WO2011097247A2

WO2011097247A2 - Tow mode transfer case with layshaft gear reduction

Info

Publication number: WO2011097247A2
Application number: PCT/US2011/023382
Authority: WO
Inventors: Carl F. Stephens; David Wenthen
Original assignee: Magna Powertrain Of America, Inc.
Priority date: 2010-02-03
Filing date: 2011-02-01
Publication date: 2011-08-11
Also published as: WO2011097244A2; WO2011097247A3; WO2011097244A3

Abstract

A transfer case comprises a first reduction gearset including a first drive gear fixed for rotation with an input shaft and a first driven gear in meshed engagement with the first drive gear. The first drive gear is fixed for rotation with a countershaft. A second reduction gearset includes a second drive gear fixed for rotation with the countershaft and a second driven gear fixed for rotation with the first output shaft. The second drive gear is in meshed engagement with the second driven gear. A range clutch is operable to establish a drive connection between the input shaft and a first output shaft and further operable to fix the second driven gear for rotation with the first output shaft and drive it a reduced speed. A mode clutch is provided to establish a drive connection between the first output shaft and a second output shaft.

Description

TOW MODE TRANSFER CASE WITH LAYSHAFT GEAR REDUCTION

FIELD

[0001] 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, a transfer case is equipped with a two-speed range unit for providing normal and towing modes of operation.

BACKGROUND

[0002] 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, some 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.

[0003] Four wheel drive vehicles equipped with two-speed transfer cases typically include a rear drive axle configured to transfer torque to the rear wheels and provide a fixed final gear reduction. Depending on the intended use of the vehicle, original equipment vehicle manufacturers may provide the user with optional rear drive axle ratios. For example, the standard rear drive axle may have a reduction ratio of approximately 3.0 to 3.5:1 to provide increased fuel economy during highway speed use. Original equipment manufacturers may also provide an optional rear drive axle ratio as part of a trailer towing package. The rear drive axle associated with the trailer towing package will typically provide a greater final gear reduction ratio ranging from 4 to 4.66. The greater rear drive axle reduction ratio may be necessary to provide increased torque for towing. Unfortunately, the lower rear drive axle ratio also negatively impacts vehicle fuel economy. As such, a vehicle purchaser may be forced to choose between trailer towing capacity and vehicle fuel efficiency at the time of purchase.

[0004] Furthermore, typical two-speed transfer cases have been equipped with low range gear ratios of approximately 2.7:1 . This gear reduction ratio may be useful during certain high torque, low speed situations but the maximum speed of the vehicle is limited when operating the transfer case in the low-range mode. As such, vehicle users may not simply choose to operate the two-speed transfer case continuously in the low-range mode when towing.

[0005] While conventional transfer cases equipped with two-speed range capability have been commercially successful, a need may exist to develop alternative power transfer systems for particular vehicle applications while further reducing the cost and complexity of two-speed actively-controlled transfer cases. For example, it may be desirable to provide a transfer case operable in a first direct drive mode of operation as well as a gear reduced mode of operation for towing. The new transfer case may be operable in combination with a standard rear drive axle having a final drive ratio suitable for high speed or highway operation.

SUMMARY

[0006] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0007] A transfer case comprises a first reduction gearset including a first drive gear fixed for rotation with an input shaft and a first driven gear in meshed engagement with the first drive gear. The first drive gear is fixed for rotation with a countershaft. A second reduction gearset includes a second drive gear fixed for rotation with the countershaft and a second driven gear fixed for rotation with the first output shaft. The second drive gear is in meshed engagement with the second driven gear. A range clutch is operable to establish a drive connection between the input shaft and a first output shaft and further operable to fix the second driven gear for rotation with the first output shaft and drive it a reduced speed. A mode clutch is provided to establish a drive connection between the first output shaft and a second output shaft.

[0008] Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

[0009] 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:

[0010] Figure 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;

[0011] Figure 2 is an end view 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;

[0012] Figure 3 is a sectional view of the transfer case shown in Figure 2;

[0013] Figure 4 is another sectional view of the transfer case shown in Figure 2;

[0014] Figure 5 is a fragmentary perspective view of the transfer case shown in Figure 2;

[0015] Figure 6 is an enlarged partial view of Figure 2 showing various components of the two-speed range unit and the mode clutch assembly;

[0016] Figure 7 is an enlarged partial view of Figure 2 showing various components of the power-operated actuation mechanism in greater detail;

[0017] Figure 8 is a perspective view of the actuator shaft assembly associated with the power-operated actuation mechanism of the present disclosure; [0018] Figure 9 is a side view of the actuator shaft assembly shown in Figure 8; and

[0019] Figures 10-15 are sectional views taken generally along line A- A in Figure 9 showing the mode cam and the actuator shaft rotated to various positions for establishing different drive modes.

DETAILED DESCRIPTION

[0020] Referring now to Figures 1 and 2 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.

[0021] 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 control signals in response to input signals from sensors 48 and mode signals from mode select mechanism 50.

[0022] Transfer case 20 is shown to include an input shaft 54 that is adapted to be coupled for driven connection with the output shaft of transmission 18. Input shaft 54 is supported in a housing 56 by bearing assemblies 58, 59 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 by a pair of bearing assemblies 64 and 66.

[0023] As best shown in Figures 2-5, range unit 40 includes a first reduction gearset 68 and a second reduction gearset 70. First reduction gearset 68 includes a first drive gear 72 fixed for rotation with input shaft 54. In the embodiment depicted in Figure 3, first drive gear 72 is integrally formed with input shaft 54. First reduction gearset 68 also includes a first driven gear 74 positioned in constant meshed engagement with first drive gear 72. First driven gear 74 is integrally formed with and fixed for rotation with a countershaft 76. Countershaft 76 is supported for rotation within housing 56 by bearing assemblies 77, 78. Second reduction gearset 70 includes a second drive gear 80 fixed for rotation with countershaft 76 and a second driven gear 82 supported for rotation within housing 56 by bearing 60. Second drive gear 80 is in constant meshed engagement with second driven gear 82.

[0024] A dog clutch 85 includes a shift collar 86 axially moveably supported on a hub 88. Hub 88 is splined to and therefore fixed for rotation with rear output shaft 38. Shift collar 86 includes a plurality of internal clutch teeth 90 in splined engagement with hub 88. Teeth 90 are selectively engageable with either external clutch teeth 92 formed on first drive gear 72 or external clutch teeth 94 formed on second driven gear 82.

[0025] In Figures 2 and 3, shift collar 86 is shown located in a neutral

(N) range position such that its clutch teeth 90 are disengaged from both of clutch teeth 92 and clutch teeth 94. In this position, torque is not transferred between input shaft 54 and rear output shaft 38. Each of the gears of first reduction gearset 68 and second reduction gearset 70 rotate when input shaft 54 is rotated. With shift collar 86 in the neutral position, second driven gear 82 is free to rotate on rear output shaft 38 via bearings 98. [0026] To operate transfer case 20 in a high (H) range position, shift collar 86 is shifted to the left when viewing Figures 3 and 4. When shift collar 86 is in the H position, clutch teeth 90 of shift collar 86 drivingly engage clutch teeth 92 of first drive gear 72. A direct drive speed ratio or "high-range" drive connection is established between input shaft 54 and rear output shaft 38.

[0027] Shift collar 86 is axially moveable from its H range position through the neutral (N) position to a tow (Tow) range position. When shift collar 86 is in the Tow-range position, clutch teeth 90 of shift collar 86 drivingly engage clutch teeth 94 of second driven gear 82. At this time, clutch teeth 90 of shift collar 86 are disengaged from clutch teeth 92 of first drive gear 72. When input shaft 54 is rotated, torque is transferred through first drive gear 72, first driven gear 74, countershaft 76, second drive gear 80, second driven gear 82, shift collar 86 and hub 88 to rear output shaft 38. Once the Tow-range drive connection has been established, a reduced speed ratio is provided between input shaft 54 and rear output shaft 38. It is contemplated that one exemplary Tow-mode drive ratio is 1 .35:1 and may be accomplished by constructing first drive gear 72 with 59 teeth, first driven gear 74 with 63 teeth, second drive gear 80 with 49 teeth and second driven gear 82 with 62 teeth.

[0028] It should be appreciated that the parallel-shaft dual reduction gear arrangement of range unit 40 is equipped with non-synchronized dog clutch 85 to provide transfer case 20 with a two-speed feature. However, the non- synchronized range shift clutch as disclosed could be modified with a synchronized range shift system to permit "on-the-move" range shifting between the high range and the Tow-range drive modes without the need to stop the motor vehicle. Furthermore, other two-speed reduction units having a shift member axially moveable to establish first and second drive connections between input shaft 54 and rear output shaft 38 are considered to be within the scope of this disclosure.

[0029] Referring primarily to Figure 6, mode clutch assembly 42 is shown to include a clutch hub 120 fixed via a spline connection 122 for rotation with rear output shaft 38, a clutch drum 124 and a multi-plate clutch pack 126 operably disposed between hub 120 and drum 124. As seen, clutch pack 126 includes a set of inner clutch plates splined to a cylindrical rim segment 128 of clutch hub 120 and which are alternately interleaved with a set of outer clutch plates splined to a cylindrical rim segment 130 of drum 124. Clutch pack 126 is retained for limited sliding movement between a reaction plate segment 132 of clutch hub 120 and a pressure plate 134. Pressure plate 134 has a face surface 136 adapted to engage and apply a compressive clutch engagement force on clutch pack 126. Pressure plate 134 is splined to rim segment 128 for common rotation with clutch hub 120 and is further supported for sliding movement on a tubular sleeve segment 138 of clutch hub 120. A return spring 140 is provided between hub 120 and pressure plate 134 for normally biasing pressure plate 134 away from engagement with clutch pack 126.

[0030] Upon engagement of mode clutch assembly 42, drive torque is transmitted from rear output shaft 38 through clutch pack 126 and a transfer assembly 142 to front output shaft 30. Transfer assembly 142 includes a first sprocket 144 rotatably supported by bearing assemblies 146 on rear output shaft 38, a second sprocket 148 fixed via a spline connection 150 to front output shaft 30 and a power chain 152 encircling sprockets 144 and 148. Clutch drum 124 is fixed for rotation with first sprocket 144 such that drive torque transferred through clutch pack 126 is transmitted through transfer assembly 142 to front output shaft 30.

[0031] Pressure plate 134 is axially moveable relative to clutch pack 126 between a first or "released" position and a second or "locked" position. With pressure plate 134 in its released position, a minimum clutch engagement force is exerted on clutch pack 126 such that virtually no drive torque is transferred through mode clutch assembly 42 so as to establish a two-wheel drive mode. Return spring 140 is arranged to normally urge pressure plate 134 toward its released position. In contrast, location of pressure plate 134 in its locked position causes a maximum clutch engagement force to be applied to clutch pack 126 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 134 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.

[0032] Power-operated actuation mechanism 44 is operable to coordinate movement of shift collar 86 between its three distinct range positions with movement of pressure plate 134 between its released and locked positions. In its most basic form, actuation mechanism 44 includes an electric motor 156, an actuator shaft 158 driven by electric motor 156, a range actuator assembly 160 and a mode actuator assembly 162. Actuator shaft 158 has its opposite ends supported by a pair of laterally-spaced bearing assemblies 164 for rotation in housing 56 about a third rotary axis. A reduction geartrain 166 provides a drive connection between a rotary output of electric motor 156 and actuator shaft 158. Reduction geartrain 166 includes a worm gearset (not shown) that is driven by the rotary output of electric motor 156 and a spur gearset 168. Actuation of electric motor 156 causes the worm gearset to drive a drive gear 170 associated with gearset 168. Specifically, drive gear 170 is a small diameter gear supported for rotation on an idler shaft 169 and which is meshed with a large diameter driven gear 172 fixed for rotation with actuator shaft 158. In particular, driven gear 172 includes a tubular hub segment 174 that is fixed via a spline connection 176 to actuator shaft 158 between a radial shaft flange 178 and rear bearing assembly 164. The cumulative reduction ratio provided by geartrain 166 permits the use of a smaller, low power electric motor. An angular position sensor or encoder 180 is mounted to an end portion of actuator shaft 158 for providing ECU 52 with an input signal indicative of the angular position of actuator shaft 158.

[0033] Range actuator assembly 160 is operable to convert bidirectional rotary motion of actuator shaft 158 into bi-directional translational movement of shift collar 86 between its three distinct range positions. Referring primarily to Figures 7-9, range actuator assembly 1 60 is shown to generally include a range cam 184, a range fork 186 and a spring-biasing unit 188. Range cam 184 is a tubular member having an inner diameter surface 190 journalled for sliding movement on actuator shaft 158. An elongated shift slot 192 is formed in range cam 184 and receives a follower pin 194 that is fixed for rotation with actuator shaft 158. Slot 192 includes a high-range dwell segment 196, a tow- range dwell segment 198 and a helical shift segment 200 interconnecting dwell segments 196 and 198. Range fork 186 includes a sleeve segment 202 supported for sliding movement on actuator shaft 158 and a fork segment 204 which extends from sleeve segment 202 into an annular groove 206 formed in shift collar 86. Sleeve segment 202 defines an interior chamber 208 within which range cam 184 and spring-biasing unit 188 are located. Spring-biasing unit 188 is operably disposed between range cam 184 and sleeve segment 202 of range fork 186. Spring-biasing unit 188 functions to urge range fork 186 to move axially in response to axial movement of range cam 184 while its spring compliance accommodates tooth "block" conditions that can occur between shift collar clutch teeth 90 and first drive gear clutch teeth 92 or second driven gear clutch teeth 94. As such, spring-biasing unit 188 assures that range fork 186 will complete axial movement of shift collar 86 into its H and Tow range positions upon elimination of any such tooth block condition.

[0034] Range actuator assembly 160 is arranged such that axial movement of range cam 184 results from movement of follower pin 194 within shift segment 200 of slot 192 in response to rotation of actuator shaft 158. As noted, such movement of range cam 184 causes range fork 186 to move shift collar 86 between its three distinct range positions. Specifically, when it is desired to shift range unit 40 into its high-range drive mode, electric motor 156 rotates actuator shaft 158 in a first direction which, in turn, causes concurrent rotation of follower pin 194. Such rotation causes follower pin 194 to move within shift segment 200 of slot 192 for axially moving range cam 184 and range fork 186 until shift collar 86 is located in its H range position. With shift collar 86 in its H range position, the high-range drive connection is established between input shaft 54 and rear output shaft 38. Continued rotation of actuator shaft 158 in the first direction causes follower pin 194 to exit shift segment 200 of shift slot 192 and enter high-range dwell segment 196 for preventing further axial movement of range cam 184, thereby maintaining shift collar 86 in its H range position. The length of high-range dwell segment 196 of shift slot 192 is selected to permit sufficient additional rotation of actuator shaft 158 in the first rotary direction to accommodate actuation of mode clutch assembly 42 by mode actuator assembly 162.

[0035] With shift collar 86 in its H range position, subsequent rotation of actuator shaft 158 in the opposite or second direction causes follower pin 194 to exit high-range dwell segment 196 and re-enter helical shift segment 200 of shift slot 192 for causing range cam 184 to begin moving shift collar 86 from its H range position toward its Tow range position. Upon continued rotation of actuator shaft 158 in the second direction, follower pin 194 exits shift segment 200 of shift slot 192 and enters tow-range dwell segment 198 for locating and maintaining shift collar 86 in its Tow range position, whereby the Tow-range drive connection between second driven gear 82 and rear output shaft 38 is established. Again, the length of tow-range dwell segment 198 of shift slot 192 is selected to permit additional rotation of actuator shaft 158 in the second rotary direction required to accommodate complete actuation of mode clutch assembly 42.

[0036] Mode actuator assembly 162 is operable to convert bidirectional rotary motion of actuator shaft 158 into bi-directional translational movement of pressure plate 134 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 162 includes a ballramp unit 212 and a mode cam 214. Ballramp unit 212 is supported on rear output shaft 38 between a radial shaft flange 216 and pressure plate 134. Ballramp unit 212 includes a first cam member 218, a second cam member 220 and balls 222 disposed in aligned sets of tapered grooves 224 and 226 formed in corresponding face surfaces of cam members 218 and 220. In particular, grooves 224 are formed in a first face surface 228 on a cam ring segment 230 of first cam member 218. As seen, a thrust bearing assembly 232 is disposed between shaft flange 216 and a second face surface 234 of cam ring segment 230. First cam member 218 further includes a tubular sleeve segment 236 and an elongated lever segment 238. Sleeve segment 236 is supported on rear output shaft 38 via a bearing assembly 240. Lever segment 238 has a terminal end portion engaging a spacer collar 242 that is piloted on an and able to rotate relative to actuator shaft 158. Mode cam 214 is fixed via a spline connection 245 for common rotation with actuator shaft 158. A lock ring 246 axially locates spacer collar 242 and mode cam 214 relative to a radial shaft flange 248.

[0037] Second cam member 220 of ballramp unit 212 has its grooves

226 formed in a first face surface 250 of a cam ring segment 252 that is shown to generally surround portions of sleeve segment 236 of first cam member 218 and sleeve segment 138 of clutch hub 120. A thrust bearing assembly 254 and thrust ring 256 are disposed between a second face surface 258 of cam ring segment 252 and a face surface 260 of pressure plate 134. Second cam member 220 further includes an elongated lever segment 262 having a mode follower 264 mounted at its terminal end that rollingly engages a cam surface 266 formed on an outer peripheral edge of mode cam 214. As will be detailed, the contour of cam surface 266 on mode cam 214 functions to control angular movement of second cam member 220 relative to first cam member 218 in response to rotation of actuator shaft 158. Such relative angular movement between cam members 218 and 220 causes balls 222 to travel along tapered grooves 224 and 226 which, in turn, causes axial movement of second cam member 220. Such axial movement of second cam member 220 functions to cause corresponding axial movement of pressure plate 134 between its released and locked positions, thereby controlling the magnitude of the clutch engagement force applied to clutch pack 126.

[0038] As seen, lever segment 262 of second cam member 220 is located on one side of actuator shaft 158 while lever segment 238 of first cam member 218 is located on the opposite side of actuator shaft 158. Due to engagement of mode follower 264 with cam surface 266 on mode cam 214, second cam member 220 is angularly moveable relative to first cam member 218 between a first or "retracted" position and a second or "extended" position in response to rotation of actuator shaft 158. With second cam member 220 rotated to its retracted position, return spring 140 biases pressure plate 134 to its released position which, in turn, urges balls 222 to be located in deep end portions of aligned grooves 224 and 226. Thus, such movement of second cam member 220 to its angularly retracted position relative to first cam member 218 also functions to locate second cam member 220 in an axially retracted position relative to clutch pack 126. While not shown, a biasing unit may be provided between lever segments 238 and 262 to assist return spring 140 in normally urging second cam member 220 toward its retracted position. In contrast, angular movement of second cam member 220 to its extended position causes balls 222 to be located in shallow end portions of aligned grooves 224 and 226 which causes axial movement of second cam member 220 to an axially extended position relative to clutch pack 126. Such axial movement of second cam member 220 causes pressure plate 134 to be moved to its locked position in opposition to the biasing exerted thereon by return spring 140. Accordingly, control of angular movement of second cam member 220 between its retracted and extended positions functions to control concurrent movement of pressure plate 134 between its released and locked positions.

[0039] As previously noted, cam surface 266 of mode cam 214 and shift slot 192 of range cam 184 are configured to coordinate movement of shift collar 86 and pressure plate 134 in response to rotation of actuator shaft 158 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 tow-range drive mode. Specifically, control system 46 functions to control the rotated position of actuator shaft 158 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.

[0040] Figure 10 illustrates actuator shaft 158 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 86 is located in its H range position and pressure plate 134 is located in its released position relative to clutch pack 126. 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 264 is shown engaging a detent portion of a first cam segment 266A of cam surface 266 on mode cam 214 which functions to locate second cam member 220 in its retracted position.

[0041] If the on-demand four-wheel high-range drive mode is thereafter selected, electric motor 156 is energized to initially rotate actuator shaft 158 in a first (i.e., clockwise) direction from its 2H position to the "ADAPT- H" position shown in Figure 1 1 . In this rotated position of actuator shaft 158, follower pin 194 is located within high-range dwell segment 196 of shift slot 192 in range cam 184 such that shift collar 86 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 actuator shaft 158 to its ADAPT-H position causes concurrent rotation of mode cam 214 to the position shown which, in turn, causes mode follower 264 to engage a first end portion of a second cam segment 266B of mode cam surface 266. Such movement of mode follower 264 from first cam segment 266A to second cam segment 266B causes second cam member 220 to move angularly relative to first cam member 218 from its retracted position to an intermediate or "ready" position. With second cam member 220 rotated to its ready position, ballramp unit 212 causes pressure plate 134 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 126. The adapt position of pressure plate 134 provides a low level of torque transfer across mode clutch assembly 42 that is required to take-up clearances in clutch pack 126 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 134 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. [0042] The limits of adaptive control in the on-demand four-wheel high- range drive mode are established by controlling bi-directional rotation of actuator shaft 158 between its ADAPT-H position of Figure 1 1 and its "LOCK-H" position shown in Figure 12. With actuator shaft 158 in its LOCK-H position, second segment 266B of mode cam surface 266 causes second cam member 220 to move to its extended position, thereby causing pressure plate 134 to move to its locked position for fully engaging mode clutch assembly 42. This range of angular travel of actuator shaft 158 causes follower pin 194 to travel within high- range dwell segment 196 of shift slot 192 so as to maintain shift collar 86 in its H range position. However, such rotation of actuator shaft 158 results in mode follower 264 riding along second segment 266B of cam surface 266 which, in turn, is configured to control angular movement of second cam member 220 between its ready position and its extended position. Bi-directional rotation of actuator shaft 158 within this range of travel is controlled by ECU 52 actuating electric motor 156 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 disclosure.

[0043] If the vehicle operator selects the part-time four-wheel high- range drive mode, electric motor 156 is energized to rotate actuator shaft 158 in the first direction to its LOCK-H position shown in Figure 12. As such, shift collar 86 is maintained in its H range position and mode cam 214 causes second cam member 220 to move to its extended position which, in turn, moves pressure plate 134 to its locked position for fully engaging mode clutch assembly 42. To limit the on-time service requirements of electric motor 156, a power-off brake 270 associated with electric motor 156 can be engaged to brake rotation of the motor output so as to prevent back-driving of geartrain 166 for holding pressure plate 134 in its locked position. In this manner, electric motor 156 can be shut- off after the part-time four-wheel high-range drive mode has been established.

[0044] If the Neutral mode is selected, electric motor 156 is energized to rotate actuator shaft 158 in a second (i.e., counterclockwise) direction to the Neutral position shown in Figure 13. Such rotation of actuator shaft 158 causes follower pin 194 to exit high-range dwell segment 196 and ride within shift segment 200 of shift slot 192 until shift collar 86 is located in its N position. Concurrently, rotation of mode cam 214 causes mode follower 264 to engage a portion of first segment 266A of cam surface 266 that is configured to move second cam member 220 to a position displaced from its retracted position. Such movement of second cam member 220 results in limited axial movement of pressure plate 134 from its released position toward clutch pack 126. Preferably, such movement of pressure plate 134 does not result in any drive torque being transferred through mode clutch assembly 42 to front driveline 12.

[0045] Figures 14 and 15 illustrate continued rotation of actuator shaft

158 in the second direction which occurs when the part-time four-wheel Tow- range drive mode is selected. In particular, Figure 14 shows an intermediate "ADAPT-T" position of actuator shaft 158 whereat follower pin 194 enters Tow- range dwell segment 198 of shift slot 192 for locating shift collar 86 in its Tow range position. Mode cam 214 has likewise been rotated for locating mode follower 264 at the interface between first segment 266A of cam surface 266 and a third segment 266C thereof. The contour of third segment 266C is configured such that second cam member 220 will be rotated to its ready position when mode follower 264 is in the position shown. As previously noted, movement of second cam member 220 to its ready position causes pressure plate 134 to move axially to its adapt position. However, selection of the part-time four-wheel Tow-range drive mode causes continued rotation of actuator shaft 158 to its LOCK-T position shown in Figure 15. Tow-range dwell segment 198 in shift slot 192 maintains shift collar 86 in its Tow range position while third segment 266C of mode cam surface 266 causes mode follower 264 to move second cam member 220 to its extended position, thereby moving pressure plate 134 to its locked position for fully engaging mode clutch assembly 42. Again, power-off brake 270 can be actuated to maintain actuator shaft 158 in its LOCK-T position.

[0046] Based on the preferred arrangement disclosed for actuation mechanism 44, actuator shaft 158 is rotatable through a first range of angular travel to accommodate range shifting of shift collar 86 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 actuator shaft 158 is established between its ADAPT-H and ADAPT-T positions. The second range of travel for actuator shaft 158 is defined between its ADAPT-H and LOCK-H positions to permit adaptive control of mode clutch 42 with shift collar 86 in the H range position. Likewise, the third range of actuator shaft travel is defined between its ADAPT-T and LOCK-T positions to permit actuation of mode clutch 42 while shift collar 86 is in its T range position.

[0047] The above referenced embodiment clearly sets forth the novel and unobvious features, structure and/or function of the present disclosure. However, one skilled in the art will appreciate that equivalent elements and/or arrangements made be used which will be covered by the scope of the following claims.

Claims

CLAIMS What Is Claimed Is:

1 . A transfer case comprising:

an input shaft;

a countershaft offset from the input shaft;

first and second output shafts;

a first reduction gearset including a first drive gear fixed for rotation with the input shaft, and a first driven gear in meshed engagement with the first drive gear and fixed for rotation with the countershaft;

a second reduction gearset including a second drive gear fixed for rotation with the countershaft and a second driven gear fixed for rotation with the first output shaft, the second drive gear being in meshed engagement with the second driven gear;

a range clutch operable in a first range position to establish a drive connection between the input shaft and the first output shaft and further operable in a second range position to fix the second driven gear for rotation with the first output shaft and drive the first output shaft at a reduced speed;

a mode clutch operable in a first mode position to disengage the second output shaft from driven engagement with the first output shaft and further operable in a second mode position to establish a drive connection between the first output shaft and the second output shaft;

a range actuator for moving the range clutch between its first and second range positions;

a mode actuator for moving the mode clutch between its first and second mode positions; and

a control system to control the range clutch and the mode clutch.

2. The transfer case of claim 1 , wherein the range clutch includes a range sleeve drivingly coupled to and axially moveable relative to the first output shaft, the range sleeve including a set of clutch teeth engaging the input shaft when the range clutch is in the first range position.

3. The transfer case of claim 2, wherein the range sleeve set of clutch teeth engage the second driven gear when the range clutch is in the second range position.

4. The transfer case of claim 3, wherein the range clutch is operable in a neutral third range position to allow the input shaft to rotate without torque transfer to the first output shaft.

5. The transfer case of claim 3, further including a housing supporting the input shaft and the first output shaft for rotation along a common axis.

6. The transfer case of claim 1 , further including an electric motor driving an actuator shaft, the range actuator and the mode actuator being driven by the actuator shaft.

7. The transfer case of claim 6, wherein the control system controls the magnitude and direction of rotation of the actuator shaft so as to coordinate movement of the range clutch and the mode clutch.

8. The transfer case of claim 1 , wherein the first and second reduction gearsets provide a speed reduction ratio between the input shaft and the first output shaft of substantially 1 .3 to 1 when the range clutch is in the second range position.

9. The transfer case of claim 1 , wherein the range actuator comprises: a follower fixed for rotation with the actuator shaft;

a range cam supported on the actuator shaft and having a shift slot within which the follower is disposed, the shift slot is configured to convert rotary movement of the actuator shaft into axial movement of the range cam;

a shift fork coupled to the range clutch; and a biasing mechanism interconnecting the shift fork to the range cam which is operable to convert axial movement of the range cam into axial movement of the shift fork for moving the range clutch between its first and second range positions.

10. The transfer case of claim 9, wherein the shift slot in the range cam includes a first dwell segment, a second dwell segment and a shift segment interconnecting the first and second dwell segments, the shift segment is configured to cause axial movement of the range clutch between its first and second range positions during rotation of the actuator shaft through a first range of rotary travel, the first dwell segment is configured to maintain the range clutch in its first range position during rotation of the actuator shaft through a second range of rotary travel, and the second dwell segment is configured to maintain the range clutch in its second range position during rotation of the actuator shaft through a third range of rotary travel.

1 1 . The transfer case of claim 2, wherein the mode clutch includes a clutch pack operably disposed between the first and second output shafts and a pressure plate moveable between the first mode position whereat a minimum clutch engagement force is exerted on the clutch pack and the second mode position whereat a maximum clutch engagement force is exerted on the clutch pack.

12. The transfer case of claim 1 1 , wherein the mode actuator comprises: a mode cam driven by the actuator shaft and having a cam surface; and a ballramp unit having a first cam member, a second cam member disposed for rotation and axial movement relative to the first cam member and rollers disposed in cam grooves formed between the first and second cam members, wherein the pressure plate is moveable between its first and second mode positions in response to movement of the second cam member between a retracted position and an extended position relative to the first cam member, and wherein the cam surface is configured to cause movement of the second cam member between its retracted and extended position in response to rotation of the mode cam with the actuator shaft.

13. The transfer case of claim 12, wherein the first cam member includes a first lever segment disposed on one side of the actuator shaft, and wherein the second cam member includes a second lever segment disposed on an opposite side of the actuator shaft and having a mode follower engaging the cam surface on the mode cam.