HK1097308B - Air-operated brake actuator with control valve - Google Patents

Description

Pneumatic brake actuator with control valve

Technical Field

The present invention relates to pneumatic diaphragm brakes for vehicles and more particularly to service brake and spring brake actuator assemblies and a built-in vent valve mechanism to provide a sealed spring cavity that protects the spring from direct exposure to atmosphere and environmental contamination.

Background

An air brake system for vehicles such as buses, trucks, trailers, and other heavy-duty vehicles includes a brake shoe and brake drum assembly that is actuated by a method of selectively applying compressed air to operate an actuator assembly. Conventional air brake actuators have both a service brake actuator, which is used to actuate the brakes under normal driving conditions by applying compressed air, and a spring-type emergency brake actuator, which causes the brakes to actuate when air pressure has been released. The emergency brake actuator includes a strong compression spring that forcibly applies the brake when air is released. This is commonly referred to as a spring brake.

The pneumatic brake actuator is of the piston or diaphragm type. In diaphragm brake actuators, two pneumatic diaphragm brake actuators are typically arranged in a tandem configuration, including a pneumatic service brake actuator for applying the vehicle's normal service brakes and a spring brake actuator for applying the vehicle's parking or emergency brakes. Both the service brake actuator and the spring brake actuator include a housing having an elastomeric diaphragm that divides the interior of the housing into two distinct fluid chambers. On the other hand, piston brake actuators operate under substantially the same principles as described above, except that instead of a diaphragm, a piston reciprocates in a cylinder to apply the normal and/or parking brakes of the vehicle.

In a typical service brake actuator, the service brake housing is divided into a pressure chamber and a pushrod chamber. A pressure chamber is fluidly connected to a source of pressurized air, and a pushrod chamber mounts a pushrod coupled to the brake assembly, whereby ingress and egress of pressurized air into the pressure chamber reciprocates the pushrod into and out of the housing to apply and release a braking operation.

In a typical spring brake actuator, the spring brake housing is divided into a pressure chamber and a spring chamber. The pressure disc is located within the spring chamber between the diaphragm and the high force compression spring, with the opposite end of the high force compression spring in contact with the pressure disc abutting the housing. In one known construction, the actuator rod extends through the pressure plate and diaphragm into the pressure chamber and through a dividing wall that separates the spring brake actuator from the service brake actuator. The ends of the actuators are fluidly connected to the pressure chambers of the service brake actuators.

When the parking brake is applied, the spring brake actuator pressure is vented from the pressure chamber, and the strong compression spring urges the pressure disc and diaphragm toward the dividing wall between the spring brake actuator and the service brake actuator. In this position, an actuator lever connected to the pressure plate is pushed to apply the parking or emergency brake, forcing the vehicle to no longer move. To release the parking brake, the pressure chamber is closed to atmosphere and compressed air is introduced into the pressure chamber of the spring brake actuator, which expands the pressure chamber to move the diaphragm and pressure plate toward the opposite end of the spring brake actuator housing, thereby compressing the strong compression spring.

One known problem with spring brake actuators of this design is that as the high force compression spring is compressed, the pressure chamber volume increases and the spring chamber volume decreases, resulting in a pressure increase within the spring chamber unless the spring chamber includes a particular system for relieving the increased pressure within the spring chamber. The pressure rise in the spring chamber when the brake is released is highly undesirable, because in order to compress the spring sufficiently to release the brake sufficiently, the pressure rise in the spring chamber must be counteracted by the pressure increase in the pressure chamber.

The pressure rise within the spring chamber is exacerbated by the fact that most compressed air systems of heavy vehicles operate at industry standard maximum pressures. The combined pressure of the spring and the increase in air pressure in the spring chamber cannot approach the maximum value for proper brake operation. When the combined force associated with the spring pressure and the pressure rise in the spring chamber approaches the force applied by the maximum pressure, the brake cannot be released, only partially released, or released very slowly, all of which are undesirable.

One typical solution to the problem of pressure rise in the spring chamber is to provide an appropriate vent design in the spring chamber. The most common venting mechanism in diaphragm brake actuators is to provide a plurality of holes in the housing around the spring chamber. The main disadvantage of such a venting opening is that the interior of the spring chamber is thus exposed to the environment. Foreign matter, such as dust, salt particles, and moisture, can enter the spring cavity, accelerating wear, corrosion, or wear of various internal brake components (e.g., springs). Damage to internal brake components by foreign matter can cause increased maintenance or premature failure of the spring and subsequent replacement of the brake actuator.

Another problem with venting the spring chamber directly outward is that vehicles (e.g., tractors/trailers) are often parked in parking lots adjacent the dock for extended periods of time. These parking lots are typically sloped and below ground. Under heavy rain or snow conditions, the parking lot can be filled with water to reach the height exceeding the exhaust hole, and flows into the interior of the spring cavity. While water will typically drain from the spring chamber through the vent hole when the brake is released, this flooding can accelerate corrosion and present other environmental hazards. Under certain environmental conditions, water can freeze, which can prevent the brake from fully releasing.

Because of the problems associated with foreign matter entering the spring chamber through the vent holes, attempts have been made to seal the spring chamber to prevent the entry of various foreign matter. However, sealing the spring chamber presents other problems, as there is a tendency for a vacuum or depression to form in the spring chamber when applying the parking brake, unless a system is provided to compensate or relieve the depression. If the depression is low enough, it can slow down the response time of the parking brake, which is undesirable.

Several known attempts to eliminate the pressure rise and vacuum generation within the spring chamber while preventing the ingress of foreign matter include, for example, fluidly connecting the spring chamber of the spring brake actuator to either chamber of the service brake actuator, providing a filter at the vent, and providing an internal fluid passageway from the spring chamber, through the actuator rod, and into the service brake pressure chamber. All these solutions are compromises in that they do not provide a complete solution or introduce other problems there. For example, a vent hole provided with a filter inherently allows ambient air to enter the brake, creating a brake that is not fully sealed. As long as the filter is open, foreign matter may enter the brake through the filter, for example if the brake actuator is submerged in a water-filled parking lot.

An example of a vent provided with a filter is provided in U.S. patent No.6029447 issued on 29/2/2000. The internal fluid passage extending through the actuator requires a complex designed two-way valve that controls fluid flow to relieve the pressure rise in the spring chamber while allowing the ingress of pressurized fluid to prevent a vacuum in the spring chamber. Examples of such two-way valves are also disclosed in U.S. patent No.5,722,311 issued at 3/1998 and U.S. patent No.5372059 issued at 13/12 1994.

On the other hand, an example of a one-way valve is disclosed in U.S. patent No.6,588,314 issued on 8.7.2003, the entire disclosure of which is incorporated herein by reference. The vent design provides an effective solution to the problem of pressure rise in the spring chamber by allowing internal air to vent from the spring chamber to the service brake pressure chamber. However, the compression spring within the spring chamber must be selected to have a much stronger spring than conventional spring chamber springs to overcome the low pressure or vacuum created within the spring chamber when the spring brake is applied or otherwise when the spring brake is not properly applied for the desired application time.

It would be desirable to have a pneumatic brake actuator that includes a spring brake actuator that is sealed and eliminates pressure buildup and vacuum formation without requiring complex or high maintenance valve and filter systems and/or without requiring very strong springs within the spring chambers.

Disclosure of Invention

The present invention relates generally to pneumatic service brake actuator systems and spring brake actuator systems having a sealed chamber and a control valve mechanism that provide protection from direct exposure to atmospheric and environmental contamination while providing a two-way vent valve function for controlling pressure buildup and vacuum buildup in the spring chamber of the spring brake actuator.

According to an aspect of the present invention, a pneumatic brake actuator for applying a parking brake of a vehicle includes: a sealed housing including a first end wall, a second end wall opposite the first end wall, a peripheral side wall extending between the first end wall and the second end wall, the first end wall, the second end wall, and the peripheral side wall together defining an interior cavity therein; a diaphragm spanning the interior chamber and dividing the interior chamber into a spring chamber between the diaphragm and the first end wall and a pressure chamber between the diaphragm and the second end wall, the diaphragm being in a first position when the pressure chamber is pressurized with fluid and in a second position when the pressure chamber is vented; a spring disposed in the spring cavity and biasing the diaphragm in a direction toward the second end wall; a hollow actuator rod having one end connected to the central aperture of the diaphragm and the other end extending through the second end wall, wherein the hollow actuator rod is in a position to apply the parking brake when the diaphragm is in the second position and in a position to release the parking brake when the diaphragm is in the first position; a control valve located within the hollow actuator stem, the control valve including a valve body having an opening formed therein for allowing fluid communication through the hollow actuator stem between the spring chamber and a side of the valve body opposite the second end wall and the pressure chamber, the control valve including a membrane formed of an elastomeric material and having a vent hole formed therethrough, the membrane being located within the valve body opening and configured to seal the opening except for the vent hole when the membrane is not subjected to fluid pressure exceeding a predetermined level.

Preferably, the valve body opening has a generally circular cross-section and the diaphragm has a circular diaphragm portion configured to abut against a peripheral wall defined by the valve body opening. The opening of the valve body typically includes an axial cylindrical bore formed in the lower interior region of the valve body, a conical bore extending axially from the cylindrical bore, and a small central bore extending from the conical bore to the top surface of the valve body, with the diaphragm being located within the opening at the intersection of the cylindrical bore and the conical bore.

Preferably, the control valve further comprises a cage securely received within the lower portion of the cylindrical bore, the cage having a generally axial fluid passage defined therearound for permitting fluid flow therethrough. The retainer generally has a circular shape with two flat sides formed on opposite sides of the circular retainer, and the fluid passage of the retainer is formed at a gap defined between the flat sides and the cylindrical hole of the valve body. The valve body of the control valve may include a fluid passage formed in a bottom surface of the valve body in a radial direction and corresponding to a gap defined by the flat edge and the cylindrical hole of the valve body.

Preferably, the control valve further includes a filter element attached to a top surface of the valve body, and the valve body of the control valve has a central recess portion formed in the top surface of the valve body.

In a preferred embodiment of the invention, when the fluid pressure within the spring chamber and inside the hollow actuator rod reaches a threshold point, the circular membrane portion of the elastic membrane deforms axially toward the cage to allow fluid to flow around the circumference of the circular membrane portion and relieve the pressure buildup within the spring chamber.

In another preferred embodiment of the invention, the membrane of the control valve comprises a ball head portion formed in the centre of the membrane and configured to close the small central hole of the valve body when the membrane is deformed in the respective direction. Thus, when the fluid pressure on the side of the second end wall opposite the pressure chamber reaches a threshold pressure, the diaphragm is urged against the inner wall defined by the conical opening, with its ball portion closing the small central bore of the valve body to seal the opening of the control valve.

In accordance with one aspect of the present invention, a control valve for use in a vehicle pneumatic brake actuator system is disclosed, wherein the brake actuator system includes a spring brake actuator and a service brake actuator arranged in series, the spring brake actuator including a sealed housing with a spring chamber and a pressure chamber, a hollow actuator rod disposed generally within the spring brake actuator and axially movable toward the service brake actuator to apply a parking brake, the service brake actuator including a service pressure chamber for applying the service brake. The control valve of the present invention comprises: a valve body positionable within the hollow actuator stem, the valve body including an opening formed therein for allowing fluid communication between the spring chamber of the spring brake actuator and the service pressure chamber of the service brake actuator through the hollow actuator stem, the control valve including a membrane formed of an elastomeric material and having a vent hole formed therethrough, the membrane being located within the valve body opening and configured to seal the opening except for the vent hole when the membrane is not subjected to fluid pressure exceeding a predetermined level.

In a preferred embodiment of the invention, when the fluid pressure within the spring chamber and inside the hollow actuator rod reaches a predetermined point, the elastic membrane deforms axially to allow fluid to flow around the circumference of the membrane, thereby enabling the pressure build-up within the spring chamber to be relieved.

In another preferred embodiment of the invention, the membrane of the control valve is configured to close the entire opening of the valve body when the membrane is circumferentially deformed against the wall of the valve body within the opening. Thus, when the fluid pressure within the primary pressure chamber reaches a threshold value, the diaphragm is urged against the inner wall of the opening to close and seal the opening in the control valve body.

Drawings

FIG. 1 is a cross-sectional view of a tandem pneumatic brake actuator according to the present invention showing a two-way control valve for controlling fluid flow between a spring chamber and a main pressure chamber in which a check bolt is in a retracted position;

FIG. 2 is a cross-sectional view of the tandem pneumatic brake actuator of FIG. 1, shown when the spring brake is not actuated and the check bolt is in an extended position;

FIG. 3 is an enlarged bottom view of the control valve of FIG. 1;

FIG. 4 is an enlarged cross-sectional view of the control valve taken along line A-A of FIG. 3, showing the diaphragm contained within the control valve in a normal condition not subjected to fluid pressure above a certain level;

FIG. 5 is an enlarged cross-sectional view of the control valve taken along line A-A of FIG. 3, showing a diaphragm housed within the control valve in a condition exposed to fluid pressure from a spring chamber of the spring brake actuator;

FIG. 6 is an enlarged cross-sectional view of the control valve taken along line A-A of FIG. 3, showing a diaphragm housed within the control valve in a position subject to fluid pressure from a service brake actuator pressure chamber.

Detailed Description

Fig. 1 and 2 show a tandem pneumatic brake actuator 10 including a service brake actuator 12 in combination with a spring brake actuator 14. The service brake actuator 12 applies and releases the service or service brakes of the vehicle. The spring brake actuator 14 is used to apply the emergency or parking brakes of the vehicle.

Both the service brake actuator 12 and the spring brake actuator 14 include housings 16, 18 formed by a connecting shell 20 connected to a service brake cover 22 and a spring brake cover 24, respectively. The interface housing 20 defines a common dividing wall that separates the service brake housing 16 from the spring brake housing 18 while forming a portion thereof. It is within the scope of the present invention to replace the connection housing 20 with discrete covering elements much like the main brake cover 22 and the spring brake cover 24.

The movable member of this embodiment includes elastomeric diaphragms 30, 32 that span the interior of the service and spring brake housings 16, 18, respectively, by having the outer edges of the diaphragms compressibly retained between the coupling housing 20 and the respective service and spring brake covers 22, 24. It will be appreciated that the invention is also applicable to piston brakes that replace diaphragms, the piston spanning the interior of a cylindrical spring brake housing.

With particular attention to the service brake actuator 12, the diaphragm 30 fluidly divides the service brake actuator 12 into a pushrod chamber 36 and a pressure chamber 38. A pushrod 40 having a pressure plate 42 disposed at one end is disposed within the pushrod chamber 36, the pressure plate 42 being adjacent the diaphragm 30, the pushrod 40 extending through a bearing 44 disposed within an opening 46 in the service brake cover 22. A return spring 48 is disposed between the bearing 44 and the pressure disc 42 to help bias the pressure disc 42, along with the push rod 40, into the interior of the service brake housing 16. Although not shown, in an S-cam brake assembly, the end of the push rod 40 is connected to a slack adjuster of the S-cam brake assembly, whereby reciprocation of the push rod 40 relative to the service brake housing 16 causes application and release of the service brakes.

The pressure chamber 38 is fluidly connected to a source of compressed air through an air inlet 50. When the vehicle operator depresses the brake pedal, compressed air is drawn from the compression chamber 38 through the intake ports 50 to reciprocate the push rod 40. The increase in compressed air within the pressure chamber 38 urges the pressure disc 42 and pushrod 40 from the coupling housing 20 toward the service brake cover 22 to provide service braking.

Focusing more closely on the spring brake actuator 14, the diaphragm 32 fluidly divides the spring brake housing 18 into a pressure chamber 56 and a spring chamber 58. The pressure chamber 56 is fluidly connected to a source of pressurized air through an aperture, not shown but substantially identical to the air inlet aperture 50. Typically, the compressed air system that supplies pressure chamber 56 is physically distinct from the compressed air system that supplies service brake actuator 12.

A pressure disc 60 is disposed within the spring chamber 58, and a high force compression spring 62 is disposed between the pressure disc 60 and the spring brake cover 24. The pressure disc 60 includes an annular groove 63, the annular groove 63 receiving an inner radial edge 64 of the diaphragm 32 therein. A retaining ring 66 may be provided that is press fit adjacent annular groove 63 to retain an inner edge 64 of diaphragm 32 to pressure plate 60. Pressure plate 60 also includes an axially stepped bore 68, with, for example, an actuator rod shoulder and a bearing shoulder 72 defined within axially stepped bore 68. The axial bore 68 is aligned with an aperture 74 in the spring brake cover 24.

An annular bearing or flange guide 76 is mounted within axial bore 68 and is positioned by contacting bearing shoulder 72. A hollow actuating rod 78 is press fit at one end within the axial bore 68 and is positioned by abutting the flange portion of the annular bearing 76. The structure of the diaphragm 32 and the actuator rod 78 to the pressure plate 60 may vary from that described above depending on the particular design of the spring brake actuator 14. The annular bearing 76 and the pressure disc 60 define an air passage or gap (not shown) therein along the limit bolt 94 to allow air to flow so that air can flow back and forth between the spring chamber 58 and the interior chamber of the hollow actuator rod 78.

The other end of the actuator rod 78 extends through a bearing seal assembly 80 disposed in an aperture 81 formed in the coupling housing 20. Back-up seal assembly 80 is well known.

A transfer plate 82 closes the end of the actuating rod 78 opposite the pressure plate 60. The valve plate 82 includes a threaded protrusion 84 that is threadably received within the actuator rod 78. The valve plate 82 and the protrusion 84 together form a valve body and house a two-way vent valve (or control valve) 86 according to the present invention. A radially extending fluid passage 87 (see fig. 3 and 4) is formed in the lower face of the valve plate 82, the lower face of the valve plate 82 being preferably sized to be received within a recess 88 of the coupling housing.

The brake actuator further includes a limit bolt assembly 90, the limit bolt assembly 90 including an adjustment nut 92 threadably mounted and permanently secured to a limit bolt 94, the limit bolt 94 terminating in a limit bolt tip 96. The limit bolt assembly connects the pressure plate and actuator rod to the spring brake actuator 14 by placing the limit bolt 94 and limit bolt top 96 inside the actuator rod 78, then extending the other end of the limit bolt 94 through the axial bore 68, then threading the limit bolt through a cap or collar 97, and permanently securing the adjustment nut 92 to the limit bolt, wherein the cap or collar 97 is riveted to the spring brake top 24 in a substantially sealed manner. Since the nut 92 and the stop bolt top 96 have a larger diameter than the smallest diameter of the passage 68, the stop bolt connects the pressure disc to the spring brake top 24.

The caging bolt head 96 preferably includes a bearing 98 disposed between opposing collars 100. The bearing 98 contacts the inner surface of the actuator rod 78 to prevent the collar 100 and limit bolt 94 from contacting the interior of the actuator rod 78 while helping to guide the reciprocating motion of the actuator during application and release of the emergency brake. Axial grooves 99 are formed in the bearing surface to form fluid flow paths around the bearing.

The caging bolt assembly 90 serves to mechanically contract and hold the high force compression spring 62 in compression (as particularly shown in figure 1). By turning the adjusting nut 92, the limit bolt can be threadably withdrawn from the spring brake housing 18. As the stop bolt is withdrawn, the stop bolt top 96 contacts the bearing 76 at the upper end of the actuator rod 78 to withdraw the actuator and pressure plate with the stop bolt, thereby compressing the spring. Locking members of powerful compression springs are known, and are generally used during assembly of the brake actuator and/or for mechanically releasing the brake in the event of a failure or absence of the compressed air system.

Referring now to fig. 3 and 4, control valve 86 is described in more detail. The bolt projection 84 and the valve plate 82 together effectively function as a valve body and define an axial cylindrical bore 110 in a lower interior region thereof. The retainer 112 is press fit in the lower end of the bore 110, preferably against an annular shoulder in the bore, in a manner that substantially closes off the opening except for a small hole or passage defined therein for allowing fluid communication therethrough. As shown in fig. 3, the cage 112 may be a circular profile having two flat sides 113 formed on opposite sides of the circular cage 112, the flat sides 113 leaving a side gap 115 in the cylindrical bore 110. The gap 115 is aligned with the radially extending fluid passage 87 to provide a continuous radial path through the cage 112 and along the lower face of the valve plate 82. The bottom surface of the retainer 112 is flat and flush with the bottom surface of the valve plate 82 and includes a groove 114 that can be used to guide the retainer 112 assembly into the bore 110. The top surface of the retainer 112 is also preferably flat and includes radially extending slots 116 at the flat edges 113 that align with the passages 87 when the retainer 112 is assembled into the valve body 82 and the valve body 84. The slot 116, flat edge 113 and channel 87 form a continuous air path through the side of the retainer 112 and along the valve plate 82. However, the shape of the cage and the fluid passage are not limited to the illustrated configuration, and other shapes may be employed as long as they sufficiently close the hole 110 with the restricted fluid passage that allows air to flow around. Above the cylindrical bore 110 is a conical cavity 118 extending upwardly from the bore 110, with a small central bore 120 formed through the top surface of the valve body 84. The top surface of the valve body 84 preferably includes a central recessed portion 122 and a filter element 126 is securely attached to the top surface of the valve body 84 overlying the axial bore. The filter element is preferably made of synthetic polyester or other materialsThe filter material is made and attached to the valve body, for example, with a pressure sensitive adhesive backing to secure the filter element to the valve body. Preferably, the filter element 126 is a porous, air permeable polyester film having hydrophobic and oleophobic properties and having a pore size of about one micron. Acceptable filter elements include treated, expanded Polytetrafluoroethylene (PTFE), available from W.L. Gorea and Associates and Gore-The product under the brand is obtained.

As shown in FIG. 4, a diaphragm or membrane 130 is mounted within the cavity defined by the upper portion of the cylindrical bore 110 and the conical cavity 118, with its circular periphery abutting the junction of the wall of the conical cavity 118 and the cylindrical cavity 110 to seal the aperture. The membrane 130 includes a ball portion 132 and a circular membrane portion 134 extending radially outward from the ball portion 132, and in order for the membrane portion 134 to be elastically deformed when air pressure applied to the membrane 130 exceeds a certain level, the membrane 130 is preferably made of a rubber or polymer material having suitable strength and elasticity. When the membrane 132 is installed in the cavity of the valve body 84 without being subjected to any external air pressure, the rounded bottom of the top portion 132 contacts the top surface of the retainer 112 and forms a small annular space 136 between the bottom surface of the membrane portion 134 and the top surface of the retainer 112, which provides a clearance that allows the membrane portion 134 therein to elastically deform. Membrane portion 134 also includes one or more through-holes (or vent holes) 140 that are sized much smaller than central bore 120 of valve body 84.

Referring now to fig. 4-6, the operation of the brake actuator and control valve 86 is described. As shown in FIG. 4, the membrane portion 134 of the elastic membrane 130 is naturally biased against the sidewall at the intersection of the cylindrical bore 110 and the conical bore 118, so it seals the opening to prevent air from passing therethrough to freely flow between the spring chamber 58 and the main actuator pressure chamber 38. In this case, however, only a limited amount of airflow is allowed through the small through-holes 140 of the membrane 130.

When the parking brake is released, compressed air enters the pressure chamber 56 of the spring brake actuator 14. As the volume of the pressure chamber 56 increases, the volume of the spring chamber 58 decreases, thereby increasing the pressure of the air contained therein. The compressed air in the spring chamber is fluidly connected to the control valve 86 through the axial bore 68 of the pressure plate 60 and through the inner chamber of the actuator rod 78. As the pressure in the spring chamber 58 increases, it reaches a point where it overcomes the elastic force of the diaphragm 130 with its periphery abutting the inner wall of the valve body 84, and pushes the peripheral region of the elastic diaphragm 130 axially against the elastic force of the diaphragm portion 134 (as shown in fig. 5). This allows air to flow from the central bore 120 around the periphery of the diaphragm 130, through the fluid path formed by the valve body opening or side gap 115, along the passage 87, and into the service brake pressure chamber 38, as indicated by arrow "a" in FIG. 5. In this manner, the pressure buildup within the "sealed" spring chamber 58 is effectively relieved by operation of the control valve of the present invention without the provision of a vent hole in the spring chamber as in the conventional spring brake actuators described above.

When the elevated pressure is released from the spring chamber 58, the membrane 130 returns to the original shape shown in FIG. 4, again sealing the opening of the control valve 86 except for the small vent hole 140 of the membrane. Fig. 2 shows a state when the pressure is accumulated after the parking brake actuator is released.

Conversely, when the parking brake is applied by exhausting compressed air from the pressure chamber 56 of the spring brake actuator, the compression spring 62 urges the pressure plate 60 and diaphragm 32 toward the dividing wall between the spring brake actuator 14 and the service brake actuator 12. As a result, the actuator lever 78 connected to the pressure plate 60 is pushed to apply the park or emergency brake, thereby forcing the vehicle to no longer move in the manner described above or as is known in the art. However, the vacuum or depression created by the forced movement of the diaphragm 32 during expansion of the spring cavity 58 may delay the application time of the spring brake or present other hazards to the proper operation of the spring brake. In this operation, the diaphragm 130 of the control valve is in a normal or non-pressurized state as shown in FIG. 4 prior to application of the parking brake. In this case, any air flow through the control valve 86 must only pass through the small apertures 140 in the membrane. The small holes 140 allow some airflow to fill the expanding volume of the spring chamber 58 when the spring brake is applied. Such "breathing" air may preferably be provided through a corresponding orifice, such as orifice 50, when the service brake actuator 12 is released. By selecting the appropriate size of the aperture 140 in accordance with the particular specifications of the vehicle service brake and spring brake system, the problem of vacuum generation within the spring chamber can be effectively addressed.

Referring specifically to FIG. 6, another operational scenario of the braking system of the present invention is described herein. When the service brakes are applied, compressed air flows into the pressure chamber 38 of the service brake actuator 12. Here, the pressure of the compressed air within the main pressure chamber 38 exceeds a threshold value, the diaphragm 130 is urged against the conical wall of the opening 118, the top portion 132 seats against the central bore 140 of the valve body 84 and the membrane portion 134 deforms to contact the conical wall, thereby sealing the entire opening of the control valve 86. In this manner, the spring cavity 58 may avoid undesirable fluid pressure buildup therein. Conversely, when the service brakes are released, the control valve 86 again returns to the normal state as shown in fig. 4.

As mentioned above, one advantage of the sealed pneumatic brake actuator 10 according to the present invention is that the spring brake actuator 14 is completely sealed from the atmosphere. A control valve with bi-directional communication or venting capability allows for the release of pressure within the seal spring chamber and also allows for vacuum generation problems within the spring chamber in response to brake application and release. The construction and construction of the control valve is relatively simple compared to the conventional two-way or one-way valves described above, wherein the brake system of the present invention does not require a vent hole in the spring chamber and a coil spring retained within the control valve that often fails over long periods of use of the brake system. Additionally, it is not necessary to select a strong compression spring 62 with an enhanced spring force in order to overcome the vacuum effect that occurs within the spring cavity 58 when applying the parking brake, such as the prior art brake system disclosed in U.S. patent No. 6588314.

While the invention has been described in detail with respect to specific embodiments thereof, it is to be understood that this is by way of illustration and not of limitation, and that various changes and modifications in form and detail thereof may be made, and the scope of the appended claims should be construed as broadly as the prior art will permit.

Claims (22)

1. A pneumatic brake actuator for applying a vehicle parking brake, comprising:

a sealed housing including a first end wall, a second end wall opposite the first end wall, a peripheral side wall extending between the first end wall and the second end wall, the first end wall, the second end wall, and the peripheral side wall together defining an interior cavity therein;

a diaphragm spanning the interior cavity and dividing the interior cavity into a spring chamber between the diaphragm and the first end wall and a pressure chamber between the diaphragm and the second end wall, the diaphragm being in a first position when the pressure chamber is pressurized with fluid and in a second position when the pressure chamber is vented;

a spring disposed in the spring cavity and biasing the diaphragm toward a second position;

a hollow actuator rod having one end connected to the central bore of the diaphragm and the other end extending through the second end wall, wherein the hollow actuator rod is in a position to apply the parking brake when the diaphragm is in the second position and in a position to release the parking brake when the diaphragm is in the first position; and

a control valve located within the hollow actuator stem, the control valve including a valve body having an aperture formed therein for allowing fluid communication through the hollow actuator stem between the spring chamber and a side opposite the second end wall and the pressure chamber, the control valve including a membrane formed of an elastomeric material and having a vent hole formed therethrough, the membrane being located within the valve body aperture and configured to seal the aperture except for the vent hole when the membrane is subjected to a fluid pressure below a predetermined level.

2. The brake actuator of claim 1 wherein the control valve allows fluid to pass through the control valve to relieve pressure buildup in the spring chamber when the diaphragm is subjected to fluid pressure exceeding a threshold value.

3. The brake actuator of claim 1 wherein the valve body opening has a circular cross-section and the diaphragm has a circular diaphragm portion configured to abut against a peripheral wall defined by the valve body opening.

4. The brake actuator of claim 3 wherein said valve body opening comprises an axial cylindrical bore formed in a lower interior region of said valve body, a conical bore extending axially from said cylindrical bore, and a small central bore extending from said conical bore to a top surface of said valve body, said diaphragm being located within said opening at the intersection of said cylindrical bore and said conical bore.

5. The brake actuator according to claim 4 wherein said control valve further comprises a cage securely received within a lower portion of said cylindrical bore, said cage having an axial fluid passage defined therearound for allowing fluid flow through said fluid passage.

6. The brake actuator according to claim 5, wherein the cage has a circular shape formed with two flat sides at opposite sides of the circular cage, the fluid passage of the cage being formed at a gap defined between the flat sides and the cylindrical hole of the valve body.

7. The brake actuator according to claim 6, wherein the valve body of the control valve includes a fluid passage formed in a radial direction in a bottom surface of the valve body and corresponding to a gap defined by the flat edge and the cylindrical hole of the valve body.

8. The brake actuator of claim 5 wherein the control valve further comprises a filter element attached to a top surface of the valve body.

9. The brake actuator according to claim 8, wherein the valve body of the control valve has a central recess formed in a top surface of the valve body.

10. The brake actuator of claim 5 wherein the circular membrane portion of the resilient membrane is axially deformable toward the cage when fluid pressure within the spring cavity and inside the hollow actuator rod reaches a threshold level to allow fluid to flow around the circumference of the circular membrane portion and release pressure buildup in the spring cavity.

11. The brake actuator according to claim 5, wherein the diaphragm of the control valve includes a ball portion formed at a center of the diaphragm and configured to close the small center hole of the valve body when the diaphragm is deformed in the corresponding direction.

12. The brake actuator of claim 11 wherein when fluid pressure on the side of said second end wall opposite said pressure chamber reaches a threshold level, said diaphragm is urged against an inner wall defined by said conical opening, said ball portion closing the small central bore of the valve body to seal the opening of the control valve.

13. A control valve for use in a vehicle pneumatic brake actuator system, the brake actuator system including a spring brake actuator and a service brake actuator arranged in series, the spring brake actuator including a sealed housing with a spring chamber and a pressure chamber, a hollow brake lever normally disposed within the spring brake actuator and axially movable toward the service brake actuator to apply a parking brake, the service brake actuator including a service pressure chamber for applying a service brake, the control valve comprising:

a valve body disposed within the hollow actuator stem, the valve body including an opening formed therein for allowing fluid communication between a spring chamber of the spring brake actuator and a service pressure chamber of the service brake actuator through the hollow actuator stem, the control valve including a membrane formed of an elastomeric material and having a vent hole formed therethrough, the membrane being located within the valve body opening and configured to seal the opening except for the vent hole when the membrane is subjected to a fluid pressure below a threshold level.

14. The control valve of claim 13, wherein the control valve allows fluid to pass through the control valve to relieve pressure buildup within the spring chamber when the diaphragm is subjected to fluid pressure exceeding a threshold level.

15. The control valve of claim 13, wherein the valve body opening has a generally circular cross-section, the diaphragm having a circular diaphragm portion configured to abut against a surrounding wall defined by the valve body opening.

16. The control valve of claim 15, wherein said valve body opening comprises an axial cylindrical bore formed in a lower interior region of said valve body, a conical bore extending axially from said cylindrical bore, and a small central bore extending from said conical bore to a top surface of said valve body, said diaphragm being located within said opening at an intersection of said cylindrical bore and said conical bore.

17. The control valve of claim 16, wherein the control valve further comprises a cage securely received within the lower portion of the cylindrical bore, the cage having an axial fluid passage defined therearound for allowing fluid to flow therethrough.

18. The control valve of claim 17, wherein the cage has a circular shape with two flat sides formed on opposite sides of the circular cage, the fluid passage of the cage being formed at a gap defined between the flat sides and the cylindrical bore of the valve body.

19. The control valve according to claim 18, wherein the valve body of the control valve includes a fluid passage formed in a radial direction at a bottom surface of the valve body and corresponding to a gap defined by the flat edge and the cylindrical hole of the valve body.

20. The control valve of claim 17, wherein the control valve further comprises a filter element attached to a top surface of the valve body.

21. The control valve of claim 17, wherein the circular membrane portion of the elastic membrane is axially deformable toward the cage to allow fluid to flow around the circumference of the circular membrane portion and relieve pressure buildup within the spring chamber when fluid pressure within the spring chamber and the interior of the hollow actuator stem reaches a threshold level.

22. The control valve of claim 17, wherein the diaphragm of the control valve includes a ball portion formed in the center of the diaphragm, the diaphragm being urged against an inner wall defined by the conical opening when fluid pressure within the primary pressure chamber reaches a threshold level, the ball portion closing a small central opening of the valve body to seal all openings formed in the control valve.