Abstract:
A shift fork actuation system includes an actuator housing defining a piston cavity and a piston assembly having three pistons with three different surface areas responsive to fluid pressure to establish positions of the piston assembly within the piston cavity. A stopping mechanism, such as a shoulder formed by the actuator housing at the interface of different bores in the piston cavity, interferes with one of the surface areas to prevent movement of one of the pistons past the stopping mechanism. A neutral position of a shift fork is established by interference of the piston assembly with the stopping mechanism. A slot in the piston assembly captures a finger extension from the shift rail to act as an anti-rotation feature, ensuring that a magnet embedded in the piston assembly as part of a sensor assembly is properly positioned.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application 60/940,822, filed May 30, 2007, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a shift fork actuation system for controlling the positions of a synchronizer in an automated layshaft transmission. 
       BACKGROUND OF THE INVENTION 
       [0003]    Layshaft transmissions typically use synchronizers to synchronize the speed of gears with a layshaft before engaging the gears with the layshaft to complete powerflow through the transmission. Synchronizers are moved by a hydraulically or mechanically actuated shift fork between a neutral position, in which an adjacent gear is not engaged with the layshaft, and an engaged position in which the gear is engaged with the layshaft. In the case of a dual synchronizer, there are two different engaged positions for two different gears mounted concentrically with the layshaft on either side of the synchronizer. Precise and rapid positioning of the synchronizer enables reliable and smooth shifting of the transmission. 
       SUMMARY OF THE INVENTION 
       [0004]    A compact, fast-responding shift fork actuation system, especially suited for use on an automated layshaft transmission is provided. The shift fork actuation system includes an actuator housing defining a piston cavity and a piston assembly having three pistons each preferably aluminum alloy and each with a different surface area responsive to fluid pressure to establish different positions of the piston assembly within the piston cavity. A stopping mechanism, such as a shoulder formed by the actuator housing at the interface of different bores of the piston cavity, interferes with one of the pistons to prevent movement of the piston past the stopping mechanism. A shift fork operatively connects to the piston assembly. The three positions of the piston assembly correspond with different positions of the shift fork and the synchronizer connected thereto. Preferably, a neutral position of the synchronizer is established by the interference of the piston assembly with the stopping mechanism. In a preferred embodiment, there are four piston assemblies, each operatively connected with a different double synchronizer. 
         [0005]    A first of the three pistons is an annular piston. The second and third pistons are integrally connected, cylindrical pistons with differing outer diameters forming a plug. A slot extends partially transversely through the plug between the second and third pistons. The shift fork actuation system also includes an elongated shift rail and a shift fork extending from the shift rail in operative engagement with the synchronizer. Preferably, the piston assembly is generally axially centered with respect to the shift rail, minimizing the length of hydraulic fluid paths to the piston assembly. A finger extension is spaced from the shift fork and extends from the shift rail into the slot such that movement of the pistons moves the finger extension to shift the synchronizer. 
         [0006]    Preferably, a sensor assembly is provided that includes a magnet integrally connected to (e.g., embedded in) each piston assembly. The sensor assembly also includes a sensor pick-up positioned on the actuator housing in operative communication with the magnet. The sensor pickup is operable to generate an electrical signal corresponding with the position of magnet and thereby of the piston assembly and, most importantly, of the synchronizer. The finger extension contacts the integrally connected second and third pistons within the slot and minimizes rotation of the pistons within the cavity, thereby ensuring that the magnet is always properly positioned for operative communication with the sensor pick-up. Additionally, an optional biasing element such as a preloaded spring may be retained in an aperture in the connected second and third pistons within the slot to rest between the finger extension and the integrally connected second and third pistons to further reduce piston rotation and thereby ensure an accurate magnet position and corresponding signal generated by the sensor pickup. 
         [0007]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic illustration of a powertrain with a dual clutch layshaft transmission having four dual synchronizers; 
           [0009]      FIG. 2  is a schematic perspective illustration in fragmentary view of a shift fork actuation system (with a portion of an actuator housing cut away to show an actuator piston assembly) for controlling the position of the synchronizers in the transmission of  FIG. 1 ; 
           [0010]      FIG. 3  is a schematic perspective illustration of the shift fork actuation system of  FIG. 2  with the actuator piston assembly and actuator housing removed to show four shift rods with actuator finger extensions and shift forks; 
           [0011]      FIG. 4  is a schematic plan view of one of the piston assemblies in a piston cavity of the actuator housing of  FIG. 2 ; 
           [0012]      FIG. 5A  is a schematic cross-sectional illustration of the shift fork actuation system taken at the arrows  5 A- 5 A shown in  FIG. 2  and rotated 180 degrees, showing a magnet embedded in the piston assembly and a sensor mechanism secured to the actuator housing; 
           [0013]      FIG. 5B  is a schematic partially cross-sectional and fragmentary illustration taken at the arrows  5 B- 5 B shown in  FIG. 2  and rotated 180 degrees, with an optional spring held within a slot of the piston assembly and showing an actuator finger extension within the slot; and 
           [0014]      FIG. 6  is a schematic perspective illustration of the actuator housing of  FIGS. 2 ,  4  and  5 A. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    Referring to the drawings, wherein like reference numbers refer to like components,  FIG. 1  shows a powertrain  10  having a transmission  12  with an input shaft  14  connected to an engine  16  through a torque converter  18 . Dual input clutches CE, CO are operatively connected to the input shaft  14  and are alternately selectively engagable to connect the input shaft  14  to a first intermediate shaft  22  and a second intermediate shaft  24 , respectively. Four double synchronizers  26 ,  28 ,  30  and  32  have synchronizer sleeves  34 A,  34 B,  34 C and  34 D that are selectively movable by respective shift forks (shown and described in  FIGS. 2 and 3 ) to selectively engage different gears with respective countershafts  36 ,  38  (also referred to herein as layshafts) to permit dynamic shifting, i.e., preselection of a gear to engage the gear with the respective layshaft prior to the layshaft carrying a torque load, i.e., prior to engagement of the input clutch CO or CE, as is known. Each respective shift fork, as described in  FIGS. 2 and 3 , is selectively movable to move a respective one of the synchronizer sleeves  34 A,  34 B,  34 C and  34 D to a neutral position (shown in  FIG. 1 ), in one direction (i.e., to the left) to engage a gear to the left with a respective layshaft, and in an opposing direction (i.e., to the right) to engage another gear to the right with the layshaft, to establish different powerflow paths from the input member  14  to an output member  40  to deliver torque at different speed ratios to a final drive mechanism  42  through the transmission  12 . For example, when synchronizer sleeve  34 A is shifted to the left, gear  44  is connected for common rotation with countershaft  36 . When synchronizer sleeve  34 A is shifted to the right, gear  46  is connected for common rotation with countershaft  36 . When synchronizer sleeve  34 B is shifted to the left, gear  48  is connected for common rotation with countershaft  36 . When synchronizer sleeve  34 B is shifted to the right, gear  50  is connected for common rotation with countershaft  36 . When synchronizer sleeve  34 C is shifted to the left, gear  52  is connected for common rotation with countershaft  38 . When synchronizer sleeve  34 C is shifted to the right, gear  54  is connected for common rotation with countershaft  38 . When synchronizer sleeve  34 D is shifted to the left, gear  56  is connected for common rotation with countershaft  38 . When synchronizer sleeve  34 D is shifted to the right, gear  58  is connected for common rotation with countershaft  38 . Each synchronizer sleeve  34 A,  34 B,  34 C and  34 D is independently shiftable from the others, to selectively engage the adjacent gears with the respective countershafts  36 ,  38 . 
         [0016]    Referring now to  FIGS. 2 and 3 , the transmission  12  is shown with a shift fork actuation system  60  that has shift forks engaged with each of the sleeves. Shift forks  62 A,  62 B,  62 C and  62 D are mounted to separate, generally parallel shift rails  64 A,  64 B,  64 C and  64 D, which in turn are supported by low resistance linear ball bearings (not shown) at the center support  66  and the case of the transmission  12 . In  FIG. 2 , shift fork  62 C is operatively engaged with synchronizer sleeve  34 C and shift fork  62 D is operatively engaged with synchronizer sleeve  34 D. In  FIG. 2 , the gears  52 ,  54 ,  56 , and  58  that are selectively engagable for common rotation with the countershaft  38  are also shown. 
         [0017]    Each shift rail  64 A,  64 B,  64 C, and  64 D has a respective finger extension  68 A,  68 B,  68 C and  68 D mounted thereto spaced from and extending in a different direction than the respective shift fork  62 A,  62 B,  62 C and  62 D on the shift rail. The finger extensions  68 A,  68 B,  68 C, and  68 D are permanently attached to the shift rails  64 A,  64 B,  64 C, and  64 D, such as by laser welding. The shift rails  64 A,  64 B,  64 C, and  64 D move with the forks  62 A,  62 B,  62 C, and  62 D and the finger extensions  68 A,  68 B,  68 C, and  68 D. Each finger extension  68 A,  68 B,  68 C and  68 D extends through an opening  70  in an actuator housing  72  and into a slot  73  in a piston assembly (piston assembly  82 D shown, three other like piston assemblies being housed in separate piston cavities  78 A,  78 B, and  78 C shown in  FIG. 6 ). The piston assemblies  82 A- 82 D and the housing  72  may be referred to as an actuator assembly  76 . Force is applied directly to the finger extensions  68 A,  68 B,  68 C, and  68 D by the respective actuator piston assemblies (actuator piston assembly  82 D shown in  FIGS. 4 and 5A  and actuator piston assembly  82 A shown in  FIG. 6 ). Ribbing and material specifications for the actuator finger extensions  68 A,  68 B,  68 C, and  68 D may be developed through the use of finite element analysis. 
         [0018]    Referring to  FIG. 6 , the actuator assembly  76  is shown in more detail. The actuator housing  72  houses the four separate piston cavities  78 A,  78 B,  78 C, and  78 D, each selectively fed pressurized fluid on either end by a valve body assembly  80  to selectively move different pistons of each respective different piston assembly (only piston assembly  82 A shown in  FIG. 6 ) to determine the position of the respective shift fork  62 A,  62 B,  62 C, and  62 D shown in  FIG. 3  operatively engaged with the respective piston assembly through the opening  70 . 
         [0019]    Referring to  FIG. 5A , a position sensor assembly includes a magnet embedded in each respective piston assembly (magnet  84 D shown embedded in piston assembly  82 D). As shown in  FIG. 5A , the position sensor assembly also includes a sensor pickup  86 D, in the form of a Hall Effect sensor, secured to the actuator housing  72 , and directly aligned with the piston assembly  82 D. The magnitude of an electric current in the sensor pickup  86 D is in relation to the position of the magnet  84 D. Because the position of the piston assembly  82 D is directly related to the position of the corresponding synchronizer shift fork  62 D of  FIG. 3 , the magnet  84 D and sensor pickup  86 D provide positioning feedback related to the time to engagement of a selected gear, allowing hydraulic pressure to be controlled to prevent abrupt shifting. Like magnets and sensor pickups are positioned in like manner and function the same as those shown with respect to piston assembly  82 D to provide position feedback of the other piston assemblies. In lieu of being embedded in the piston assemblies, the magnets  84 D could be secured to the outer surface of the respective piston assemblies, or in some other way made integral with the piston assemblies for rotation therewith. Many other types of sensor assemblies known in the art may be employed in lieu of a Hall Effect sensor in combination with a magnet. 
         [0020]    Referring to  FIGS. 2 and 5A , the slot  73  in the piston assembly  82 D extends only partway through the piston assembly  82 D. The finger extension  68 D (see  FIG. 2 ) traverses an opening  70  in the actuator housing  72  and substantially occupies the entire slot  73  to prevent the piston assembly  82 D from rotating. Thus, the slot  73  acts as both a mechanism to transfer movement from the finger extension  68 D to the piston assembly  82 D and as an anti-rotation feature for the piston assembly  82 D that ensures that the magnet  84 D is positioned on a portion of the circumference of the piston cavity  78 D that is always in general alignment with the sensor pickup  86 D (shown in  FIG. 5A ). 
         [0021]    Referring to  FIG. 4 , one of the piston assemblies  82 D is shown in greater detail nested within piston cavity  78 D of the actuator housing  72 . The piston assembly  82 D includes three pistons  90 ,  92  and  94 , each preferably of an aluminum alloy. The first piston  90  is an annular piston with an outer diameter  96  and an inner diameter  98  at an inner bore that extends substantially the length of the piston  90 , ending at a portion having an end opening  100 . The outer diameter  96  fits within a first bore  101  of the piston cavity  78 D. The second piston  92  has an outer diameter  102  that fits within a second bore  104  of the piston cavity  78 D. The outer diameter  96  of the first piston  90  is larger than the second bore  104 , and the piston  90  is mechanically prevented from traveling further to the right than shown in  FIG. 4  by a shoulder  106  of the actuator housing  72  formed where the first and second bores  101 ,  104  interface. The second piston  92  is connected for movement with the third piston  94 , with the slot  73  formed therebetween. 
         [0022]    Referring to  FIG. 5B , the finger extension  68 D is shown in the slot  73  of the connected pistons  92 ,  94  (only piston  94  shown in the cross-section of the piston assembly  82 D; piston  90  and housing  72  not shown for purposes of clarity in the drawing). Optionally, in order to reduce a clearance  85  between the finger extension  68 D and the piston assembly  82 D at the slot  73 , an aperture  87  is provided in the joined pistons  92 ,  94  (see  FIG. 5A ; aperture  87  also shown in  FIG. 4 ) and retains a biasing element  89  such as a preloaded spring (biasing element  89  not in cross-sectional view in  FIG. 5B  and not shown in  FIG. 5A  for purposes of clarity in the drawing). The biasing element  89  further reduces rotation of the piston assembly  82 D of  FIG. 4  to further ensure proper alignment of the sensor pickup  86 D and the magnet  84 D shown in  FIG. 5A  to ensure an accurate magnet position and corresponding signal generated by the sensor pickup  86 D. The biasing element could be made of any material, such as plastic, rubber, spring steel, stainless steel, or a combination of these. 
         [0023]    As shown, channels  108 ,  110  allow hydraulic fluid to be fed to or exhausted from the portions of the piston cavity  78 D to the left of the piston  94 , referred to as chamber  112 , and to the right of piston  92 , referred to as chamber  114 . The pistons  90 ,  92 ,  94  are in the positions shown due to hydraulic pressure applied to both a first surface area  116  and a second surface area  118  and the finger extension  68 D shown in  FIG. 2  that extends in the slot  73  will be in a first position corresponding with a neutral position of the synchronizer sleeve  34 D of  FIG. 2 . Because the neutral position is defined by the shoulder  106 , neutral is a fixed, mechanically-precise position that prevents undesired partial engagement or drag of the synchronizer  32  of  FIG. 1 , as required for low spin losses and longer synchronizer life. If the fluid in chamber  114  is exhausted, the integrally connected pistons  92 ,  94  move to the right, with hydraulic pressure being applied to a first surface area  116  of piston  94 , and the finger extension  68 D extending in slot  73  will be in a second position in which the operatively connected synchronizer sleeve  34 D moves to the right to engage the gear  58  with countershaft  38 . If the fluid in chamber  112  is exhausted instead, hydraulic pressure acts on second surface area  118  and the piston  90  and the connected pistons  92 ,  94  move together to the left to a position corresponding with the third position of the synchronizer sleeve  34 D of  FIG. 1  in which the synchronizer sleeve  34 D engages gear  56  for rotation with countershaft  36 . Thus, the hydraulic fluid passages from the valve body  80  (shown in  FIG. 6 ) to the piston assembly  82 D are relatively short, especially in comparison to systems having actuator pistons at the ends of the shift rails. Because the piston assemblies are not at the ends of the shift rails  64 A- 64 D (see, e.g., piston assembly  82 D in  FIG. 2 ), the overall axial length of the actuator system  60  is shorter compared to such systems. Additionally, the shorter path for pressurized fluid enables quick response time of the actuators and enables a compact arrangement of the shift fork actuator system  60 . 
         [0024]    Referring again to  FIG. 2 , a shift rail detent  120  is shown formed at the end of each respective shift rail (shown in shift rails  64 C and  64 D, but also formed on shift rails  64 A and  64 B shown in  FIG. 3 ). A neutral detent pin  122  is associated with each respective detent  120  and has a spring-biased ball that is captured in the respective detent  120  when the respective shift rail  64 A,  64 B,  64 C,  64 D is in the neutral position. When the shift rail moves to either engaged position (i.e., to the right or to the left), the ball of the detent pin  122  is snapped over a ridge defining the detent  120 , helping to quickly move the connected respective shift fork and synchronizer to the desired engaged position. The detents  120  as well as the mechanical shoulder  106  of each piston assembly (see  FIG. 4 ) help to ensure a neutral position of the piston assembly, such as piston assembly  82 D, without requiring pressurization of the piston assembly. 
         [0025]    While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.