High output spring brake actuator

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.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 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.
 2. Description of the Prior Art
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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).
 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.
 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
 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.
 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.
 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%).
 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.
 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%).
 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 (21/2) times the thickness
 of the diaphragm material, or even smaller (e.g 21/4times 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.
 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.
 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 (21/2) 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 (41/2) 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.
 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
 41/2 times the thickness of the diaphragm). This gap may be reduced, as
 above, in the present invention down to as small as 21/2 times the
 diaphragm thickness, or even smaller (e.g. 21/2.times.0.125=0.3125 inches;
 21/4.times.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 0.110
 inches, then the gap may be further reduced to 21/2.times.0.110=0.275
 inches (or 21/4.times.0.110=0.2475 inches), providing even more room for a
 larger pressure plate.
 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.
 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.
 Historically, the effective surface area of spring brake pressure plates
 has been standardized into different types (9, 12, 1620, 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 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.
 Additional objects of the invention will be apparent from the detailed
 descriptions and the claims herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 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.
 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.
 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.
 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.
 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.
 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 (21/2) times the width (thickness) of
 diaphragm 51, or smaller. This allows sufficient space for diaphragm 51 to
 expand or fold over itself (i.e. twice its width) as plate 59 moves up and
 down, plus a small additional space (1/2 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.
 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 (21/2) times the width (thickness) of diaphragm 31, or smaller. This
 allows sufficient space for diaphragm 31 to expand or fold over itself
 (i.e. twice its width) as plate 29 moves up and down, plus a small
 additional space (1/2 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.
 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 0.110 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
 21/2 times this thickness) could be as small as 0.3125 inches. Reducing
 the diaphragm thickness to 0.110 inches results in a gap G or G' as small
 as 0.275 inches. Using a low friction material may allow a gap G or G' of
 21/4 times its thickness which, for the 0.110 diaphragm would result in a
 very small gap G or G' of 0.2475 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.
 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.
 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=R.sup.2 where (R) is the radius of the circle defined by the pressure
 plate, and =approximately 3.14159. This formula may also be stated as
 A=1/4D.sup.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.
 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 (R-2.5x).sup.2 to as small as about (R4.56x).sup.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 1/4(D-5x).sup.2 to about
 1/4(D-9.12x).sup.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 [1/4(7.375).sup.2 to 11/4(6.86).sup.2 ]. Employing a
 diaphragm 51 having a thickness "x" of 0.110 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.
 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
 (R'-25x).sup.2 to as small as about (R'4.56x).sup.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 1/4(D'-5x).sup.2 to about
 1/4(D'-9.12x).sup.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.
 The circumference of plate 59 (or 29) is defined by the formula 2 R(2 R'
 for plate 29) or D(D' for plate 29). Using this formula, the range of
 circumference for plate 59 ranges from as large as about 2 (R-2.5x) to as
 small as about 2(R-4.56x) 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 2 (R'-2.5x')to as small as about 2(R'-4.56x').
 Stated with the other formula, the range in circumference for the plate is
 from about (D-5x) to about (D'-912x). 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.
 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.
 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.