BACKGROUND OF THE INVENTION

a. Field of the Invention

[0001] This invention relates to a braking system for a vehicle. In particular, the invention relates to a braking system that enables application and release of a parking brake with fluid control and that also maintains appl ication of the brake during a loss of fluid pressure— all without using spring applied braking and using only a single fluid control conduit. b. Background Art.

[0002] Conventional commercial vehicle braking systems typically include spring- actuated/fluid released parking brake actuators in which a mechanical spring actuates or applies the parkmg brake and pressurized fluid (typically pneumatic fluid) is used to release the brake. The parking brake remains released as long as fluid pressure is maintained in the parking brake actuator. The braking system typically has a common source of fluid pressure connected to actuators disposed at each wheel end of the of vehicle such that fluid pressure in a common control line simultaneously releases the parking brake for each wheel.

[0003] The use of spring-actuated/fluid released parking brake actuators has several drawbacks. An unintended loss of fluid pressure due to, for example, a leak or rupture in the fluid control system results in actuation of the parking brakes on each wheel. While this action is desirable-and even required in certain circumstances-to meet safety concerns, the action is undesirable in many other circumstances. For example, in military applications it is not uncommon for the fluid control system to become damaged as a result of damage to a single wheel from an improvised explosive device (IED). Actuation of the parking brakes in this circumstance immobilizes the vehicle and may trap the vehicle occupants in a hostile environment. In other applications, movement of the vehicle may be required because the immobilized vehicle is in a dangerous location (e.g., with high traffic volumes), is blocking the passage of other vehicles including potential emergency service vehicles, or is in a remote location far from potential repair facilities (e.g., a logging vehicle in a remote forest). The spring in the actuator is relatively heavy, highly stressed to produce the parking force and also tends to corrode and fatigue over time. The corrosion makes repair and replacement different. More importantly, because of the significant spring forces applied in the actuator, unexpected failure of the spring may resul t in significant injury to maintenance and other personnel. Finally, constant fluid pressure is required to maintain the parking brake in a release position.

[0004] in order to overcome some of the problems associated with sprmg-aetuated/fluid released parking brake actuators, some actuators have been developed that use fluid pressure to actuate the parking brake. One example is the brake actuator previously offered for sale under the trademark DD-3 by the applicant Bendix Commercial Vehicle Systems LLC. For proper functioning of the parking brake, fluid actuated parking brake actuators must include means to maintain application of the parking brake despite any loss in fluid pressure. In the case of the DD-3 and similar brake actuators, a separate fluid control line has been used to actuate the locking mechanism. The requirement for an additional fluid control line, however, increases the number of parts, cost, and complexity of the braking system .

[0005] The inventors herein have recognized a need for a braking system that will minimize and/or eliminate one or more of the above-identified deficiencies.

BRIEF SUMMARY OF THE INVENTION

[0006] This invention relates to a braking system for a vehicle. In particular, the invention relates to a braking system that enables application and release of a parking brake with fluid control and that also maintains application of the brake during a loss of fluid pressure al l without using spring applied brakmg and using only a single fluid control conduit.

[0007J A braking system in accordance with one embodiment of the present invention includes a brake actuator and a fluid control system. The brake actuator includes a housing defining a sendee brake chamber and a parking brake chamber. The brake actuator further includes a service diaphragm disposed within the service brake chamber and a service pushrod disposed on a first side of the service diaphragm. Introduction of fluid on a second side of the service diaphragm urges the service pushrod away from a service release position towards a service apply position. The brake actuator further includes a parking diaphragm disposed within the parking brake chamber and having a first side facing the service diaphragm. Introduction of fluid on a second side of the parking diaphragm urging the sendee pushrod to the sendee apply position. The fluid control system includes a relay valve comprising a delivery port in fluid communication with the parking brake chamber of the brake actuator and first and second control ports. The fluid control system further includes a first inversion valve comprising a delivery port in fluid communication with the first control port of the relay valve, a supply port in fluid communication with a fluid control conduit, and a control port in fluid communication with the fluid control conduit. The fluid control system further includes a second inversion valve comprising a delivery port in fluid communication with the second control port of the relay valve, a supply port, and a control port. The fluid control system further includes a third inversion valve comprising a delivery port in fluid communication with the supply port of the second inversion valve and the control port of the second inversion valve, a supply port in fluid communication with a fluid source and a control port in fluid communication with the fluid control conduit. A decrease in fluid pressure in the fluid control conduit enables delivery of fluid from the fluid source to the parking brake chamber and an increase in fluid pressure in the fluid control conduit enables exhaustion of fluid from the parking brake chamber.

[0ΘΘ8] A brake actuator for use in braking system in accordance with one embodiment of the present invention includes a housing defining a service brake chamber and a parking brake chamber. The actuator further includes a service diaphragm disposed within the service brake chamber and a sendee pushrod di sposed on a first side of the service diaphragm. The actuator further includes a parking diaphragm disposed within the parking brake chamber and a parking pushrod extending through the parking diaphragm. The parking pushrod has a first end disposed on a first side of the parking diaphragm and extending into the service brake chamber and disposed on a second side of the sendee diaphragm. The parking pushrod also defines a plurality of teeth. The actuator further includes a spring disposed within the sendee brake chamber on the first side of the service diaphragm. The spring biases the sendee pushrod to a service release position and the parking pushrod to a parking release position. The actuator further includes a clutch disposed about the parking pushrod and defining a clutch tooth on a radially inner surface and configured for intermittent engagement with the plurality of teeth of the parking pushrod and a clutch actuator configured to control rotation of the clutch. Introduction of fluid on the second side of the sendee diaphragm urges the sendee pushrod away from the service release position towards a service apply position. Introduction of fluid on a second side of the parking diaphragm urges the parking pushrod away from the parking release position towards a parking apply position and the service pushrod away from the service release position towards the service apply position.

[0009] A brake actuator for use in braking system in accordance with another embodiment of the present invention includes a housing defining a service brake chamber and a parking brake chamber. The actuator further includes a service diaphragm disposed within the sendee brake chamber and a service pushrod disposed on a first side of the service diaphragm. The actuator further includes a parking diaphragm disposed within the parking brake chamber and a parking pushrod extending through the parking diaphragm. The parking pushrod has a first end disposed on a first side of the parking diaphragm and extending into the serv ice brake chamber and disposed on a second side of the service diaphragm. The actuator further includes a spring disposed within the service brake chamber on the first side of the service diaphragm. The spring biases the sendee pushrod to a sendee release position and the parking pushrod to a parking release position. The actuator further includes means for holding the parking pushrod in a parking apply position, introduction of fluid on the second side of the sendee diaphragm urges the sendee pushrod away from the service release position towards a sendee apply position. Introduction of fluid on a second side of the parking diaphragm urges the parking pushrod away from the parking release position towards the parking apply position and the sendee pushrod away from the sendee release position towards the service apply position.

[0010] A fluid control system for a brake actuator for use in braking system in accordance with one embodiment of the present invention includes a relay valve comprising a delivery port, in fluid communication with a parking brake chamber of the brake actuator and first and second control ports. The fluid control system further includes a first inversion valve comprising a delivery port in fluid communication with the first control port of the rel ay valve, a supply port in fluid

communication with a fluid control conduit and a control port in fluid communication with the fluid control conduit. The fluid control system further includes a second inversion valve comprising a delivery port in fluid communication with the second control port of the relay valve, a supply port and a control port. The fluid control system further includes a third inversion valve comprising a delivery port in fluid communication with the supply port of the second inversion valve and the control port of the second inversion valve, a supply port in fluid communication with a fluid source; and, a control port in fluid communication with the fluid control conduit. A decrease in fluid pressure in the fluid control conduit enables delivery of fluid from the fluid source to the parking brake chamber. An increase in fluid pressure in the fluid control conduit enables exhaustion of fluid from the parking brake chamber.

[0011] A braking system in accordance with the present invention represents an improvement relative to conventional braking systems. The inventive braking system enables application of the parking brake using fluid control and can also maintain application of the brake during a loss of fluid pressure without using mechanical springs. As a result, the weight of the actuator is reduced, repair and replacement of the parking brake actuator is easier and safer than conventional spring- actuated parking brake actuators, and a constant fluid pressure is not required to maintain the actuator in a released state. The inventive braking system also permits selective release of the parking brake despite a leak or rupture in the fluid control system of the type that would result in actuation and locking of spring-actuated parking brake actuators. As a result, the vehicle is not immobilized and the vehicle and its occupants can be moved, if necessary, to a safer or more accessible location and/or out of the way of other vehicles. The inventive braking system also achieves the above benefits while using a conventional two-line fluid control system (i.e., one fluid control line to the service brake chamber and another fluid control line to the parking brake chamber)— unlike conventional fluid-actuated parking brake actuators that require a separate fluid control line to maintain the position of the parking brake actuator.

[0Θ12] The foregoing and other aspects, features, details, utilities, and advantages of the present invention will, be apparent from reading the following description and claims, and from reviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a diagrammatic view of a braking system in accordance with one

embodiment of the present invention.

[0Θ14] Figure 2 is a cross-sectional view of one embodiment of a brake actuator fur use in the braking system of Figure 1.

[0015] Figure 3 is a cross-sectional view of a portion of the brake actuator of Figure 2 taken along lines 3-3 in Figure 2. [0Θ16] Figure 4 is a cross-sectional view of a portion of the brake actuator of Figure 2 taken alones 4-4 in Figure 3.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views, Figure 1 illustrates a braking system 10 in accordance with one embodiment of the present invention. System 10 is provided to control actuation and release of foundation brakes (not shown) applied to one or more vehicle wheels 12. The brakes may comprise, for example, conventional drum brakes in which one or more brakes shoes are configured to engage and disengage a braking surface in a drum upon actuation and release of the brakes. In addition to conventional vehicle brakes, system 10 may include a fluid control system 14 and one or more brake actuators 16. System 10 is configured for use with commercial vehicles including tracks and buses. It should be understood, however, that system 10 can be configured for use with a variety of vehicles.

[0018] Fluid control system 14 is provided to control distribution of fluid within system 10 and delivery of fluid to and venting of fluid from actuators 16. Fluid control system 14 may include a fluid source 18 and one or more control valves 20, 22, 24, 26, 28, 30, 32 controlling distribution of fluid through system 10.

[001 ] Fluid source 18 is conventional in the art any may comprise a fluid reservoir or tank for storing pressurized fluid. A compressor (not shown) operated by the vehicle engine may be used for charging the tank with fluid pressure and one or more check valves (not shown) may be used to prevent loss of pressure from the tank in the event of an upstream or downstream pressure loss, in the illustrated embodiment the fluid comprises pneumatic fluid. Fluid source 18 is coupled to one end of a conduit 34 extending between fluid source 18 and valve 32 and at one end of a conduit 38 extending between fluid source 18 and valve 28.

[0020] Valve 20 comprises a conventional foot control pedal valve for use in actuating and releasing the service brake. When the pedal is depressed, the valve 20 opens to allow pressurized fluid to fiow ^r from a fluid source (not shown) through valve 20 to a control port on relay valve 30. The increase in fluid pressure at the control port on valve 30 causes fluid pressure to flow from a fluid source (not shown ) through supply and delivery ports on valve 30 to actuator 16 in order the actuate the sendee brake. When the pedal is released, valves 20 and 30 close and fluid pressure is exhausted to release the service brake. Valve 20 is coupled at one end of conduit 40 extending between valve 20 and valve 30.

[0021] Valve 22 comprises a control valve for use in actuating and releasing the parking brake. Valve 22 may be mounted on the dashboard in a truck cab and may comprise a manually operable push-pull button valve. Vaive 22 may include a supply port through which fluid enters vaive 22 and a delivery port through which fluid exits vaive 22. Valve 22 is coupled at one end of conduit 42 extending between valve 22 and valves 24, 28. Manually pulling the button on valve 22 exhausts fluid from conduit 42 to actuate the parking brake as described in greater detail below. Manually pressing the button causes delivery of fluid pressure from a fluid source (not shown) through valve 22 and conduit 42 to release the parking brake as described in greater detail below.

[0022] Valves 24, 26, 28, comprise inversion valves (i.e. valves that deliver fluid pressure in the absence of fluid pressure at a control port of the vaive). V alves 24, 26, 28, may comprise the inversion valve offered for sale under the trademark "TR-3" by the applicant Bendix Commercial Vehicle Systems LLC. Valves 24, 26, 28 each include a supply port S, a delivery port D, a control port C and an exhaust port. In valves 24, 26, however, control port C is plugged and an internal passage connects the supply port S and control port C. The supply port S of valve 24 is coupled to one end of fluid conduit 42 extending between valve 22 and vaive 24 while the deliver} _' port D of valve 24 is coupled to one end of a fluid conduit 44 extending between valve 24 and valve 32. The supply port 8 of valve 26 is coupled to one end of a conduit 46 extending between valve 26 and the delivery port D of valve 28 while the delivery port D of valve 26 is coupled to one end of a conduit 48 extending between valve 26 and valve 32. The supply port S of valve 28 is coupled to one end of conduit 38 extending between fluid source 18 and valve 28. The delivery port D of valve 28 is coupled to one end of a conduit 46 extending between valve 28 and the supply port S of valve 26. Finally, the control port C of valve 28 is coupled to one end of the conduit 42 extending between control valve 22 and valve 28.

[0023] Valves 30, 32 comprise relay valves. Valves 30, 32 are provided to increase the speed of application and release of the brakes and are conventional in the art. Valves 30, 32 may comprise the relay valve offered for sale under the trademark "R-12DC" by the applicant Bendix Commercial Vehicle Systems LLC. Valve 30 has a supply port coupled to a service brake fluid source (not shown), and a delivery port coupled to one end of a conduit 50 extending between valve 30 and a service port 52 on actuator 16. Valve 30 also has a control port coupled to one end of conduit 40 extending between valve 20 and valve 30. Valve 32 has a supply port coupled to one end of conduit 34 extending between fluid source 18 and valve 32 and a delivery port coupled to one end of a conduit 54 extending between valve 32 and a parking port 56 on actuator 16. Valve 32 also has one control port coupled to one end of conduit 44 extending between valve 24 and valve 32 and another control port coupled to one end of conduit 48 extending between valve 26 and valve 32.

[0024] As illustrated in Figure 1 , the fluid control system 14 of system 10 is configured to control actuation of the brakes associated with the wheels of a tractor in a tractor-trailer vehicle. It should be understood, however, that system 10 can be substantial ly replicated for use in controlling actuation of the brakes associated with the wheels of the trailer. In particular, a trailer may have a fluid source 18, valves 24, 26, 28, 30, 32 and associated fluid conduits arranged substantially similar to the arrangement on the tractor and fluid conduits extending from control valves 20, 22 can be coupled to corresponding conduits on the trailer through a conventional glad hand connection.

[0Θ25] Referring now to Figure 2, actuator 16 is provided to control actuation and release of the brakes associated with the vehicle wheels 12. Actuator 16 may include a housing 58, a service diaphragm 60, a service pushrod 62 movable between a service release position (shown in Figure 2) and sendee apply position, a spring 64, a parking diaphragm 66, a parking pushrod 68 movable between a parking release position (shown in Figure 2) and a parking apply position, means for holding the parking pushrod 68 in the parking apply position such as clutch 70 and means, such as clutch actuator 72, for rotating clutch 70.

[0Θ26] Housing 58 provides structural support for the other components of actuator 16 and protects those components from foreign objects and elements. Housing 58 further defines a service brake chamber 74 and a parking brake chamber 76. Housing 58 may include multiple housing members 78, 80, 82, 84. Members 78, 80 define service brake chamber 74 and receive service diaphragm 60 therebetween. Members 78, 80, may be held in place by a conventional clamp 86 disposed about the circumference of housing 58. Members 78, 80 defines aligned central apertures through which service pushrod 62. Member 78 also defines surrounding apertures through which mounting bolts 88 may extend for mounting actuator 16 within the vehicle. Conventional nuts and washers may be disposed about bolts 88 to securely fasten actuator 16 to the vehicle. Members 80, 82 define parking brake chamber 76 and receive parking diaphragm 66 therebetween. Members 80, 82 may be held in place by crimping one or both of members 80, 82 or through the use of a cl amp as described above or other conventional fasteners. Members 80, 82 define aligned central apertures through which parking pushrod 68 may extend. Members 82, 84 further defines a chamber 90 configured to receive clutch 70 and clutch actuator 72.

[0Θ27] Service diaphragm 60 is provided to urge sendee pushrod 62 outward from actuator 16 in response to fluid pressure in order to cause application of the vehicle brakes. Diaphragm 60 is conventional in the art. Diaphragm 60 is disposed within chamber 74 and may be held between members 78, 80 of housing 58. Fluid pressure may be introduced to sendee chamber 74 on one side of diaphragm 60 opposite the side on which sendee pushrod 62 is disposed through sendee port 52 via fluid conduit 50 (see Figure 1 ). Introduction of fluid pressure to chamber 74 urges service pushrod 62 to a sendee apply position.

[0Θ28] Service pushrod 62 transfers forces generated in actuator 16 to the vehicle brakes in order to apply or release the brakes. Pushrod 62 is disposed about and moves along a longitudinal axis 92. Pushrod 62 may define a yoke 94 at one end of the pushrod 62 disposed outside of housing 58. In a conventional drum brake, for example, the yoke 94 may be coupled to a slack adjuster (not shown) that is disposed about a camshaft (not shown) and translates linear movement of the service pushrod 62 into rotational movement of the camshaft and a cam (not shown) mounted on one end of the camshaft. Rotation of the cam then causes corresponding movement of brakes shoes into and out of engagement with a braking surface in a drum. Pushrod 62 may define a pressure plate 96 at an opposite end from yoke 94. Plate 96 provides a surface on one side configured to engage diaphragm 60 and a surface on the opposite side configured to act as a spring seat for spring 64.

[0029] Spring 64 is provided to bias service pushrod 62 to a service release position and parking pushrod 68 to a parking release position. Spring 64 is conventional in the art and is disposed within sendee brake chamber 74 on one side of service diaphragm 60. Spring 64 is disposed about service pushrod 62 and is seated between member 78 of housing 58 and pressure plate 94 of pushrod 62. [0Θ30] Parking diaphragm 66 is provided to urge parking pushrod 68 in one direction along axis 92 away from the parking release position and towards the parking apply position in response to fluid pressure in order to cause application of the vehicle brakes. Diaphragm 66 is conventional in the art. Diaphragm 66 is disposed within chamber 76 and may be held between members 80, 82 of housing 58. Fluid pressure may be introduced to parking brake chamber 76 on one side of diaphragm 66 through a parking port 56 via fluid conduit 54 (see Figure 1 ). Introduction of fluid pressure urges parking pushrod 68 to a parking apply position.

[0Θ31] Parking pushrod 68 is provided to cause movement of sendee diaphragm 60 and service pushrod 62 in response to the introduction of fluid pressure in parking brake chamber 76 in order to apply or release the brakes. Pushrod 68 is disposed about and moves along axis 92 and may be axially aligned with pushrod 62. Pushrod 68 extends through parking diaphragm 66 and is configured for movement with parking diaphragm 66. Pushrod 68 may comprise multiple members

98, 100 generally disposed on opposite sides of diaphragm 66. Member 98 of pushrod 68 is disposed on one side of diaphragm 66 and extends into service brake chamber 74. Member 98 defines a pressure plate 102 at one longitudinal end of pushrod 68 configured for selective engagement with one side of sendee diaphragm 60. Pushrod 68 further defines another pressure plate 104 intermediate the longitudinal ends of pushrod 68. Plate 104 is disposed on one side of parking diaphragm 66. Introduction of fluid pressure on the opposite side of diaphragm 66 urges pushrod 68 to a parking apply position and therefore urges service pushrod 62 to the service apply position via the force exerted on service diaphragm 60 by pl ate 102 of parking pushrod 68. In accordance with one aspect of the present invention, member 100 of parking pushrod 68 may define a plurality of teeth 106 for a purpose described below. Referring to Figures 2-4, in one

embodiment, two sets of teeth 106 may be arranged in separate longitudinally extending rows on pushrod 68. Each row of teeth 106 extends over only a circumferential portion of the outer circumferential surface of the pushrod 68, Referring to Figure 3, the two rows of teeth 106 may be diametrically opposite one another and the circumferential portions of the outer circumferential surface of pushrod 68 between the rows of teeth 106 may be un toothed or smooth.

[0032] Clutch 70 is provided to hold parking pushrod 68 in the parking apply position. Clutch

70 is annular in construction and is disposed within chamber 90 about pushrod 68. Clutch 70 is a one-way or ratchet clutch. Referring to Figures 3-4, clutch 70 defines at least one or more clutch teeth 108 on a radially inner surface of the clutch 70. Teeth 108 may formed in an axially extending row along the radially inner surface of clutch 70. Teeth 108 may also be formed on a pawl 1 10 that is disposed in a recess in the radially inner surface of clutch 70 and that is biased radially inwardly towards pushrod 68 by one or more springs 112. The clutch teeth 108 have a profile such that, during alignment of clutch teeth 108 and teeth 106 on parking pushrod 68, movement of parking pushrod 68 is permitted in one direction along axis 92 away from the parking release position and towards the parking apply position (i.e. downward in Figure 2) and movement of the parking pushrod 68 is inhibited in an opposite direction along axis 92 away from parking apply position and towards the parking release position (i.e. upward in Figure 2). As a result, fluid pressure introduced into parking brake chamber 76 will result in movement of pushrod 68 to the parking apply position and, thereafter, clutch 70 will prevent release of the parking brake (until directed as discussed below) even in the event of a loss of fluid pressure in parking brake chamber 76. Referring to Figure 3, clutch 70 also defines a plurality of ratchet teeth 1 14 on a radially outer surface of clutch 70. In the illustrated embodiment clutch 70 defines four ratchet teeth that are equally

circumferentiaily spaced about clutch 70 (i.e. every ninety degrees). Ratchet teeth 70 have a profile defining a relatively gradual slope on one side and a relatively steep slope on another side for a purpose discussed hereinbelow.

[0033] Clutch actuator 72 is provided to control rotation of clutch 70 about pushrod 68. In accordance with one embodiment of the invention, clutch 70 is rotated in ninety degree increments such that the teeth 108 on clutch 70 are moved into and out of engagement with rows of teeth 106 on pushrod 68. As a result, when pushrod 68 is in the parking apply position, the next rotation of clutch 70 disengages teeth 108 from teeth 106 on pushrod 68 and allows pushrod 68 to move to the parking release position under the force of spring 64 in service chamber 74. The next rotation of clutch 70 would then occur during actuation of the parking brake (as discussed below) and cause reengagement of teeth 108 with another row of teeth 106 on pushrod 68 to hold pushrod 68 in the parking apply position. Actuator 72 may include a piston 116, a pawl 118 and a spring 120,

[0034] One end of piston 1 16 is disposed within a fluid chamber 122 formed in housing 58.

Fluid pressure within chamber 122 urges piston 120 in one direction (upward in Figure 3). A spring

(not shown) may bias piston 120 in the opposite direction (downward in Figure 3). Piston 116 defines mounting surfaces for pawl 1 18 and spring 120.

I I [0Θ35] Pawl 118 transfers force from movement of piston 116 to clutch 70 in order to cause rotation of clutch 70. Pawl 1 18 has one end pivotally mounted on piston 1 16. The opposite, or free, end of pawl 118 is biased away from piston 1 16 by spring 120 and is configured to engage one of ratchet teeth 1 14 on clutch 70 and, in particular, the side of a tooth 1 14 having a relatively steep slope. Movement of piston 1 16 in response to fluid pressure is transferred to clutch 70 through the engagement of pawl 118 with a corresponding ratchet tooth 1 14 of clutch 70. Because of the profile of ratchet teeth 114, the illustrated clutch 70 rotates ninety degrees in order to align clutch teeth 108 on clutch 70 with either teeth 106 on pushrod 68 (when the parking brake is applied) or an untoothed portion of pushrod 68 (when the par king brake is released). Once fluid pressure in chamber 112 is removed, a spring (not shown) returns piston 116 to its original position and pawl 118 slides over the next ratchet tooth 104— in particular, the side of the tooth 104 having a relatively gradual slope— -to reposition paw 1 18 for the next desired rotation of clutch 70.

[0036] Referring to Figures 1 -2, the operation of braking system 10 will, now be described in greater detail. Actuation of the service brake is accomplished by depressing foot pedal valve 20. This action causes relay valve 30 to direct fluid from a fluid source (not shown) to service port 52 of actuator 16 and into service brake chamber 74 on one side of diaphragm 60. Fluid pressure in chamber 74 urges diaphragm 60 and sendee pushrod 62 to a service apply position against the force of spring 64 to apply the vehicle brakes. When the pedal is released, valve 20 closes and fluid pressure is exhausted from chamber 74. Spring 64 urges pushrod 62 to the service release position to release the brake.

[0037] When the vehicle operator wishes to actuate the parking brake, the operator may pull the button on control valve 22 to exhaust fluid, conduit 42. The loss in fluid pressure in conduit 42 allows fluid pressure from conduit 42 to flow from supply port 8 of valve 28 to delivery port D of valve 28 and through conduit 46 to the supply S and control ports C of valve 26. Fluid from supply port S of valve 26 flows outward from delivery port D of valve 26 to a predetermined pressure into conduit 48 to a control port of valve 32. This action causes delivery of fluid from fluid source 20 through valve 32 and conduit 54 to parking port. 56 of actuator 16 and into parking brake chamber

76 of actuator 16 on one side of parking diaphragm 66. Fluid pressure in chamber 76 parking pushrod 68 to the parking apply position and, consequently, service pushrod 62 to the sendee apply position against the force of spring 64 in order to apply the vehicle brakes. Fluid is also delivered through conduit 54 to fluid chamber 122 resulting in rotation of clutch 70 such that the clutch teeth 108 on clutch 70 are aligned with teeth 106 on pushrod 68. As a result, while pushrod 68 is able to move to the parking apply position freely, pushrod 68 is temporarily prevented from returning to the parking release position. Once the fluid pressure in chamber 76 reaches a predetermined level, valve 26 closes and the remaining fluid pressure in conduits 48, 54 is exhausted. Pressure is also vented from parking brake chamber 76. Because clutch 70 retains the position of parking pushrod 68, constant fluid pressure is not required for continued application of the parking brake. Further, the inventive braking system allows variation in the applied parking brake force without varying the size of the parking brake chamber. Rather, valve 26 can be adjusted to control the predetermined pressure at which further delivery of fluid is stopped.

[0038] When the vehicle operator wants to release the parking brake, the operator pushes in the control button on control valve 22 which increases fluid pressure in conduit 42. Because valve 24 is open (due to the prior absence of fluid pressure in conduit 42), fluid from conduit 42 initially flows through supply port S on valve 24 to delivery port D on valve 24 and into conduit 44. Fluid flow into conduit 44 occurs until a predetermined fluid pressure is reached and valve 24 closes and exhausts to atmosphere. Fluid in conduit 44 is delivered to a control port of valve 32. This action causes delivery of fluid from fluid source 20 through valve 32 and conduit 54 to parking port 56 and into parking brake chamber 76 of actuator 16 and also into fluid chamber 122 thereby causing rotation of clutch 70 such that clutch teeth 108 on clutch 70 are no longer aligned with the teeth 106 on pushrod 68. Once the predetermined pressure is reached, valves 24, 32 close and conduits 44, 54

(and parking brake chamber 76) are vented. Because the clutch teeth 108 on clutch 70 are no longer aligned with the teeth 106 on pushrod 68, spring 64 urges parking pushrod 68 to the parking release position (and service pushrod 62 to the service release position) to release the parking brake.

[0Θ39] A braking system 10 in accordance with the present invention represents an

improvement relative to conventional braking systems. The inventive braking system 10 enables application of the parking brake using fluid control rather than mechanical springs and can also maintain application of the parking brake during a loss of fluid pressure. As a result, the weight of the actuator is reduced, repair and replacement of the parking brake actuator is easier and safer than conventional spring-actuated parking brake actuators, and a constant fluid pressure is not required to maintain the actuator in a released state. A diaphragm is more durable than a parking spring which equates to longer life. The inventive braking system also permits selective release of the parking brake despite a leak or rupture in the fluid control system of the type that would result in actuation and locking of spring-actuated parking brake actuators. As a result, the vehicle is not immobilized and the vehicle and its occupants can be moved, if necessary, to a safer or more accessible location and/or out of the way of other vehicles. The inventive braking system also achieves the above benefits while using a conventional two-line fluid control system (i.e., one fluid control line to the service brake chamber and another fluid control line to the parking brake chamber)— unlike conventional fluid-actuated parking brake actuators that require a separate fluid control line to maintain the position of the parking brake actuator.

[0Θ40] While the invention has been shown and described with reference to one or more particular embodiments thereof, it will be understood by those of skill in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.