Abstract:
A pneumatic actuator includes a housing defining a chamber. A diaphragm is positioned within the chamber. A rod member is attached to the diaphragm and is movable between a first position and a second position. A cam member is attached to the diaphragm and to the rod member. The cam member has a cam surface. A deflection rod has a first end portion and a second end portion, where the first end portion of the deflection rod is attached to the housing. A follower is attached to the second end portion of the deflection rod. The follower engages the cam surface of the cam member so as to urge the rod member towards the second position.

Description:
CLAIM OF PRIORITY 
     This application claims priority to U.S. Provisional Patent Application No. 61/660,131, filed Jun. 15, 2012, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to air brake systems for vehicles and, in particular, to a deflection spring for the pneumatic actuator of such a system. 
     BACKGROUND 
     Heavy trucks, trailers and other commercial vehicles typically use an air brake system to provide the braking forces necessary to stop the vehicle. Such a system typically includes a brake pedal positioned on the floor of the driver&#39;s cab or compartment of the vehicle that, upon actuation, provides air from an air reservoir to an air chamber. The air chamber acts as a pneumatic actuator in that it features an actuator rod that either extends out of or retracts into the air chamber so as to activate the mechanism that pushes the brake lining material of the brake shoes against the vehicle brake drum at each vehicle wheel-end. The mechanism typically includes a slack adjustor which turns a cam roller via a camshaft so as to force the brake shoes to engage the brake drum so as to stop the vehicle. 
     An example of a prior art pneumatic or air chamber of such an air brake system is described in U.S. Pat. No. 5,829,339 to Smith, the contents of which are hereby incorporated by reference. 
     Cross-sectional views of a prior art air chamber are also provided in  FIGS. 1A-1D . As explained in greater detail below, with reference to  FIGS. 1A-1D , a large main compression spring  10  (also known as a parking spring or a power spring) serves as a mechanical means to prevent the vehicle from rolling when there is no air in the brake system and when the vehicle is stationary or parked. This spring supplies the parking force needed to hold the vehicle stationary. A larger or stronger spring typically means that a larger parking force can be achieved. 
     One problem with such a design is that a great deal of air pressure is needed to keep the main spring from applying the brake and thereby maintain the spring in a compressed state (illustrated in  FIG. 1A ). Also as the brake applies and the main spring  10  extends (as illustrated in  FIG. 1C ), one is not able to capitalize on the high amount of force that the spring exhibits in the compressed state (due to the equation Spring Force=K×X, where K is the spring constant and X is the compression distance of the spring). To this end, the main spring is subjected to very high compressive forces when in the condition of  FIGS. 1A, 1B and 1D  that are never translated to the parking brake force for the vehicle. 
     In addition, while in the compressed state ( FIG. 1A ) the main spring coils are close together and could be touching. This contact, combined with the vibrations experienced by the axle and vehicle as it drives, could cause an increase in wear in the spring coils. This wear could possibly break through the spring plating and damage the spring surface creating high stress areas and, without a protective coating, the spring would be subject to corrosion. The resulting rust pits become stress risers that will shorten the life of the spring. 
     A coil spring failure can result in a punctured diaphragm or a reduction in stroke, parking force, or the inability to completely release a brake for a given wheel-end of the vehicle. As such, much work must be put into protecting the spring from corrosion and also from individual coil contact. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1D  are cross-sectional views of a prior art air chamber or pneumatic actuator with the parking chamber pressurized, both the parking and service chambers pressurized, the main spring engaged (with neither the parking nor the service chambers pressurized) and the main spring mechanically caged with a release tool, respectively; 
         FIGS. 2A and 2B  are cross-sectional views of a pneumatic actuator according to a first embodiment of the invention showing the deflected and non-deflected spring conditions, respectively; 
         FIGS. 3A and 3B  are cross-sectional views of the top portion of an air chamber equipped with a second embodiment of the deflection spring of the invention showing the deflected and non-deflected spring conditions, respectively; 
         FIGS. 4A and 4B  are cross-sectional views of a top portion of an air chamber equipped with a third embodiment of the deflection spring of the invention showing the deflected and non-deflected spring conditions, respectively; 
         FIG. 5  is an enlarged perspective view of the cam member and push rod of  FIGS. 2A and 2B and 3A and 3B ; 
         FIG. 6  is an enlarged perspective view of the cup-shaped cam member and central sleeve of  FIGS. 4A and 4B ; 
         FIG. 7  is a graph of a force curve illustrating the parking force provided by a prior art main spring; 
         FIG. 8  is a graph of a force curve illustrating the parking force provided by an embodiment of the deflection spring of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     As noted above,  FIGS. 1A-1D  provide cross-sectional views of a prior art air chamber or pneumatic actuator, indicated in general at  12 . While the invention is described in terms an air chamber for an air brake system of a vehicle, it is to be understood that the deflection spring of the present invention may be used in other types of pneumatic actuators for a variety of purposes. 
     The air chamber includes an actuator rod  14  and a housing  16  that defines a parking chamber  18  and a service chamber  20  ( FIGS. 1A, 1C and 1D ). The bottom end of the actuator rod is connected to a lever arm  22  that attaches the actuator rod to a slack adjustor or a camshaft upon which the cam roller (for actuating the brake shoes) is positioned. An upper diaphragm  24  is positioned within the parking chamber while a lower diaphragm  26  is positioned in the service chamber. The main compression spring  10  is positioned between an upper plate  30 , which is mounted to the top end of a push rod  32  ( FIGS. 1C and 1D ) and the upper diaphragm, and the top of the housing  16 . A lower compression spring  34  is mounted between a lower plate  36 , which is mounted to the top end of the actuator rod and the lower diaphragm, and the bottom of housing  16 . A chamber divider wall  38  positioned within the housing separates the parking and service chambers and has a central opening  40  through which the push rod  32  passes. An upper compression spring  44  ( FIG. 1D ) is positioned between the top of the chamber divider wall  38  and the bottom of upper diaphragm  24  and upper plate  30 . 
     In operation, as illustrated in  FIG. 1A , when parking chamber  18  is pressurized, upper diaphragm  24 , upper plate  30 , push rod  32 , lower diaphragm  26  and lower plate  36 , and thus the actuator rod  14 , move upwards, as indicated by arrow  46 . This causes main spring  10  to be compressed, while the upper and lower springs extend. As a result, the vehicle brakes are released and the vehicle may be driven. When the vehicle driver presses down on the vehicle brake pedal, the service chamber is pressurized, as illustrated in  FIG. 1B . This cause the lower diaphragm  26  to move down and the lower spring  34  to be compressed. As a result, actuator rod  14  moves down, as indicated by arrow  48 , and the vehicle brakes are applied. 
     The situation when the vehicle is off or air is otherwise evacuated from the air brake system is illustrated in  FIG. 1C . When this occurs, air is absent from the parking and service chambers  18  and  20  and the main spring  10  pushes the push rod  32  and actuating rod  14  downward in the direction of arrow  52  so that the vehicle brakes are applied. As illustrated in  FIG. 1D , the actuating rod may be raised, as indicated by arrow  54 , and the main spring mechanically caged, through use of a release tool so that the vehicle may be moved even though there is no air in the vehicle brake system. 
     In accordance with the present invention, the main spring  10  of  FIGS. 1A-1D  is replaced with a deflection spring assembly. More specifically, in accordance with the present invention, a pneumatic actuator, indicated in general at  100  in  FIGS. 2A and 2B , includes a number of deflection beams or rods  102   a - 102   d . While four deflection rods are illustrated, an alternative number advantageously (as explained below) could be used. 
     In addition to the deflection rods  102   a - 102   d , the deflection spring assembly, indicated in general at  103 , includes a cam member  101  having a convex hemispherical cam surface  104  ( FIGS. 2A, 2B and 5 ) positioned on top of, and connected to or otherwise provided with, a push rod  105  and upper diaphragm  124  (which corresponds to upper diaphragm  24  of  FIGS. 1A-1C ). Furthermore, the bottom ends of the deflection rods  102   a - 102   e  are provided with followers  106   a - 106   d  that engage and travel along the cam surface  104 . The followers preferably feature a disc-shaped construction and are attached to the ends of the deflection rod. The followers may be mounted to the ends of the deflection rods in a fixed fashion, being either integrally formed with the deflection rods (as illustrated in  FIGS. 2A and 2B ) or independently formed and joined to the rods. 
     The deflection springs and associated components of  FIGS. 2A and 2B  are positioned within a parking chamber housing  116  that features top cap  117 . The parking chamber housing also includes a port  115  ( FIG. 2A ) through which pressurized air may selectively be introduced into the parking chamber from a pressurized air source, such as the air brake system of a truck. As illustrated in  FIGS. 2A and 2B , the top cap features a number of recesses  119  that receive the top ends of the deflection rods  102   a - 102   d . The forces acting on the deflection rods  102   a - 102   d  (explained below) keep the top ends of the deflection rods within the recesses of top cap  117 , and/or they may be secured in place with adhesive, welding or some other fastening arrangement. 
     The remaining components of the pneumatic actuator  100  of  FIGS. 2A and 2B  are similar to the components of the pneumatic actuator of  FIGS. 1A-1D  and function in the same manner. The parking chamber housing  116  defines a parking chamber  118 , while a service chamber housing  121  defines a service chamber  120 . The service chamber housing is provided with a port  123  ( FIG. 2B ) through which pressurized air may be selectively introduced into the service chamber from a source of pressurized air, such as the air brake system of a vehicle. The bottom end of an actuator rod  114  is connected to a lever arm (not shown) of a braking system. The upper diaphragm  134  is positioned within the parking chamber while a lower diaphragm  126  is positioned in the service chamber. A lower compression spring  134  is mounted between a lower plate  136 , which is mounted to the top end of the actuator rod  114  and the lower diaphragm, and the bottom of the service chamber housing  121 . A divider wall  138  separates the parking and service chambers and has a central opening through which the push rod  105  passes. An upper compression spring  145  is positioned between the top side of the divider wall  138  and the bottom of upper diaphragm  124  and the cam member  101 . 
     In an alternative embodiment, illustrated in  FIGS. 3A and 3B , three followers  206   a - 206   c  are mounted upon three deflection rods  202   a - 202   c . In this embodiment, the followers take the form of spherical rollers that pivot, rotate or otherwise turn about an axle as illustrated at  208  for follower  206   c ) with respect to the bottom ends of the deflection rods. Alternatively, the spherical followers may be mounted to or integrally formed with the deflection rods in a fixed fashion as illustrated by follower  206   a ). In the embodiment of  FIGS. 3A and 3B , the deflection springs and associated components are positioned within an air chamber housing  216 . The top ends of the deflection rods  202   a - 202   c  are mounted in such a way that they rest inside a recess or recesses formed within the inner surface of the top of the housing  216 , deformations ( 218 ) or an annular projection or ridge formed in or on the interior surface near the center of the top of the housing. The resulting forces keep the top ends of the deflection rods in place within the housing. Alternatively, the top ends of the deflection rods may be mechanically fastened or joined to the housing using, as examples, only, a fastener, such as a screw or bolt ( 209  in  FIG. 2A ), adhesive or welding. In addition, in the embodiment of  FIGS. 3A and 3B , the push rod  205  is integrally formed with the cam member  201 , which has a convex hemispherical cam surface  204  (like cam member  101  of  FIGS. 2A, 2B and 5 ). 
     In yet another alternative embodiment of the invention, illustrated in  FIGS. 4A and 4B , the cam member having a hemispherical cam surface of  FIGS. 2A and 2B  is replaced with a cup-shaped cam member  220  (also shown in  FIG. 6 ) having walls with internal surfaces that form a concave annular cam surface  222 . The cup-shaped cam member has a central sleeve  223  that acts as a push rod ( 32  of  FIGS. 1A-1D ) and receives the top end of the actuator rod ( 225  in  FIG. 4A ). Similar to the embodiment of  FIGS. 2A and 2B , deflection rods  132   a - 132   c  are provided with followers  234   a - 234   c  which are shaped to engage and travel along the concave annular cam surface  222  of the cup-shaped cam member  220 . While three deflection rods are shown, an alternative number may advantageously be used. In addition, while the followers  234   a - 234   c  are shown as fixed to the bottom ends of the deflection rods  232   a - 232   c , they alternatively may take the form of rollers, as illustrated for follower  206   c  in the embodiment of  FIGS. 3A and 3B . The housing  226  of the air chamber features a central recess  236  that limits the upwards travel of the cup-shaped cam member  220 . In this embodiment, the cup-shaped cam member  220  seals against the interior surface of the sides of the cylindrical housing  226  via O-rings  237   a  and  237   b  (illustrated in phantom in  FIG. 4B ) positioned in annular grooves  239   a  and  239   b  (also shown in  FIG. 6 ). As a result, the cup-shaped cam member  220  serves as the upper diaphragm for the assembly so that the separate upper diaphragm  124  in the embodiment of  FIGS. 2A and 2B  (or  24  of  FIGS. 1A-1D ) is not required. 
     The top ends of the deflection rods  232   a - 232   e  are mounted in such a way that they rest inside an annular recess or recesses ( 238   a - 238   c ), deformations or an annular projection or ridge formed in or on the interior surface near the periphery of the top of the housing. The resulting forces keep the top ends of the deflection rods in place within the housing. Alternatively, the top ends of the deflection rods may be mechanically fastened or joined to the housing. 
     In operation, the deflection rods resist the deflected condition illustrated in  FIGS. 2A, 3A and 4A  so that the cam surface, and thus the push rod, is urged downward into the positions illustrated in  FIGS. 2B, 3B and 4B .  FIGS. 2A, 3A and 4A  correspond to the brake system configurations of  FIGS. 1A, 1B and 1D , while  FIGS. 2B, 3B and 4B  correspond to the brake system configuration of  FIG. 1C . 
     In choosing the size of followers  106   a - 106   d  ( FIGS. 2A and 2B ),  206   a - 206   c  ( FIGS. 3A and 3B ) or  234   a - 234   c  ( FIGS. 4A and 4B ), one must keep in mind that with a static system, any spherical object with an applied force on it has a resulting force through the centroid of the sphere and normal to the plane or surface (convex cam surface  104  of  FIGS. 2A and 2B , convex cam surface  204  of  FIGS. 3A and 3B  or concave cam surface  222  of  FIGS. 4A and 4B ) that is touching it. With this in mind, it is important to choose an appropriate size spherical or dome-shaped contact that will allow the attaching rod or beam (deflection rods) to not contact the cam surface such that the resulting free body diagrams can be simplified. 
     When the follower (sphere or dome-shaped surface) and deflection rod are in their resting state (no deflection), there is no horizontal force component and everything is in the vertical direction. As this is not a stable condition, the sphere and rod are installed such that there is a slight deflection of the deflection rods when in the condition of  FIGS. 2B, 3B and 4B  thereby assuring the proper direction of travel. The resulting small horizontal force is relatively negligible in comparison to the vertical component. 
     As the cam surface moves upward between the positions of  2 B and  2 A,  3 B and  3 A or  4 B and  4 A, thereby releasing the brake, the vertical force component of the roller against the cam surface gets smaller and smaller while the horizontal component gets larger and larger. Care must be taken to not pass the point where all of the force goes to the horizontal components as this would be another unstable condition and the cam surface could then continue up and then the beam and sphere assembly would be physically holding the brake off. 
     The embodiment of the invention described above therefore takes advantage of the deflection of a beam or rod and the resulting horizontal and vertical force components seen by a cam surface and a follower type of contact. These deflection rods or beams produce a vertical or applied force curve that is the inverse of the current spring design, as illustrated in  FIGS. 7 and 8  where the spring curves of a prior art coil compression spring and an embodiment of the deflection spring of the invention are illustrated, respectively. As a result, the more distance the actuator rod travels downward (so as to drop or extend) in the embodiments of  FIGS. 2A-6 , the stronger the deflection spring force ( FIG. 8 ) gets, as opposed to the prior art compression coil spring which loses force ( FIG. 7 ) the more the actuator rod drops or extends. In addition, when the brake is released ( FIGS. 2A, 3A and 4A ), the vertical force required to hold off the brake is minimal as most of the force components exerted by the followers on the cam surface are now in the horizontal direction. 
     The use of deflection rods or beams also enables one to better control the desired force output of the unit by simply adding or removing deflection rods. Corrosion of the spring is not as relevant as the preferred deflection rod material includes carbon fiber or aramid reinforced materials, which exhibit high tensile strengths and flexibility. Other materials that exhibit at least a semi-elastic behavior may be alternatively used. By using an embodiment of the deflection spring of the invention, there is also a possibility of reducing the overall weight of the air chamber, which lowers the amount of stress seen by the mounting bolts. 
     While the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.