Abstract:
A shift system for a transfer case including a spring-loaded range fork assembly operable for shifting a range sleeve between two speed range positions. The range fork assembly includes a bracket, a range fork, and a spring assembly. The spring assembly is compressed and inserted into chambers formed in both the bracket and range fork. The range fork assembly is slidably maintained on a shift rail and the range fork is coupled to the range sleeve. An actuator mechanism is provided for causing selective axial movement of the range fork assembly on the rail. The spring assembly allows the bracket to shift and apply a shift force on the range fork. This shift force causes the range fork to slide the range sleeve to the desired range position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Serial No. 60/280,273, filed Mar. 30, 2001. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to shift systems for transmissions and transfer cases of the type used in the driveline of motor vehicles. Specifically, the present invention is directed to a spring-loaded shift fork assembly for use in such shift systems. 
     It is known in the automobile industry to equip power transfer assemblies (i.e., manual transmissions, transfer cases, etc.) with a shift system having spring-loaded shift devices for completing a delayed gear or mode shift once speed synchronization or a torque break occurs. Examples of conventional spring-loaded shift systems are disclosed in U.S. Pat. Nos. 4,529,080, 4,770,280 and 5,517,876. In each of these patents, a pair of springs are used to provide a bi-directional preload function for effectuating coupling of a dog-type shift sleeve with a desired gearset. While such arrangements are satisfactory for their intended purpose, a need exists to develop simpler, more cost-effective alternatives that provide the desired function while advancing the art. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved shift system for a power transmission device having a spring-loaded shift fork assembly. 
     As a related object, the shift system of the present invention is adapted for use with the range shift mechanism of a four-wheel drive transfer case. 
     As a still further object, the shift system of the present invention is adapted for use with a gearshift mechanism of a multi-speed transmission or transaxle. 
     According to a preferred embodiment of the present invention, a shift system for a transfer case includes a spring-load range fork assembly operable for shifting a range sleeve between two speed range positions. The range fork assembly includes a bracket, a range fork, and a spring assembly. The spring assembly is compressed and inserted into chambers formed in both the bracket and range fork. The range fork assembly is slidably maintained on a shift rail and the range fork is coupled to the range sleeve. An actuator mechanism is provided for causing selective axial movement of the range fork assembly on the rail. During operation of the transfer case, the transmission of drive torque while shifting into either speed range may create a resistance force which impedes the axial movement of the range sleeve. However, the spring assembly allows the bracket to shift and apply a shift force on the range fork. When a torque reversal occurs, the shift force causes the range fork to slide the range sleeve to the desired position. 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further objects, features and advantages of the present invention will become apparent from the following detailed specification and the appended claims which, in conjunction with drawings, set forth the best mode now contemplated for carrying out the invention. Referring to the drawings: 
     FIG. 1 is a sectional view of an exemplary four-wheel drive transfer case with which the shift system of the present invention may be utilized; and 
     FIGS. 2 and 3 are exploded perspective views of the spring-load shift fork assembly associated with the shift system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In general the present invention is directed to a shift system of the type used in motor vehicle power transmission devices for effectuating axial movement of a coupling member (i.e., a shift sleeve) to shift between gear ratios or drive modes. Thus, while the present invention is shown specifically associated with the range shift system of a two-speed transfer case, it will be appreciated that the present invention is also applicable for use with the mode shift system of the transfer case as well as for use with the gearshift system of multi-speed gear-change transmissions. 
     Referring to FIG. 1, an exemplary construction for a two-speed transfer case  10  is shown to be equipped with a shift system  12  according to the present invention. Transfer case  10  also includes: a housing  14 ; an input shaft  16  rotatably supported from housing  14 ; a rear output shaft  18  rotatably supported between input shaft  16  and housing  14 ; a front output shaft  20  rotatably supported from housing  14 ; a planetary gearset  22  driven by input shaft  16 ; a range clutch  24  for selectively coupling one of a high-range output and a low-range output of planetary gearset  22  to rear output shaft  18 , a transfer mechanism  26  driven by front output shaft  20 ; and a mode clutch  28  for selectively coupling transfer mechanism  26  to rear output shaft  18 . As will be detailed, shift system  12  controls actuation of range clutch  24  and mode clutch  28  for establishing various operational drive modes. 
     Planetary gearset  22  includes: a sun gear  30  driven by input shaft  16 ; a ring gear  32  non-rotatably fixed to housing  14 ; a planet carrier  34 ; and a set of planet gears  36  rotatably supported on pins  38  mounted to planet carrier  34  and which are meshed with sun gear  30  and ring gear  32 . Range clutch  24  includes a range sleeve  40  which is splined for rotation with rear output shaft  18  and axial sliding movement thereon between a high-range (H) position, a neutral (N) position, and a low-range (L) position. In the high-range position, clutch teeth  42  on range sleeve  40  are meshed with clutch teeth  44  on sun gear  30  for establishing a first or direct ratio drive connection between input shaft  16  and rear output shaft  18  such that transfer case  10  operates in a High-Range drive mode. In the low-range position, clutch teeth  42  on range sleeve  40  are meshed with clutch teeth  46  on planet carrier  34  for establishing a second or reduced ratio drive connection between input shaft  16  and rear output shaft  18  such that transfer case  10  operates in a Low-Range drive mode. Finally, with range sleeve  40  in its neutral position clutch teeth  42  are disengaged from clutch teeth  44  on stubshaft  31  and clutch teeth  46  on planet carrier  34  for establishing a non-driven Neutral mode for transfer case  10 . 
     Transfer mechanism  26  is shown to include a first sprocket  50  rotatably supported on rear output shaft  18 , a second sprocket  52  fixed to front output shaft  20 , and a power chain  54  connecting first sprocket  50  to second sprocket  52 . Mode shift mechanism  28  includes a clutch hub  56  fixed to rear output shaft  18 , a clutch gear  58  fixed to first sprocket  50 , a synchronizer  60  disposed between clutch hub  56  and clutch gear  58 , and a mode sleeve  62  splined for rotation with clutch hub  56  and axial movement thereon between a two-wheel drive (2WD) position and a four-wheel drive (4WD) position. In its 2WD position, mode sleeve  62  is disengaged from clutch gear  58  and transfer mechanism  26  is uncoupled from rear output shaft  18  such that transfer case  10  is operating in a Two-Wheel Drive mode. When mode sleeve  62  is slid axially to its 4WD position, synchronizer  60  is energized to synchronize the speed of first sprocket  50  to that of rear output shaft  18 . Once the synchronization process is complete, mode sleeve  62  is permitted to move into coupled engagement with clutch gear  58  for coupling transfer mechanism  26  to rear output shaft  18  and establishing the Four-Wheel Drive mode. 
     To provide means for coordinating the axial movement of range sleeve  40  between its three distinct range positions and mode sleeve  62  between its two distinct mode positions, shift system  12  includes: a shift rail  70  mounted to housing  14 ; a spring-loaded range fork assembly  72  supported on shift rail  70 ; a mode fork assembly  74  supported on shift rail  70 ; a sector plate  76  operably coupled to range fork assembly  72  and mode fork assembly  74 ; and a shift actuator  78  for causing controlled rotary movement of sector plate  76 . As seen best from FIG. 1, mode fork assembly  74  includes a mode fork  80  and a biasing spring  82 . Mode fork  80  has a tubular sleeve segment  84  journalled on shift rail  70  and a fork segment  86  extending from sleeve segment  82  with a C-shaped end portion  88  retained in an annular groove formed in mode sleeve  62 . A mode pin  90  is secured to sleeve segment  84  and bears against a mode cam surface  92  formed along an outer edge of sector plate  76 . Cam surface  92  is contoured such that rotation of sector plate  76  via actuation of shift actuator  78  causes corresponding axial sliding movement of mode fork  80  on shift rail  70 . Such axial movement of mode fork  80  results in corresponding axial movement of mode sleeve  62  between its 2WD and 4WD positions. Spring  82  is coaxially mounted on shift rail  70  and acts on mode fork  80  to maintain engagement of mode pin  90  with mode cam surface  92 . 
     Referring now primarily to FIGS. 2 and 3, range fork assembly  72  is shown to include a range fork  94 , a bracket  96 , and a spring assembly  98 . Range fork  94  includes a cylindrical tubular body segment  100  and a fork segment  102  extending orthogonally from body segment  100  with its C-shaped end portion  104  adapted for retention in an annular groove formed in range sleeve  40 . A pair of disc-like annular end flanges  106  and  108  are formed at opposite ends of body segment  100 . Apertures  110  and  112  are formed through end flanges  106  and  108 , respectively, and are sized to permit sliding insertion of shift rail  70  therethrough. A pair of truncated flanges  114  and  116  are formed between end flanges  106  and  108  and include arcuate support surfaces  114   a  and  116   a , respectively, adapted to support shift rail  70  thereon. Thus, body segment  100  of range fork  94  defines three distinct cavities, namely, a first end cavity  118 , a central cavity  120 , and a second end cavity  122 . Gussets  124  extend between body segment  100  and fork segment  102  to stiffen range fork  94  and minimize bending. 
     Bracket  96  of range fork assembly  72  is shown to include a base segment  126  and a pair of laterally-spaced lug segments  128  and  130 . Lug segment  128  includes a disc-like end flange  132  with an aperture  134  therethrough, and a truncated flange  136  having an arcuate support surface  136   a . Similarly, lug segment  130  includes a disc-like end flange  138  with an aperture  140  therethrough, and a truncated flange  142  having an arcuate support surface  142   a . Apertures  134  and  140  are adapted to permit sliding insertion of shift rail  70  therethrough while support surfaces  136   a  and  142   a  of truncated flanges  136  and  142  are adapted to support shift rail  70 . In addition, a spring cavity  144  is formed between truncated flanges  136  and  142 . 
     Spring assembly  98  includes a coil spring  150  and a pair of tubular washer sleeves  152  which are inserted into opposite ends of coil spring  150 . Each washer sleeve  152  has a thin-walled tubular body segment  154  and a radial flange segment  156  extending from one end of body segment  154 . The outer diameter of body segment  154  for each washer sleeve  152  is sized to fit inside coil spring  150  while its inner diameter is sized to permit shift rail  70  to extend therethrough. Thus, body segments  154  act as spring guides for the opposite ends of coil spring  150 . In addition, the end surfaces of coil spring  150  are adapted to engage flange segments  156  of washer sleeves  152 . 
     The components of range fork assembly  72  are pre-assembled prior to mounting on shift rail  70 . Specifically, spring assembly  98  is compressed and placed in spring cavity  144  of bracket  96  such that a portion of the outer face surface of flange segment  156  on each washer sleeve  152  engages a corresponding inner face surface  136   b  and  142   b  of truncated flanges  136  and  142 , respectively. Thereafter, bracket  96  is brought into mating engagement with body segment  100  of range fork  94  such that spring cavity  144  is aligned with central cavity  120  to define an enclosed spring chamber. As such, a portion of the outer face surfaces of flange segments  156  on each washer sleeve  152  also engages a corresponding inner face surface  114   b  and  116   b  of truncated flanges  114  and  116 , respectively, for retaining spring assembly  98  within the spring chamber. In this assembled arrangement, end flange  132  of lug segment  128  is positioned within first end cavity  118  and end flange  138  of lug segment  130  is positioned within second end cavity  122 . Moreover, lug apertures  134  and  140  are colinearly aligned with end flange apertures  110  and  112  as well as with the apertures through washer sleeves  152  so as to permit shift rail  70  to be slid through the aligned apertures for mounting range fork assembly  72  thereon for sliding movement. Since coil spring  150  is compressed prior to installation into spring cavity  144  of bracket  96 , it is preloaded for generating a “self-centering” feature whereby truncated flanges  114  and  116  on range fork  94  are radially aligned with truncated flanges  136  and  142  on bracket  96 , as shown in FIG.  1 . Optionally, spring assembly  98  can initially be installed in center cavity  120  of range fork  94  with bracket  96  thereafter assembled with range fork  94 . 
     A range pin  160  is secured to base segment  126  of bracket  96  and is retained in a range cam slot  162  formed in sector plate  76 . Thus, rotation of sector plate  76  is adapted to cause sliding axial movement of range fork assembly  72  on shift rail  70  which, in turn, results in axial movement of range sleeve  42  between its H, N and L range positions. Sector plate  76  has mode cam surface  90  and range cam slot  162  arranged to provide coordinated axial movement of mode fork assembly  74  and range fork assembly  72  in response to rotation of an output member  164  of shift actuator  78 . Preferably, sector plate  76  can be rotated to four distinct positions for establishing a Two-Wheel High-Range drive mode (2WD-H), the Four-Wheel High-Range drive mode (4WD-H), the Neutral mode (N) and the Four-Wheel Low-Range drive mode (4WD-L). Shift actuator  78  is shown as a gearmotor/encoder assembly  166  operable to receive an electric shift signal which is indicative of the selected drive mode from a mode selector (not shown) that is controlled by the vehicle operator. Based on the selected mode, shift actuator  78  causes sector plate  76  to be rotated to the desired position. However, the spring-loaded feature of range fork assembly  72  allows axial movement of range fork  94  to lag behind that of bracket  96 , via compression of coil spring  150 , when residual drag or an instantaneous torque lock condition prevents engagement of clutch teeth  42  on range sleeve  40  with the clutch teeth on sun gear  30  or planet carrier  34 . For example, if the vehicle operator desires to shift transfer case  10  from the 4WD-H drive mode into the 4WD-L drive mode, a suitable signal is sent to gearmotor/encoder assembly  166  which causes sector plate  76  to rotate to the corresponding sector position. Such sector rotation does not cause movement of mode sleeve  62  out of its 4WD position but does cause bracket  96  to move axially due to the travel of range pin  160  in range cam slot  162 . Coil spring  150  urges range fork  94  to move axially in concert with bracket  96 . However, if misalignment of clutch teeth  46  on planet carrier  34  with clutch teeth  42  on range sleeve  40  prevents movement of range sleeve  40  to its L position, coil spring  150  is compressed in excess of its preload for applying a biasing load on range fork  94 . Once the misalignment is eliminated, coil spring  150  forces continued axial movement of range fork  94  which, in turn, causes range sleeve  40  to move into its L position with range fork  94  being again centered relative to bracket  96 . 
     Thus, the single spring configuration of the present invention provides a bi-directional spring-loaded function for accommodating shifts into and out of all of the available ranges. Moreover, this arrangement prevents potential damage to gearmotor/encoder assembly  166  by preventing excessive motor current when a shift can not be immediately completed since sector plate  76  is permitted to rotate to the desired sector position while coil loading spring  150  to subsequently cause movement of range fork  94  and effectuate coupling of range sleeve  40 . In manually-shifted systems, a shift lever can be moved by the vehicle operator to rotate sector plate  76 . While disclosed in association with transfer case  10 , spring-biased range fork assembly  72  can also be used in automatically-shifted (“automated”) manual transmissions and transaxles where a power-operated (i.e., electrical, hydraulic) actuator is used to move a shift sleeve into and out of engagement with constant-mesh gearsets to effectuate a gear change. 
     While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the description of the appended claims.