Abstract:
A park brake assembly includes a base. A park brake lever is pivotal about the base. A pawl is pivotal about a pawl pivot. The pawl is linked to the brake lever. A sector is attached to the base and the pawl releasably locks and unlocks relative to the sector. The pawl includes a pivot slot that receives the pawl pivot and wherein the pawl translates about the pawl pivot changing an angle of engagement of the pawl relative to the sector when subjected to a force.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. Provisional Patent Application 61/430,744 filed Jan. 7, 2011, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to parking brakes. 
     BACKGROUND OF THE INVENTION 
     Parking brakes may include a pawl and sector to lock the parking brake in a desired position. In operation the pawl tip is positioned in a root of the sector teeth to lock the pawl and sector. The pawl rotates under the bias of a spring to enter into the root of the sector teeth as the pawl travels across the sector. One may want an enhanced locking function of a park brake. 
     There is therefore a need in the art for a parking brake that provides an enhanced locking function. There is also a need in the art for a park brake that provides an enhanced locking function and utilizes a minimum number of components resulting in a cost effective and easily manufactured parking brake. 
     SUMMARY OF THE INVENTION 
     In one aspect there is disclosed a park brake assembly that includes a base. A park brake lever is pivotal about the base. The park brake lever is linked to a pawl. A sector releasably mates with the pawl to releasably lock the park brake in position. The pawl includes an angle of engagement to initially lock with the sector to provide a locking position wherein the pawl translates about a pawl pivot to change the angle of engagement of the pawl and sector when subjected to a force. 
     In another aspect there is disclosed a park brake assembly that includes a base. A park brake lever is pivotal about the base. The park brake lever is linked to a pawl. A sector releasably mates with the pawl to releasably lock the park brake in position. The pawl includes a first and second pivot axis wherein the pawl shifts between the first and second pivot axes to change the angle of engagement of the pawl and sector when subjected to a force. 
     In a further aspect there is disclosed a park brake assembly that includes a base. A park brake lever is pivotal about the base. A pawl is pivotal about a pawl pivot. The pawl is linked to the brake lever. A sector is attached to the base and the pawl releasably locks and unlocks relative to the sector. The pawl includes a pivot slot that receives the pawl pivot and wherein the pawl translates about the pawl pivot changing an angle of engagement of the pawl relative to the sector when subjected to a force. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of one embodiment of a park brake assembly; 
         FIG. 2  is a partial perspective view including the brake lever, pawl, sector and biasing member; 
         FIG. 3  is a partial perspective view of the brake lever, rod pawl and biasing member; 
         FIG. 4  is a partial perspective view of the pawl, sector and base; 
         FIG. 5  is a partial perspective view of a pawl and sector in a half lock state; 
         FIG. 6  is a partial perspective view of the pawl translated about the pivot. 
         FIG. 7  is a force and moment free body diagram detailing a pawl and sector. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Force and Moment calculations 
     Referring to  FIG. 7 , the following specifications and definitions are described. 
     
         
         Sector: Primary gear which has a moment applied to it. 
         Pawl: Secondary gear which applies an opposing moment on the Sector. 
         Contact Point: The point which the Pawl Tip is contacting the Sector Tooth. 
         F T : The line of force tangent to the arc of Sector tooth rotation, through the contact point 
         Fp: The line of force from the Pawl pivot the contact point. 
         F N : The line of force applied perpendicular or normal, to the Sector tooth at the contact point. 
         F 11 : The line of force on the Pawl tip, due to the friction between the Pawl and Sector teeth. 
         F 12 : The line of force on the Pawl pivot hole, due to the friction between the Pawl and Rivet or Bushing. 
       
    
     The forces described in more below may be made with reference to the free body diagram of  FIG. 7 . 
     Referring to the Figures there is shown one embodiment of a park brake assembly  10 . The park brake assembly  10  includes a brake lever  12  that is pivotally mounted to a base  14  to actuate the park brake  10  between locked and released positions. The brake lever  12  is pivotal about a pivot pin  16  that is attached to the base  14 . The brake lever  12  includes a rod  18  that is coupled to an actuator  20  on one end of the rod  18  and to a locking pawl  22  at another end of the rod  18 . A mounting bracket  24  is attached to the brake lever  12  and is rotatable about the pivot  16 . The mounting bracket  24  may be connected to a cable mechanism  25  that links to the brake of a vehicle. 
     The park brake assembly  10  includes a sector  26  that is attached to the base  14 . The sector  26  includes teeth  28  formed thereon that mate with the pawl  22 . The pawl  22  includes a pivot slot  29  formed therein and is rotatable about a pawl pivot  30  that defines a pivot axis  32  of the pawl  22 . In one aspect, the pivot slot  29  is oversized relative to the pivot  30  allowing the pawl  22  to translate about the pivot  30 , as will be described in more detail below. The pawl slot  29  may also have a shape relative to the pivot  30  that allows for translation of the pawl  22 . In another aspect, the pivot  30  may have a shape that allows for translation of the pawl  22 . The pawl slot  29  may have a larger diameter or size than the pivot  30 . As referenced above, the pivot slot may include a predetermined shape to allow translation of the pawl  22 . For example, the pivot slot may be ovoid or out of round or have other shapes that allow for the translation or movement of the pawl  22 . 
     In one aspect, the pawl  22  translates relative to the pivot in an amount such that a half lock conditioned is prevented while the pawl does not move to a position where a no lock condition occurs with the sector  26 . In one aspect, the pawl  22  may translate such that an angular shift of the pawl  22  is less than 0.1 millimeters. 
     The pawl  22  also includes a tip  31  that enters into the space or root between adjacent teeth  28  on the sector  26 . The tip  31  may include various configurations such as the double tip  31  of  FIG. 2  or the single tip  31  of  FIG. 5 . 
     The pawl  22  also includes a spring member  34  that contacts the pawl  22  at one end of the spring member  34  and contacts the mounting bracket  24  at another end of the spring member  34 . The spring member  34  biases the pawl  22  about the pivot  30  in a desired direction as well as biases against the actuator  20  coupled to the rod  18 . The biasing force applied by the spring member  34  forces the pawl  22  into the root of the sector  26  between adjacent teeth  28  of the sector  26 . The spring member  34  may be of various types and may be mounted at various locations to induce the desired bias on the pawl  22 . In the depicted embodiment of  FIG. 2 , the spring member  34  may be a torsion spring that imparts a clockwise bias to the pawl  22  forcing the tip  31  of the pawl  22  into the root of the sector  26 . 
     Referring to  FIGS. 5 and 6  there is shown a partial view of a pawl  22  and sector  26  in a half lock and translated position respectively. In  FIG. 5  the spring member  34  force is represented by the direction A arrow which rotates the pawl  22  clockwise about the pivot  30  and pushes the pawl  22  against the pivot  30  in the direction A. When the pawl  22  moves in an opposing direction or clockwise, the sum of the moments about the pawl  22  results in a contact angle between the pawl  22  and sector  26  such that there is a tip to tip relationship where the friction forces (F 11  above) of the pawl  22  and sector  26  prevent the pawl tip  31  from dropping into the root of the sector  26  between adjacent teeth  28 . In this position the sector  26  exerts a force on the pawl  22  shown by the direction B. 
     The force applied by the sector  26  in the direction B causes the pawl  22  to translate about the pivot  30 , as shown in  FIG. 6 . The pawl  22  has moved or translated from the pivot axis  32  (R 1 ) shown in  FIG. 5  to the pivot axis  32  (R 2 ) shown in  FIG. 6 . The pivot slot  29  is oversized or has a predetermined shape allowing the pivot to move from one portion of the pivot slot  29  to another portion of the pivot slot  29 . Once the pawl has translated its position or moved from a first pivot axis  32  in  FIG. 5  to a second pivot axis  32  in  FIG. 6 , the sum of the forces or moments changes such that the angle of the force or direction of the force causing the rotation of the pawl  22  changes overcoming the resistance of the tip to tip frictional forces of the half lock condition of  FIG. 5 . 
     In use the park brake  10  may be moved from an initial unlocked position as an operator depresses the actuator  20  against the biasing force of the spring member  34 . The brake lever  12  is moved about the pivot  16  such that the pawl  22  travels over the sector  26 . When the operator releases the actuator  20 , the spring member  34  exerts a rotational force on the pawl  22  such that the pawl  22  rotates about the pawl pivot  30 . The tip or tips  31  of the pawl  22  is urged into the root of the sector  26  between adjacent teeth  28  of the sector  26  locking the pawl  22  relative to the sector  26 . 
     The sector  26  may exert a force on the pawl  22  such that the pawl  22  translates about the pawl pivot  30 , as described above. The translation of the pawl  22  about the pawl pivot  30  allows the spring member to rotate the pawl  22  into the sector  26  preventing a half lock condition. 
     The park brake  10  is now releasably locked in position. To remove or unlock the park brake  10  an operator may lift the brake lever  12  unloading the engagement of the pawl  22  and sector  26  and again depress the actuator  20  rotating the pawl  22  out of the sector  26  such that the brake lever  12  may be pivoted about the pivot  16  to the initial starting or disengaged position. 
     The invention has been described in an illustrative manner. It is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than limitation. Many modifications and variations of the invention are possible in light of the above teachings. Therefore, within the scope of the appended claims, the invention may be practiced other than as specifically described.