Abstract:
A brake caliper with an integral parking brake. The brake caliper includes a housing having a bore with tapered walls, a piston having a rod mounted within the bore, a wedge element positioned between the rod and the tapered walls and displaceable to a locking position and a release position, a resilient element positioned to displace the wedge element to the release position, and a locking element for selectively displacing the wedge element to the locking position. When the locking element is momentarily actuated while a hydraulic pressure is present, the wedge element is urged to the locking position, thereby holding the brake caliper in a braking position when the hydraulic pressure is removed and then the locking element is de-actuated. A hydraulic pressure sufficient to cause the resilient element to urge the wedge element to the release position is effective to release the caliper.

Description:
FIELD 
     The present invention relates generally to brakes and, more particularly, to hydraulically actuated brakes having calipers and an integral parking brake. 
     BACKGROUND 
     Disc brake calipers are well known in the art of vehicle braking systems. In such systems, a master cylinder generates a hydraulic brake signal that is communicated to several brake actuators via brake lines. The signal drives a piston disposed within the caliper, which in turn forces an inboard brake pad into frictional engagement with a rotatable brake disc or “rotor.” A reaction force is also generated, which causes the caliper to move inboard, drawing an outboard brake pad into frictional engagement with the rotor. Upon removal of the brake signal, the brake pads retract from the rotor to prevent frictional driving losses and undue wear on the braking components. 
     It is also known to utilize the brakes of a vehicle for both service and parking braking. Such brakes are typically actuated hydraulically when used for service braking. A separate mechanical actuation means is used to actuate the brakes for use as a parking brake. For example, a shaft or lever may be used to rotatably transmit a parking brake force to a clutch or cable that moves a piston to an extended, brake-engaging direction. Rotation of the lever in the opposite direction releases the clutch or cable and permits retraction of the piston. 
     Electrical actuation of a parking brake is also available in the art. In these systems an electric motor applies a force to a mechanical parking brake system by acting on either a drum or disc brake. Advantages of electrically-actuated parking brakes include ease of operation and consistent application of a predetermined amount of braking force. However, electrically-actuated parking brakes typically require large electric motor actuators that in turn require a relatively large amount of electrical power to actuate. 
     A particular problem with mechanical or electrical actuation of a hydraulic service brake for use as a parking brake occurs when a separate hydraulic pressure source is unable to supply adequate hydraulic pressure for applying and releasing the parking brake. Under such conditions, insufficient parking brake clamp force may be present to keep the vehicle from moving, particularly if the vehicle is parked on an incline. 
     There is a need for a hydraulic service brake that can provide sufficient clamp force for use as a parking brake under all vehicle operating conditions. There is a further need for a parking brake system that does not require a relatively large amount of electrical power to actuate. 
     SUMMARY 
     A brake caliper with an integral parking brake is disclosed according to an embodiment of the present invention. The brake caliper includes a housing having a bore with tapered walls, a piston having a rod mounted within the bore, a wedge element positioned between the rod and the tapered walls, a locking element for urging the wedge element to a locking position and a resilient element positioned to displace the wedge element to a release position. Accordingly, the piston rod may be locked in position relative to the housing, thereby serving as a parking brake, by actuation of the locking element, which displaces the wedge element to the locking position in which it jams between the piston rod and the tapered walls of the housing. The resilient element acts against the wedge element to hold it away from contact with the tapered walls in a release position until acted upon by the locking element. 
     In a preferred embodiment, the caliper includes a secondary piston. The primary and secondary pistons are acted on by the available hydraulic pressure, increasing the total clamp force of the caliper as compared with the force available from a single piston. With this arrangement, a cumulative clamp force is generated by the primary and secondary pistons as pressure is applied to the caliper. The parking brake is applied from a hydraulic pressure source and then electro-mechanically latched by a non-continuous electrical signal. 
     The wedge element preferably comprises a series of rollers rotatably mounted on a plunger or sleeve surrounding and slidably receiving the piston link, the locking element, an electromagnetic coil, and the release element. As the pistons traverse the bores of a caliper to engage the brake in response to hydraulic pressure, they move a hardened steel square piston link past the rollers. In this piston direction the rollers are allowed to rotate freely. To apply the parking brake, before the hydraulic pressure is released, the electromagnetic coil is energized, causing the plunger to be retracted and the rollers to engage the tapered walls such that the rollers become wedged between the piston link and the walls. The reaction force applied to the caliper by the pistons is thus maintained because the primary piston is held in place by the rollers, which are not able to rotate due to their wedged state. The electromagnetic coil can be de-energized after the rollers are wedged without affecting the clamping force of the brake. 
     To release the parking brake, a hydraulic pressure slightly greater than that originally applied to the pistons is applied. The increased pressure causes the pistons and the piston link to extend slightly, allowing the rollers of the locking assembly to disengage from their wedged state and rotate freely. The spring urges the rollers away from wedged engagement with the tapered walls. The freed pistons are allowed to retract into the caliper bore, relieving the caliper clamp force. 
     An object of the present invention is to provide a vehicle brake assembly of the type including a disc brake caliper positioned in relation to a rotor whereby friction pads are applied to opposite sides of the rotor when the brakes are applied. 
     Another object of the invention is to provide a method for operating a vehicle brake assembly of the type including a disc brake caliper positioned in relation to a rotor whereby friction pads are applied to opposite sides of the rotor when the brakes are applied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features of the inventive embodiments will become apparent to those skilled in the art to which the embodiments relate from reading the specification and claims with reference to the accompanying drawings, in which: 
         FIG. 1  is a side elevation in section of the general arrangement of a vehicle brake assembly according to an embodiment of the present invention; 
         FIG. 2  is a side elevational view of a locking assembly of the assembly of  FIG. 1 ; and 
         FIG. 3  is a perspective view of the locking assembly of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , a vehicle brake assembly, generally designated  10 , includes the caliper  12  of the present invention and a rotor  14  operatively secured to a vehicle wheel (not shown). Rotor  14  has first and second opposed friction braking surfaces  16  and  18 , respectively. Caliper  12  is mounted on a vehicle by a suitable mounting bracket (not shown) so that it is slidably movable during brake engagement and release. The vehicle brake system is schematically illustrated in  FIG. 1  as also including a master cylinder  20  actuated by a brake pedal  22  pivotally mounted on a vehicle  24 . Master cylinder  20  is connected to caliper  12  by a conduit  26 . 
     Caliper  12  includes a caliper body  28  formed to provide an outboard caliper leg  30 , an inboard caliper leg  32  and a bridge section  34  joining the two caliper legs and extending over the outer periphery of rotor  14 . Inboard caliper leg  32  is formed to generally define a cylinder  35  having a first bore  33  with an interior wall  38  and a second bore  37  with an interior wall  36 . An open end of first bore  33  opens towards caliper outboard leg  30 . One skilled in the art will recognize that first bore  33  and second bore  37  may be combined to form a single bore without departing from the scope of the invention. 
     An outboard brake shoe assembly  40 , composed of a backing plate  42  and any conventional friction material known in the art, is formed to provide a friction pad  44  and is suitably mounted on outboard caliper leg  30  so that the friction pad may engage first disc friction braking surface  16  when the brake is actuated. Similarly, an inboard brake shoe assembly  46 , composed of backing plate  48  and any conventional friction material known in the art, is formed to provide a friction pad  50 . Friction pad  50  is suitably mounted to be engaged by a piston head  52  for movement with a primary piston  54  so that the friction pad engages disc friction braking surface  18  when brake assembly  10  is actuated. Likewise, friction pad  50  retracts with primary piston  54  when the brake is released. Inboard shoe assembly  46  engages piston head  52  to prevent rotation of primary piston  54  in the cylinder  36 , in any manner known in the art. 
     Primary piston  54  is generally cylindrical and is disposed to reciprocate within wall  38  of first bore  33  with piston head  52  extending through the open end of the first bore and facing second disc friction braking surface  18 . Primary piston  54  includes a piston rod portion  56  having a generally planar end  57 . Primary piston  54  further includes a first pressure chamber  58  coupled to a fluid inlet  60 . A suitable seal  62  and a boot  64  are provided to seal primary piston  54  and cylinder wall  38 , and to prevent the entry of dust or other contaminants into the cylinder. 
     A stationary collar  66  is disposed in cylinder wall  38 , generally coaxial to primary piston  54 . Collar  66  comprises a generally cylindrical exterior surface  63  such that an interior cylindrical sidewall  67  of primary piston  54  is reciprocably slidable in relation to the collar. Collar  66  also includes a generally square or rectangular inner surface  65  having tapered walls  68 . Collar  66 , and particularly ramp  68 , may be made of any suitable material, such as steel and sintered powdered metal, and may further be hardened by any conventional hardening process. A fluid seal  70  of collar  66  prevents ingress of brake fluid. 
     In an alternate embodiment of the present invention, stationary collar  66  may be eliminated, in which case tapered walls  68  are formed by tapering walls  38  of first bore  33  in any conventional manner, such as casting and machining. 
     A piston link  72  is disposed in cylinder wall  38  adjacent to primary piston  54  such that a generally planar first end  73  of the piston link contacts planar end  57  of piston rod  56  and is at least partially surrounded by a portion of the inner surface of collar  68 . A plurality of side walls  75  of piston link  72  form a generally square or rectangular shape. A second, opposing end  77  of piston link  72  has a generally concave surface. Piston link  72  may be made of any suitable material, such as steel, and may be hardened by any conventional hardening process. In an alternate embodiment, piston link  72  may be attached to or made part of piston rod  56 . 
     A secondary piston  74  may optionally be disposed to reciprocate adjacent to and in contact with the second end  77  of piston link  72  in cylinder wall  36  of second bore  37 . A first end  79  of secondary piston  74  is convex in shape and is adapted to couple to the concave surface of end  77  of piston link  72 . A second, opposing end  81  of secondary piston  74  is generally planar. A second pressure chamber  76  is delimited by the planar end  81  of secondary piston  74  and cylinder  36 . A seal  78  prevents egress of fluid from second pressure chamber  76 . 
     Secondary piston  74  is preferably not attached to piston rod  56 , either directly or through piston link  72 . This allows piston rod  56  and secondary piston  74  to have less influence on the alignment of piston link  72  as it passes through a locking assembly  80  (discussed below). In addition, the arrangement of secondary piston  74  as disclosed herein also makes the concentricity variation of the centerlines of first bore  33  and second bore  37  to be less critical with respect to each other for the alignment and installation of primary and secondary pistons  54 ,  74 . However, one skilled in the art will recognize that secondary piston  74  and piston rod  56  may be attached directly or through piston link  72  without departing from the scope of the invention. 
     With reference now to  FIGS. 1–3  in combination, the caliper  12  includes a locking assembly  80 . Locking assembly  80  comprises a locking element such as an electromagnetic coil  82 , a compression spring  84 , a plunger  86 , and a wedge element in the form of a plurality of rollers  88  held in a generally square or rectangular shape and rotatably mounted on a separator  90 . Plunger  86  may be made of any suitable material including, without limitation, ferrous material. Rollers  88  may be made of any suitable material, such as steel, and may be hardened by any conventional hardening process. Other forms of wedge elements, such as ball bearings and prism-shaped wedges, may be used without departing from the scope of the invention. A lower guide portion  92  of plunger  86  is generally square or rectangular, and is adapted to receive piston link  72 . Spring  84  is preferably a helical type, though any conventional form of resilient element may be used including, without limitation, Belleville washers, leaf springs, compression springs, expansion springs and elastics. 
     Locking assembly  80  is disposed to reciprocate in cylinder wall  38  such that rollers  88  are positioned proximate ramp  68  of collar  66  and are in rolling contact with the side walls  75  of piston link  72 , and lower guide portion  92  receives the piston link. Electromagnetic coil  82  is a conventional wound coil and is connected to a source  93  of electric power. Coil  82 , when energized, creates a magnetic field that attracts plunger  86 , compressing spring  84 . When coil  82  is de-energized, spring  84  urges plunger  86  away from the coil. 
     As stated above, in alternate embodiments of the present invention other types of wedge elements may be used in place of locking assembly  80 . For example, a wedge-shaped piece resembling separator  90  but lacking rollers  88  (see  FIG. 2 ) may be positioned between piston rod  56  and ramp  68 , and made displaceable between a locking position wherein the wedge-shaped piece engages the ramp and a release position wherein the wedge-shaped piece is positioned away from the ramp. A resilient element such as spring  84  may be positioned to displace the wedge-shaped piece to the release position while a locking element such as electromagnetic coil  82  selectively displaces the wedge-shaped piece to the locking position. The wedge-shaped piece may be made of any suitable material including, without limitation, ferrous material, and may optionally be hardened by any conventional hardening process. In other alternate embodiments the wedge-shaped piece may be plated or coated with a plastic or TEFLON material to aid the slidable movement of the piece. 
     Referring to  FIG. 1 , during service braking actuation, master cylinder  20  is actuated by depressing brake pedal  22  and a brake signal comprising brake fluid under pressure is delivered to fluid inlet  60  by means of brake conduit  26 . The brake fluid is communicated to first and second pressure chambers  58 ,  76 , respectively, acting against primary piston  54  and secondary piston  74  to extend the primary and secondary pistons. Movement of primary piston  54  is aided by secondary piston  74 , which slidably urges piston link  72  against the planar end  57  of piston rod  56 . With this arrangement, a cumulative extending force of primary piston  54  is generated by the primary piston and secondary piston  74 . Piston head  52  and inboard brake shoe assembly  46  are in turn slidably urged toward rotor  14  until friction pad  50  engages second disc friction braking surface  18 . Pressurization in first and second pressure chambers  58 ,  76  respectively, also causes caliper housing  28  to move in the opposite direction of inboard brake shoe assembly  46  to engage friction pad  44  of outboard brake shoe assembly  40  with first disc friction braking surface  16 . The resulting friction between friction pad  50  and braking surface  18 , and between friction pad  44  and braking surface  16 , generates braking action in a well-known manner. 
     Upon release of the hydraulic pressure, the hydraulic signal is removed and the pressure in first and second pressure chambers  58 ,  76 , respectively, is released. Primary piston  54  and secondary piston  74  slidably retract, actuated by seal  62  in a well-known manner. During retraction, primary piston  54 , actuated by seal  62 , slidably presses against piston link  72 . Piston link  72  in turn slidably urges secondary piston  74  to move slidably to a retracted position. 
     Referring again to  FIGS. 1–3 , brake assembly  10  may additionally function as a parking brake. To engage the parking brake, an operator actuates a hydraulic pressure source such as brake pedal  22  and master cylinder  20 . Sources of hydraulic pressure include, but are not limited to, an antilock braking system, traction control modulator, or central brake system, to generate a first hydraulic signal to actuate caliper  12  in the manner described above. While the first hydraulic signal is present to hold caliper  12  in the engaged position, the operator actuates a brake switch  104  that supplies electrical power from a source  93  (e.g., the vehicle battery) to electromagnetic coil  82 , which causes plunger  86  to move toward the coil. The retracting movement of plunger  86  causes rollers  88  to displace along ramp  68  of collar  66 , engaging the rollers in a wedge formed by the ramp and side walls  75  of piston link  72 . The operator may then remove the first hydraulic signal. Caliper  12  remains engaged to rotor  14  because primary piston  54  remains in an extended position, prevented from retracting by piston link  72 , which is in turn captively held by the wedged rollers  88 . The operator may then remove power from coil  82 . Rollers  88  will remain wedged in place due to the retracting force exerted on primary piston  54  by seal  62 . 
     To release the parking brake, the operator again actuates a hydraulic pressure source, transmitting a second hydraulic signal to fluid inlet  60  by means of brake conduit  26 . The second hydraulic signal preferably is at a slightly higher pressure as compared to the first hydraulic signal that was applied to engage the parking brake. The brake fluid is communicated to first and second pressure chambers  58 ,  76 , acting against primary piston  54  and secondary piston  74 . Primary piston  54  is slidably urged to a slightly greater extended position by the pressure of the brake fluid. Movement of primary piston  54  is aided by secondary piston  74 , which slidably urges piston link  72  against planar end  57  of piston rod  56 . The extending movement of primary piston  54 , piston link  72 , and secondary piston  74  acts to release rollers  88  from their wedged position. Spring  84 , coupled between plunger  86  and cylinder  36 , is free to expand, urging rollers  88  away from ramp  68 . When the operator subsequently releases hydraulic pressure, removing the second hydraulic signal, primary piston  54 , piston link  72  and secondary piston  74  are free to retract, urged by seal  62 , causing caliper  12  to release rotor  14 . 
     While this invention has been shown and described with respect to a detailed embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the scope of the claims of the invention.