Abstract:
Vacuum brake booster for motor vehicles comprising a booster housing  7  whose interior is subdivided by a movable wall  4  into a vacuum chamber  30  and a working chamber  20 , a control valve  1  that controls a pressure difference acting upon the movable wall  4  and is accommodated in a control housing  2  that carries the movable wall  4 , said control valve being composed of two concentrically arranged sealing seats  9, 10  and an elastically deformable valve member  11 , wherein the control housing  2  includes a cavity  40  through which a cross member  60  extends and delimits the axial movability of a valve piston  16  relative to the control housing  2 , and the cross member  60  includes at least one projection  62, 80  made of an elastic material in the area where the cross member  60  rests against the booster housing  7.    
     The core of the invention is that at least one projection  66, 84, 85, 88, 90  made of an elastic material is provided in the area of points of abutment between the control housing  2  and the cross member  60.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates to a vacuum brake booster for motor vehicles comprising a booster housing whose interior is subdivided by a movable wall into a vacuum chamber and a working chamber, a control valve that controls a pressure difference acting upon the movable wall and is accommodated in a control housing that carries the movable wall, said control valve being composed of two concentrically arranged sealing seats and an elastically deformable valve member, wherein the control housing includes a cavity through which a cross member extends and delimits the axial movability of a valve piston relative to the control housing, and the cross member includes at least one projection made of an elastic material in the area where the cross member rests against the booster housing.  
       BACKGROUND OF THE INVENTION  
       [0002]     GB 2 253 018 A discloses a vacuum brake booster of this type. A cross member described in said publication includes two legs extending into a groove of a valve piston. Two other arcuate legs are provided with elastic projections in order to cushion the abutment of the cross member on a booster housing.  
       BRIEF SUMMARY OF THE INVENTION  
       [0003]     An object of the invention is to provide a vacuum brake booster wherein damping of the abutment noises of the cross member is further improved.  
         [0004]     According to the invention, this object is achieved in that at least one projection made of an elastic material is provided in the area of points of abutment between the control housing and the cross member. The term ‘projection’ herein refers to an elastic component mounted on the cross member and projecting a distance from the abutment surface of the cross member outside the release position or the normal position that exists in the non-assembled condition. The part, which moves to abut on the cross member, will initially come to rest against the projection that is thereby compressed.  
         [0005]     In a preferred aspect of the invention, points of abutment between the valve piston and the cross member also include at least one projection made of an elastic material, whereby the abutment of the two parts on each other is damped.  
         [0006]     It is important for the functions of the cross member that the parts abutting on each other have a defined distance in the release position. This demand is most reliably satisfied when the parts are in direct abutment on each other, meaning the distance is zero. To achieve this condition, the height of the projection and/or the material of the projection is so chosen in a favorable embodiment that in the release position of the vacuum brake booster the elevation of the projection with respect to its surroundings is considerably smaller than in the normal condition, preferably is negligibly small.  
         [0007]     Preferably, the projection is arranged on the cross member in such a fashion that it is able to yield an opposite abutment surface in the release position so that the material of the control housing or of the valve piston and the booster housing rests against the material of the cross member in an essentially direct fashion.  
         [0008]     To provide the projection with sufficient space to yield, the projection is arranged in a recess of the cross member according to a favorable improvement. This applies in particular when the cross-sectional surface A 1  of the recess is chosen to be larger than the cross-sectional surface A 2  of the projection.  
         [0009]     Preferably, the projection is attached to the cross member by cementing, casting on, fitting in or vulcanizing. When, according to a favorable improvement, the projection is attached to a lateral surface of the cross member and the projection protrudes from the abutment surface of the cross member, the protruding portion of the projection can swing away or be bent off laterally.  
         [0010]     In order not to deteriorate the guiding of the parts movable relative each other in the direction of the longitudinal axis of the valve piston, it is advisable to arrange the projection in the center of the respective abutment area. This arrangement prevents relative tilting of the parts.  
         [0011]     The projection is arranged over the entire length of a leg of the cross member according to a favorable improvement.  
         [0012]     A corresponding shaping and an appropriate design of the projection is necessary to compensate a change in the condition of a reaction plate when heated and the resultant change in position of the parts resting against each other. As the reaction plate is enclosed in a space formed by the control housing and a push rod, the growth of the reaction plate upon heating manifests itself by expansion of the plate in the longitudinal direction of the push rod, thereby displacing a master brake cylinder piston by a certain distance towards the inlet of the master brake cylinder irrespective of the force exerted by the vacuum brake booster. Said displacement can be compensated by the effect that, upon corresponding heating of the projection, said becomes more yielding when exposed to force and flows into the encompassing free space. It is this way possible that the projection, when subjected to higher temperatures, ensures a smaller distance between abutting parts than at lower temperatures where the projection has a comparatively solid condition. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a view of the longitudinal cross-section taken through the control housing assembly of a vacuum brake booster.  
         [0014]      FIGS. 2 and 3  show a first embodiment of a cross member.  
         [0015]     FIGS.  4  to  6  show a second embodiment of a cross member.  
         [0016]     FIGS.  7  to  9  show a third embodiment of a cross member.  
         [0017]      FIGS. 10 and 11  show a fourth embodiment of a cross member.  
         [0018]      FIGS. 12 and 13  show a fifth embodiment of a cross member. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0019]      FIG. 1  shows the longitudinal cross-section through a control housing assembly of a brake booster in the release position. A booster housing  7  is comprised of two housing shells interconnected by lanced indentations at a joint  22 , with only the housing shell close to the brake pedal being shown (for the sake of clarity) . The interior of the booster housing  7  is subdivided by a booster piston  19  into a vacuum chamber  30 , which is connected to a vacuum source by way of a connecting bore (not shown), and a working chamber  20 . The booster piston  19  includes a rolling diaphragm  21  abutting in the working chamber  20  on the booster piston  19  and a cylindrically designed control housing  2  connected to the booster piston  19  and the rolling diaphragm  21 . Rolling diaphragm  21  is pressure-tightly compressed at the joint  22  and embraces with its inner portion an inside edge  43  of the booster piston  19 , sealing said in relation to the control housing  2 . Control housing  2  projects with a cylindrical neck  2 ′ from the booster housing  7  and is protected against contamination of its surface by means of a pleated bellows  23 . The control housing  2  seals the working chamber  20  towards the outside by means of a sliding guide ring  24 . In the interior of the control housing  2 , the control rod composed of piston rod  14  and valve piston  16  is axially slidably arranged and connectable to a brake pedal of a motor vehicle by way of a fork head (not illustrated). Further, the control housing  2 ,  2 ′ contains a control valve  1  which is actuated by the valve piston  16  and controls the pressure difference between vacuum chamber  30  and working chamber  20  by way of channels  28 ,  29 .  
         [0020]     In addition, the part of the control housing  2 ,  2 ′ arranged in the vacuum chamber  30  includes a stepped bore  31  retained in which is a reaction plate  32  and a clamping washer  34  by way of a clamping sleeve  35 . A push rod  33  actuates a master brake cylinder (not shown) secured to the frontal end of the bottom of the booster housing  7 .  
         [0021]     To reset the booster piston  19 , a restoring spring is provided (not shown) that is compressed between the clamping sleeve  35  of the booster piston  19  and the bottom of the booster housing  7 .  
         [0022]     A cross member  60  is slipped from the side into a cavity  40  that is transversely arranged and open radially outwards relative to the working chamber  20 . The purpose of the cross member  60  is to limit the axial displacement of the valve piston  16  relative to the control housing  2 ,  2 ′. Besides, the cross member  60  ensures that the control housing  2 ,  2 ′ in the release position moves back into a defined position, shown in more detail in the drawing, relative to the booster housing  7 , i.e. it moves so far in the direction of the brake pedal until the control housing  2 , by way of the cross member  60 , abuts on a stationary stop on the booster housing  7  being configured as bead  8 .  
         [0023]     The drawing shows the control assembly of the brake booster in the release position, i.e. in a position in which the two chambers  30  and  20  are in connection through a channel  28  and a narrow slot  52  between the control valve  1  and a sealing seat  9  of the control housing  2 , while the inflow of atmospheric air into the working chamber  20  is additionally prevented.  
         [0024]      FIG. 2  shows a cross member  60  configured as a wedge and generally having a U-shape, thus including two legs  61 . Projections  62  in the shape of rubber knobs are cast on at the ends of the two legs  61 , namely at lateral surfaces  63 . Said projections  62  extend vertically relative to the viewing plane of  FIG. 2  and project a portion from a surface  64  of the cross member  60 , that means in an upward and downward direction from the viewing plane. The resultant projecting length forms an elevation relative to the surface  64  of the cross member  60 . Corresponding projections  62  are provided at a bottom part  65  of the cross member  60  of the legs  61 . The booster housing  7  comes to rest against these projections  62 .  
         [0025]     Additional projections  66  configured as knobs are arranged in the mean zone of legs  61  having the same design as described hereinabove. These additional projections  66  can be secured to an inside surface  70  of legs  61 . Projections  66  damp an abutment of the valve piston  16  having a circumferential groove  68  into which the cross member  60  with its leg  61  extends. The abutment surface then forms a lateral wall  69  of groove  68 . To prevent the projections  66  disposed at the inside surface  70  of legs  61  from being sheared off by a bottom  71  of groove  68  when the cross member  60  is slipped in, the projections are laterally offset relative to the center line that projects from the plane of projection of the valve piston  16 . The arrangement of the projections  66  as described is appropriate when the diameter of the valve piston  16  is smaller than the width of the cross member  60 .  
         [0026]     If this is not the case, it is advisable, as indicated in  FIG. 2 , to arrange the additional projections  66  at the lateral surface  63  of the legs.  
         [0027]      FIG. 3  depicts a cross-section in the plane C-C taken through the cross member  60  shown in  FIG. 2  in the direction of the arrows. As is known, the cross member  60  also moves to bear against the control housing  2 . Said control housing  2  is generally made of plastics so that the noise of the stop is reduced. In case of need, it is possible to provide further projections to cushion the abutment of the control housing  2 .  
         [0028]      FIG. 4  shows a perspective view of a cross member  60  wherein projections  80  are provided at the ends of legs  61  and at the bottom portion  65 . Projections  80  are inserted into recesses  81  that have a cross-sectional surface A 1  larger than the cross-sectional surface A 2  of projections  80 . A circumferential space  83  is thus produced, and the projection  80  may flow laterally into the circumferential space  83  under the effect of a force that acts from above on the projection  80  in  FIGS. 4 and 5 .  
         [0029]      FIG. 5  shows a cross-section in the plane A-A in  FIG. 4  where the correlations described are clearly shown. In this arrangement, the projection  80  can project both beyond the top and the bottom surface  64  of the cross member  60  obviating the need for an aligned assembly. As becomes apparent from  FIG. 5 , two equal portions of projection  80  are integrally connected by way of a mid-portion, and the projection  80  may be made in a casting process. The projection extends through a through-hole  59  in the cross member  60 .  
         [0030]     It is, however, also possible to insert two equal parts of projection, coming from opposite directions, into the associated recess  81  and attach them there. Attachment can be by cementing or static friction, e.g. by press fitting attachments at the projection parts into the through-hole. Blind-end bores can also be used for this purpose in order not to weaken the material unnecessarily.  
         [0031]      FIG. 6  depicts a cross-section along the plane B-B in  FIG. 4 . An elastic layer  84  is applied to the surface  64  of the cross member  60  to cushion the abutment of the valve piston  16  and/or the control housing  2 .  FIG. 5  shows the projecting length of the projection  80  beyond the surface  64  of the cross member  60 .  
         [0032]      FIG. 7  and the cross-section through the plane D-D illustrated in  FIG. 8  show another embodiment of a cross member  60 , with the legs  61  of the cross member  60  being provided with cylindrical recesses  86  that extend from the top to the bottom surface  64  of the cross member  60  and almost over the entire length of the legs  61 . By way of these recesses  86 , elastic projections  85 , e.g. elastomeric membranes, are fitted to the surfaces  64 , said projections being able to yield into air chambers  87  produced by the recesses  86  in the release position of the vacuum brake booster. It is possible, as shown in  FIG. 9 , to not arrange the projections  85  over the entire length of the legs  61  but to provide several projections  85  at the respective points of abutment of the cross member  60 .  
         [0033]      FIGS. 10 and 11  show another embodiment of a cross member  60 . It becomes apparent from the cross-section taken through plane E-E that conical projections  88  are mounted in recesses  89  disposed on the surfaces  64  of legs  61 , said projections protruding from the surface  64 . The diameter of the recesses  89  exceeds the diameter of the projections  88  so that the projections  88  can yield into the recesses  89  in the release position of the vacuum brake booster.  
         [0034]      FIG. 12  and  FIG. 13  representing a cross-section taken through plane F-F show a fifth embodiment of a cross member  60  wherein an elastic projection  90  arranged in a recess  91  protrudes from the lateral surface  63  and surface  64 . The resultant projecting length of the projection  90  will yield laterally in the release position.  
         [0035]     As described in  FIG. 7 , it is possible in the embodiments according to FIGS.  10  to  13  to arrange the elastic projections  88 ,  90  over the entire length of the legs  61  or to mount several projections  88 ,  90  according to the points of abutment of the cross member  60 .  
         [0036]     The following statements can be made with respect to the embodiments illustrated in FIGS.  2  to  13 .  
         [0037]     Currently, there are two functional criteria in the brake actuating unit which need improvement.  
         [0038]     Criterion 1: lost travel  
         [0039]     Criterion 2: mechanical wedge abutment noise of the vacuum brake booster when released.  
         [0040]     Ad criterion 1: the lost travel depends, among others, on the 
        growth of the reaction plate under heat (0.2-0.6 mm, depending on the type of the reaction plate)     elongation of the valve member of the control valve when exposed to heat and under high pressure (roughly 0.2 mm).        
 
         [0043]     To ensure opening of the control valve at high temperatures and under high pressure, it is necessary to arrange for roughly 0.8 mm, which become apparent as lost travel under normal conditions (e.g. room temperature).  
         [0044]     Ad criterion 2: when the vacuum brake booster is released, abutment of the valve piston on the wedge (cross member), on the one hand, and abutment of the wedge on the control housing and the booster housing, on the other hand, cause mechanical abutment noises.  
         [0045]     The embodiment of FIGS.  2  to  14  has the following design: In a conventional (or optionally broader) wedge, projections are vulcanized or mounted thereon which counteract both criteria by tuning shape and hardness of the projections as the decisive criterion. These features eliminate the shortcomings described above.  
         [heading-0046]     Ad Criterion 1:  
         [0047]     The projections in the area wedge/booster housing are so configured with respect to shape and hardness that they consolidate in the extent the reaction plate grows.  
         [0048]     Thus, growth of the reaction plate can be compensated and need not be taken into account in the analysis of the lost travel. When the rating allows further pressurization of the master cylinder of roughly 0.2 mm, the elongation of the control valve under pressure can also be compensated and removed from the analysis of the lost travel.  
         [0049]     It is possible to compensate a variation of the dimension of the plug-shaped push rod end part during adjustment on the assembly line, which is related to this provision, by way of increasing the dimensional tolerance. However, this would eliminate again a fraction of the lost travel reduction (roughly 0.2 mm).  
         [heading-0050]     Ad Criterion 2:  
         [0051]     Projections in the area wedge/valve piston and wedge/control housing must be configured such with respect to shape and hardness that they bring about noise damping upon a first contact and cause metal/metal or metal/plastics contact in the event of full release of the vacuum brake booster. This rating is suitable to maintain a defined dimension in the vacuum brake booster. Damping by means of a rubber layer is alternatively feasible herein.