Abstract:
Disclosed is a diaphragm-based spring brake actuator assembly which allows for the delivery of more force to the push rod without increasing the size of the actuator unit. The invention allows for the use of a stronger heavy main compression spring in the emergency brake chamber to provide greater emergency or parking brake force to the push rod. The invention also allows the service brake chamber to operate more efficiently when braking pressure is introduced. These functions are accomplished through modifications in the design of the actuators which allow for the deployment of a larger pressure plate inside either the emergency housing or the service brake housing, or both, allowing delivery of more force to the push rod of the actuator; and are made possible in actuator units having the same dimensional profile as existing weaker units.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to braking systems for heavy duty vehicles, and in particular to an improved diaphragm-based spring brake actuator which provides significantly increased braking force from a spring brake assembly having a size that is the same as or smaller than existing brake assemblies.  
           [0003]    2. Description of the Prior Art  
           [0004]    Various forms of pneumatic vehicle spring brake actuators have been introduced over the years primarily for use in the trucking industry. A typical double diaphragm air brake actuator includes two portions: an operator controlled service brake portion which is used for slowing or stopping a vehicle, and an emergency or parking brake portion which automatically engages when air pressure is removed. A typical service brake portion is characterized by a closed housing which contains a movable diaphragm stretched across the inside. One side of the diaphragm is closely associated with a centrally located pressure plate attached to a slidable push rod which extends out of the housing for attachment to the brakes of the vehicle. On the other side of the diaphragm a sealed chamber is formed within the housing.  
           [0005]    An opening is provided in the sealed service brake chamber for connection to a pneumatic (air) pressure source usually provided by an on-board air compressor. The brakes of the vehicle can be applied by introducing sufficient pneumatic pressure into the sealed chamber to act against the service brake diaphragm which moves the plate, pushing the push rod out. A small return spring is ordinarily provided inside the service brake housing around the push rod and adjacent to the pressure plate to urge it to retract when the air pressure behind the diaphragm is reduced.  
           [0006]    A typical emergency brake portion of an air brake actuator is attached in axial alignment with or made a part of the service brake assembly. The emergency brake is a separate closed housing which contains a heavy main compression spring and a second movable diaphragm creating a second sealed chamber. The emergency brake diaphragm is in contact with a second pressure plate which is also attached to or directly associated with the slidable central push rod of the service brake.  
           [0007]    The second sealed chamber is formed inside the emergency brake housing on one side of the diaphragm, and the heavy main compression spring is deployed on the opposite side. As with the service brake, the sealed chamber of the emergency brake is connected to the on-board pneumatic source of the vehicle. As long as sufficient air pressure is provided to the sealed chamber, the diaphragm in the emergency brake will remain fully extended thereby compressing the large spring. However, should pressure fall, or should there be a leak in the sealed chamber, the diaphragm will be unable to hold the large compression spring in place. When this occurs, either slowly or quickly, the large compression spring will move the second pressure plate causing the push rod to be extended out thereby applying the brakes of the vehicle.  
           [0008]    Under normal conditions, when the vehicle is parked, the air pressure to the emergency brake portion is cut off causing the large compression spring to apply the brakes.  
           [0009]    In the transportation industry, it is becoming ever more desirable to provide more powerful spring brake actuators without changing their size. Increasing load sizes, new regulations and other factors have created a need for additional power in a spring brake with the same dimensional profile as existing double diaphragm spring brakes.  
           [0010]    A stronger spring brake which takes up the same or a smaller space can result in great savings in the transportation industry. Under present regulations, a loaded truck must be able to apply its brakes and hold its position on a twenty percent (20%) grade. For many heavy vehicles, in order to accomplish this requires additional brake actuators and/or additional axles with brake actuators on them. With stronger brake actuators, fewer of them are needed to bring or hold such a vehicle at rest, thereby saving the cost of the additional brake actuators and/or additional axles.  
           [0011]    There is also a need for a more powerful spring brake which fits into a smaller space. This need is driven by such factors as the installation of vehicle air suspensions, lowered floor heights, shorter wheel bases, and the addition of new and bulky chassis equipment. All of these factors compete for the same space occupied by the spring brake.  
           [0012]    A spring brake assembly of smaller size which provides the same power as a larger assembly will also reduce weight and cost. A truck tractor and semi trailer may use 8 spring brake actuators on its axles. Replacing these with smaller units having the same strength that are two pounds lighter will reduce the weight by 16 pounds. While this may not seem significant at first blush, a liquid hauling vehicle is frequently loaded to the exact legal limit. Over the life of that vehicle, the 16 pound reduction will convert to thousands of dollars of hauling revenue.  
           [0013]    Stronger brake assemblies deployed in the same space can improve the stopping characteristics of a vehicle thereby potentially increasing the gross vehicle weight allowance for the vehicle (i.e. more payload).  
           [0014]    Existing service brake assemblies have been designed for attachment to the brake system of a heavy duty vehicle. The end of the service brake push rod is typically attached to a clevis which is, in turn, attached to the end of a slack adjuster arm located on a cam shaft which makes up part of the foundation brake of the vehicle. The push rod is moved in and out of the service brake assembly using pneumatic pressure as described above in order to operate the brakes of the vehicle. As this occurs, in some situations the push rod and clevis move the end of the slack adjuster through a slightly arcuate path around the cam.  
           [0015]    For decades, the pressure plates used in existing diaphragm-based spring brake actuators have been relatively small in comparison to the overall profiles of these units. In a typical brake actuator, the pressure plate in the service brake chamber has approximately the same diameter as the pressure plate in the emergency brake chamber. The edges of such pressure plates have traditionally been restricted to the central portion of the brake chamber, presumably to allow sufficient space around the edges of the plates for the diaphragm to fold over itself. However, these traditional wide tolerances that have developed over time are far more than is necessary for the diaphragm to function properly, and have unnecessarily limited the sizes of the pressure plates used, and therefore unnecessarily inhibit the potential force that can be delivered to the push rod by the spring brake actuator.  
         SUMMARY OF THE INVENTION  
         [0016]    The present invention is a departure from traditional diaphragm-based spring brake actuator assemblies which allows for the delivery of more force to the push rod without increasing the size of the actuator unit. One embodiment of the invention allows for the use of a stronger heavy main compression spring in the emergency brake chamber to provide greater emergency or parking brake force to the push rod. This is accomplished through novel changes to the design of the emergency brake chamber which allow it to more efficiently hold off the spring. A stronger emergency spring gives the brake actuator a greater capacity to hold a vehicle in place while parked on a grade. Another embodiment of the invention employs similar novel changes to the design of the service brake chamber which allow it to operate more efficiently when braking pressure is introduced.  
           [0017]    In the present invention, the pressure plate deployed inside either the emergency housing or the service brake housing, or both, is significantly larger than the corresponding plate(s) found in existing units having the same dimensional profile. The plate(s) of the present invention have a greater diameter and a larger circumference thereby defining a larger area.  
           [0018]    With respect to the emergency spring brake, the size of the pressure plate is directly proportional to the amount of force needed to hold off the large compression spring in the emergency brake housing. According to the formula F=PA, the force (F) exerted against the compression spring is equal to the amount of pressure (P) exerted by the chamber multiplied by the area (A) of the pressure plate over which it is exerted. Thus, increasing the size of the pressure plate increases the area (A) over which the pressure (P) is exerted, thereby increasing the force (F) against the spring. For illustrative purposes and by way of example only, and without limiting the scope of the appended claims herein, a pressure (P) of 60 pounds per square inch (60 psi) exerted against a pressure plate in the emergency brake housing having an area of 30 square inches results in a force of 1,800 pounds. In this example, if the area of the pressure plate is increased to 35 square inches, the resulting force of the spring that may be held off increases to 2,100 pounds. Thus, by simply increasing the surface area of the pressure plate, in this example an emergency brake spring that is over 14% stronger may be used (i.e. held off). Typical increases provided by the present invention are in the range of about twenty percent (20%).  
           [0019]    The availability of higher pressure (P) will also increase the amount of force (F) available to hold off the emergency brake spring. Thus, by increasing the surface area (A) of the pressure plate alone or in conjunction with increasing the available pressure (P), a much stronger spring may be used in the emergency brake.  
           [0020]    With respect to the service brake, the size of the pressure plate therein is directly proportional to the amount of force applied to the push rod. Again, using the formula F=PA, the force (F) applied to the push rod is equal to the amount of pressure (P) exerted by the chamber multiplied by the area (A) of the pressure plate over which it is exerted. Thus, increasing the size of the pressure plate increases the area (A) over which the pressure (P) is exerted, thereby increasing the force (F) applied to the push rod. For illustrative purposes and by way of example only, and without limiting the scope of the appended claims herein, a pressure (P) of 60 pounds per square inch (60 psi) exerted against a pressure plate in the service brake housing having an area of 30 square inches results in a force of 1,800 pounds applied to the push rod. In this example, if the area of the pressure plate is increased to 35 square inches, the resulting force applied to the push rod increases to 2,100 pounds. Thus, by simply increasing the surface area of the pressure plate, in this example the service brake becomes 14% more efficient (i.e. stronger). Typical increases provided by the present invention are in the range of about twenty percent (20%).  
           [0021]    The present invention facilitates increasing the size of the either the emergency brake pressure plate or the service brake pressure plate, or both, by incorporating one or more of the following features. First, the cylindrical walls of the spring brake housing may be made more vertical, more parallel to the orientation of the push rod, and/or more nearly perpendicular to the orientation of the pressure plate inside the housing. Next, the space between the outside circumferential edge of the pressure plate and the inside of the cylindrical wall of the brake housing (this space sometimes hereafter referred to as the “gap”) may be reduced to a size that is as small as about two and one half (2½) times the thickness of the diaphragm material, or even smaller (e.g 2¼ times said thickness), thereby providing room for a larger pressure plate. Next, the diaphragm itself may be made of very thin material in order to further minimize the size of the above described gap in order to maximize the size of the pressure plate. Next, axial movement of the main compression spring may be minimized by minimizing side load exerted by said spring. This is accomplished by grinding down a portion of the surfaces of the end spring coils (the coils at the top and at the bottom of the spring) so that these coils seat more predictably against the housing and pressure plate. Finally, configuring the shape of the pressure plate to nest with an adaptor plate located on the central shaft of the brake actuator helps keep the pressure plate in central alignment. A bushing/seal retainer may also be employed in the center of the spring housing to help align the larger pressure plate in order to prevent it from drifting sideways. Each of these features, used alone or in conjunction with each other, allows for deployment of a larger pressure plate which can then be used to hold off a stronger spring in the emergency brake housing, or to provide more force to the push rod in the service brake housing.  
           [0022]    The use of more vertical cylindrical walls in the present invention increases the interior cross sectional area of the emergency brake housing, thereby allowing for the surface area of the pressure plate to also be increased. This is accomplished without raising the height or width of the cylinder; thus, the overall profile of the brake actuator remains the same.  
           [0023]    Through experimentation, it has been determined that the size of the pressure plate may be increased until the above-described gap between the circumferential edge of the pressure plate and the inside wall of the housing is as small as two and one half (2½) times the thickness of the diaphragm material without any significant degradation in diaphragm performance. Existing brake actuators unnecessarily provide much larger gaps between the edges of the pressure plate and the walls of the housing which range from four and one half (4½) up to seven (7) times the thickness of the diaphragm material. In the present invention, the surface area of the pressure plate that is gained by closing this gap is substantial. When combined with more vertical cylindrical walls, even more space is made available for the pressure plate.  
           [0024]    The use of thinner diaphragm material allows the edges of the pressure plate to extend even closer to the cylindrical walls of the housing, thereby allowing for an even greater increase in the surface area of the pressure plate. Existing brake actuators use diaphragm materials having an average thickness of 0.125 inches, a tight one having a gap of 0.57 inches between the edge of the pressure plate and the wall of the housing (about 4½ times the thickness of the diaphragm). This gap may be reduced, as above, in the present invention down to as small as 2½ times the diaphragm thickness, or even smaller (e.g. 2½×0.125=0.3125 inches; 2¼×0.125=0.2813 inches). For illustrative purposes and by way of example only, and without limiting the scope of the appended claims herein, if the thickness of the diaphragm material is reduced to 0.09 inches, then the gap may be further reduced to 2½×0.09=0.225 inches (or 2¼×0.09=0.2025 inches), providing even more room for a larger pressure plate.  
           [0025]    Maintaining proper alignment of the larger pressure plate of the present invention is important. This may be accomplished in one or more of several ways. First, an adaptor plate may be employed on the central shaft of the brake actuator on one side of the diaphragm which works in conjunction with a recessed area on the underside of the pressure plate on the other side of the diaphragm. As the pressure plate moves up and down, this adaptor plate nestles through the diaphragm into the recessed area, keeping the pressure plate in central alignment. Alignment may also be improved through the use of a bushing/seal retainer in the center of the spring housing. Alignment may be further improved by reducing the side load of the main compression spring by grinding down the exterior surfaces of the end coils of the spring. Traditionally, such springs have a side load of 6 to 8 percent; in the present invention, reducing this load to 2 or 3 percent greatly improves alignment of the main spring in the emergency housing.  
           [0026]    It is to be noted that the improved performance of the diaphragm-based brake actuators of the present invention is accomplished using the same circumferential dimensions as existing brake actuators using common membrane diaphragm materials. The diaphragm is not attached in the center of the actuator, it does not use a moving wall, and it does not have any opening or hole in the center thereof.  
           [0027]    Historically, the effective surface area of spring brake pressure plates has been standardized into different types ( 9 ,  12 ,  16   20 ,  24 ,  30  and  36 ), each type providing an incrementally larger braking strength. This allows for standard components and parts to be manufactured for each type. For each type, there is also an incrementally larger associated profile (size) for the brake actuator. Using the design of the present invention, a smaller type (e.g.  24 ) having a smaller profile may have the strength of a larger type with a larger profile (e.g.  30 ). A smaller unit utilizing the features of the present invention may be employed as a replacement for a larger type, but requiring a smaller space. In addition, the present invention now makes a new type  43  unit available in the space occupied by a type  36 .  
           [0028]    It is therefore a primary object of the present invention to provide a stronger diaphragm-based spring brake actuator unit without increasing the overall size of the unit.  
           [0029]    It is also a primary object of the present invention to provide a diaphragm-based spring brake actuator unit that is able to hold of a stronger emergency brake spring without increasing the overall size of the unit.  
           [0030]    It is a further primary object of the present invention to provide a diaphragm-based spring brake actuator unit that is able to provide more force to the push rod from the service brake assembly without increasing the overall size of the unit.  
           [0031]    It is a further important object of the present invention to provide a stronger diaphragm-based spring brake actuator unit having at larger pressure plate inside of either the emergency brake housing, the service brake housing, or both.  
           [0032]    It is a further important object of the present invention to provide a stronger diaphragm-based spring brake actuator unit having more vertical cylindrical walls on the brake housing to accommodate a larger pressure plate inside.  
           [0033]    It is a further important object of the present invention to provide a stronger diaphragm-based spring brake actuator unit having a very tight gap between the outside circumferential edge of the pressure plate and the inside of the cylindrical wall of the emergency brake housing to provide more room for a larger pressure plate inside.  
           [0034]    It is a further important object of the present invention to provide a stronger diaphragm-based spring brake actuator unit having a diaphragm made of thinner material in the emergency brake housing to provide more room for a larger pressure plate inside.  
           [0035]    It is a further object of the present invention to provide a smaller, stronger diaphragm-based spring brake actuator unit in order to allow for more room for air suspensions and other parts underneath the vehicle to which it is attached.  
           [0036]    It is a further object of the present invention to provide a stronger diaphragm-based spring brake actuator unit that is retrofittable onto existing brake assemblies.  
           [0037]    Additional objects of the invention will be apparent from the detailed descriptions and the claims herein. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]    [0038]FIG. 1 is a cross sectional side view of a typical brake actuator in the normal driving position, the emergency spring being held off by the pressure of the upper chamber.  
         [0039]    [0039]FIG. 2 is a cross sectional side view of a typical brake actuator in the normal driving position with the service brake being applied by the vehicle operator.  
         [0040]    [0040]FIG. 3 is a cross sectional side view of a typical brake actuator showing it in the parked condition, with the emergency spring brake activated.  
         [0041]    [0041]FIG. 4 is a cross sectional side view of a combined emergency and service brake actuator of the present invention.  
         [0042]    [0042]FIG. 5 is a cross sectional side view of a combined emergency and service brake actuator of the present invention showing the descending positions of the pressure plates of each actuator.  
         [0043]    [0043]FIG. 6 is a side view of the present invention comparing its profile to an existing competitive brake unit (shown in phantom lines).  
         [0044]    [0044]FIG. 7 is an enlarged cross sectional side view of an emergency brake chamber of the present invention showing the curling of the diaphragm inside the housing as the pressure plate descends.  
         [0045]    [0045]FIG. 8 is an enlarged cross sectional side view of a service brake chamber of the present invention.  
         [0046]    [0046]FIG. 9 is an enlarged cross sectional side view of a typical prior art actuator. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0047]    Referring to the drawings wherein like reference characters designate like or corresponding parts throughout the several views, and referring particularly to the prior art actuator shown in FIGS.  1 - 3 , it is seen that a typical dual diaphragm air brake actuator, generally  20 , is comprised of a service brake assembly, generally  30 , and an emergency brake assembly, generally  50 . While each of these assemblies may be deployed independently of the other, when combined the service brake assembly and emergency brake assembly are in axial alignment with each other along the path of a push rod  21  which extends out from the center of one end of the service brake assembly.  
         [0048]    The distal end of push rod  21  extends out from lower service brake housing cup  25  and is attached to a clevis  23  which is, in turn, attached to a slack adjuster  24  attached to a rod or cam  26  associated with the brakes of a vehicle. Thus, as push rod  21  moves in and out of the service brake assembly  30 , it exerts force to the brakes of the vehicle. The proximal end of push rod  21  is attached to or closely associated with a pressure plate  29  located inside the service brake assembly  30 . A flexible service brake diaphragm  31  is provided inside assembly  30  above plate  29 , and is sealed at its edges to define a chamber  33  in conjunction with the upper exterior housing  35 .  
         [0049]    When pressurized air is introduced into chamber  33 , diaphragm  31  expands exerting pressure against plate  29  thereby pushing rod  21  out of assembly  30  as shown in FIG. 2. The application of pressure to chamber  33  of the service brake assembly  30  is controlled by the operator of the vehicle through normal operation of the brakes. The amount of pressure applied to chamber  33  may be varied resulting in a greater or lesser extension of rod  21 , and a greater or lesser application of the vehicle brakes. A retraction spring  37  is provided in lower housing  25  around rod  21  to urge plate  29  and rod  21  back inside assembly  30  when air pressure is removed from chamber  33 , as shown in FIG. 1.  
         [0050]    The emergency brake assembly  50  includes a lower housing cup  45  and an upper housing cup  55 . A diaphragm  51  is provided inside assembly  50 , sealingly attached at its edges-between upper and lower cups  45  and  55 , to define a chamber  53  in conjunction with the lower housing cup  45 . An extension rod  61  having the same diameter and characteristics as push rod  21  is provided in the center of assembly  50  and inside chamber  53 , axially aligned with push rod  21 . The distal end of rod  61  extends through a sealed opening at the center of lower housing cup  45  and into the upper housing cup  35  of the service brake assembly  50 . The distal end of rod  61  is attached to a small plate  52  inside service assembly  30 .  
         [0051]    The proximal end of rod  61  is also attached to a small adaptor push rod plate  62  located inside chamber  53 . Adaptor plate  62  is in contact with diaphragm  51 . Above diaphragm  51  is the pressure plate  59  of the emergency brake, above which the main compression spring  58  is located. The lower surface of pressure plate  59  includes a relief area  54 . By fully pressurizing chamber  53 , diaphragm  51  is expanded and pressed against pressure plate  59 , compressing main spring  58  into the upper end of housing cup  55 , as shown in FIGS. 1 and 2. When pressure is released from chamber  53  either by the stopping of the vehicle or from a failure in the pressure system, main spring  58  presses down against plate  59  pushing diaphragm  51 , plate  62  and rod  61  downward. The force of main spring  58  against plate  59  is transmitted through diaphragm  51  to plate  62 , rod  61 , plate  52 , diaphragm  31 , plate  29  and rod  21  to the brakes of the vehicle, as shown in FIG. 3. When such force is exerted, plate  62  nests within the relief area  54  of pressure plate  59 .  
         [0052]    Referring to FIGS. 4 and 5, it is seen that the upper pressure plate  59  of the present invention is large, and that the edges of plate  59  come very close to the cylindrical side walls of cups  45  and  55 . These side walls are vertical or nearly vertical (i.e. they are parallel or nearly parallel to rod  61 , and perpendicular or nearly perpendicular to plate  59 ). The cylindrical wall of cup  55  is only tapered above the uppermost position of plate  59  as shown in FIG. 4. The upright cylindrical walls of cups  45  and  55  provide a consistently wider space inside assembly  50  through which pressure plate  59  may be raised and lowered (see FIG. 5). The gap between the outside circumferential edge of plate  59  and the inside of the cylindrical walls of cups  45  and  55  is depicted as “G” in FIG. 5. This gap may be as small as two and one half (2½) times the width (thickness) of diaphragm  51 , or smaller. This allows sufficient space for diaphragm  51  to fold over itself (i.e. twice its width) as plate  59  moves up and down, plus a small additional space (½ its width, or less) to avoid unnecessary friction. See FIG. 7. A lesser additional space (resulting in an even larger pressure plate) may be available with certain low friction diaphragm materials.  
         [0053]    With respect to the service brake housing shown in FIGS. 4 and 5, it is seen that the upper pressure plate  29  of the present invention is also very large, and that the edges of plate  29  come very close to the cylindrical side walls of cups  25  and  35 . These side walls are also vertical or nearly vertical (i.e. they are parallel or nearly parallel to rod  21 , and perpendicular or nearly perpendicular to plate  59 ). The upright cylindrical walls of cups  35  and  25  provide a consistently wider space inside assembly  30  through which pressure plate  29  may be raised and lowered (see FIG. 5). The gap between the outside circumferential edge of plate  29  and the inside of the cylindrical walls of cups  25  and  35  is depicted as G′ in FIG. 5. This gap may also be as small as two and one half (2½) times the width (thickness) of diaphragm  31 , or smaller. This allows sufficient space for diaphragm  31  to fold over itself (i.e. twice its width) as plate  29  moves up and down, plus a small additional space (½ its width, or less) to avoid unnecessary friction. A lesser additional space (resulting in an even larger pressure plate) may be available with certain low friction diaphragm materials.  
         [0054]    Diaphragms  31  and  51  may be made of a very thin material. Instead of an average width of 0.125 inches, diaphragm materials as thin as 0.09 inches have been successfully used, and even thinner diaphragm materials may also be used. Using a standard diaphragm of 0.125 inches, gaps G and G′ (at 2½ times this thickness) could be as small as 0.3125 inches. Reducing the diaphragm thickness to 0.09 inches results in a gap G or G′ as small as 0.225 inches. Using a low friction material may allow a gap G or G′ of 2¼ times its thickness which, for the 0.09 diaphragm would result in a very small gap G or G′ of 0.2025 inches. A thinner diaphragm material will reduce gap G or G′ even further. Each of these modifications, used together or separately, allows for a larger pressure plate to be installed inside the housing.  
         [0055]    The smallest known gap G or G′ in an existing brake actuator is 0.57 inches using a diaphragm having a thickness of 0.125 inches (see FIG. 9). This gives a ratio of diaphragm thickness to gap size of 1:4.56. The present invention provides a smaller ratio which can be as low as 1:2.5 or lower. Embodiments having a ratio of diaphragm thickness to gap size ranging from 1:4.56 to 1:2.5 or smaller will allow ever increasing space for larger and larger pressure plates  29  and  59 . This provides for a range of pressure plate sizes and corresponding strengths for the main compression spring  58  within the emergency housing, and for the available braking strength in the service housing.  
         [0056]    The area (A) of a circle such as the pressure plates  29  and  59  of the present invention is determined according to the well known formula A=πR 2  where (R) is the radius of the circle defined by the pressure plate, and π=approximately 3.14159. This formula may also be stated as A=¼πD 2  where (D) is the diameter of the circle defined by the pressure plate. See FIG. 5 where D and R are used for plate  59 , and D′ and R′ are used for plate  29 . The inside sectional area of the cylindrical housing cups ( 55  and  45  in the emergency brake housing, and  35  and  25  in the service brake housing) may also be defined by the same formulas, where D is the diameter of the available inside circumferential wall of cups  55  and  45  in the emergency brake housing, and D′ for the cups  35  and  25  in the service brake housing.  
         [0057]    Using the above formulas, the possible areas (A) for pressure plate  59  relative to gap G defined by the present invention range from as large as about π(R-2.5 x) 2  to as small as about π(R-4.56 x) 2  where “R” is the radius of the inside circumferential wall of cups  55  and  45  through which plate  59  travels, and “x” is the thickness of the diaphragm  51 . Stated with the other formula, the range is from about ¼π(D-5 x) 2  to about ¼π(D-9.12 x) 2 . For illustrative purposes and by way of example only, and without limiting the scope of the appended claims herein, if the available inside diameter (D) of the housing cups  45  and  55  is eight inches (8″), and a diaphragm  51  having a thickness “x” of one eighth inch (0.125″) is employed, then the possible area (A) sizes for plate  59  defined by the present invention range from about 42.718 to about 36.961 square inches [¼π(7.375) 2  to ¼π(6.86) 2 ]. Employing a diaphragm  51  having a thickness “x” of 0.09 inches in this example results in a larger area (A) range for plate  59  of between about 44.77 and about 40.48 square inches. Straightening the outside walls of cups  45  and  55  to create an available diameter of more than eight inches will increase the available area (A) for plate  59  even more.  
         [0058]    These same principles also apply to the service brake assembly  30  defined by cups  25  and  35 , and using pressure plate  29 . Using the above formulas, the possible areas (A) for the pressure plate  29  relative to gap G′ defined by the present invention range from as large as about π(R′-2.5 x) 2  to as small as about π(R′-4.566 x) 2  where R′ is the radius of the inside circumferential wall of cups  35  and  25  through which plate  29  travels, and “x” is the thickness of the diaphragm  31 . Stated with the other formula, the range is from about ¼π(D′-5 x) 2  to about  {fraction (1/4)}π(D′- 9.12 x) 2 . Employing a thinner diaphragm  31  and/or straightening the outside walls of cups  35  and  25  to create a larger available inside diameter will increase the available area (A) for plate  29  even more.  
         [0059]    The circumference of plate  59  (or  29 ) is defined by the formula 2πR (2πR′ for plate  29 ) or πD (πD′ for plate  29 ). Using this formula, the range of circumference for plate  59  ranges from as large as about 2π(R-2.5 x) to as small as about 2π(R-4.56 x) where R is the radius of the inside circumferential wall of cups  45  and  55  through which plate  59  travels, and “x” is the thickness of the diaphragm  51 . The same formulas apply to the service brake using R′ for the inside circumferential wall of cups  25  and  35  through which plate  29  travels, and “x′” for the thickness of diaphragm  31 : about 2π(R′-2.5 x′) to as small as about 2π(R′-4.563 x′).  
         [0060]    Stated with the other formula, the range in circumference for the plate is from about π(D-5 x) to about π(D′-9.12 x). Employing a thinner diaphragm  31  (or  51 ) and/or straightening the outside walls of cups  35  and  25  (or  45  and  55 ) to create a larger available inside diameter will increase the available circumference for plate  29  (or  59 ) even more.  
         [0061]    The improvements of the present invention may be applied to a single-diaphragm stand alone service brake actuator, to a single-diaphragm stand alone emergency brake actuator, or to a combined service and emergency brake actuator.  
         [0062]    It is to be understood that variations and modifications of the present invention may be made without departing from the scope thereof. It is also to be understood that the present invention is not to be limited by the specific embodiments disclosed herein, but only in accordance with the appended claims when read in light of the foregoing specification.