention. 
     If the vehicle operator selects the part-time four-wheel high-range drive mode, electric motor  304  is energized to rotate cam plate  306  in the first direction to its LOCK-H position shown in  FIG. 10 . As such, shift collar  80  is maintained in its H range position and mode cam  312  causes second cam member  190  to move to its extended position which, in turn, moves pressure plate  104  to its locked position for fully engaging mode clutch assembly  42 . To limit the on-time service requirements of electric motor  304 , a power-off brake  318  associated with electric motor  304  can be engaged to brake rotation of the motor output so as to prevent back-driving of cam plate  306  for holding pressure plate  104  in its locked position. In this manner, electric motor  304  can be shut-off after the part-time four-wheel high-range drive mode has been established. 
     If the Neutral mode is selected, electric motor  304  is energized to rotate cam plate  306  in a second direction to the Neutral position shown in  FIG. 11 . Such rotation of cam plate  306  causes follower pin  164  to exit high-range dwell segment  166  and ride within shift segment  170  of shift slot  162  until shift collar  80  is located in its N position. Concurrently, rotation of mode cam  312  causes mode follower  314  to engage a portion of first segment  316 A of cam surface  316  that is configured to move second cam member  190  to a position displaced from its retracted position. Such movement of second cam member  190  results in limited axial movement of pressure plate  104  from its released position toward clutch pack  96 . Preferably, such movement of pressure plate  104  does not result in any drive torque being transferred through mode clutch assembly  42  to front driveline  12 . 
       FIGS. 12 and 13  illustrate continued rotation of cam plate in the second direction which occurs when the part-time four-wheel low-range drive mode is selected. In particular,  FIG. 12  shows an intermediate “ADAPT-L” position of cam plate  306  whereat follower pin  164  enters low-range dwell segment  168  of shift slot  162  for locating shift collar  80  in its L range position. Mode cam  312  has likewise been rotated for locating mode follower  314  at the interface between first segment  316 A of cam surface  316  and a third segment  316 C thereof. The contour of third segment  316 C is configured such that first cam member  188  is rotated to move second cam member  190  to its ready position. As previously noted, movement of second cam member  190  to its ready position causes pressure plate  104  to move axially to its adapt position. However, selection of the part-time four-wheel low-range drive mode causes continued rotation of cam plate  306  to its LOCK-L position shown in  FIG. 13 . Low-range dwell segment  168  in shift slot  162  maintains shift collar  80  in its L range position while third segment  316 C of mode cam surface  316  causes mode follower  314  to move second cam member  190  to its extended position, thereby moving pressure plate  104  to its locked position for fully engaging mode clutch assembly  42 . Again, power-off brake  318  can be actuated to maintain cam plate  306  in its LOCK-L position. 
     Based on the preferred arrangement disclosed for actuation mechanism  302 , cam plate  306  is rotatable through a first range of angular travel to accommodate range shifting of shift collar  80  as well as second and third ranges of angular travel to accommodate engagement of mode clutch assembly  42 . In particular, the first range of angular travel for cam plate  306  is established between its ADAPT-H and ADAPT-L positions. The second range of travel for cam plate  306  is defined between its ADAPT-H and LOCK-H positions to permit adaptive control of mode clutch assembly  42  with shift collar  80  in the H range position. Likewise, the third range of cam plate travel is defined between its ADAPT-L and LOCK-L positions to permit actuation of mode clutch assembly  42  while shift collar  80  is in its L range position. 
       FIG. 14  depicts another transfer case  400 . Transfer case  400  is substantially similar to transfer case  300 . Accordingly, like elements will retain their previously introduced reference numerals. Transfer case  400  includes an electric motor  402  having a driven shaft  404  rotatable about an axis  406 . Axis  406  extends substantially parallel to and offset from an axis of rotation of rear output shaft  38 . Cam plate  306  continues to be rotatable about an axis extending substantially perpendicular to the axis about which rear output shaft  38  rotates as previously described. A worm  408  is fixed to driven shaft  404 . Worm  408  is in meshed driving engagement with a worm gear  410  formed on an outer peripheral surface of cam plate  306 . Accordingly, energization of electric motor  402  causes driven shaft  404  to rotate in one of two directions. Worm  408  rotates in the same direction as driven shaft  404  to cause cam plate  306  to rotate in response to worm gear  410  being driven by worm  408 . As previously described, follower  164  is axially translatable in response to rotation of cam plate  306 . Additionally, mode follower  314  follows the contour of cam surface  316  thereby selectively actuating mode clutch assembly  42  as previously described. The arrangement of electric motor  402 , driven shaft  404  and cam plate  306  allows a designer to best utilize the space available for the transfer case by positioning electric motor  402  near rear output shaft  38  at a more aft location, if desired. 
       FIG. 15  depicts a portion of an alternate power-operated actuation mechanism  500 . Actuation mechanism  500  is substantially similar to actuation mechanism  302 . Accordingly, like elements will retain their previously introduced reference numerals. Actuation mechanism  500  includes a cam plate  502  driven by an electric motor (not shown), a range actuator assembly  504  and a mode actuator assembly  506 . Rotation of cam plate  502  causes follower pin  164  to translate within shift slot  162 . Range shuttle  154  is fixed to follower pin  164  to cause range fork  156  to translate as previously described. 
     A range cam portion  508  of cam plate  502  includes shift slot  162  and has an outer diameter larger than a mode cam  510  portion of cam plate  502 . To achieve a compact overall size of actuation mechanism  500 , a cam member  512  of a mode actuator  514  includes a curved portion  516  to reach around range cam portion  508 . A roller  518  is rotatably coupled to a distal end of cam member  512 . Roller  518  is in driven engagement with mode cam  510 . The relatively compact package is formed through the use of the curved arm of cam member  512 . 
     It should be appreciated that the various drive elements including worm gear drives, planetary gearsets, face cams, edge cams, and ball ramp actuators may be combined with one another to define a transfer case contemplated by the inventor but not particularly described in detail or shown in any one of the particular Figures. 
     Furthermore, the foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations may be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.