Patent Publication Number: US-2017363168-A1

Title: Anti-tilt feature for torsion spring for default-to-park lever

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of co-pending U.S. application Ser. No. 14/938,178, filed Nov. 11, 2015, which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to a shift-by-wire transmission, and, more specifically, to a bias spring operable in a default-to-park mechanism. 
     In a shift-by-wire transmission arrangement, the traditional mechanical connection between a transmission gear selector and the transmission is eliminated. Instead, a transmission control device transmits an electrical signal to an electronic controller which directs separate actuators to apply or release the various friction elements of the transmission to obtain a desired gear ratio. The control device is no longer necessarily in the form of a lever because the control device is no longer moving a mechanical connection for controlling the transmission. Instead, the control device is typically an electro-mechanical interface (e.g., a series of buttons, lever, or knob) that is used to instruct the transmission to switch between the transmission ranges. 
     In order to ensure that a vehicle enters an immobile state when a shift-by-wire system enters a Park state, a failsafe system may be implemented known as a default-to-park system. In some embodiments, the transmission may be provided with a park pawl operated by an actuator in order to selectably engage with at least one gear in the transmission to inhibit vehicle motion or to release from the at least one gear to permit vehicle motion. The park pawl may be engaged in response to a driver shifting the transmission gear selector to PARK. 
     The actuator for a default-to-park system may be a hydraulic actuator (i.e., valve), for example. Under normal vehicle operation, an engine drives a transmission pump to supply hydraulic pressure to the actuator and enable application or release of the park pawl. When the engine is turned off or in the event of certain component failures, the hydraulic pressure is lost. A return element such as a bias spring is used to automatically return the park pawl into engagement with the transmission to act as a transmission brake or lock so that vehicle movement is halted by default under those conditions. 
     The bias spring is typically comprised of a torsion spring, as shown in U.S. Patent Application Publication US2015/0308571, published Oct. 29, 2015, entitled “Parking Override Device for a Shift-By-Wire Transmission,” which is incorporated herein by reference in its entirety. Torsion springs are used in many applications where package space or other considerations may preclude the use of an extension or compression spring. One potential problem with torsion springs in the known configurations is the tendency for the ends where the spring is anchored (to either a stationary or a moving component) to “tilt” as the spring is loaded (i.e., wound-up) by motion of the components. If great enough, this tilt can cause the spring end to become disconnected from the component it is intended to remain in contact with. This tendency to tilt is usually countered by increasing the wire diameter of the spring itself. However, increasing the wire diameter has the effect of increasing the rate of the spring, so that as the spring is deflected the spring force rapidly increases. This increase in force from the spring is often undesirable. The desired state for the spring as used in the default-to-park mechanism is to provide an initial loading force (i.e., an “installed force”) when the components are in their resting state (to keep the park pawl engaged), with the force increasing as little as possible as the spring is deflected during operation of the components so that the capacity (and cost) of the actuator can be kept low. 
     SUMMARY OF THE INVENTION 
     The present invention provides a torsion spring configuration that prevents twisting because of a counterbalancing of the twisting force due to a special shaping of the attachment of one end of the spring to a lever member in the default-to-park system. Thus, the invention allows the use of a spring with a smaller wire diameter, and consequently a lower spring rate. Besides enabling the use of a lighter and less expensive spring, the power of the actuator needed to overcome the spring force is reduced. 
     In one aspect of the invention, a default-to-park system for a transmission comprises a lever member having a cylindrical hub configured to rotate about a spindle. The lever member has a transverse plate including a lever arm engageable with a park release actuator and has a receiver engageable with a link to a park pawl of a transmission brake. A bias spring comprises torsion coils disposed over the hub, a fixed end, and a drive end. The drive end has a heel segment bearing against a transverse surface of the transverse plate facing the torsion coils and a loading hook wrapped around an edge of the transverse plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of one typical example of a shift-by-wire system for incorporating the invention. 
         FIG. 2  is a perspective view of a lever mechanism for a default-to-park system using a bias spring. 
         FIG. 3  is a side view of a lever mechanism according to one embodiment of the invention. 
         FIG. 4  is a perspective view of the lever mechanism of  FIG. 4  installed in a transmission case. 
         FIGS. 5 and 6  are perspective views of the lever mechanism of  FIG. 4 . 
         FIG. 7  is a perspective view of the bias spring of  FIG. 4 . 
         FIG. 8  is another perspective view of the lever mechanism of  FIG. 4  installed in the transmission case. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a shift-by-wire system  10  for an automatic transmission  11  includes a gear shift module (GSM)  12  converting a manually set position of a shift control element into an electrical signal and a powertrain control module (PCM)  13 . Automatic transmission  11  further includes a hydraulic actuator  17  and a range sensor  19 . A default-to-park system  15  includes a park pawl  16 , a lever mechanism  18  (driven by actuator  17 ), a link  24 , and a bias spring  14 . 
     GSM  12  may be located in the passenger compartment of a vehicle where it is operated manually by the vehicle operator, who selects the desired automatic transmission range by actuating a button or shift lever corresponding to the desired range: Park (P), Reverse (R), Neutral (N), Drive (D) and Low (L). These input commands are translated into signals representing the vehicle operator&#39;s requests for the desired transmission gear or range. PCM  13  is the decision maker, managing all of the interlocks for shift-by-wire system  10 . Hydraulic actuator  17 , part of automatic transmission  11 , executes the operator&#39;s requests received via PCM  13  to shift automatic transmission  11  to the appropriate gear or range after activating hydraulic actuator  17  so that park pawl  16  first disengages from transmission  11 , with resulting transmission range reported by range sensor  19 . Modules  12  and  13  communicate over a communication area network (CAN). When the vehicle is parked or a fault occurs for which the vehicle should be made immobile, PCM  13  deactivates hydraulic actuator  17  which returns link  24  and park pawl  16  to the engaged position under the influence of bias spring  14 . 
       FIG. 2  shows lever mechanism  18  including a lever member  20  having a transverse lever plate  21  and a cylindrical hub  25  which are configured to rotate about a fixed spindle or shaft  26 . Lever plate  21  includes a lever arm  22  engageable with a park release actuator (not shown) and a receiver  23  engaged with a link  24  to the park pawl of the transmission brake. Spindle  26  is mounted transverse to the axial operating direction of the park release actuator and link  24  and has one end  27  adapted to be fixed to a transmission case (not shown). Another hub  28  may be mounted on spindle  26  inside cylindrical hub  25  as part of an override mechanism as described in US patent application publication US2015/0308571A1. 
     A bias spring  30  mounted over cylindrical hub  25  has torsion coils  31 , a fixed end  32 , and a drive end  33 . Fixed end  32  is captured by an appropriate surface on the transmission case (not shown). Drive end  33  is captured in a slot  34  formed into an edge of lever plate  21 . When the park release actuator moves lever arm  22  forward to wind-up bias spring  30 , the potential exists for spring  30  to tilt (i.e., rotate about an axis perpendicular to the axis of coils  31 ), especially at drive end  33 . Consequently, drive end  33  could dislodge from slot  34  unless the wire used to form spring  30  is sufficiently rigid to resist the tilt. To obtain the desired stiffness, the wire diameter could be increased which leads to the undesirable results described above. 
     The present invention adopts a modified interface between the drive end of the bias spring and the transverse lever plate as shown in  FIGS. 3-8 .  FIG. 3  is a side view looking along the rotational axis of the hub, upon which a bias spring  40  is mounted. A transverse lever plate  35  has a lever arm  36  interfaced with a park release actuator  37 . A receiver  38  (e.g., a mounting hole) in lever plate  35  connects to one end of link  24 . Solid lines in  FIG. 3  show the position of the lever mechanism in its engaged state in which the park pawl prevents movement of the transmission. Upon activation of actuator  37 , it moves in the direction of arrow  41  to a position shown by dashed line  42 . Transverse lever plate  35  moves to the position shown at dashed line  43 , whereby link  24  moves in the direction of arrow  44  to the position shown by the dashed line  45 . When actuator  37  is deactivated, bias spring  40  moves the lever mechanism back to the position show in solid lines. 
       FIG. 4  is a perspective view of the lever mechanism installed in a transmission case  46 . The park release actuator is removed for clarity. Case  46  includes a boss  47  for receiving the spindle and for providing a bearing surface for a fixed end  48  of bias spring  40 . Spring  40  includes torsion coils  50  disposed over cylindrical hub  52  which is fixedly to lever plate  35 . Spring  40  has a drive end  51  which is wound up by movement of lever arm  36  to the left in  FIG. 4  and which unwinds upon deactivation of the park release actuator in order to move link  24  to the left so that the transmission becomes locked. 
     Referring to  FIG. 5 , drive end  51  of spring  40  includes a heel segment  54  bearing against a transverse plate surface  55  of plate  35  (i.e., the surface that faces coils  50 ). Drive end  51  further includes a loading hook  56  wrapped around the edge of lever plate  35 . As shown in  FIG. 6 , loading hook  56  includes a wraparound end  58  bearing against a second surface  57  of plate  35 . An override mechanism  59  may also be mounted over spindle  53 . 
     Referring again to  FIG. 5 , the configuration of heel segment  54  and loading hook  56  provide a counterbalance for bias spring  40  that directly resists twisting. Heel segment  54  lies flat against surface  55  for a predetermined length which is sufficient to resist twisting. In addition, friction between heel segment  54  and lever plate  35  limits lateral movement of the drive end of the spring. The wraparound shape of loading hook  56  and end  58  further limit the twisting while ensuring that the direction of load transfer between spring  40  and lever plate  35  is aligned in the plane of rotation of the lever mechanism. 
       FIG. 7  shows spring  40  in greater detail. Heel segment  54  may be separated from torsion coils  50  by a bridge segment  60  which extends coaxially with respect to the spindle. Bridge segment  60  is provided to accommodate a hub capturing mechanism as shown in  FIG. 8 . Thus, cylindrical hub  52  includes a guide slot  61  to accommodate a guide pin  63  which is secured to the spindle (or to an internal hub of an override mechanism) upon which hub  52  rotates. Guide pin  63  thus maintains proper alignment for hub  52  and lever plate  35  at a desired location along the spindle. Since guide pin  63  is installed after hub  52  with bias spring  40  have been installed, guide slot  61  must remain accessible (i.e., not be blocked by spring  40 ). Thus, bridge segment  60  is utilized between the torsion coils and the drive end of spring  40 . However, the presence of bridge segment  60  increases the potential for tilting of bias spring  40 . Nevertheless, such tendency to tilt is counterbalanced by the action of heel segment  54  and loading hook  56 . Consequently, a bias spring of a small diameter wire can be used without any associated risk of detachment of the spring due to twisting. Therefore, a more desirable spring rate can be obtained along with lowering costs and weight for the default-to-park system.