Abstract:
A parking pawl is implemented on a series type, hybrid electric vehicle using a pair of parallel worm gear rollers. The worm gear rollers are placed on opposite sides of a transmission gear from one another and mounted for rotation along their major axes allowing the worm gear rollers to be rotated to present either a planed side or a threaded section to the gear. The gear is circumferentially slotted to cooperate with the threaded section of the rollers to prevent rotation of the gear, but to turn freely when the planed faces of the worm gear roller are adjacent the gear.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a hybrid electric vehicle and more particularly to a park pawl for the transmission of such a vehicle.  
         BACKGROUND OF THE INVENTION  
         [0002]    Series type hybrid electric vehicles have an electric generator driven by a thermal engine to supply electrical power to the vehicles&#39; battery and electrical power distribution systems, which in turn support operation of an electric drive motor. Unlike parallel type electric vehicles which have a drive line which can be driven directly by the thermal engine, series type electric vehicles are driven by only the electric drive motor. The term series refers to the path of energy from the thermal engines to the drive line and hence to a powered axle and wheels. Several advantages follow from this arrangement. For example, if the drive motor does not require power and the thermal engine is running, then all electrical power can be used to charge the battery, and run other electrical power using systems. Further, the drive motor and thermal engine may be positioned on the vehicle chassis without consideration of other&#39;s position.  
           [0003]    In a series type hybrid electric vehicle, the drive motor may be connected to the driven axle through a gear reduction transmission and a drive train. The transmission is typically based on a ring or planetary gear set comprising several rotatable elements. Park brakes have been provided in such vehicles through a mechanical, non-fluid operated mechanism in proximity to the a portion, or portions, of the drive train, such as illustrated in U.S. Pat. No. 6,186,253. However, park pawls, such as commonly found in automatic transmissions, which provide a back up to park brakes, are not readily duplicated with off the shelf motors and with the gear reduction devices used as transmissions. Nor is the possibility of leaving the vehicle in gear to use the thermal engine as a brake available. What is needed is a mechanically reliable park pawl easily implemented with electric vehicles.  
         SUMMARY OF THE INVENTION  
         [0004]    According to the invention there is provided a park pawl for a transmission for an electric motor. The transmission comprises gears which may be engaged with the drive motor, to propel a vehicle. The park pawl engages a gear to prevent its rotation relative to the frame of the vehicle. The gear is circular with pawl engagement points around its circumference. The pawl proper comprises a pair of worm gear rollers arrayed on opposite sides of the gear from one another and oriented to have parallel axes of rotation in the plane of the gear and perpendicular to the axis of rotation of the gear. The worm gear rollers have planed faces parallel to their respective axes of rotation with the worm gear rollers being positionable to present the planed faces to the gear and thereby allowing the gear to rotate. The worm gear rollers are further positionable to bring their respective gear threads into engagement with the pawl engagement points of the gear by rotation of the rollers, preventing rotation of the gear.  
           [0005]    Additional effects, features and advantages will be apparent in the written description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0007]    [0007]FIG. 1 is a rear quarter perspective view of a chassis of a series type hybrid electric vehicle.  
         [0008]    [0008]FIG. 2 is a front elevation of a traction motor transmission illustrating the park pawl in a disengaged state.  
         [0009]    [0009]FIG. 3 is a top view of the traction motor transmission of FIG. 2.  
         [0010]    [0010]FIG. 4 is a front elevation of the transmission and park pawl showing the park pawl moved to an engaged position.  
         [0011]    [0011]FIG. 5 is a top view of the transmission and park pawl showing the park pawl moved to an engaged position. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    Referring now to the drawings in greater detail, in FIGS.  1  to  5 , there is shown a series type hybrid electric vehicle  101  with an electric drive motor  11  and transmission  23 . Vehicle  101  has a chassis  102  with two frame rails  103   a  and  103   b . There is a thermal engine  104 , preferably a diesel, and an electric generator  105  supported from frame rails  103   a  and  103   b  and positioned relatively forward on the vehicle  101 . The generator  105  is driven by motor  104  and is electrically engaged by cables  111  to an electric control system (not shown) and batteries (not shown). The batteries are located within a battery box  108  which hangs from right frame rail  103   b . The electric control system and batteries are electrically engaged by cables  111  to electric drive motor  11 . The electric drive motor  11  is also supported from frame rails  103   a  and  103   b  in part from a chassis cross member  17  and other similar components (not shown).  
         [0013]    Electric drive motor  11  is mechanically engaged to drive a drive or rear axle assembly  110  with rear wheels  112  through transmission  23  and a prop or drive shaft  113 . Although described and shown as rear drive, the invention may also be applied in a front drive configuration where a drive axle assembly is positioned forward on vehicle  101 . Indeed, the invention may be applied to non-vehicle applications where it is desirable to provide a means to positionally lock a drive line. In a vehicle application, when motor  11  is energized and rotates, drive shaft  113  rotates and this rotational motion is coupled to rear wheels  112  through transmission  23  and rear axle assembly  110 . Transmission  23  steps down the output of motor  11  to provide the mechanical advantage required for propelling vehicle  101 . As described below, transmission  23  also provides a lock feature operating as a highly secure, and reliable, park pawl.  
         [0014]    The park pawl  20  of the preferred embodiment is implemented by modifying a ring gear  22  by milling (or casting) the gear&#39;s circumferential edge into a plurality of teeth  30 , with the gaps  31  between the teeth providing a plurality of pawl engagement points. The pawls are provided by the threads  27  and  29  of two worm gear rollers  26  and  28  (or similar structures), which are disposed on opposite sides from one another with respect to gear  22 . Worm gear rollers  26  and  28  are milled to provide flat faces  44  and  46 , which may be positioned adjacent gear  22  and facing one another upon rotation of the rollers. In this position of rollers  26  and  28 , gear  22  is free to rotate on its output shaft  24 . Approximately three quarters of the circumference of rollers  26  and  28  is circumscribed by exterior threads  27  and  29 , and it is these partial threads which provide the pawl structures for insertion into the gaps  31  between teeth  30  on the gear&#39;s circumference when the worm gear rollers are rotated as illustrated in FIGS. 4 and 5.  
         [0015]    In the preferred embodiment, the rotation of worm gear rollers  26  and  28  is coordinated, with worm gear roller  26  rotating counterclockwise as viewed from the top and indicated by the letter C in FIG. 3 and worm gear roller  28  rotating clockwise as indicated by the letter B to engage with gear  22 . Worm gear rollers  26  and  28  are interconnected to one another in part through a crank arm  32  which rotates on a central shaft  57 . Shaft  57  has an axis of rotation parallel to the axes of rotation of rollers  26  and  28  and is displaced to one side of one of the major surfaces of gear  22 . Crank arm  32  is coupled to rollers  26  and  28  by push rods  50  and  52 , each of which are attached at one end thereof by pivoting mounts  62  and  63  to opposite ends of the crank arm. The opposite, free ends of push rods  50  and  52  are attached at pivot points  60  and  62  to the ends of levers  54  and  56 . Levers  54  and  56  extend to one side of worm gear rollers  26  and  28  away from and perpendicular to the axes of rotation of the worm gear rollers  26  and  28 .  
         [0016]    With rotation of crank arm  32  in the clockwise direction indicated by the letter A in FIG. 3, push rods  50  and  52  push levers  54  and  56  outwardly turning worm gear rollers  26  and  28  in opposite directions to bring the threaded portion of the rollers into engagement with gear  22 . Reversing the direction of rotation of crank arm  32  pulls levers  54  and  56  back to parallel positions and returns the flat or milled faces  44  and  46  to positions facing the edge of gear  22 . A spring  34  biases park pawl  20  toward the position illustrated in FIG. 5, so that should the threads  27  and  29  initially impact against teeth  30  instead of sliding into the slots or gaps  31  between the teeth, the threads will be urged into the gaps by the spring force upon any further rotation of gear  22 .  
         [0017]    Spring  34  may be incorporated into park pawl  20  in a number of ways. As illustrated, spring  34  is shown coupled between crank arm  57  and a fixed point and urges the crank arm to turn in the direction indicated by the letter A. Alternatively, a compression spring could be connected between crank arm  57  and push rods  50  and  52  to bias the rods to a more open position. Still other combinations will occur to those skilled in the art.  
         [0018]    Activation of the park pawl  20  is under a vehicle operator&#39;s control, which may be done either through a gear shift lever  40  or by a handle  42  installed in a vehicle cab. Where a handle is used crank arm positioning  36  is a direct mechanical linkage for rotating shaft  57 . An indent position for handle  42  may be provided for countering the spring bias of spring  34  when it is desired to operate the vehicle. Alternatively, propulsion control  38  may be used to operate a crank positioning motor  36  when a gear shift selector is moved from park to drive, neutral or reverse.  
         [0019]    The park pawl of the present invention provides secure locking of a transmission, even under shock loading. Positioning of the pawl can be effected using a simple combination of a spring and mechanical linkage. The cost is low and the modification required of the off the shelf components minimal.  
         [0020]    While the invention is shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit and scope of the invention.