Parking brake actuator with internal flow control

Various examples of brake actuators are provided. In one example, a brake actuator includes a parking brake chamber defined by a spring housing and an adapter base. The parking brake chamber includes a parking diaphragm that has an opening and the parking diaphragm is disposed between the spring housing and the adapter base which divides the brake spring chamber into a spring volume and a release volume, with the spring volume of the brake spring chamber being sealed from atmosphere. The parking brake chamber also includes a piston that is disposed in the spring volume and is in contact with the parking diaphragm, and the piston includes a piston passageway through the piston to allow air flow between the release volume and the spring volume.

BACKGROUND

The present invention relates to a brake actuator in an air brake system. More specifically, the present invention relates to a parking brake actuator with a parking brake spring.

Parking brakes that operate with a spring to actuate the brake include a parking brake diaphragm that moves when air flows in and out of the parking brake chamber. Air enters the release volume of the parking brake spring chamber and the expansion compresses the brake spring during the parking brake “release” cycle. During the parking brake “apply” cycle air exits the release volume of the parking brake spring chamber and the spring volume of the parking brake spring chamber increases as the spring extends.

Many conventional brake actuators are open to the environment. These brake actuators have multiple internal components that are vulnerable to contamination in the applied position. Contamination of the spring volume causes corrosion of the power spring and other internal components, eventually causing brake chamber failure.

Other conventional brake actuators are configured to allow air out of but not into the spring volume of the brake spring chamber to minimize or eliminate the possibility of contamination of the components. However, during operation of actuators which are sealed in this manner, a vacuum can develop as the parking brake is applied and the spring volume within the brake spring chamber increases. The vacuum created during the brake apply cycle works against the brake actuator's delivery of full parking force for the particular spring type and spring stroke of the brake actuator. Thus, it would be beneficial to improve the brake actuator's delivery of full parking force while also preventing contamination inside the brake actuator.

SUMMARY

Various examples of a brake actuator are disclosed. In accordance with one aspect, a brake actuator includes a parking brake chamber defined by a spring housing and an adapter base. The parking brake chamber includes a parking diaphragm that has an opening and the parking diaphragm is disposed between the spring housing and the adapter base which divides the brake spring chamber into a spring volume and a release volume, with the spring volume of the brake spring chamber being sealed from the atmosphere. The parking brake chamber also includes a piston that is disposed in the spring volume and is in contact with the parking diaphragm, and the piston includes a piston passageway through the piston to allow air flow between the release volume and the spring volume.

In another aspect, the brake actuator includes a parking brake chamber defined by a spring housing and an adapter base. The brake actuator includes a parking diaphragm having an opening and is disposed between the spring housing and the adapter base and divides the brake spring chamber into a spring volume and a release volume, with the spring volume of the brake spring chamber being sealed from the inflow of environmental air. The parking brake chamber also includes a piston that contacts the parking diaphragm and is disposed in the spring volume portion of the parking brake chamber, the piston comprising an open passageway through the piston. The brake actuator also includes a means for moving the parking diaphragm away from the piston passageway to allow air flow from the release volume to the spring volume of the brake spring chamber.

In another aspect, a method for controlling fluid flow in a brake actuator, the method includes passing fluid from a release volume of a brake spring chamber through an opening of a parking diaphragm and through a passageway of a piston to a spring volume of the spring chamber, wherein the spring volume is sealed from the inflow of environmental air.

DETAILED DESCRIPTION

Example embodiments of the present invention are directed to a brake actuator, such as that used in a truck vehicle or other commercial vehicles. Although the examples explained herein relate to a brake actuator on trucks or other commercial vehicles with pneumatic brakes, it is understood that the brake actuator can be used in alternative applications. In addition, although the examples explained herein often describe fluid within the brake actuator as being air, it is understood that the various examples of a brake actuator can be used in alternative applications with alternative fluids.

FIG. 1illustrates a schematic, cross-sectional view of a parking brake actuator10in a parking brake “release” state, according to an example embodiment of the present invention. Brake actuator10is provided to control actuation and release of brakes, for example a drum brake or a disk brake, associated with one or more vehicle wheels. Brake actuator10includes a plurality of housing members that define and protect components inside brake actuator10. For example, spring housing14and adapter base16define a parking brake chamber18. Parking brake chamber18includes a parking brake diaphragm20, a piston22, a parking spring24, a biasing member70and an adapter base push rod26which are movable between a brake “release” position (FIG. 1) and a brake “apply” position (FIG. 5), and from a brake apply to a brake release position in the brake apply and brake release cycles described below. Parking brake piston22includes piston head21and piston flange23. A bead flange28of spring housing16is rolled over and crimped to secure the spring housing16to adapter base18. The parking actuator optionally includes follower plate66which is disposed between biasing member70and parking diaphragm20, and washer68which is disposed between parking diaphragm20and adapter base push rod26.

Brake actuator10also includes a service brake chamber40defined by service housing42and adapter base16. Service brake chamber42includes a service brake diaphragm44, a service brake spring46, and a push rod48which are movable between a service brake apply position (FIG. 1) and a service brake release position (FIG. 5). The service brakes are operated by the driver with the vehicle foot pedal and service brake control is independent of the parking brake control except that when the parking brake is applied, the service brake components are also used in application of the parking brakes. Service brake diaphragm44is held between service housing40and adapter base16and secured with clamping ring49or other fasteners can be used for field service. In another example a bead flange such as a bead flange28or other fasteners can be used for attaching housing42to adapter base16.

Still referring toFIG. 1, adapter base16defines a central opening50and piston22defines a central cavity52, both of which are centered about axis, Y, and through which the adapter base push rod26extends. The adapter base16optionally includes bushings51and53to support adapter base push rod26and an o-ring55to provide a tight seal for the parking brake chamber18. Service housing40defines a central opening54centered about axis, Y, and through which the service push rod48extends. Brake actuator10is typically mounted on a vehicle axle with threaded mounting studs56, for example, and brake unit (not shown) such that service push rod48is connected to an operating member of the brake and transfers forces generated in actuator10to the vehicle brakes in order to apply or release the parking brakes and release or apply the service brakes.

As mentioned above, parking diaphragm20is disposed within parking brake spring chamber18between spring housing14and adapter base16. Parking diaphragm20divides the parking brake chamber18into a spring volume17and a release volume19on opposite sides of parking diaphragm20. Fluid, for example air fluid, is introduced to the release volume through port60in a wall of adapter base16. During the parking brake “release” cycle pressurized air, expands parking diaphragm20and the pressure urges piston22and adapter base push rod26which is connected to the piston22in one direction along axis, Y and away from the adapter base16to increase the release volume19. This pressure on and by the diaphragm forces the parking brake spring24to compress to release the parking brake in the “brake release” position as shown inFIG. 1. In the brake release state piston22may contact spring housing14. Brake actuator10also includes a vent62, for example a one-way valve, for the flow of air out of the spring volume portion of parking brake chamber18. The one-way valve or vent62eliminates the need to provide vent holes in the spring housing14and protects the spring volume from external contaminants. The one-way valve or vent62also seals the spring volume17portion of the parking brake chamber18to protect the internal components from foreign objects and elements.

Parking brake spring24, for example a coil spring, has one end resting on spring housing14, and an opposite end resting on parking brake piston22. In this example, the piston head21has a geometry and height designed to control the stroke, or the extent of retraction, of the parking brake piston22in the parking brake “release” state which also controls the extent of compression of the parking brake spring24. The piston abuts the spring housing14and transfers pressure load from the parking brake piston22to the spring housing when the parking brake piston and spring reach the fully-retracted position as shown inFIG. 1. Sufficient pneumatic pressure in the release volume19overcomes the spring force of the parking brake spring24. Accordingly, brake actuator10is in the brake “release” state inFIG. 1which is the normal mode of brake actuator10while the vehicle is driving in motion. The air pressure in the release volume19of the spring chamber is greater than the air pressure in the spring volume17and the parking diaphragm20is inflated and pressed against the piston22and biasing member70.

FIG. 2is a close-up view of the dotted portion of parking brake actuator ofFIG. 1in the brake release state. The air pressure acting on the large parking diaphragm20during the brake release cycle presses the parking diaphragm20against parking piston22and against the piston passageways80,82to create a leak-free seal that prevents air flow from the release volume19to the spring volume17of the parking spring chamber18. Parking diaphragm20is made of a pliable material, for example, a flexible thermoplastic or flexible thermoset, including reinforced thermoplastics or thermosets and blends thereof. Suitable materials for parking brake diaphragm20include, but are not limited to, natural rubber, neoprene, and mixtures thereof, for example. Piston passage80, for example, extends from a spring-side surface81of piston to a diaphragm-side surface83of piston and defines a channel for fluid flow between the release volume and spring volume. The pressure in the release volume of the parking brake chamber is typically greater than in the spring volume during all cycles. The spring volume pressure is typically limited to no more than slightly above atmospheric pressure as governed by the setting of one-way vent62(FIG. 1).

In accordance with another aspect of the present invention, the parking diaphragm20is movable to control fluid or air flow between the release volume19and the spring volume17of the parking brake chamber based on differential pressure in those volumes. Parking diaphragm20has an opening71in alignment with axis Y through which adaptor base push rod26extends, and through which air can flow from release volume19to spring volume17. Opening71has a diameter, d1which surrounds adapter base push rod26having a smaller diameter d2. The difference between diameter d1and d2creates annular opening71through which air from release volume19can flow to spring volume17during parking brake “apply” cycle as will be further described. In the brake release state as shown, the force on the parking diaphragm20against piston22created by the air pressure in the release volume19overcomes the force of the biasing member70, for example a valve spring, on parking diaphragm20. Force exerted by air pressure onto parking diaphragm20is transferred to biasing member70which is shown compressed against piston22. The tight seal of parking diaphragm against piston22and piston passageways80,82prevents the flow of air from release volume19to spring volume17of parking brake chamber18.

Biasing member70urges parking diaphragm20away from passageways80,82when the pressure differential PDacross the diaphragm, is less than a predetermined threshold pressure, PT. The predetermined threshold pressure is a design parameter that is based on, but is not limited to, at least one of: the stiffness and or height of biasing member70, the diameter of the diaphragm, the stiffness of the diaphragm, for example, etc. The pressure differential, PD, in question is defined as the difference between the pressure in the release volume, PR, and the pressure in the spring volume, PS, as described below:
PD=PR−PS;
wherein the predetermined threshold pressure PT, is determined based on the particular brake actuator10being considered. Accordingly, as pressure drops in release volume17, a predetermined threshold pressure PT, is eventually reached and biasing member70forces parking diaphragm away from passageways80,82.

It is to be understood that the biasing member70is not limited to a spring or even a coil spring, but includes any elastic member which provides the energy storage and return function required to urge the diaphragm when the pressure differential, PD, is less than predetermined threshold pressure, PT. For example, alternative spring configurations, including multiple coil springs, leaf springs, cantilevered springs, etc., and alternative elements such as resilient blocks or chargeable high pressure bladders, are within the scope of the present invention. The biasing member can be constructed from a variety of materials, for example, metal, polymers, and wires and/or natural and synthetic fibers made of metal or polymer or both.

FIG. 2also illustrates the detail of adapter base push rod26which contacts piston22at shoulder72. A second shoulder74is spaced a distance d3away from shoulder72of piston22. In such case, distance d3is greater than the combined thickness of diaphragm20and the biasing member70, in a compressed state as shown. This creates a clearance or gap85for movement of parking diaphragm20away from piston22. Therefore, in one example embodiment, biasing member70and parking diaphragm20are pressed tightly against piston22and a gap85is formed between the parking diaphragm20and the adapter plate push rod26. In another example embodiment, follower plate66or washer68, or both, are present. Follower plate66is an optional component which protects the parking diaphragm from wear by contact from biasing member70. Washer68is can be used to limit travel of diaphragm20. In an example where both follower plate66and washer68are utilized as shown, follower plate66is spaced a distance d4away from washer68and the distance between them, d4, is greater than the thickness of parking diaphragm20. A clearance or gap85remains between parking diaphragm20and washer68as shown, and gap85varies in size. Gap85allows room for biasing member70to expand so that at least a portion of parking diaphragm20can be urged away from the piston22by biasing member70when the differential pressure, PD, is below the predetermined threshold pressure, PT, as illustrated inFIGS. 3, 4 and 5.

As mentioned above, parking brake actuator10is in the parking brake release state when the vehicle is being driven and is in motion. When the vehicle operator engages the parking brake control to apply the parking brake, a valve in pneumatic communication with port60is opened and air in the release volume flows out of the port while the brake spring pushes the piston and diaphragm toward the release volume and away from the plug and one-way valve. Since the air pressure inside the release volume exceeds the pressure inside the brake spring volume, the diaphragm is pushed against the piston until the spring force of biasing member, for example, valve spring70exceeds the force on the parking diaphragm20imparted by the air pressure differential between the release volume and the spring volume.

FIG. 3is a close-up view of the dotted portion of parking brake actuator ofFIG. 1during parking brake application. Biasing member70, for example, a valve spring, is disposed between the piston and the diaphragm and is in transition from a compressed mode to a fully expanded mode. Even when partially expanded, biasing member70urges parking diaphragm20away from piston22when the pressure differential, PD, across the diaphragm, reaches the predetermined threshold pressure, PT. That is, during the brake apply cycle, biasing member70provides a biasing force on parking diaphragm20urging it to a floating position between the piston and washer that permits clean dry air in the parking brake release volume19to pass, as indicated by arrow90, through opening71of parking diaphragm20and through passageway80, as indicated by arrow92, into the spring volume17when the pressure differential, PD, across the parking diaphragm20has fallen below the predetermined threshold pressure, PT. Air enters passage80on the diaphragm-side surface83of piston22and exits adjacent spring-side surface81of piston22.

The predetermined threshold pressure, PT, is a pressure at which air can begin to pass over to spring volume17and is preferably low enough to prevent excessive amount of air to flow from release volume19into spring volume17during the brake apply or brake release cycle. The predetermined threshold pressure is also determined to permit air passage to spring volume17to alleviate the vacuum created in spring volume17by the application of parking brakes. The predetermined threshold pressure, PT, can depend on a variety of factors such as the vacuum created during spring volume expansion and other factors known to those of skill in the art. In another example, the predetermined threshold pressure, PT, can be slightly greater than the absolute value of the vacuum created in spring volume17. For example, if a vacuum of −8 psi is created in spring volume17of a particular brake actuator at full stroke, the predetermined threshold pressure, PT, should be slightly greater than 8 psi to alleviate the vacuum. In this example, biasing member70would urge parking diaphragm away from passage80when pressure in release volume19relative to spring volume17dropped from a high pressure, for example, 100-130 psi, to a threshold pressure that approaches, and is greater than 8 psi. Accordingly, in one example the predetermined threshold pressure at which the parking diaphragm20moves away from piston22, is less than about 120% of the absolute value of the vacuum pressure in spring volume17, and in another example, less than about 110% of the absolute value of vacuum pressure created in spring volume17for a given brake actuator design. In another example, the predetermined threshold pressure, PT, can range from about 5 psi to about 15 psi.

Biasing member70is designed to overcome the positive predetermined threshold pressure and move parking diaphragm away from the passageway80to permit air flow into spring volume17. For example, a biasing member70that is a valve spring as shown, will have a spring constant that is great enough such that in the compressed position it generates sufficient force to overcome the positive pressure in release volume19. The spring force of the valve spring is designed based on the pressure differential, PD, across parking diaphragm20, decreasing below the predetermined threshold pressure, PT, and should not be too strong or else, for example, a substantial volume of air from the release chamber will flow into the spring chamber and leak out of the system through the vent or one-way valve62during brake release.

FIGS. 4 and 5illustrate parking brake actuator10in the parking brake applied state. Biasing member70can be in a partially extended or fully extended position and is shown pressed indirectly against parking diaphragm20and indirectly against follower plate66which can be optionally employed to protect the surface of parking diaphragm. Parking diaphragm20is shown pressed against optional washer68and indirectly against shoulder74of the adapter base push rod26.FIG. 5is a schematic cross-sectional, full view of a parking brake actuator in a brake applied state. As shown, parking brake spring24is in the extended position, the default brake “apply” position. Spring volume17is larger than in the parking brake “release” state (FIG. 1). Parking diaphragm20is deflated with release volume19having little remaining pressure. Piston22is dropped a stroke distance, ls, from spring housing14. The example brake actuators described herein prevents or eliminates vacuum in spring volume17which allows a greater portion of force stored by parking brake spring24to be delivered to the foundation brake.

FIG. 6is a cross-sectional view of piston22illustrating several features. Piston22has an annular protrusion or annular ring84disposed between central cavity52and piston passageways80,82,86and88and at a radial distance from central cavity52which is less than the radial distance of the piston passageways from central cavity52. Protrusion84locates and retains the biasing member, preventing slippage from position. Annular protrusion84preferably has a smooth surface to facilitate an improved seal of parking diaphragm20against diaphragm-side surface83of piston22. The height of annular protrusion84is at least as great as the height of the biasing member70, for example the height of compressed biasing member70, for example, valve spring, in the parking brake release state as shown (FIG. 2) to facilitate improved sealing of parking diaphragm20against piston passageways.

FIG. 7is a diaphragm-side view of the parking piston22taken across lines7-7ofFIG. 6. The piston flange23can have an area that is significantly greater than the piston head21. Piston22has a plurality of passageways,80,82,86and88, to increase or evenly distribute the flow of air between the release volume19and spring volume17. Annular protrusion84is radially disposed between central opening52and passageways80,82,86and88.

The embodiments described herein provide for a brake actuator with internal flow control of clean air within the parking brake chamber. A biasing member is capable of forcing the parking diaphragm away from the parking piston when there is low air pressure in the parking release chamber, allowing clean system air which resides in the release volume to flow to the spring volume of the parking brake chamber. This prevents or eliminates any buildup of a vacuum in the spring volume which allows a larger portion of the parking spring's full force to be delivered to the foundation brake. The sealed spring chamber design guards against external contaminants, such salt and gravel.