Abstract:
In order to minimize the vacuum-dependent force component acting upon the valve body of a vacuum brake booster that is equipped with a pneumatically pressure-compensated control valve without requiring constructive modifications of the valve gear casing that accommodates the control valve, the invention includes a radially inner limit as well as the radially outer limit of a pressure compensation chamber in the valve gear casing, which is limited by the valve body, is formed by a guide part that guides the valve body.

Description:
TECHNICAL FIELD 
   The invention relates to a vacuum brake booster for motor vehicles. 
   BACKGROUND OF THE INVENTION 
   One vacuum brake booster of this general type is, for example, known from DE-OS 42 27 479 A1. The valve body of this known brake booster, which is prestressed in the direction of the sealing seats by means of a valve spring, is provided with passages within the region of its sealing surface. These passages originate at an annular chamber that is connected to the work chamber and end in the pneumatic chamber, namely on the side of the valve body that faces away from the sealing seats. Consequently, a continuous pressure compensation takes place between this chamber and the work chamber. In this case, the valve body has two pneumatically effective surfaces. The first effective surface or annular surface is limited by the radially inner sealing seat (atmospheric sealing seat) of the control valve as well a radially inner sealing lip of the valve body which cooperates with the guide part that limits the pneumatic chamber. Consequently, this effective surface or annular surface is subjected to a pneumatic differential pressure between the atmospheric pressure and the vacuum that exists in the pneumatic chamber in the release position. This results in a force component that is directed opposite to the force generated by the valve spring and decreases during the actuation of the brake booster as the degree of ventilation of the pneumatic chamber increases, namely until said force component is reduced to zero once the control point of the brake booster is reached or the work chamber is entirely ventilated, i.e., the first annular surface is pressure-compensated. 
   The second effective surface or annular surface is limited by the radially outer sealing seat (vacuum sealing seat) of the control valve as well as a radially outer sealing lip of the valve body which cooperates with the inner wall of the valve gear casing which limits the pneumatic chamber, i.e., this effective surface or annular surface is pressure-compensated in the release position of the brake booster and subjected to a pneumatic differential pressure when the brake booster is actuated. Consequently, a vacuum-dependent force component that boosts the effect of the aforementioned valve spring is generated. 
   However, one disadvantage associated with previously disclosed vacuum brake boosters is that constructive modifications in the valve gear casing are required for minimizing the vacuum-dependent sealing force component that occurs when higher actuation forces are applied. 
   The present invention make it possible to minimize the aforementioned sealing force component without requiring constructive modifications of the valve gear casing. 
   According to the invention, this objective is attained by forming the radially outer limitation of the pneumatic chamber by the guide part. 
   According to one additional advantageous development of the invention in which the valve body comprises a radially outer sealing lip as well as a radially inner sealing lip, and in which the radially inner sealing lip cooperates with an inner, tubular region of the guide part, it is proposed that the radially outer sealing lip cooperates with a radially outer region of the guide part, which is realized as a cylinder and radially adjoins the valve gear casing. 
   One additional advantageous embodiment of the invention in which the valve body comprises a first annular surface that is limited by the radially inner sealing lip and the radially inner sealing seat as well as a second annular surface that is limited by the radially outer sealing lip and the radially outer sealing seat is characterized by the fact that both annular surfaces have the same size. Due to this measure, the vacuum brake booster according to the invention behaves in a pressure-compensated fashion, in particular, in its moderate operating range, namely because the forces that act in opposite directions are neutralized. 
   Another advantageous embodiment of the object of the invention is characterized by the fact that the first annular surface is larger than the second annular surface. This embodiment is particularly advantageous in the realization of control valves that can be controlled proportionally and, for example, are used in brake boosters that are actuated independently or electromechanically. 
   In this case, it is advantageous if the guide part is made of plastic or metal. 
   A simplification in the assembly of the control components in which the guide part is sealed relative to the valve gear casing by means of a ring seal can be attained with one additional embodiment in which the ring seal is arranged in a radial groove in the guide part. 
   A reduction of the axial length, in particular, the axial length of the brake booster control components in which an air filter, as well as a readjusting spring that prestresses the radially inner sealing seat (atmospheric sealing seat) opposite the actuating direction, is arranged within the air intake region of the valve gear casing, can be attained if the air filter axially adjoins the guide part and the readjusting spring is realized as a cylinder and arranged radially outside as well as coaxial to the air filter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial longitudinal section through the vacuum brake booster of the present invention. 
       FIG. 2  is and enlarged, partial cross sectional view of the control components of the vacuum brake booster of  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The schematically illustrated booster housing  1  of the vacuum brake booster according to the invention shown in  FIG. 1  is divided into a work chamber  3  and a vacuum chamber  4  by means of an axially movable wall  2 . The axially movable wall  2  consists of a deep-drawn diaphragm disk  8  and a flexible membrane  18  that adjoins the diaphragm disk. This flexible membrane forms a roll-type membrane seal between the outer circumference of the diaphragm disk  8  and the booster housing  1 . 
   A control valve  12  that is actuated by means of an actuating rod  7  is accommodated in a valve gear casing  5  that carries the movable wall  2  and is guided in the booster housing  1  in a sealed fashion. This control valve consists of a first sealing seat  15  that is arranged on the valve gear casing  5 , a second sealing seat  16  that is arranged on a valve piston  9  that is connected to the actuating rod  7 , as well as an annular valve body  10  that cooperates with both sealing seats  15 , 16 . This valve body is guided in a guide part  21  that is arranged in the valve gear casing  5  in a sealed fashion and pressed against the sealing seats  15 , 16  by means of a valve spring  22  that is supported on the guide part  21 . The work chamber  3  can be connected to the vacuum chamber  4  by way of channels  28  that extend laterally in the valve gear casing  5 . 
   The brake force is transmitted to the actuating piston of a master cylinder (not shown) of the brake system by way of an elastic reaction disk  6  that adjoins the base of the valve gear casing  5  as well as a plunger rod  14  that comprises a top flange  23 . The master cylinder of the brake system is arranged on the vacuum side of the vacuum brake booster. The force introduced by way of the actuating rod  7  is transmitted to the reaction disk  6  by means of the valve piston  9 . 
   A readjusting spring  26  that is schematically illustrated in  FIG. 1  and supported on the face wall of the booster housing  1  on the vacuum side, namely on a flange (not shown), holds the movable wall  2  in the normal position shown. In addition, a piston rod pull-back spring  27  that is arranged between the actuating rod  7  and the guide part  21  such that it radially encompasses an air filter  33  that axially adjoins the guide part  21  is provided. The force of this piston rod pull-back spring ensures a certain prestress of the valve piston  9  or its sealing seat  16  relative to the valve body  10 . 
   In order to connect the work chamber  3  to the atmosphere during the actuation of the control valve  12 , a channel  29  that approximately extends radially is arranged in the valve gear casing  5 . The return movement of the valve piston  9  at the end of a brake maneuver is limited by a transverse element  11  that, in the release position of the vacuum brake booster shown in the figures, adjoins a sliding ring seal  13  which guides the valve gear casing  5  in the booster housing  1  in a sealed fashion. 
   The figures also show that the valve body  10  limits a pneumatic chamber  17  in the guide part  21 . This pneumatic chamber is connected to an annular chamber  24  that is limited by the sealing seats  15 , 16  by way of passages  19  ( FIG. 2 ) arranged in the valve body  10 . The aforementioned pneumatic channels  29  end in this annular chamber such that the pneumatic chamber  17  formed on the side of the valve body  10  that faces away from the sealing surface  20  is continuously connected to the work chamber  3 , i.e., the valve body  10  is pressure-compensated. 
     FIG. 2  shows that the valve body  10  comprises a radially outer first sealing lip  31  that adjoins a cylindrical, radially outer region  25  of the guide part  21  in a sealed fashion as well as a radially inner second sealing lip  32  that cooperates with a tubular, radially inner region  30  of the guide part  21 . A ring seal  34  that is arranged in a circumferential radial groove  35  of the guide part  21  effectively seals the guide part  21  relative to the valve gear casing  5 . 
   In the release position shown in  FIG. 2 , the pressure in the vacuum channels  28 , the annular chamber  24 , and the pneumatic pressure compensation chamber  17  is approximately identical or corresponds to the vacuum that exists in the vacuum chamber  4 . The valve spring  22  generates the force required for pressing the valve body  10  against the sealing seats  15 , 16 . Since atmospheric pressure exists in the chamber that is limited by the valve piston  9 , as well as the radially inner region of the valve body  10 , a pneumatic differential pressure acts upon the first annular surface A 1  of the valve body  10  between an imaginary circle corresponding to points opposite the second (atmospheric) sealing seat  16  and the radially inner sealing lip  32 . Consequently, a force component which counteracts the sealing force generated by the valve spring  22  and depends on the vacuum in the pressure compensation chamber  17  is generated. 
   During the actuation of the brake booster, i.e., when the second sealing seat  16  is lifted off the valve body  10  and the work chamber  3  is ventilated, the pressure compensation chamber  17  is simultaneously ventilated such that a pneumatic differential pressure acts upon the second annular surface A 2  of the valve body  10  between an imaginary circle corresponding to points opposite the first (vacuum) sealing seat  15  and the radially outer sealing lip  31 . Consequently, a force component that is directed toward the sealing seats  15 , 16  and boosts the effect of the valve spring  22  is generated. During the continued ventilation of the pressure compensation chamber  17 , the force component that acts upon the first annular surface A 1  decreases until it is reduced to zero once the control point of the brake booster is reached or the brake booster is entirely ventilated. 
   The previous description indicates that the behavior of the vacuum brake booster according to the invention can be influenced by suitably adapting the two annular surfaces A 1  and A 2 . For example, if both annular surfaces A 1  and A 2  have the same size, a pressure-compensated behavior of the brake booster is attained within the moderate range of the actuating forces, namely due to the fact that both force components are neutralized. However, if the radially outer annular surface A 2  is larger than the first annular surface A 1 , a proportional control of the control valve  12 , which is particularly practical in independently or electromagnetically actuated brake boosters, can be attained.