Abstract:
A brake actuating assembly includes a cam member that has first and second cam surfaces. A first piston in a first hydraulic circuit is selectively enabled to axially translate the cam member from a released position to an actuation position while an actuation piston moves friction surfaces into engagement with a rotor to effect a brake application. During a brake application, pressurized fluid is diverted away from a second piston in a second hydraulic circuit that is later enabled to return the cam member to the released position. A resilient assembly selectively engages the second cam surface to obliquely move the cam member toward the actuation piston during a brake application. An electrically actuable valve supplies pressure fluid that acts on and moves the resilient assembly away from the second cam surface and thereafter allow a second hydraulic piston to return cam member to the released position.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to a parking brake for a hydraulic braking systems wherein service brakes are hydraulically applied to effect a brake application in the wheel brakes and resiliently retained in an applied condition without hydraulic pressure until released by a reapplication of hydraulic pressure. 
   2. Description of the Related Art 
   It is known to control the brake function of a parking brake through a cable control of a cam or lever to mechanically apply and release a service brake. In addition, electrical control of parking brake systems could also be achieved by a spring apply and a hydraulic or air release using a two-position switch to supply current to activate a solenoid actuated valve. The parking brake being applied by a spring that acts through a linkage to provide a mechanical force that moves friction linings into engagement with a drum or disc brake to effect a parking brake application such as disclosed in shown in U.S. Pat. No. 5,370,449. A solenoid latching mechanism for a parking brake is shown in U.S. Pat. No. 5,443,132 and a hydraulically applied and released parking brake mechanism has been suggested in U.S. Pat. No. 5,161,650. This patented arrangement provides rigid coupling forcing brake friction surfaces into engagement. In addition, it is known to provide for a hydraulic apply, but a spring-loaded release of a parking brake mechanism. Unfortunately in such a spring loaded release design it could be susceptible to an unintended release in cases where the clamp load of the brake caliper might be reduced and as a result the cam could move to the released position under the force of the release spring. This would cause the parking brake to release unintentionally. 
   It is desirable to provide a certain amount of compliance or resilience to a parking brake assembly to avoid brake release or undue strain on the braking system due to dimensional variations in the brake mechanism as might, for example, be caused by temperature induced expansion or contraction of brake mechanism components. 
   SUMMARY OF INVENTION 
   The present invention provides solutions to the above problems by providing a resilient or compliant biasing force for maintaining vehicle brake friction surfaces engaged to provide a parking brake function. 
   The present invention improves upon previous designs in several ways including structure for a hydraulic apply and a hydraulic release while maintaining the apply function in a mechanically compliant fashion to prevents a possibility of an inadvertent release. The structure provides two distinct and positively held positions (applied and unapplied) that do not change during a variance in clamp load. The addition of parking brake compliance will prevent roll-away, reduce system clamping pressure, and provide a corresponding reduction of caliper stresses, while importantly not adding to caliper compliance during service brake applies which would increase pedal travel and degrade pedal feel. 
   The invention comprises, in one form thereof, a vehicle parking brake actuating assembly with a first hydraulic circuit for initially applying the vehicle parking brake and a second hydraulic circuit for returning the vehicle parking brake to an unapplied state. Once applied, a resilient bias assembly retains the vehicle parking brake in the applied state until the second hydraulic circuit is enabled. 
   In more particular detail, a vehicle parking brake actuating assembly has a hydraulic circuit that may be selectively enabled to initially apply the vehicle parking brake, a resilient bias assembly for retaining the vehicle parking brake in the applied state, and another hydraulic circuit that may be selectively enabled to return the vehicle parking brake to an unapplied state. The resilient bias assembly may include a brake piston for actuating the brake, a Belleville piston, a Belleville spring engaging the Belleville piston to move the Belleville piston toward the brake piston, a cam member disposed intermediate the Belleville piston and brake piston, and an arrangement including the hydraulic circuits for reciprocally shuttling the cam member between a brake applied position where the cam member transfers a biasing force from the Belleville spring the brake piston, and a brake unapplied position where application of Belleville spring force to the brake piston is precluded by the cam member. The hydraulic circuits may each include a hydraulic piston engaging a corresponding end of the cam member and responsive to applied hydraulic pressure to translate the cam member. The hydraulic circuits may each further include a displacement piston resiliently biased to displace fluid away from the corresponding hydraulic piston subsequent to enablement of the corresponding hydraulic circuit. 
   An advantage of the present invention resided in structure wherein brake caliper overstress is limited and yet inadvertent parking brake release is prevented to avoid vehicle roll after a parking brake has been applied. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a cross-sectional view of the brake actuating assembly according to the invention in one form in a brake released configuration; 
       FIGS. 2   a – 2   e  are cross-sectional views of the cam and adjoining parts of  FIG. 1  illustrating brake operation; and 
       FIG. 3  is a cross-sectional view of the brake actuating assembly of  FIG. 1 , but in a brake applied configuration. 
   

   Corresponding reference characters indicate corresponding parts throughout the several drawing views. 
   DETAILED DESCRIPTION 
   Referring now to the drawings and particularly to  FIG. 1 , there is shown an illustrative portion of a disk brake caliper assembly  11  including a caliper assembly housing  13  containing a service brake piston  15  which is movable upwardly upon command from the brake released position shown to retain a rotatable brake disk between first and second friction pads in a manner known in the prior art. A parking brake adjuster assembly  17  engages the service brake piston  15  near one end (upper as shown) and selectively engages a shuttle or cam member  23  near the other end. The cam member  23  is also selectively engaged from below by one end of a Belleville piston  21  that is moved into engagement with the cam member  23  by a Belleville spring or washer stack  19 . The opposed ends of the cam member  23  are engaged by an apply piston  25  and a release piston  27  respectively and outboard of the apply and release pistons are located a pair of spring biased displacement pistons  29  and  31 . Appropriate seals such as  45 ,  66  and  67  may be provided. An apply solenoid valve  37  provides, upon energization, a path for pressure fluid from fluid channel  33  by way of conduit  35  to move the apply piston  25  and the cam member  23  rightwardly as shown. When energized, the apply solenoid valve  37  also provides fluid flow through a fluid passageway  39  to force and move Belleville piston  21  downwardly out of engagement with cam notch  47  and compressing Belleville spring  19 . Similarly, a release solenoid valve  41  may be energized to provide a pressure fluid path from the fluid channel  33  by way of conduits  69  and  71  to move the release piston  27  toward the left as viewed. 
   In the caliper brake assembly  11  cross-sectional view shown in  FIG. 1 , the service and parking brake is shown in the unapplied or released state. Service brake applies are accomplished when fluid under pressure as commanded by the driver from the actuation system enters the fluid channel  33  and, by way of conduit  69 , causes the service brake piston  15  and parking brake adjuster assembly  17  to move outboard (upward as oriented in  FIG. 1 ) and thus apply the service brake. The parking brake is applied when fluid under pressure enters the designated fluid channel  33  and, as in the service brake application, acts upon the service brake piston  15  and parking brake adjuster assembly  17 , causing it to move outboard (upwardly) to affect a brake apply. This motion disengages the lower legs of adjuster assembly  17  from a pair of cam member  23  notches  49  and  51 . Fluid is then simultaneously or subsequently supplied to the apply piston  25  and Belleville piston  21  via the apply solenoid valve  37  that allows communication of the service brake circuit fluid pressure to the apply piston  25  by way of conduit  35 . This apply of the cam member  23 , via the hydraulic apply piston  25 , has been facilitated by the simultaneous or previous action of the service brake piston  15  to move the service brake piston and parking brake adjuster assembly  17  away from and out of the way of the apply path of the cam. The fluid pressure applied to the Belleville piston  21  by way of conduit  39  has also moved the Belleville piston  21  away from the cam and out of engagement with cam notch  47  while compressing the stack of Belleville washers  19 . Freeing the cam member  23  for axial translation is seen by comparing  FIGS. 2   a  and  2   b.  Axial movement of the cam member  23  is seen by comparing  FIGS. 2   b  and  2   c.  Room for the cam member  23  to move to the applied position of  FIGS. 2   d,    2   e  and  3  is provided by the movement of a displacement piston  29 . These displacement pistons  29  and  31  are disposed on each end of the cam member  23  outboard of the pistons  25  and  27  to ensure a reserved volume into which the cam can move when applied or released. The springs such as  53  on the backside of the displacement pistons are used to displace any fluid back to the actuation system when an apply or release has been accomplished and the system returns to a non-pressurized state. These displacement pistons are a place/volume keeper for the fluid that normally would be pushed back to a system reservoir during an apply or release. However, since only one fluid channel is connected between the brake and the actuation system, it is desirable to store this fluid during pressurization of the fluid channel, until such time that system pressure is returned to zero and then the spring loaded displacement piston can push this fluid back into the actuation system. Once the cam member  23  has moved into the apply position of  FIG. 3 , the service brake circuit fluid pressure throughout the entire brake, can then be reduced to zero. This will allow the service brake piston  15  and adjuster assembly  17  to relax back against the cam and correspondingly for the Belleville piston  21  to be pushed against the cam by the force of the Belleville washers. It also allows spring  53  to force displacement piston  31  to return fluid to the supply by way of channel  33 , all as illustrated by comparing  FIGS. 2   c  and  2   d.  In this apply position, the cam member  23  has a larger cross-section presented in between the service brake piston and adjuster assembly, and the Belleville piston  21  shown at  59  in  FIG. 2   d  than it did in the released position shown at  57  in  FIG. 2   a.  This causes the compressive load of the Belleville washer stack  19  to act directly against the service brake piston  15  and adjuster assembly  17  through the larger cross-section  59  of the cam. The cam is configured such that it can move freely transversely to its direction of reciprocation (up and down as viewed) by a distance illustrated at  55  in  FIG. 2   b  in the cam bore to allow the compressive force of the Belleville washer stack to act directly against the service brake piston and adjuster assembly. The cam member  23  is shown in the parking brake released position in  FIGS. 2   a – 2   b,  while  FIGS. 2   d  and  2   e  show the parking brake applied position. The service brake caliper acting as the brake component of the parking brake, is then held in the apply position by the cam and Belleville piston. The engagement of the Belleville piston into optional additional indents in the cam as shown at  43  in  FIG. 2   e  will enhance the prevention of longitudinal movement of the cam and thus prevent an unintended parking brake release. 
   This interaction between the Belleville washer stack  19 , and the service brake piston  15  and adjuster assembly  17  adds compliance to the parking brake components. This ensures that in cases, such as thermal contraction of the friction material, the resulting lost travel can be compensated by the movement of the Belleville washer stack to maintain a very consistent parking brake caliper clamp load on the shoe and lining assemblies to maintain the required level of torque. This added compliance is only present within the caliper during the parking brake application and therefore does not effect the compliance or pedal feel of the service brake system. Belleville washers are preferred for their size to force efficiency and for their ability under load to demonstrate a low spring rate. This low spring rate provides for a reduced fall-off in a brake clamp load and brake torque in cases such as thermal contraction of the friction material. 
   Release of the parking brake is accomplished when fluid under pressure enters the designated fluid channel  33  and, as in the service brake application, acts upon the service brake piston  15  and parking brake adjuster assembly  17  to move it outboard (upwardly as shown). This allows the Belleville piston  21  to move upward until it reaches its physical stop in the housing  13 . However, the gap created by the apply of the service brake piston and parking brake adjuster assembly is sufficient to allow longitudinal movement of the cam member  23  when fluid under pressure from the service brake circuit is communicated through conduit  69  to the release piston  27  via energization of the release solenoid valve  41 . This movement results in the cam member  23  shuttling back to the released position of  FIGS. 1 ,  2   a  and  2   b.  The attendant reduction in cross-section from  59  to  57  also acts to prevent longitudinal movement of the cam and would prevent the latching/reinforcing of an unintended parking brake apply. The pressure to the service brake piston and parking brake adjuster assembly is then released and the total release of the brake is accomplished as the released position of the cam presents a smaller cross-section  57  to the service brake piston and parking brake adjuster assembly, allowing it to fully retract to its released position. If the Belleville washer stack or spring  19 , or other resilient component, were not present in the design, then the alternative for compensating for brake lining relaxation would be to use the existing caliper bridge compliance. This would necessitate pressurizing the caliper to a much higher pressure to additionally stretch the caliper to provide for the lost travel capability. This would require an inordinately high pressure and provide excessive stress to the caliper. The incorporation of the Belleville stack into the design precludes these problems. 
   In summary, a wheeled vehicle parking brake is applied and released by first hydraulically forcing the vehicle service brake piston  15  into a vehicle wheel rotation braking condition by supplying pressure fluid by way of fluid channel  33 , and conduit  69  forcing the piston and adjuster assembly upwardly as viewed. A supplemental resilient bias is then applied to retain the vehicle service brake in the wheel rotation braking condition by initially temporarily applying hydraulic pressure by way of channel  33 , conduit  35 , the enabled valve  37  and conduit  39  to force Belleville piston  21  downwardly as viewed thereby temporarily overpowering the resilient bias as shown in  FIG. 2   b.  Cam member  23  along with apply piston  25 , release piston  27  and displacement piston  29  are then translated along a path generally oblique to the direction in which the resilient bias is applied as illustrated in the transition from  FIG. 2   b  to  FIG. 2   c.  The hydraulic force is then relieved while the supplemental resilient bias of spring  19  remains allowing Belleville piston  21  to move upwardly into engagement with cam surface  63 . Cam member  23  is displaced by Belleville piston  21  in the direction in which the resilient bias is applied as illustrated by the transition from  FIG. 2   c  to  FIG. 2   d.  Here, the adjuster assembly  17  is moved into engagement with cam surface  61  by spring  65  and brake piston  15  from above and by spring  19  and Belleville piston  21  from below. Removal of the hydraulic pressure also allows displacement piston  31  to settle back against piston  25  providing space for a subsequent return of the cam and piston. In  FIG. 2   d,  the effective length of the mechanical coupling between spring  19  and vehicle service brake applying piston  15  has varied from height  57  shown in  FIG. 2   a  to height  59  to allow spring  19  to sustain a desired parking brake condition in the service brake. 
   Release of the parking brake essentially reverses these steps, but the displacement piston  31  remains in contact with piston  25  while displacement piston  29  temporarily separates from piston  27 . The hydraulic force which hydraulically moves the vehicle service brake toward the vehicle wheel rotation braking condition is re-established, the supplemental resilient bias removed by translation of cam member  23  to the left as viewed, and the hydraulic force relieved thereby establishing a parking brake released condition in the service brake.