Abstract:
A vehicle brake system has a gas pressure accumulator ( 10 ) comprising a housing ( 12 ), the interior of which is divided by metal bellows ( 16 ) into a gas-sensed gas chamber ( 20 ) and a fluid chamber ( 22 ). Via a feed line ( 24 ) a fluid may be supplied under pressure to and removed from the fluid chamber ( 22 ), wherein provided between the fluid chamber ( 22 ) and the feed line ( 24 ) is a valve arrangement ( 74 ), which closes when the pressure in the feed line ( 24 ) drops below a minimum value and opens when the pressure exceeds the minimum value. To increase the operational reliability of the gas pressure accumulator ( 10 ), the valve arrangement ( 74 ) closes when the pressure in the feed line ( 24 ) exceeds a maximum value and opens when the pressure drops below the maximum value.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/EP00/10809 filed Nov. 2, 2000, which claims priority to German Patent Application No. 19954326.7 filed Nov. 11, 1999, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a vehicle brake system having a gas pressure accumulator, which comprises a housing, the interior of which is divided by metal bellows into a gas-filled gas chamber and a fluid chamber, to which via a feed line a fluid may be supplied under pressure and removed. In said case, disposed between the fluid chamber and the feed line is a valve arrangement, which closes when the pressure in the feed line drops below a minimum value and opens when the pressure exceeds the minimum value. The invention further relates to such a gas pressure accumulator. 
     The fluid chamber of such gas pressure accumulators is filled, during operation of the vehicle brake system, counter to the pressure in the gas chamber partially or completely with brake fluid, in order to store the latter. 
     From DE 39 01 261 A1 a pressure accumulator for hydraulic systems is known, which comprises a housing, the interior of which is subdivided by two metal bellows into a gas chamber and a fluid chamber, wherein the latter is connected to the hydraulic system by a valve actuated by metal bellows. In said case, a valve body is fastened by a retaining body to one of the metal bellows, which moves the valve body onto a valve seat when the maximum admissible quantity of fluid has been removed from the fluid chamber. The valve body therefore closes the fluid chamber. With said valve it is possible merely to ensure that the metal bellows are not damaged in the event of a further drop of pressure in the hydraulic system. DE 39 01 261 A1 does admittedly provide a so-called accumulator charging valve but its function and mode of operation are not explained in said printed publication. 
     Particularly high standards are demanded of vehicle brake systems with regard to the operability and reliability of the equipment. 
     The object of the invention is therefore to overcome the previously described drawbacks and design the vehicle brake system having a gas pressure accumulator in such a way that the gas pressure accumulator remains operable even in the event of failure of another device of the vehicle brake system (e.g. the accumulator charging valve described in DE 39 01 261 A1). 
     SUMMARY OF THE INVENTION 
     Solution According to the Invention 
     The object is achieved according to the invention by a vehicle brake system of the type described initially having a gas pressure accumulator, in which the valve arrangement closes when the pressure in the feed line exceeds a maximum value and opens when the pressure drops below the maximum value. The object is further achieved by such a gas pressure accumulator. 
     By virtue of the design according to the invention the pressure in the fluid chamber of the gas pressure accumulator is limited to a maximum value, with the result that the metal bellows themselves remain operable even in the event of extremely high pressure in the feed line. The valve arrangement in said case performs a dual function. It closes the fluid chamber when the pressure in the feed line is below minimum pressure or above maximum pressure and opens it when the pressure is between minimum and maximum pressure. 
     An advantageous development of the gas pressure accumulator provides that the metal bellows during supply and removal of the fluid executes a stroke motion, by means of which the valve arrangement is actuated. Thus, closing of the fluid chamber is linked directly to the motion of the metal bellows, with the result that a self-contained safety system is formed. 
     In a first advantageous refinement of the invention, the valve arrangement comprises a piston, which is provided on the metal bellows and may be displaced along an axis between two sealing seats, which are arranged axially spaced-apart inside a hollow cylinder provided on the housing. In an alternative advantageous refinement, the valve arrangement comprises a hollow cylinder, which is provided on the metal bellows, is directed along an axis, has two internally disposed, axially spaced-apart sealing seats and is disposed in an axially displaceable manner around a piston provided on the housing. In said refinements the dual function of the valve arrangement is realized in a particularly simple manner. 
     An advantageous development of the sealing seats provides that the latter comprise paraxial sealing surfaces. The piston may slide against said sealing surfaces during axial displacement of the piston and/or of the hollow cylinder. The piston in said case retains the sealing function. As a result of elasticity or thermal expansion the volume of the gas and the fluid may change. The housing or the valve arrangement may moreover deform. In said case, the volume of the gas chamber and fluid chamber is slightly altered. In the case of the braking equipment according to the invention, the metal bellows are displaceable along the sealing surfaces and hence may compensate the differential pressures, which arise, without being damaged. 
     There is advantageously connected to the hollow cylinder a coaxial mandrel, on which the piston is guided or which is guided in the piston. By said means a guided movement of the piston relative to the sealing seats is possible and, at the same time, a compact form of construction of the gas pressure accumulator is achieved. 
     According to a development, the metal bellows are substantially in the shape of a hollow cylinder and the piston as well as the hollow cylinder are disposed radially inside the metal bellows, with the result that a particularly compact form of construction is achieved. 
     An advantageous refinement provides that the stroke motion of the metal bellows is delimited by two end stops in order to select defined end positions for the movable components. In the end positions the valve arrangement is in both cases closed. 
     A seal or a sealing seat is advantageously formed on at least one end stop. On the end stop, therefore, a redundant seal is formed, which enables particularly good sealing. In a particularly advantageous manner the redundant seal is disposed on the end stop delimiting the normal position of the piston. Thus, the gas pressure accumulator is sealed particularly well when the pressure in the feed line is lower than the admissible minimum pressure. The pressure in the feed line, the so-called system pressure of the vehicle brake system, may drop below said minimum pressure, the so-called gas admission pressure, especially during extended stationary periods of the vehicle. 
     The gas pressure accumulator may alternatively be provided with a valve arrangement, which is provided with at least one redundant seal on an end stop but does not have the dual function described above. Given such a valve arrangement, the piston as closing element during a closing motion first contacts a first sealing seat and effects sealing there. Then the piston contacts a second sealing seat, which forms an end stop for the closing element, and effects redundant sealing there. The first sealing seat may correspond to one of the sealing surfaces described above. 
     To guarantee the necessary sealing of the valve arrangement, at least one seal is advantageously disposed on the piston and may effect sealing against at least one sealing seat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and properties are explained in the description of two embodiments with reference to the accompanying drawings. 
     FIG. 1 shows a first embodiment of a gas pressure accumulator according to the invention in longitudinal section. 
     FIG. 2 shows a second embodiment of a gas pressure accumulator according to the invention in longitudinal section. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A gas pressure accumulator  10  illustrated in FIGS. 1 and 2 comprises a can-shaped housing  12 , which is closed by a cover  14 . The interior of the housing  12  is divided by metal bellows  16 , which adjoin the cover  14 , and by a disk  18  fastened in a gastight manner thereto into a gas chamber  20  and a fluid chamber  22 . The gas chamber  20  is filled with a pressurized gas. The cover  14  is penetrated by a feed line  24 , through which a fluid is supplied to the fluid chamber  22  when the pressure in the feed line  24  rises. The fluid is stored in the fluid chamber  22  and removed from the latter when the pressure in the feed line  24  drops. 
     The housing  12  has a cylindrical outer wall  26  with a longitudinal axis  28 . Adjoining the outer wall  26  is a disk-shaped end wall  30 , formed coaxially in which is a threaded bore  32 , through which the gas may be supplied at a so-called gas admission pressure into the gas chamber  20 . The threaded bore  32  is closed by a screw plug  34 , which rests against a sealing washer  36 . 
     The cover  14  has a disk-shaped closing portion  38 , which by means of a shoulder  40  formed on the circumference thereof is centred in and supported against the outer wall  26  of the housing  12 . The disk-shaped closing portion  38  is connected in a gastight manner to the outer wall  26  by a weld seam  42 . 
     In the embodiment illustrated in FIG. 1, a hollow cylinder  44  and a mandrel  46  are integrally formed coaxially on the side of the closing portion  38  directed towards the interior of the housing  12 . Integrally formed coaxially on the outside of the closing portion  38  is a connection  48 , which is connected by substantially axially directed bores  50 ,  52  and  54  to the interior of the housing  12 . 
     The metal bellows  16  are folded, substantially cylindrical and connected at both axial ends by weld seams  56  and  58  in a gastight manner to the closing portion  38  and the disk  18  respectively. 
     The disk  18  is directed normally to the axis and integrally connected to a coaxial rod  60 , formed in which is an axial bore  62 , by means of which the rod  60  is guided on the mandrel  46 . Integrally adjoining the rod  60  is a piston  64 , the diameter of which is greater than that of the rod  60 . 
     Formed on the inner periphery of the hollow cylinder  44  are two axially spaced-apart paraxial sealing surfaces  66  and  68 , which are axially aligned and each form a sealing seat. Axially between the sealing surfaces  66  and  68  a recess  70  is formed in the inner periphery of the hollow cylinder  44  so that the diameter of the latter in said region is greater than the diameter of the sealing seats on the sealing surfaces  66  and  68 . 
     The piston  64  has a circumferential groove, in which a seal  72  in the form of a sealing ring is inserted or injected. The seal  72  is designed in such a way that it cooperates with the sealing surface  66  or  68  and hence forms a valve arrangement  74 , which may effect dual sealing in a fluid-tight manner. 
     FIG. 1 shows the metal bellows  16  in a position, in which virtually no fluid is stored in the gas pressure accumulator  10 , i.e. the pressure in the fluid chamber  22  has reached its minimum value, the gas admission pressure. The piston  64  in said case is situated almost in a normal position, in which the seal  72  rests against the sealing surface  66  and effects sealing there. Between the piston  64 , the hollow cylinder  44  and the closing portion  38  of the cover  14  a so-called admission chamber  76  is therefore created, which is connected only by the bore  52  to the connection  48  but is otherwise closed. The valve arrangement is therefore closed between the feed line  24  and the fluid chamber  22 . As no fluid may pass from the fluid chamber  22  into the admission chamber  76 , even in the event of a drop of the pressure at the connection  48  the pressure in the fluid chamber  22  remains constant and limited to the minimum value. The metal bellows  16  are therefore reliably protected from damage in the event of a pressure drop. 
     When the pressure at the connection  48  and/or the feed line  24  rises, the pressure in the admission chamber  76  is also increased and the piston  64  is moved axially, in relation to FIG. 1, upwards, wherein the metal bellows  16  are extended and the gas chamber  20  is reduced in size. In the region of the recess  70  the incoming fluid may in said case flow around the piston  64  and therefore acts directly upon the metal bellows  16  and/or the disk  18 . The rising fluid pressure moves the piston  64 , which is connected to the disk  18 , in said case virtually free of friction in the region of a stroke distance X, which corresponds to the operating stroke of the gas pressure accumulator  10 . Fluid may in said case pass through the bore  54  into the bore  62 , with the result that a pressure compensation occurs there. 
     If the pressure at the connection  48  continues to rise, at a so-called maximum pressure in the fluid chamber  22  the seal  72  of the piston  64  reaches the sealing surface  68  and effects sealing there. The piston  64  is situated almost in its end position and the valve arrangement  74  once more closes between the fluid chamber  22  and the feed line  24  and/or the admission chamber  76 . The metal bellows  16  are therefore protected from being damaged by excess pressure since no fluid may pass from the admission chamber  76  into the fluid chamber  22 . 
     At the sealing surfaces  66  and  68  the piston  64  may slide with the seal  72  along an axial stroke distance X 1  and X 2  respectively. During said stroke distances X 1  and X 2  the sealing is maintained, while a slight pressure compensation between the fluid chamber  22  and the admission chamber  76  is possible. In said manner it is possible to compensate elasticity and thermal expansion as described above. 
     To prevent the piston  64  from moving the seal  72  beyond the sealing surface  66 , in the—in FIG.  1 —axially bottom, inner end of the piston  64  a phase  78  is formed and on the closing portion  38  an end stop  80  is formed, which lies opposite the piston  64  and against which the piston  64  may rest in a defined manner. 
     Furthermore, in the region of said end stop  80  a seal  82  is inserted into the closing portion  38  and, together with an opposing sealing seat  84  formed on the piston  64 , forms a redundant seal of the piston  64  in the normal position. The seal  82  may alternatively be inserted in the piston  64 . 
     Formed on the inside of the end wall  30  is an end stop  86 , against which the disk  18  rests in the—in relation to FIG.  1 —top end position of the piston  64 . 
     FIG. 2 shows an embodiment of a gas pressure accumulator  10 , which is of a similar construction to the one illustrated in FIG.  1 . In said gas pressure accumulator  10 , however, the disk  18  is integrally connected to the rod  60  and a hollow cylinder  44 ′. The rod  60  is guided in an axially displaceable manner in a bore  54 ′ of the mandrel  46  and is penetrated by a bore  62 ′, which connects the bore  50  to the admission chamber  76 . At the end directed towards the hollow cylinder  44 ′ a piston  64 ′ is integrally formed with the mandrel  46 . 
     In said embodiment, during the stroke of the disk  18  the hollow cylinder  44 ′ is moved, while the piston  64 ′ remains stationary. Otherwise, the function of the valve arrangement is identical to that described above for FIG.  1 . 
     In contrast to the embodiment of FIG. 1, an end stop  80 ′ is formed on the piston  64 ′. Furthermore, there is disposed on the piston  64 ′ an axially directed seal  82 ′, which with an opposing sealing seat  84 ′ on the disk  18  forms a redundant seal of the piston  64 ′ in the normal position. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.