SPRING BRAKE ACTUATOR WITH FILTERED SERVICE VENTILATION OPENINGS
BACKGROUND OF THE INVENTION Field of the Invention The invention relates to air-operated diaphragm brakes for vehicles, and particularly to service brake and spring-loaded actuator assemblies in combination. STATE OF THE PRIOR ART An air brake system for a vehicle such as a bus, truck or the like typically includes a brake shoe and drum assembly that is operated by means of an actuator assembly operated by the selective application of air compressed. Conventional air brake actuators have both a service brake actuator for operating the brakes under normal driving conditions by application of compressed air, and a spring-type emergency brake actuator, which causes the brakes to be actuated when The air pressure has been released. The emergency brake actuator includes a strong compression spring that forces the brake application when air is released. This is often referred to as a spring brake. Typically, the spring brake actuator is arranged in tandem with the service brake actuator.
When full pressure is applied to the spring brake actuator, the air pressure acting against a diaphragm compresses the compression spring. A spring brake actuator rod is held in a retracted position by a relatively small return spring, thus not affecting brake operation. When the brake is to be applied during normal operation, compressed air is supplied to the service brake actuator, which, acting against a diaphragm, causes a service brake thrust rod to be extended and causes the brakes to be released. applied with an application force that is proportional to the air pressure applied to the service brake actuator. In the event of a loss of air pressure or intentional discharge of air from the spring brake actuator, the brake will be activated mechanically by the force of the compression spring acting on the spring brake actuator rod, which at its it acts on the service brake thrust rod to apply the brakes. In this way, the spring brake portion serves as both a parking brake and an emergency brake. In a typical air brake system of the prior art, the spring brake actuator and the service brake actuator are disposed in a single housing comprising a spring brake portion and a service brake portion. The service brake portion includes an air chamber defined partially by a flexible service diaphragm which acts against a service brake thrust rod and a return spring to ensure proper release of the brake when air is discharged from the air chamber. air The spring brake portion includes a spring chamber and an air chamber, both partially defined by a spring brake diaphragm which acts against a spring pressure plate to compress the compression spring in the spring chamber when it is applied. air pressure to the spring brake diaphragm in the air chamber. An actuator rod extends through the spring brake diaphragm to connect integrally with the pressure plate. In operation it is pushed out of the air chamber through an opening in the housing and a bearing provided with a pneumatic seal to link the service diaphragm and the thrust rod of the service brake, thereby causing the brakes to be applied . The spring brake diaphragm is provided with a centrally disposed opening having an annular rim and the actuator rod extends through the opening and links the annular rim to form an air-tight seal. The actuator rod is hollow, with a central bore, and a cage bolt that releases the brakes extends to the central bore. An end plate on the bolt links the spring brake pressure plate to bring the spring to a compressed state when the spring is caged by rotation of the bolt.
The service chamber on the service side of the service brake diaphragm is typically vented to the outside atmosphere to allow air to be discharged from that portion of the housing when the diaphragm is actuated by introducing compressed air to the opposite side of the diaphragm. service brake. When the service brake diaphragm is expanded, the diaphragm is moved in one direction so as to reduce the size of the service chamber that expels air out of the service chamber and through the ventilated openings in the service chamber. When the introduction of compressed air is removed, the service brake return spring returns the diaphragm to its unexpanded position. This movement may cause air to be returned to the service chamber. As the service chamber is ventilated to the outside atmosphere under the chassis of the truck, outside air brought to the service chamber is often accompanied by moisture and corrosive substances, such as road salt. Because the return spring is worn when cycling and its bare metal is thus exposed, the return spring may corrode due to airborne contaminants. Also, because the service brake return spring must exert a substantial force in order to return the service diaphragm to its unexpanded position, the service brake return spring may be subjected to great stress upon cycling . The life of the spring is inversely proportional to the amount of corrosion and stress that it experiences. In sealed housings, the spring can not be removed from the housing without permanently deforming it. In other housings, the actuator must be replaced by a new actuator and the old actuator sent to a rebuilder. In any case, the operator of the truck can not service the actuator. Often, the entire brake actuator unit must be replaced when the service chamber needs repair. A prior United States patent application of the same transferee, entitled "Sealed Brake Actuator with Filtered Vent Openings", corresponding to international application No. PCT / US95 / 02151, filed on February 21, 1995, discusses the advantages of sealing the spring brake chamber against the introduction of corrosive substances and is incorporated in this application by reference. The problem of sealing the service side of a fluid brake actuator continues to exist. SUMMARY OF THE INVENTION A brake actuator in accordance with the present invention focuses the problem of contaminated air in the service chamber by providing a filter assembly on one or more vent openings in the housing wall. When the elastomeric diaphragm is expanded to actuate the brake, air is discharged from the service chamber through the vent openings to the atmosphere, simultaneously cleaning the filter assembly. Conversely, when the return spring is expanded to return the elastomeric diaphragm to its original position, as when the service brake is released, the atmospheric air is carried through the filter assembly and the vent openings into the chamber. service, with any air contaminants being removed from the air by the filter assembly. Consequently, the air in the service chamber is kept substantially free of contaminants, thereby protecting the interior of the service brake against corrosion. As a result, the useful service life of the brake actuator assembly is considerably prolonged. In accordance with one embodiment of the invention, the vent openings are provided in the housing in communication with the internal service brake chamber and a gas permeable filter element covers the vent openings. An external frame assembly covers the filter element. Preferably, the outer frame assembly defines tortuous air passages of the filter element to the outside of the housing. Preferably, the filter element is a gas permeable, oleophobic, hydrophobic, expanded PTFE membrane. In another aspect of the invention, the filter element can be installed between a frame and a bushing. The frame may comprise an element having radial rays and an erect peripheral rim. The filter element can be disk-shaped and installed within the erect peripheral rim of the frame member, whereby the reciprocation of the movable diaphragm causes air to cycle to and from the brake chamber through the filter assembly, which keeps the chamber substantially free of contaminants. In another embodiment of the invention, the actuator rod for the service brake extends through an opening in the brake housing, which is the main opening for the air inlet or outlet of the brake chamber. A filter assembly is mounted in the housing, which surrounds the opening and is adapted to filter the air moving in and out of the housing when reciprocating the actuator rod. Preferably, the filter element has properties similar to the filter element of the previous embodiment and may comprise a flexible boot covering the opening and having the filter element there disposed. In a further aspect of the invention, the flexible boot comprises a bellows member provided with at least one accordion-like fold having therein an opening covered by a filter element. In addition, this invention contemplates the use of a filter assembly to cover the housing openings in various types of fluid-driven brakes, including movable diaphragm type brakes, rolling lobe and piston. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a cross-sectional view of a spring brake actuator with a filtered side service air-vent assembly according to the invention.; Fig. 2 is a cross-sectional view of a spring brake actuator similar to Fig. 1, showing the enlarged region 2 of Fig. 1; Figure 3 is an exploded perspective view of the filtered service side ventilator assembly of Figure 1; Figure 4 is a plan view of a frame portion of the filtered vent assembly according to the invention, taken along lines 4-4 of Figure 3; Figure 5 is a cross-sectional view of the frame portion of the filtered vent assembly, taken along lines 5-5 of Figure 4; Figure 6 is an exploded view of a bellows assembly located in the lower portion of the spring brake actuator of Figure 1; Fig. 7 is a cross-sectional view of a spring brake actuator having a second embodiment of the filtered service side ventilator assembly according to the invention; Figure 8 is an exploded view of the filtered vent assembly of Figure 7; Figure 9 is a cross-sectional view of the filtered vent assembly, taken along lines 9-9 of Figure 8;
Fig. 10 is a cross-sectional view of a piston brake actuator illustrating a third embodiment of the invention; Figure 11 is a plan view of a spring brake actuator with a fourth embodiment of the filtered vent assembly according to the invention; and Figure 12 is a cross-sectional view of the spring brake actuator of Figure 11, taken along line 12-12. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a cross-sectional view of a brake actuator unit, operated by air 10, having a general configuration well known in the art. The actuator unit 10 comprises a service brake portion 12 mounted in tandem in a spring brake or emergency brake portion 14. A service brake thrust rod 18 extends from the service brake 12 and is adapted to be connected operatively to a conventional brake shoe and drum (not shown in the drawing) in a conventional manner. The reciprocating movement of the push rod 18 will cause the brake to be applied and released alternatively. The service brake 12 comprises a pair of cup-shaped housing sections 21 and 22, each having an outwardly facing flange edge 25. The housing sections 21, 22 are gripped together at their edges of flange by means of a clamp 27. An elastomeric diaphragm 32 is compressed at its peripheral edge between the flange edges 25 of the housing sections 21, 22. A first chamber 29 is thus activated between the housing section 22 and the elastomeric diaphragm 32 and a second chamber 30 is defined between the housing section 21 and the elastomer diaphragm 32. The service brake push rod 18 extends through a central opening 34 in the housing section 21 and then to the second chamber 30, where it ends in a pressure plate 36. A return spring 40 extends between the pressure plate 36 and the inner surface of the housing section 21. A guide The push rod 42 mounted on the housing section 21 defines the central opening 34 and has a spring seat 48 that receives an end of the compression spring 40 to hold it in position around the opening 34. The spring 40 in this manner tends to urge the pressure plate 36 and the service brake push rod 18 to a fully retracted position shown in figure 1, where the first chamber 29 collapses. To operate the service brake, compressed air is introduced into the first chamber 29 through an air service gate 23 in the housing section 22 to force the diaphragm 32 and the pressure plate 36 against the force of the spring 40 to drive the push rod 18. The openings 44 are provided in the section of housing 21 to allow rapid evacuation of air from the second chamber 30 upon actuation of the brake such that air exits the second chamber 30 through a filter assembly 300. A bellows assembly 400 is provided in the opening 34 to capture any air that escapes beyond the push rod guide 42. Mounting spindles 46 are provided for mounting the brake actuator unit 10 in a vehicle. The spring brake 14 comprises a pair of cup-shaped housing sections 51, 52, which are joined at their edges to form an internal chamber 53. Typically, the housing sections 22, 51 are formed from an integral cast part. . The housing section 51 is provided with an outwardly facing circumferential flange edge 54. The housing sections 51 and 52 are gripped together by means of a peripheral curved edge 56 in the housing section 52 linking the edge 54. in the housing section 51. An elastomeric diaphragm 58 is suspended within the inner chamber 53 and is compressed at its peripheral edge between the edges 54., 56 of the housing sections 51, 52. The portion of the inner chamber 53 between the diaphragm 58 and the housing section 51 forms an air chamber 100 that is filled with compressed air, typically supplied through a service hatch. 55 in the housing section 51 when the emergency brake is in its normal, released position. The portion of the internal chamber 53 between the diaphragm 58 and the housing section 52 defines a compression spring chamber 101, containing a high force spring brake compression spring 72. An actuator rod 60, aligned with the piston rod push 18, has a distal end extending through a central opening 64 in an end wall of the housing section 51. The distal end of the actuator rod 60 terminates in a reaction plate 62 arranged in an annular seat 63 in an end wall of the housing section 22. The opening 64 is provided with a bearing 66 having at least one annular recess accommodating at least one ring seal at 0 68. The bearing 66 defines a bearing surface for the rod actuator 60, and the O-ring 68 seals the pressure chamber in the service brake 12 of the air chamber 100 in the spring brake 14. The proximal end of the actuator rod 60 extends through of a centrally disposed aperture 81 in the diaphragm 58 and terminates in a pressure plate 70 which links the spring brake compression spring 72. The pressure plate 70 comprises a substantially planar portion 73 which links an end of the spring 72 and a tubular portion 74 extending generally axially along the axis of the spring 72. The tubular portion 74 is press fit onto an end portion of the actuator rod 60 such that the pressure plate 70 and the actuator rod 60 form a integral unit. An annular recess 76 in an end wall of the housing section 52 serves as a seat for the spring 72 and also provides rigidity to the end wall against the force of the spring 72. During normal operation of the brake actuator 10, the The actuator rod 60 will be in the fully retracted position, as sketched in FIG. 1, by means of compressed air, which is maintained in the air chamber 100. When the compressed air is discharged, the compression spring 72 forces the plate of pressure 70 and the rod 60, integrally connected to the pressure plate, in the direction of the brake push rod 18 of the service brake 12. The force of the spring 72, when released, causes the actuator rod 60 to be extended through the central opening 64 which, in turn, causes the reaction plate 62 to apply a force to the diaphragm 32 and the pressure plate 36 of the service brake 12. This action causes the push rod of Service brake 18 is actuated and the brake applied. When the emergency brake is released, compressed air is again introduced into the air chamber 100. The force of the compressed air against the diaphragm 58 urges the pressure plate 70, the rod 60, and the spring 72 to the position retracted sketched in Figure 1. The actuator rod 60 is preferably a hollow tube or rod provided with a central bore 61 for accommodating a brake release cage or tool 78. The distal end is closed by the reaction plate 62. , but the proximal end is open, and an opening is provided in the pressure plate 70 to allow the caging rod 78 to extend therethrough. The caging rod 78 is used to manually release the spring brake or to ensure that the compression spring 72 remains compressed (or caged) when maintenance functions are to be performed on the brake assembly. The caging rod 78 is preferably a bolt or the like threaded in a fixed threaded opening such as the threaded collar 77 fixed by welding to the end wall of the housing section 52. A hexagonal head nut 79 is fixedly attached to the cage rod 78 to facilitate threading the bolt in and out of the chamber 14 by a common tool or similar. The end of the grout rod 78 within the actuator rod 60 has an enlarged head or plate 86. An annular shoulder directed inwardly 80 in the tubular section 74 provides positive engagement with the actuator rod 60 when the rod is actuated and further serves as a bonding surface for the head of the caging rod 86 against which the cage rod head 86 will abut when the cage rod 78 is removed., the caging rod 78 retains the compression spring 72 in its compressed or caged state. Otherwise, the caging rod 78 extends towards the central bore 61 of the actuator rod 60 at a distance sufficient to allow the actuator rod 60 to extend over its entire travel length without any connection between the shoulder 80 and the rod head. Cage 86. Figures 1 and 2 further show the filter assembly 300 disposed above the vent openings 44 in the housing section 21 in a shape defining a tortuous air path identified by the arrow A extending between the second chamber 30. and the outside atmosphere. Each opening 44 (of which there can be several) is covered by one or more filter assemblies 300. This structure effectively requires that all the air entering and leaving the chamber 30 pass through a filter assembly 300, while protecting simultaneously the openings 44 and the second chamber 30 of waste, impurities, accumulation of ice, paint, salt and the like. It will be understood that when air is drawn into the chamber 30 when the return spring 40 expands, the contaminants in the atmosphere will be trapped by the filter assembly 300. Conversely, when air is expelled from the chamber 30 when the diaphragm Service 32 expands due to the introduction of compressed air or by action of spring brake 14, is forced back through filter 300 and will effectively clean it, expelling contaminants into the atmosphere. In this way, the interior of the second chamber 30 is kept substantially free of contaminants, independently of the operation of the spring brake. One or more filter assemblies 300 may be disposed around the exterior of the housing 21, depending on the number of vent openings 44 present. As shown in Figure 1 and in greater detail in Figures 2 and 3, the filter assembly 300 comprises a frame 302 and a cover plate 304 with a filter element 306 retained therebetween. The frame 302 comprises an open grille defined by radially extending spokes 310, which terminate in an axially extending annular wall 312. The inner ends of the radial spokes 310 are secured to a cylindrical hub 314 that surrounds a central bore 316 An annular edge 318 includes several stepped portions of equal size 320 in spaced relation, which define several intermittent notches 322. The annular space between the outer surface of the cylindrical hub 314 and the inner surface of the annular wall 312 defines a receiving area filter 324. Preferably, frame 302 is injection molded of a relatively soft polymer, such as Hytrel 40, typically 40 durometer on the Shore D scale or equivalent. In this way, the internal frame 302 has some flexibility. As shown in Figures 2-3, the filter element 306 is a gas-permeable annular disk, preferably constructed of a porous polymer having hydrophobic and oleophobic properties and a pore size of about one meter. An acceptable filter element comprises an expanded, treated polytetrafluoroethylene (PTFE), available from .L. Gore and Associates, and sold under the Gore-Tex denomination. The filter element 306 has a central opening 326 dimensioned identically to the outer diameter of the central hub 314 of the frame 302 and an outer edge corresponding to the inner diameter of the annular wall 312, such that the filter element 306 is received from tightly fitting within the filter receiving area 324 of the frame 302. The cover plate 304 is a plate-shaped stamped metal plate comprising a central rounded protrusion 330 having a central opening 332 and tapering down to a flat portion , circular 334, ending additionally in a cylindrical bushing 336 extending outward from a circumferential edge thereof. The central opening 332 of the rounded protrusion 330 has the same dimensions as the openings 316 and 326 in the internal frame 302 and the filter element 306, respectively. The elements of the cover plate 304 can be further described in terms of an outer side and an inner side, where the outer side of the cover plate 304 faces away from the housing 21, and the inner side faces the filter element 306. In the assembly shown in Figures 1-3, the filter frame assembly 300 is stacked as illustrated in Figure 3 and is retained by a fastening member 338 that is inserted into the aligned aperture 332 of the cover plate. 304, the opening 326 of the filter element 306 and the central opening 316 of the frame 302. The holding member 338 can be threaded into a corresponding pin on the outside of the housing 21 or welded to the outer surface of the housing 21. Alternatively, the fastening member 338 may take the form of a rivet which is fastened to the outer surface of the housing 21 in a conventional manner or any other suitable fastening means s that will securely secure the filter frame assembly 300 to the exterior of the housing 21. The frame 302 rests against the exterior surface of the housing section 21, but the wall 336 is dimensioned such that there is a clearance 340 between its edge bottom end and the outer surface of the housing 21. Preferably, the frame 302 is spaced apart from the air openings 44 in order to avoid back pressure caused by an immediate blockage of the opening. The outer flange 312 of the frame 302 rests securely against the outer surface of the housing section 21. The filter element 306, on the other hand, can be fixedly attached to the inner surface of the frame 302, preferably by thermal staking to the inner, continuous lateral surface of the hub 314 and the cylindrical wall 312. The result is that the air passing through the frame filter assembly 300 is forced to follow a tortuous path through the vent opening 44, the open grid of the frame 302, the pores of the filter element 306, the notches 322 in the outer rim of the frame 302, and through the clearance 340 between the cover plate 304 and the housing 21. This tortuous path and the element of filter 306 ensure that debris, impurities, ice, salt, paint and the like will not clog the filter or vent openings. Apart from the filtered air openings 44, the only other gate through which atmospheric contaminants can enter the service chamber 30 is shown in Figure 1 as a central opening 34 in the push rod guide 42 through which reciprocating the push rod. A bellows assembly 400 is mounted adjacent the exterior side of the central opening 34 to prevent air and contaminants from entering through the central opening 34. As shown in FIGS. 1 and 6, the bellows assembly 400 comprises a clamp of assembly 402, a flexible boot such as bellows 404 and a bracket 406. Mounting bracket 402 is generally a piece of stamped, rigid metal, while the bellows is constructed of a resilient, flexible plastic material such as Hytrel, manufactured by DuPont. The mounting bracket 402 comprises an outer annular ring 408 and an internal annular ring 410. The inner annular ring 410 extends radially inwardly of the outer annular ring 408 and is vertically displaced in a terrace form therefrom. The bellows 404 comprises a cylindrical body 412 having several radially extending corrugations 414 located around the circumference of the body 412. A first end of the bellows 404 includes an outwardly curved radial flange 416 defining a circular lip 418. A second end of the bellows 404 includes an axially extending flange 420 having a circular lip 422. As shown in Figure 1, the bellows assembly 400 can be assembled to the housing 21 and the push rod 18 by axially placing the bellows 404 on the push rod 18 such that the circular lip 418 of the radial flange 416 abuts the end wall of the housing 21 and it extends completely over the central opening 34. The mounting bracket 402 is then placed axially on the push rod 18 and the bellows 404 such that the outer annular ring 408 abuts the end wall of the housing 21 and the inner annular ring 410 puncture the radial flange 416 of the bellows 404 between the inner annular ring 410 and the end wall of the housing 21. The mounting bracket 402 can then be secured to the housing 21 such as by welding, to permanently retain the bellows 404 in the housing 21. The opposite end of the bellows 404 is then fixed to the push rod 18 such as by a conventional grip band 424 with a threaded fastener 426. In operation In this embodiment, the bellows assembly 400 acts to prevent air from entering the second chamber 30 through the central opening 34 and ensures that atmospheric air virtually enters the second chamber only through the vent openings 44 after being first filtered by the filter assembly 300. Corrugations 414 in the bellows 404 allow the bellows 404 to contract and expand with the reciprocations of the push rod 18 without breaking. Turning to Figure 7, an alternate embodiment of a filter assembly is generally shown at 500. The filter assembly 500 comprises a mounting flange 502, a taper bellows 504 and a second mounting flange 506. As shown in FIG. shown in Figure 7 and in greater detail in Figures 8-9, the mounting flange 502 comprises first and second radially extending flanges 508 and 510, respectively, between which an annular groove 512 is defined. Annular groove 512 corresponds to an internal diameter approximate to the diameter of the central opening 34 in the housing section 21. The first mounting flange 502 further includes an axially disposed central opening 514, whose diameter is slightly larger than the external diameter of the push rod 18. The bellows 504 extends away from the first mounting flange 502 and is provided with several radially extending corrugations 516. The second mounting flange 506 buys nde an annular lip having an internal diameter equal to or just slightly greater than the external diameter of the push rod 18. Each corrugation 516 comprises a guide side 518 and a rear side 520, as shown in figure 8. The guide sides and rear 518 and 520 each include an integrally molded flow passage 522. Each flow passage 522 comprises a treated section of expanded polytetrafluoroethylene (PTFE), such as Gore-Tex, preferably formed into a film-like or film-like section. thin and integrally molded with the guide and rear sides 518 and 520 of each corrugation 516. As a whole, as shown in Figure 7, the first mounting flange 502 of the filter assembly 500 is axially placed on the push rod 18 and positioned adjacent the central opening 34 of the housing section 21. The first mounting flange 502 is partially received within the central opening 34 of the housing section. at 21, such that the inner circumferential edge 34a of the housing 21 is slidable within the annular groove 512. An adhesive or other fastener can be used to further secure the first mounting flange 502 to the inner circumferential edge 34a of the central opening 34. However, it has been found that additional fastening means may not be necessary where there is a close fit of the first mounting flange 502 to the central opening 34. The inner surface of the second flange 510 of the first mounting flange 502 also functions as a spring seat to assist centering and proper positioning of the service brake return spring 40 within the second chamber 30, thus helping to further retain the first mounting flange 502 to the inner circumferential edge 34a of the housing 21. It should also be noted that the housing 21 of this embodiment does not require any vent 44, because all the air is divided. to pass through the filter assembly 500. The second mounting flange 506 is positioned axially around the push rod 18 and fixedly secured thereto by fastening means such as the hose clamp 524 and the threaded fastener 526, as shown in FIG. shows in figures 7-8. When this assembly is complete, the only path by which the fill air can enter and exit the second chamber 30 is through the respirable flow passages 522 arranged on the guide and rear sides 518 and 520 of each corrugation 516 in the bellows 504. During operation, it will be understood that when air is drawn into the second chamber 30 when the return spring 40 expands so as to return the service diaphragm to its retracted position adjacent to the housing section 22, the contaminants in the atmosphere will be trapped by the filter assembly 500. Conversely, when air is expelled from the second chamber 30 when the service diaphragm 32 urges the push rod 18 out of the housing 21 due to the introduction of compressed air through the gate 23, the air in the chamber 30 is forced back through the filter assembly 500 and will effectively clean it, expelling the contaminants trapped by the filter assembly towards the atmosphere. In this way, the interior of the chamber 30 is kept substantially free of contaminants, independently of the operation of the brake actuator. The assemblies of the first and second mounting flanges 502 and 506, respectively, are sufficiently tight to prevent any contaminant from entering the second chamber 30 or preventing any leakage therefrom. The flow passage membranes 522 effectively trap any contaminants on their external surface and only allow clean air to enter the second chamber 30. Referring to FIG. 10, an alternate embodiment of a spring-loaded brake is shown by means of of fluid 600 for attachment to an existing brake system in a vehicle (not shown), to provide emergency brake and parking service. The spring brake actuator 600 comprises a cylindrical head 602 formed of steel and having a cylindrical portion 603, an end wall 604 and an opposite open end 606. An aluminum or forged steel adapter 608 is received within the open end of the head 606. The adapter 608 has an outer annular wall 610 that bears three annular grooves 612, 614 and 616 and a portion of the head 602 is deformed towards the two external outer annular grooves 614 and 616 to permanently secure the head 602 to the adapter 608. The adapter 608 has a central tubular guide, which has an internal bore 670. A bearing lobe diaphragm 618 divides the inner chamber of the brake actuator 600 into a pressure chamber 620 and a spring chamber 622. The shape of the diaphragm 618 is generally tubular, with one end terminating in an annular eyebrow 624. The inner end annular groove 612 receives the annular eyebrow 624 so that the diaphragm 618 is maintained between the adapter 608 and the head 602 in a sealing relationship. A plastic pressure plate 628 is attached to the diaphragm 618 in the spring chamber 622. A resilient compression spring 626 extends between the head end wall 604 and the pressure plate 628, providing a bearing surface for the spring 626 A hollow actuator rod 630 is press fit into the pressure plate 628 and extends out of the pressure chamber 620 through an opening 632 in the adapter 608. The bearing 688 and the O-ring 690 are mounted in an annular groove 617 for guiding and sealing the actuator rod 630 as it passes through an opening 632 to provide an air-tight seal between the actuator rod 630 and the adapter 608. The pressure plate 628 has a flat plate portion 629, a central tubular flange 634 and an inner tubular hub 646 that receives the actuator rod 630. It also has an annular flange 636 extending axially towards the head end wall 604. The flange 636 terminates in an annular guide flange 638 which extends radially outwardly from the flange 636 towards the cylindrical portion 603 of the head 602. As seen in Figure 1, the return flange of pressure plate 636 is spaced radially inward from the cylindrical portion 603 of the head 602 for forming a free annular space 666. The guide tab 638 has a series of transverse holes (not shown) for venting the free annular space 666. The flange 638 bears against the internal cylindrical portion 603 of the head 602 in reciprocal motion. The inner tubular hub 646 defines a first bore 648, which ends in a shoulder 660 and a second bore 650. The outer diameter of the inner tubular hub 646 is slightly smaller in an upper portion that forms the bore 650 than in a lower portion which forms the first perforation 648. The actuator rod 630 comprises a proximal end 644 fixed to the pressure plate 628 and a distal end 647 extending from the opening 632 of the actuator rod. The proximal end of the actuator rod 644 is received in the first bore 648 of the pressure plate. A steel sheave 658 lies against shoulder 660. The proximal end 644 of the actuator rod is snapped into the first bore 648, resting against sheave 658 to keep sheave 658 securely in place. A disc-shaped retaining plate 662 and pressure plate 628 wall diaphragm 618 to secure diaphragm 618 to pressure plate 628. Various pins (not shown) pass through mating holes (not shown) in the 618 bearing diaphragm and retaining plate 662 and ultrasonically swabbed to the underside of retaining plate 662 to lock the pressure plate 628, rolling the lobe diaphragm 618 and retaining plate 662 together. Alternatively, threaded fasteners (not shown) can be used to join the pressure plate 628, the bearing diaphragm 618 and the retaining plate 662 together. The compression spring 626 is nested in the pressure plate 628 between the annular flange 636 and the central tubular flange 634. A collar 676 is mounted on the end wall 604 and has a threaded bore in which a cage bolt is screwed 672. A hexagon nut 678 is staked on the outer end of the caging bolt 672 for rotation of the bolt 672 by a wrench. A head 670 is formed at the inner end of the bolt 676 to engage the sheave 658 and manually retract the diaphragm 618 and the pressure plate 628 to compress the spring 626 to release the brake pressure, if desired, when in the operating mode. emergency or parking.
During normal operation of the brake actuator 600, the actuator rod 630 will be in the fully retracted position by means of compressed air which is maintained in the pressure chamber 620. When the compressed air is discharged, the compression spring 626 is expanded to forcing the pressure plate 628 and the actuator rod 630 integrally connected downwards through the adapter opening 632 to drive a brake (not shown), in emergency or parking mode. The guide tab 638 and the guide 640 guide the movement of the pressure plate 628 when reciprocating inside the head 602. As the pressure plate 628 moves towards the adapter 608, the diaphragm 618 is folded on itself or rolls, mode of being received within the free annular space 666. When the brake is released, the compressed air is once again introduced into the pressure chamber 620. The force of the compressed air against the bearing diaphragm 618 and the retaining plate 662 exceeds the compression force of the spring 626 and returns the pressure plate 628 and the actuator rod 630 to the position observed in figure 10. A vent opening 671 is located along the end wall 604 of the cylindrical head 602 A filter assembly 300, as shown in Figures 2-5, is mounted on the vent opening 671 in a previously described manner. In operation, upon reciprocating the pressure plate 628 during the operation of the brake, the air is sucked in and out of the vent opening 761, and thereby through the filter assembly 300. As disclosed in embodiments Prior to this invention, the filter assembly 300 mounted on the vent opening 671 prevents contaminants from entering the spring chamber 622 and damaging the compression spring 626. Another alternative embodiment of the filter side service is shown in Figures 11 and 12. The filter assembly, generally shown at 700, is shown as an annular ring disposed over the vent openings 44 in the housing section 21 in a shape defining a tortuous air path shown. by the arrow A which extends between the air chamber 30 and the external atmosphere. In this embodiment, the vent openings 44 (of which there can be several) are disposed in a radial relationship spaced around a common horizontal circumference of the housing section 21 so that all of the vent openings 44 can be covered. for a single annular accommodation. As shown in Figure 11, the filter assembly 700 is mounted on the housing portion 21 by several fasteners 702. This structure, as in the previous embodiments, requires that all air in and out of the service chamber 30 to pass through the filter assembly 700 while simultaneously protecting the vent openings 44 and the interior chamber 30 of debris, impurities, ice buildup, paint, salt and the like. It will be understood that when air is drawn into the chamber 30 when the return spring 40 expands, atmospheric contaminants will be trapped by the filter assembly 700. Conversely, when air is expelled from the chamber 30, when the service diaphragm 32 It expands due to the introduction of compressed air, is forced back through the filter assembly 700 and will effectively clean it, expelling contaminants into the atmosphere. In this way, the inner chamber 30 is kept substantially free of contaminants. Because the vent openings 44 in this embodiment are disposed in the described radial relationship, a single filter assembly 700 can be arranged around the outside of the housing 21, regardless of the number of vent openings 44 present. The filter assembly 700 comprises an internal frame 704, an external frame 706, an element of filter 708 and a cover plate 710. Internal frame 704 comprises an annular plate 712 having several spaced openings 714 therein located. The annular plate 712 further includes a pair of outwardly extending radial tabs 716 and 718 located at opposite upper and lower edges of the plate 712 and an inwardly extending radial flange 720 oppositely disposed along an edge exterior of the annular plate 712 adjacent the radial flange 718. In this embodiment, "upper" and "lower" are relative to the orientation of Figures 11 and 12 and do not reflect the orientation of the respective structures with respect to the orientation of the spring brake actuator in use. The distal portions of the flanges 716 and 718 include a crenellated portion defined by alternating solid and notched portions shown at 717 and 719, respectively. The flanges 716 and 718 of the inner frame 704 further define right-angle shoulders 744 and 746, respectively. The outer frame 706 comprises an annular plate 722 having several spaced openings 724 there disposed. The annular plate 722 further includes a pair of flanges 726 and 728 which are located on each outer edge of the annular plate 722 and extend radially both inwardly and outwardly of the annular plate 722 so that the outer frame 706 has a section H-shaped cross section, as shown in Figure 12. The distal portion of the flanges 726 and 728 that extend outwardly from the surface outwardly of the annular plate includes a crenellated surface defined by alternating solid and alternating portions, shown by way of example at 730 and 732, respectively. The outer frame 706 is dimensioned to be received between the flanges 716 and 718 adjacent the outer surface of the annular plate 712 of the inner frame 704. The filter element 708 is an annular strip, permeable to gas, preferably constructed from a porous polymer having hydrophobic and oleophobic properties and a pore size of about 1 miera. An acceptable filter element, as in the previous embodiments, comprises a Gore-Tex membrane dimensioned substantially to the outer diameter of the annular plate 712 of the inner frame 704 and having a width substantially corresponding to the distance between the inner surfaces of the radial tabs 716 and 718 thereof. The cover plate 710 is an annular stamped metal channel, having a substantially U-shaped cross section defined by an outer wall 734 having various openings 736 there disposed. The outer wall 734 terminates in the upper and lower side walls extending inwards 738 and 740, respectively. One or both of the upper and lower side walls 738 and 740 may include several spaced openings shown in Figure 12 at 742 and 743, respectively. The opening 736 in the cover plate 710 is axially aligned with vent openings 44 or pins (not shown) in the housing portion 21. The opening 724 in the outer frame 706 and the opening 714 in the inner frame 704 are also sufficient diameter to allow a fastener 702 to pass therethrough. As a whole, as shown in Figures 11-12, the inwardly extending portions of the flanges 726 and 728 of the outer frame 706 are received within the portions that they extend outwardly from the flanges 716 and 718 of the inner frame 704 in a stacked relationship with the filter element 708 retained therebetween. The cover plate 710 is mounted to one or both of the internal frames 704 and the outer frame 706 to complete the assembly of the filter assembly 700. The filter assembly 700 is then mounted to the housing 21 by one or more fastening members 702 , each of which is inserted into an aligned aperture 736 of the cover plate 710, the opening 724 of the external frame 706 and the opening 714 of the internal frame 704. The fastening member 702 can be threaded into an aperture of corresponding vent 44 or plug (not shown) on the outside of the housing portion 21 or welded to the exterior surface of the housing 21. Alternatively, the fastening member 702 may take the form of a rivet which is secured to the outer surface of housing 21 in a conventional manner or any other suitable fastening means that will securely secure the filter assembly 700 to the exterior of the housing 21. Alignment of the filter assembly 700 with respect to the housing 21 can be further enhanced by providing a pair of annular grooves 748 and 750 on each side of the vent openings 44 so that the shoulders 744 and 746 of the internal frame 704 can be received. The internal frame 704 rests against the external surface of the housing portion 21, but the radial flange 720 causes a clearance 752 to exist between the lower bottom edge of the annular plate 712 and the outer surface of the housing portion 21. Preferably, the inner frame 704 is spaced from the vent openings 44 to so as to avoid back pressure caused by an intermediate blockage of the vent opening 44. The outer frame 706 is spaced from the inner frame 704 in a similar manner. The result is that the air passing through the filter assembly 700 is forced to follow a tortuous path (arrow A) through the vent opening 44, the opening 714 in the inner frame 704, the pores of the filter element 708, the opening 724 in the outer frame 706, the notched portions 732 of the outer frame 706, the notched portions 719 of the inner frame 704, and through the openings 742 and 743 in the cover plate 710. This tortuous path and the filter element 708 ensures that debris, impurities, salt, ice, paint and the like do not clog the filter or vent openings 44. Apart from the filtered air openings 44, the only other gate through which atmospheric contaminants can enter in the service chamber 30 is shown in figure 11 as the central opening 34 in the guide 42 of the push rod through which reciprocates the push rod 18. A bellows assembly 400, as shown in FIG. shown in Figures 1 and 6, is mounted adjacent the outer side of the central opening 34 to prevent air and contaminants from entering therethrough, as shown in the drawings and in the description of the service brake of Figure 1.
The invention encompasses other variations and modifications not illustrated in the drawings, but clearly evident to those skilled in the art. For example, the filter element can be separated from the housing and in communication with a single vent opening or multiple vent openings by one or more conduits, as the case may be. In this way, the filter element can be placed in a protected area of the vehicle chassis. It will be understood that other variations and modifications of the above embodiments may be designed by those skilled in the art without departing from the scope of the invention, as defined by the appended claims.