Abstract:
A torque based park lock assembly for motor vehicles, especially passenger cars and trucks equipped with automatic transmissions, includes a lobed wheel secured to a transmission output shaft and a rotation restricting flexible lever which may be selectively moved into engagement with the lobed wheel by a cam. A bi-directional motor rotates the cam. A return spring translates the lever away from the lobed wheel when the cam is rotated into a disengaged (non-Park) position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/970,778 filed Jan. 8, 2008. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates generally to park mechanisms for motor vehicles and more particularly to an improved torque based park lock system utilizing a lobed wheel secured to the output shaft of a transmission. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art. 
     In vehicles equipped with automatic transmissions, a mechanism must be provided to lock the transmission and vehicle drive wheels in the Park or P position of the transmission to preclude rotation of the drive wheels and motion of the vehicle when it is parked. 
     While there are many variations, the basic transmission park mechanism typically includes a cogged or toothed wheel which rotates with the automatic transmission output shaft and a pawl that may be selectively engaged with the teeth of the wheel to prevent its rotation and thus motion of the vehicle. 
     Because a simple pawl and cog mechanism could be damaged or rendered inoperable if engaged while the vehicle is moving, current designs allow for this contingency by establishing a lock speed. Below the lock speed, the pawl engages and renders the drive line and vehicle stationary. Above the lock speed, the mechanism allows drive line and vehicle motion. If the chosen lock speed is high, loads on the system are high; if the chosen lock speed is low, a vehicle parked on a grade may roll away. Obviously, therefore, selection of the proper lock speed is both important and a balance of several engineering and performance parameters. 
     The typical cog and pawl park mechanism also comprehends several components such as an actuator, an actuator spring, a return spring, a pawl, a guide and a cogged or toothed park gear. In view of the foregoing, the present inventors have determined that improvements to park lock mechanisms are both desirable and possible. 
     SUMMARY 
     The present invention provides an improved, torque based park lock assembly for motor vehicles, especially passenger cars and trucks equipped with automatic transmissions. The assembly includes a lobed wheel secured to a transmission output shaft and a rotation restricting flexible lever or arm which may be selectively moved into engagement with the lobed wheel by a cam rotated by a bi-directional motor. A return spring translates the lever away from the lobed wheel when the cam is rotated into a disengaged (non-Park, i.e., operating) position. In an alternate embodiment, the lobed wheel includes rollers on each lobe to reduce friction. 
     Another aspect of the present invention provides an improved, torque based park lock assembly for automatic transmissions. 
     Another aspect of the present invention provides a park lock assembly for automatic transmissions having a lobed wheel and a flexible lever which may be selectively moved into engagement with the wheel. 
     Another aspect of the present invention provides a park lock assembly for automatic transmissions having a lobed wheel with a friction reducing roller on each lobe and a flexible arm which may be selectively moved into engagement with the rollers. 
     Another aspect of the present invention provides a park lock assembly for automatic transmissions having a lobed wheel and a flexible lever which may be selectively moved into engagement with the wheel by a cam and a bi-directional motor. 
     Another aspect of the present invention provides a park lock assembly for automatic transmissions having a lobed wheel with a friction reducing roller on each lobe and a flexible arm which may be selectively moved into engagement with the rollers by a cam and bi-directional motor. 
     Another aspect of the present invention provides a park lock assembly for automatic transmissions having a lobed wheel and a flexible lever which is moved out of engagement by a return spring. 
     Another aspect of the present invention provides a park lock assembly for automatic transmissions having a lobed wheel with a friction reducing roller on each lobe and a flexible arm which is moved out of engagement by a return spring. 
     Further advantages and areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples provided are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a perspective view of a park lock assembly according to a first embodiment of the present invention in an automatic transmission and motor vehicle drive train; 
         FIG. 2  a side elevational view of a park lock assembly according to another embodiment of the present invention in a disengaged (non-Park) state or operating mode; and 
         FIG. 3  a side elevational view of a park lock assembly according to another embodiment of the present invention in an engaged (Park) state or operating mode. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     With reference now to  FIG. 1 , a motor vehicle drive train is illustrated and generally designated by the reference number  10 . The motor vehicle drive train  10  includes a prime mover such as an internal combustion engine  12  which may be either gas, E85 or Diesel fueled. Alternatively, the engine  12  may be a full electric or a hybrid power plant utilizing both one or more electric motors and an internal combustion engine. Typically, the output of the engine  12  is directly coupled to and drives an input of a torque converter  14  and the output of the torque converter  14  is directly coupled to and drives an input of a multiple speed automatic transmission assembly  16 . The automatic transmission assembly  16  will typically include a plurality of planetary gear sets, clutches, brakes and actuators that are selectively engaged to provide a plurality of forward gear ratios and one reverse gear ratio. Power from the automatic transmission assembly  16  is delivered by an output shaft  18  with which a park lock assembly  20  according to the present invention is associated. The automatic transmission assembly  16  and the park lock assembly  20  are received within a metal housing  22  which is typically cast aluminum or magnesium. A resilient seal  24  is disposed between the output shaft  18  and the housing  22  and provides a fluid tight seal therebetween to retain transmission fluid (not illustrated) in the housing  22 . The output shaft  18  is coupled to and drives a final drive assembly  26  which may include a prop shaft, a differential, a pair of axles and wheels and tires (all not illustrated). 
     The park lock assembly  20  includes a lobed wheel  30  having a plurality of radially projecting, rounded fingers or lobes  32  alternating with rounded, reduced diameter recesses  34 . The lobed wheel  30  is secured to the output shaft  18  by any suitable means such as welding, staking, interengaging splines and snap rings or a key (all not illustrated) and rotates therewith. While seven of the lobes  32  are illustrated, it should be appreciated that the number of lobes  32  may be adjusted up or down if a suitable dimensional difference between the radial distance to the tips of the lobes  32  and the radial distance to a chordal reference line extending between two ends of two adjacent lobes  32  is great enough that the park lock assembly  20  will operate properly, i.e., provide sufficient restraining torque to the output shaft  18  when engaged to inhibit vehicle motion. Fewer lobes  32  facilitate generation of greater restraining torque but may cause undesirably abrupt engagement of Park whereas more lobes  32  will provide smoother engagement but may not generate sufficient restraining torque. 
     Aligned with the lobed wheel  30  is a stiffly resilient lever arm  36  having a first enlarged circular end  38   a  which defines a first through opening  42   a  and a second enlarged circular end  38   b  which defines a second through opening  42   b . The first through opening  42   a  receives a pivot pin  44  which may be secured to and extend from the housing  22  which pivotally supports the lever arm  36 . A snap ring  46  or other retaining feature or treatment such as staking maintains the lever arm  36  on the pivot pin  44 . The pivot pin  44  may include a shoulder (not illustrated) to further axially locate the lever arm  36 . 
     The second enlarged circular end  38   b  of the lever arm  36  is disposed operably adjacent and engages a rotatable cam  50 . The rotatable cam  50  defines a surface  52  having a unitary, i.e., smoothly increasing (or decreasing), profile and a single, radially extending wall or shoulder  54 . It should be appreciated that the exterior surface of the first enlarged circular end  38   a  of the lever arm  36  is essentially unimportant whereas the first through opening  42   a  provides a pivoting mounting. Likewise, the second through opening  42   b  is essentially unimportant whereas the second enlarged circular end  38   b  provides an appropriate curved surface which functions as a cam follower engaging the surface  52  of the rotatable cam  50 . The lever arm  36  is preferably fabricated in this configuration, however, to simplify and speed manufacture of the park lock assembly  20  as it may be installed and will function in any orientation. 
     The rotatable cam  50  is secured to and rotates with an output shaft  56  of a bi-directional motor and gear reduction assembly  60  which is supplied with electrical energy through a multiple conductor cable  62 . The motor and gear reduction assembly  60  are preferably configured, either electrically or mechanically, to rotate the output shaft  56  and the cam  50  approximately 320° to 330° and in any event less than 360°. 
     An ear or projection  64  extending from the inner surface of the housing  22  receives and supports a return or compression spring  66  which engages a side of the lever arm  36  opposite the rotatable cam  50 . The return spring  66  provides a biasing force to the lever arm  36  and ensures that it moves away from and out of contact with the lobed wheel  30  when the rotatable cam  50  is in its non-Park position illustrated in  FIG. 1 . Alternatively, the compression spring  66  may be replaced with a tension spring (not illustrated) disposed on the opposite side of the lever arm  36  or a torsion spring (not illustrated) disposed about the axis of the pivot pin  44 . 
     Turning now to  FIGS. 2 and 3 , an alternate embodiment of the lobed wheel  30  is illustrated and designated by the reference number  30 ′. The alternate embodiment lobed wheel  30 ′ is designed and intended to be a lower friction device and thus includes a plurality of friction reducing rollers  70  rotatably disposed in aligned pairs of circular openings  72  proximate the ends of the lobes  32 ′ such that the lever arm  36  engages the rollers  70  rather than the surfaces of the lobes  32 . 
     Operation of the park lock assembly  20  according to the present invention will now be described with reference to  FIGS. 2 and 3 . It should be understood that this description applies equally to the embodiment having the lobed wheel  30  illustrated in  FIG. 1 . In  FIG. 2 , the rotatable cam  50  is in a non-Park position or operating mode. As such, the lever arm  36  is biased and moved up by the return spring  66  and the lever arm  36  is out of contact with the rollers  72  on the lobed wheel  30 ′ which is therefore free to rotate. This is the position of the lobed wheel  30 ′ and the lever arm  36  during operation of the motor vehicle drive train  10  in forward and reverse gears and neutral. 
     In  FIG. 3 , the gear selector (not illustrated) has been moved to Park or P and the rotatable cam  50  has rotated into the Park position, translating the second end  38   b  of the lever arm  36  down such that the center portion of the lever arm  36  engages the rollers  72  of the lobed wheel  30 ′. The force exerted by the lever arm  36  on the rollers  72  of the lobed wheel  30 ′ will inhibit rotation of the output shaft  18  up to a maximum torque value at which point the lever arm  36  will flex sufficiently that the output shaft  18  will rotate until the torque on the output shaft  18  drops below the maximum torque value. When this occurs, the lever arm  36  will once again prevent rotation of the output shaft  18 . 
     It will thus be appreciated that the park lock assembly  20  according to the present invention represents an improvement from many operating perspectives. First of all, the assembly  20  is torque based rather than engagement speed based as are current designs. Second of all, the assembly  20  is symmetrical in configuration which thus provides the same performance during both forward and reverse engagement. Third, the assembly  20  ratchets more smoothly and quietly than current designs if engaged while the vehicle is in motion. Lastly, the assembly  20  is more rugged than current designs since engagement at high vehicle speeds causes the same flexing of the lever arm  36  as does engagement at lower speeds. 
     As noted, the foregoing description of the invention is merely exemplary in nature and variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.