Abstract:
A vacuum booster and method utilize a power piston apparatus including a power piston operatively mounted within a booster housing for movement along a longitudinal axis of the booster, and having a sliding seal fixedly attached thereto for slidingly engaging a sealing surface of an annular wall of the booster housing and axially dividing the closed booster cavity into a high pressure cavity and a low pressure cavity. The sliding seal is mounted on an outer periphery of a seal support flange extending radially outward from the power piston. A convoluted divider provides an air passage connecting primary and secondary high pressure cavities in the booster, without the use of air tubes.

Description:
TECHNICAL FIELD OF THE INVENTION  
       [0001]     This invention relates to vacuum brake boosters, and more particularly to a vacuum brake booster that does not use rolling diaphragms for sealing high and low pressure cavities within the booster from one another.  
       BACKGROUND OF THE INVENTION  
       [0002]     Vehicles such as automobiles, trucks, buses, and motor homes typically include a dashboard at the front of the passenger compartment, having a power brake booster on the front of the dashboard connected by a push rod to a brake pedal mounted on the rear of the dashboard in the passenger compartment. Such power brake boosters typically include a power piston that is sealed the inside of a booster housing by one or more rolling diaphragms. In some boosters having two or more diaphragms and a divider, forming a tandem brake booster, air passages or conduits must be provided to conduct air past the primary diaphragm. The use of rolling diaphragms, and air passages through the diaphragms creates certain disadvantages, and adds undesirable complexity and cost to the booster.  
         [0003]      FIG. 1  shows a typical prior tandem vacuum brake booster  100 . The booster  100  includes a housing assembly  102 , having a rear housing  104  adapted for connection to the front of the dashboard, and a front housing  106  adapted to receive and provide a mounting surface for a master cylinder  108  of the brake system. The housing assembly  102  of the vacuum booster  100  includes a divider  110  that divides the interior of the housing assembly into a primary chamber  112  and a secondary chamber  114 , and provides sliding support for an axially movable booster power piston  116  that is connected via the push rod  118  to the brake pedal  120 .  
         [0004]     Extending radially outward from the power piston  116 , in the secondary chamber  114  of the housing  102 , is a secondary diaphragm support  122 . In similar fashion, a primary diaphragm support  124  extends radially outward from the power piston  116  in the primary chamber  112 . The primary and secondary diaphragm supports  122 ,  124  are fixed to the power piston  116  and move axially along an axis of motion  126  with the power piston  116 .  
         [0005]     A flexible rolling secondary diaphragm  128  has an outer periphery sealed to the inner walls of the secondary chamber  114  of the front housing  106 , an inner periphery sealed to the power piston  116 , and a skirt extending along the secondary diaphragm support  124  between the inner and outer peripheries of the secondary diaphragm  128 , to thereby form a secondary low pressure chamber  130  between the secondary diaphragm  128  and the front wall  132  of the front housing  106  and a secondary high pressure chamber  134  between the secondary diaphragm  128  and the divider  110 .  
         [0006]     A flexible rolling primary diaphragm  136  has an outer periphery sealed to the inner wall of the primary chamber  112  of the rear housing  102 , an inner periphery sealed to the power piston  116 , an inner periphery sealed to the power piston  116 , and a skirt extending between the inner and outer peripheries of the primary diaphragm  136 , along the secondary diaphragm support  122 , to thereby form a primary low pressure chamber  138  between the primary diaphragm  136  and the divider  110 , and a primary high pressure chamber  140  between the primary diaphragm  136  and the rear wall  142  of the rear housing  102 .  
         [0007]     One or more air tubes  144  extend through the primary low pressure chamber  138  to connect the primary and secondary high pressure chambers  140 ,  134 . The primary and secondary diaphragms  136 ,  128  include integrally formed grommets that provide a sliding seal between the air tubes  144  and the primary and secondary diaphragms  136 ,  128 . The primary and secondary low pressure chambers  138 ,  130  are connected by holes  146  passing through the power piston  116 .  
         [0008]     The booster  100  includes valve elements, indicated generally by arrow  148 , operably attached to the push rod  118  within the power piston  116 , for selectively connecting all four chambers  138 ,  140 ,  130 ,  134  (i.e. the primary low pressure, secondary low pressure, primary high pressure, and secondary high pressure chambers) to a source of vacuum (not shown), such as the interior passages of an engine intake manifold, when the brake pedal  120  is not depressed. When the brake pedal  120  is depressed, the push rod  118  moves the valve elements  148  to a position where the primary and secondary low pressure chambers  138 ,  130  remain connected to the source of vacuum, but the primary and secondary high pressure chambers  140 ,  134  are connected to atmospheric air pressure around the brake booster  100 .  
         [0009]     The difference in pressure between the atmospheric pressure operating against the rear side of the primary and secondary diaphragms  136 ,  128 , and the vacuum operating against the front side of the primary and secondary diaphragms  136 ,  128 , generates a force against the primary and secondary diaphragm supports  124 ,  122  that drives the power piston  116  forward, (to the left in  FIG. 1 ) and augments the force exerted through the push rod  118  from the brake pedal  120 , acting through an internal booster output rod  150  in moving a hydraulic piston  152  in the master cylinder  108  to generate hydraulic pressure in the brake system for applying the brakes. The action of the brake booster  100  thus allows the pedal force required to generate a desired hydraulic pressure in the master cylinder  108  to be significantly less than the pedal force that would be required without the booster  100 .  
         [0010]     When the brake pedal  120  is released, after a braking event, a booster return spring  154  disposed between the front housing  106  and the power piston  116  causes the power piston  116  to move back to poise position, illustrated in  FIG. 1 . As the return spring  154  drives the power piston  116  back to the poise position, the valve elements  148  are momentarily positioned, as a result of the motion of the power piston and the action of springs within the valve elements, to allow the air in the primary and secondary high pressure chambers  140 ,  130  to escape through the valve elements  148 . Once the air has escaped, the valve elements  148  return to a poised position, as shown in  FIG. 1 , that allows the primary and secondary high pressure chambers  140 ,  130  to be evacuated by the source of vacuum, to thereby equalize pressure across the primary and secondary diaphragms  136 ,  128 .  
         [0011]     Having the air tubes  144  pass through the divider  110 , primary diaphragm  136 , and primary diaphragm support  124 , in order to allow sealed passage through the primary low pressure (vacuum) chamber  138  between the primary and secondary high pressure chambers  140 ,  134 , add significant undesirable complexity to the booster  100 .  
         [0012]      FIGS. 2   a ,  2   b , and  3   a ,  3   b  illustrate a potential opportunity for enhancing performance the primary and secondary diaphragms  136 ,  128  respectively, in comparison to prior boosters. As shown in  FIGS. 2   a  and  3   a , air pressure within the primary high pressure chamber  134  fills the folded portions  156 ,  158  of the primary and secondary diaphragms  136 ,  128 , with pressure acting on the interior of the folded portion  156  in such a manner that the effective area of the primary and secondary diaphragms  136 , 128  is less than the internal cross sectional area bounded by the inner surfaces  160 ,  162  of the booster housing  102 .  
         [0013]     Having the effective area of the primary and secondary diaphragms  136 ,  128  be less than the internal cross section of the booster housing  102 , results in a reduction in the force that is generated by the power piston  116 , or conversely, in the booster housing  102  having an outer diameter that is larger than would otherwise be required if the effective area equaled the internal cross sectional area of the booster housing  102 . In a booster  100  of typical construction for a vehicle such as an automobile, having the effective diameter of the rolling diaphragm be less than the internal cross sectional are of the booster  100  results in a missed opportunity for generating additional assist force with the primary and secondary diaphragms  136 ,  128 .  
         [0014]     As shown in  FIGS. 2   b  and  3   b , rolling diaphragms present another disadvantage in that the folded portion  156 ,  158  of the primary and secondary diaphragms does not remain neatly folded, as shown in  FIGS. 2   a  and  3   a , as air is admitted into the primary and secondary high pressure chambers  140 ,  138 . The folded portions  156 ,  158  of the diaphragms  136 ,  128  bulge around the edge of the primary and secondary diaphragm supports  124 ,  122 , and try to form a cross section of the folded portions  156 ,  158  that is toroidal rather than folded. As the folded portions  156 ,  158  bulge around the diaphragm supports  124 ,  122 , the net effect of the air pressure acting on the inside of the bulged folded portions  156 ,  158  generates a small undesirable force, on the diaphragm supports  124 ,  122 , that acts to resist the desired motion and force of the power piston  116 .  
         [0015]     Large rolling diaphragms, having large thin wall sections, such as the primary and secondary diaphragms  136 ,  128  shown in  FIGS. 1, 2   a - 2   b ,  3   a - 3   b , are also difficult to manufacture. These diaphragms are typically fabricated from a rubber compound. Such rubber compounds inherently include hard particles of carbon black, that can result in localized weakness and tearing of the diaphragm in the area where the hard particles are located. Manufacturing procedures for rolling diaphragms made from rubber typically include filtration measures to reduce the presence of hard particles of carbon black, but experience has shown that even with filtration, it is difficult to produce rolling diaphragms with large thin wall sections in which the incidence of hard particles of carbon black is reduced to an acceptable level.  
         [0016]     It is desirable, therefore, to provide a booster in which the effective area of the elements producing force on the booster power piston is equal to the internal cross sectional area of the booster housing. It is also desirable to provide a booster having an improved apparatus for conducting air between the primary and secondary high pressure chambers of a tandem vacuum brake booster. It is further desirable, to provide an improved booster that does not require rolling diaphragms.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention provides an improved booster, meeting the requirements discussed above, through use of a power piston apparatus including a power piston operatively mounted within a booster housing for movement along a longitudinal axis of the booster, and having a sliding seal fixedly attached thereto for slidingly engaging a sealing surface of a annular wall of the booster housing for axially dividing the closed booster cavity into a high pressure cavity and a low pressure cavity.  
         [0018]     In one form of the invention, the power piston apparatus includes an imperforate, generally annular, seal support flange extending radially outward from the power piston and defining an outer periphery thereof adapted for attachment of the sliding seal. The seal may be a lip seal.  
         [0019]     One or more seal support flanges, according to the invention may be attached to the power piston for supporting either or both of a primary or a secondary seal. A second seal support flange, according to the invention, may include a generally annular shaped wall thereof having an outer surface in sliding sealing engagement with a sealing surface of the booster housing, a first end thereof attached to the power piston in the primary chamber, and an imperforate radially extending flange thereof attached to the opposite end of the annular shaped wall of the second seal support flange and extending radially outward to a distal peripheral edge thereof adapted for attachment of the second sliding seal.  
         [0020]     A booster according to the present invention may also include a divider having an imperforate wall thereof fixedly attached and sealed to the booster housing and including a seal for sliding passage therethrough of the power piston, the divider dividing the closed cavity into a primary chamber and a secondary chamber, with a seal support flange dividing one of the primary or secondary chambers into a high pressure and a low pressure cavity thereof. The divider may include an annular wall thereof in the primary chamber, having a radially inward facing surface thereof forming a portion of the sealing surface of the housing in the primary chamber, and the housing may include an imperforate outer shell thereof spaced radially outward from the annular wall of the divider, to thereby form an air passage between the outer shell of the housing and the annular wall of the divider, the air passage providing fluid communication between a high pressure cavity of the primary chamber and a high pressure cavity of the secondary chamber. The divider may also include one or more imperforate formed notches at the juncture of the axially facing wall and the annular wall of the divider, the one or more formed notches providing fluid communication between the air passage and a high pressure cavity of the secondary chamber.  
         [0021]     The present invention may also take the form of a method for assembling a booster or power piston apparatus, according to the invention.  
         [0022]     The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a cross section of a prior vacuum brake booster having a rolling diaphragms and air tubes;  
         [0024]      FIGS. 2   a - 2   b , and  3   a - 3   b  are enlarged partial cross sections of the rolling diaphragms of the prior booster of  FIG. 1 ;  
         [0025]      FIG. 4  is a cross sectional view of a tandem vacuum booster according to an exemplary embodiment of the present invention, including primary and secondary sliding seals, instead of rolling diaphragms, and a convoluted divider forming an air passage between high pressure cavities within the booster, rather than air tubes;  
         [0026]      FIG. 5  is perspective view of a divider, according to the invention,  FIG. 6  is an enlarged cross section of a portion of the booster of  FIG. 4 ;  
         [0027]      FIGS. 7 and 8  are enlarged cross sections of portions of the exemplary embodiment of  FIG. 4 ; and  
         [0028]      FIG. 9  is a cross section of an alternate embodiment of a secondary sliding seal support flange, according to the invention.  
     
    
       [0029]     Throughout the following description of exemplary embodiments of the invention, components and features that are substantially equivalent or similar will be identified in the drawings by the same reference numerals. For the sake of brevity, once a particular element or function of the invention has been described in relation to one exemplary embodiment, the description and function will not be repeated for elements that are substantially equivalent or similar in form and/or function to the components previously described, in those instances where the alternate exemplary embodiments will be readily understood by those skilled in the art from a comparison of the drawings showing the various exemplary embodiments in light of the description of a previously presented embodiment.  
       DETAILED DESCRIPTION  
       [0030]      FIG. 4  shows a first exemplary embodiment of the invention in the form of a tandem vacuum booster  10 , including a booster housing  12  and a power piston apparatus  14 .  
         [0031]     The booster housing  12  includes a rear housing  16 , adapted for attaching the booster  10  to a panel (not shown), a stepped front housing  18 , adapted for attachment of a master cylinder (not shown), and a divider  20 . The front and rear housings  18 ,  16  and the divider  20  are all joined and sealed from the environment at a common juncture  22  of the booster housing  12 , using a multifunctional seal  23 . The booster housing  12  defines a closed booster cavity  24 , a longitudinal axis  26  of the booster  10 , and a first and a second generally annular wall  28 ,  29  having first and second radially inward facing sealing surfaces  30 ,  32  thereof.  
         [0032]     The first annular wall  28  and first radially inward facing sealing surface  30  are provided by an annular wall  28  of the divider  20 . The annular wall  28  of the divider  20  is sealed to the housing  12  at the common juncture  22 . As shown, in  FIGS. 4 and 5 , the divider also includes an imperforate axially facing wall  34  joined at a radially outer peripheral edge thereof to the annular wall  28  of the divider  20 , at a point adjacent to the step  36  in the front housing  18 , to thereby form a juncture  38  of the axially facing wall  34  with the annular wall  28  of the divider  20 . The annular wall  34  of the divider  20  divides the internal cavity  24  of the booster housing  12  into a primary chamber  40  and a secondary chamber  42 . The term imperforate, as used herein with respect to the divider  20 , is intended to mean closed to the passage of air when the booster  10  is fully assembled. As shown, in  FIGS. 4 and 5 , the axially facing wall  34  of the divider  20  includes a central hole  44 , adapted for attachment of a piston seal  46 , for sliding passage therethrough of a power piston  48  of the power piston apparatus  14 . As shown, in  FIG. 5 , the axially facing wall  34  also includes a pair of holes  50  spaced on either side of the central hole  44  for attachment of a tie rod seal (not shown) and passage of tie rods (not shown) for mounting the booster  10  to a panel and attaching the master cylinder to the booster  10 .  
         [0033]     The front housing  18  further includes an imperforate outer shell  52  thereof, spaced radially outward from the annular wall  28  of the divider  20 , to thereby form an air passage  54  between the outer shell  52  of the housing  12  and the annular wall  28  of the divider  2 . The air passage  54  provides fluid communication between a high pressure cavity of the primary chamber  40  and the high pressure cavity of the secondary chamber  42 , in a manner described in more detail below.  
         [0034]     As shown, in  FIGS. 4 , and  5  through  7  the power piston apparatus  14  includes a primary and a secondary, imperforate, generally annular, seal support flange  56 ,  58  extending radially outward from the power piston  48  and defining outer peripheries  60 ,  62  thereof adapted for attachment of primary and secondary sliding seals  64 ,  66 , that seal the peripheries  60 ,  62  against the first and second sealing surfaces  30 ,  32  of the booster housing  12 . The primary and secondary sliding seals  64 ,  66  are preferably low friction lip seals, but other types of seals may also be used.  
         [0035]     The primary and secondary sliding seals  64 ,  66  shown in  FIGS. 4, 6  and  9  can be fabricated with less concern over the presence of hard particles of carbon black, than was the case for rolling diaphragms having thin wall sections, because the sliding seals  64 ,  66  of the present invention do not include thin wall sections. The sliding seals  64 ,  66  of the present invention are more robust than rolling diaphragms, and have thicker cross sections that can tolerate higher percentages of hard particles of carbon black without being subject to the tearing sometimes experienced in the large, thin-walled, rolling diaphragms used in prior boosters.  
         [0036]     As shown, in  FIGS. 7 and 8 , the primary and secondary seal support flanges  56 ,  58  each include a collar  68 ,  70  for attaching the seal support  56 ,  58  to the power piston  48  at the junctures of the seal supports  56 ,  58  and the power piston  48 , and a seal  72  for sealing the juncture of each seal support flanges  56 ,  58  and the power piston  48 . The seals  72  may take any appropriate form, such as the  0 -ring seal  72  shown in  FIG. 8 , or the flat washer-like seal  72  shown in  FIG. 7 . A retainer  74  may also be used for attaching the seals  72  and seal support flanges  56 ,  58  to the power piston  48 .  
         [0037]     In the same manner as described above with regard to the axially facing wall  34  of the divider  20 , the term imperforate as used with respect to the primary and secondary seal support flanges  56 ,  58 , means that once the booster  10  is assembled, the primary and secondary seal support flanges  56 ,  58  define a barrier to air flow. In embodiments of the invention including tie rods extending axially through the booster housing  12 , for example, the seal support flanges  56 ,  58  may include holes (not shown) for passage of the tie rods. Such holes are slidingly sealed to the tie rods by sliding grommet-like seals installed into the holes in the primary and secondary seal support flanges  56 ,  58 .  
         [0038]     As shown, in  FIG. 6 , the primary seal support flange  56  divides the primary chamber  40  into a primary high pressure cavity  76  and a primary low pressure cavity  78 . The secondary seal support flange  58  divides the secondary chamber  42  into a secondary high pressure cavity  80  and a secondary low pressure cavity  82 .  
         [0039]     As shown, in  FIGS. 5 and 6 , the divider  20  includes one or more imperforate formed notches  84  at the juncture  38  of the axially facing wall  34  and the annular wall  28  of the divider  20 , and the annular wall  28  includes a series of slots  86  for the passage of air, from the primary high pressure cavity  76  to the secondary high pressure cavity  80 . The formed notches  84  provide fluid communication between the air passage  54  and the secondary high pressure cavity  80 .  
         [0040]     The air passage  54  formed between the annular wall  28  of the divider  20  and the outer shell  52  eliminate the need for the air tubes  144  shown in the prior booster of  FIG. 1 . The sliding seals  64 ,  66  eliminate the need for the rolling diaphragms  128 ,  136  shown in  FIGS. 1-3 , and provide an effective area that is equal to the internal cross section of the booster  10  at the first and second sealing surfaces  30 ,  32 . The sliding seals  64 ,  66  of the present invention thus allow a higher force to be generated with a booster  10  having the same internal dimensions of the sealing surfaces  30 ,  32  as the internal surfaces  160 ,  162  of the booster  100 , or conversely would allow the outside dimensions of the booster  10  to be reduced for a booster that was only required to produce the same boost force as the prior booster  100 .  
         [0041]     Those skilled in the art will readily recognize that, while the embodiments of the invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. For example, the invention can be utilized in a tandem booster  100 , as shown in  FIG. 1 , or in other types of single stage or tandem vacuum boosters.  
         [0042]      FIG. 9  shows an alternate embodiment of a secondary seal support flange  58  including a generally annular shaped wall  88  thereof having an outer surface  90  adapted for sliding sealing engagement with the piston seal  46  in the divider  20 , a collar  70  at a first end thereof adapted for attachment to the power piston  48  in the primary chamber  40 , and an imperforate radially extending flange  92  thereof attached to the opposite end of the annular shaped wall  88  and extending radially outward to a distal peripheral edge  62  thereof adapted for attachment of the second sliding seal  66 . The secondary seal support flange  58  of the embodiment shown in  FIG. 9  thus allows a portion of the power piston  48  to be eliminated, and replaced by the annular wall  88  of the secondary seal support flange  58 , to thus simplify and reduce the cost of the booster  10 .  
         [0043]     The scope of the invention is indicated in the appended claims, and all changes or modifications within the meaning and range of equivalents are intended to be embraced therein.