Abstract:
The present invention discloses a pressure fluid accumulator with a housing having its interior subdivided into two chambers by a media-separating element, the first chamber being filled with a gas and the second chamber being filled with a liquid, and wherein in a hydraulic port a bottom valve is provided whose closure member is operable by the media-separating element and which permits filling the second chamber with liquid and prevents complete evacuation of the second chamber. In order to prevent a damage of the bottom valve and an inadvertent escape of fluid and, thus, ensure a considerable increase in the reliability in operation, according to the present invention, the closure member can be moved by the media-separating element to adopt a position in which it fulfils the function of a hydraulic piston.

Description:
TECHNICAL FIELD 
     The present invention generally relates to vehicle brake systems and more particularly relates to a pressure fluid accumulator for use in vehicle brake systems. 
     BACKGROUND OF THE INVENTION 
     A pressure fluid accumulator of this general type is disclosed in international patent application WO 98/37329. The media-separating element in the prior art pressure fluid accumulator is configured as a metallic pleated bellows, and the closure member of the bottom valve is connected to the end surface close to the hydraulic port of this bellows by means of a spring. To achieve effective closure of the hydraulic port, the closure member includes a rubber-elastic sealing element. 
     A shortcoming from which the prior art accumulator suffers is the condition that the closing slot which develops when the closure member moves to sit on the bottom is penetrated by the pressure fluid so that there is the imminent risk of damage or destruction of the sealing element and, hence, failure of the pressure fluid accumulator. Another disadvantage is seen in the escape of pressure fluid which may be caused by an expansion of the pleated bellows due to temperature variations. 
     In view of the above, an object of the present invention is to improve upon a pressure fluid accumulator of the above-mentioned type to such effect that damage of the bottom valve and inadvertent pressure fluid escape is prevented and, thus, the reliability in operation is considerably increased. 
     According to the present invention, this object is achieved in that the closure member can be moved by the media-separating element to adopt a position in which it fulfils the function of a hydraulic piston. This is achieved in that the closure member, upon approach of the end surface of the pleated bellows on the bottom, is moved into the hydraulic flow without inhibiting it, and subsequently, when floating in the hydraulic flow, is moved into abutment on a stop, with the result that the hydraulic port is closed in the way of a locked hydraulic piston. 
     To render the idea of the present invention more precise, the closure member is guided in a bore provided in the hydraulic port and includes at least one sealing element which provides a sealant vis-à-vis the wall of the bore. The bore is preferably configured as a stepped bore, and the sealing element cooperates with the small-diameter portion of the bore. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an axial cross-sectional view of a first embodiment of the pressure fluid accumulator of the present invention. 
     FIG. 2 is an axial cross-sectional view of the bottom valve used in the embodiment of FIG. 1 in its opened condition. 
     FIGS. 2 a  and  2   b  is a view of the bottom valve according to FIG. 2 in the transition condition or in the closed condition. 
     FIG. 3 is an axial cross-sectional view of a second embodiment of the pressure fluid accumulator of the present invention. 
     FIGS. 4 a  to  4   c  are axial cross-sectional views of the bottom valve used in the embodiment of FIG. 3 in different conditions. 
     FIG. 5 is an axial cross-sectional view of a third embodiment of the bottom valve. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The first embodiment of the pressure fluid accumulator of the present invention as illustrated in FIG. 1 has a housing  1 , with its interior subdivided into two pressure compartments or chambers  3 ,  4  by means of a media-separating element  2 . The media-separating element  2  is preferably formed by a thin-walled metallic pleated bellows which is connected pressure-tightly to a cover  15  that closes the housing, on the one hand, and is closed by a plate  16 , on the other hand. The interior of the pleated bellows  2  is the first chamber  3  which can be filled with a gas that is usually under high pressure by way of a fill port (not shown) provided in the cover  15 . In the bottom part of the housing  1 , a hydraulic port  5  is provided in which a bottom valve  6  is arranged whose closure member  7  projects into the second chamber  4 . The bottom valve  6  is preferably configured so that it permits filling the second chamber  4  with a pressurized fluid, such as a brake fluid, on the one hand, and prevents complete evacuation of the second chamber  4 , on the other hand. Further, the first chamber  3  houses a compression spring  17  which is compressed between the cover  15  and the above-mentioned plate  16  and, thus, preloads the pleated bellows  2  in the direction of the bottom valve  6 . This ensures that the hydraulic pressure which prevails in the second chamber  4  is always higher than the gas pressure that prevails in the first chamber  3 . To finally achieve centering of the pleated bellows  2  in the housing  1 , there is provision of a slotted ring  18  which embraces the pleated bellows  2  and, in the assembled condition, abuts on the wall of the housing  1 . 
     As can be taken from FIG. 2 in particular, the hydraulic port  5  that includes a fill or evacuation port  13  has a bore  10  which is designed as a stepped bore and is comprised of a first portion  11  of large diameter and a second portion  12  of small diameter. The transition area between the two portions  11 ,  12  is preferably a conical annular surface  9 . The above-mentioned closure member  7  is guided in the stepped bore  10  and  11 ,  12 , respectively, there being provision of a collar  19  with at least one passage  20  for guiding in the first bore portion  11 , while for guiding in the second bore portion  12  a second collar  21  is used which has several radial flow ducts  22 . The flow ducts  22  along with the above-mentioned passage  20  provide a flow connection between the second chamber  4  and the fill or evacuation port  13  of the hydraulic port  5 . An end surface of the second collar  21  which is remote from the fill or evacuation port  13  provides a flank of a radial groove  23  which receives a sealing element  8  that is a sealing cup in the embodiment shown. In the opened condition of the bottom valve  6  shown in FIG. 2, the first collar  19  bears against a stop  24  under preload by a compression spring  14 . 
     Closing of the bottom valve  6  takes place in two periods which are illustrated in FIGS. 2 a  and  2   b.  Shortly before the evacuation of the chamber  4 , the plate  16  that closes the pleated bellows  2  starts touching the end of the closure member  7  which preferably has a semispherical design. Upon continued discharge of the pressure fluid, the closure member  7  is displaced in opposition to the force generated by the compression spring  14  or urged downwards in the drawing until the outside sealing lip of the sealing cup  8  moves into contact with the conical annular surface  13  and, thus, prevents fluid circulation around the closure member  7 . The closure member  7  starts in this moment to fulfil the function of a hydraulic piston and is displaced further downwards by the residual pressure that prevails in the chamber  4 . This causes displacement of the sealing element  8  into the bore portion  12  whose diameter will not change. Only small pressure differences may occur at the sealing cup  8  in the actions so far discussed, the said differences corresponding to the spring forces, friction forces, and inertia forces which act on the closure member  7 . This situation changes as soon as the closure member  7  has reached its bottom stop and is supported on the housing  1  with a force of any rate. Due to the large pressure differences which the sealing cup  8  has to withhold, the sealing cup  8  is loaded statically with an optimally small metallic sealing slot that is, above all, constant with time. The condition which has just be described, in which the sealing element  8  fulfils the function of a non-return valve that opens in the direction of the second chamber  4  is illustrated in FIG. 2 b.    
     The bottom valve  6  is opened because liquid pressure fluid is pumped from the outside into the pressure fluid accumulator  1  according to the present invention. When the charging pressure exceeds the residual pressure or internal pressure that prevails in the chamber  4 , the external sealing lip of the sealing cup  8  turns about, thereby permitting pressure fluid to flow in through the sealing slot confined by the wall of the bore portion  12 , with the compression spring  14  simultaneously sliding back the closure member  7 . The result is that the sealing cup  8  or its outside sealing lip detaches from the bore wall and gives way to the inflowing pressure fluid. Exactly as in the closing process, the contour of the annular chamber which accommodates the sealing cup  8  changes only when the pressure difference that prevails at the sealing cup is low. The closure member  7  is urged further upwards by the compression spring  14  until it abuts on the plate  16  again that closes the pleated bellows  2 . With continued filling of the chamber  4 , the plate  16  will retreat, and the travel of the closure member  7  is limited by the upper stop  24 . 
     In the second design of the object of the present invention illustrated in FIG. 3, a sensor device  30  for sensing the movement of the media-separating element  2  is provided in the chamber  3  filled with gas. The sensor device  30  which is preferably configured as an inductive travel sensor, represents an assembly which is independent to handle and can be inserted into an opening in the cover  15 . The assembly is comprised of a two-part sensor housing  31  in which a coil  32  and a metallic pin  33  cooperating with the coil  32  are mainly arranged. The two-part sensor housing  31  is preferably composed of telescopically guided housing parts  34 ,  35 , and the part  34  close to the opening in the cover  15  takes up the coil  32 , while the second housing part  35  which partly embraces the first housing part  34  is supported on the plate  16  under the preload of a compression spring  36 . Fastened on the side of the second housing part  35  remote from the plate  16  is the above-mentioned pin  33  which is guided in the first housing part  34  and projects in part into a cylindrical chamber  37  designed inside the coil  32 . Electrical connections of the sensor device  30  are formed by the contact pins  38  which project from the sensor housing  31 . By means of a non-illustrated electronic evaluating unit connected to the electrical connections, the inductance of the coil  32  can be determined which changes in response to the depth of immersion of the metallic pin  33  into the cylindrical chamber  37  that is encompassed by the coil  32 . The position of the plate  16  and, from this, the fill condition of the pressure fluid accumulator of the present invention is determined from the measured inductance with the aid of characteristic curves stored in the electronic evaluating unit. Further, electrical measurement means (not shown) can be provided within the scope of the idea of the present invention, which means serve for measuring the electrical resistance of the coil  32  in addition to the measurement of the inductance, and the test value thereof is used to determine the temperature of the accumulator. 
     FIG. 3 shows also a modified design of the bottom valve  6 , whose closure member  40  is provided with two sealing elements  41 ,  42  arranged one behind the other in order to reduce the probability of failure. 
     As can be taken in particular from FIGS. 4 a  to  4   c  which represent the individual phases of the closing operation, the sealing elements  41 ,  42  which again are configured as sealing cups cooperate with two separated portions  43 ,  44  of a bore stepped several times (not shown in detail) that is provided in the hydraulic port  5 . The closing travels of the two sealing elements  41 ,  42  are preferably rated so that the sealing elements  41 ,  42  move into abutment on the associated bore portions  43 ,  44  offset in time. As becomes apparent from FIG. 4 b  in particular, when the closure member  40  is displaced by the above-mentioned plate  16 , the external sealing lip of the first sealing cup  41  is the first to come into contact with a first conical annular surface  45  following which is the associated bore portion  43 . The second sealing cup  42  is still at a distance from an associated second conical annular surface  46  so that the pressure fluid which propagates through a flow duct  47  provided in the hydraulic port  5  is applied to the first sealing cup  41 , and the closure member  40  is displaced further in the direction of the bottom stop by the effect of the pressure fluid. During the mentioned closing movement, the second sealing cup  42  first of all comes into contact with the conical annular surface  46  associated with it and finally, in the closing position (FIG. 4 c ) seals in relation to the associated bore portion  44 . 
     In a third design of the bottom valve illustrated in FIG. 5, flow cross-sections  27  are designed in a preferably cylindrical guide portion  26  and limited towards the outside by a sleeve  25  which forms the above-mentioned closing member. In the actuating direction of the bottom valve behind the flow cross-sections  27 , there is a sealing cup  28  which, after having been overridden by the sleeve  25 , seals in relation thereto and thus prevents further flow of pressure fluid. 
     Concludingly, it is to be noted that all embodiments of the bottom valve described hereinabove are easy to design and likewise permit simple and low-cost manufacture. The bottom valves can be installed as prefabricated, tested modules into hydraulic accumulators equipped with metallic bellows. The sealing elements or sealing cups are acted upon by pressure only in situations in which the sealing slot has adopted its final contour and will change no more. This function principle prevents the sealing elements from being damaged by parts of the sealing elements that are sheared off at metal edges. Another advantage includes that both the opened and the closed condition of the bottom valve is mechanically stable. The result is that transitions between an open and a closed condition of the bottom valve which are caused by an expansion due to temperature are avoided. Especially, no fluid is allowed to escape during storage of the pressure fluid accumulator when the pressure applied from outside is equal to zero.