Abstract:
A pressure tank, in particular hydraulic accumulator ( 3, 5 ), has a parting element ( 1 ) separating a space ( 11 ) for a first gaseous working medium from a space for a second fluid working medium in the tank. The parting element is flexible, can move under deformation and defines a domed main parting plane extending from an annular edge ( 13 ). The parting element ( 1 ) is produced from a substance having a fluoroplastic material, preferably a substance composed entirely of fluoroplastic material.

Description:
FIELD OF THE INVENTION 
     The invention relates to pressure tank, in particular to a hydraulic accumulator, with a resilient separating element movable with deformation and separating a space in the tank for a first, in particular gaseous working medium, from a space for a second working medium, in particular a fluid. The separating element defines a domed main separating plane extending from an annular edge. 
     BACKGROUND OF THE INVENTION 
     A pressure tank of this type in the form of a hydraulic accumulator is disclosed in DE 28 52 912 A1. The resilient separating element is formed of a rubber-like material (synthetic rubber, such as acrylic nitrile-butadiene rubber) in the known hydraulic accumulator and forms a membrane movable by deformation and separating the gas side from the liquid side in the accumulator housing. Two main demands must be imposed on the operating behavior of hydraulic accumulators with these membranes movable by deformation. First, the impermeability of the membrane must be ensured to prevent gas diffusion. Second, corresponding mechanical properties of the membrane are necessary, especially ease of movement and high cyclic bending strength to be maintained even under the influence of corrosive media. 
     In the aforementioned known hydraulic accumulator, these requirements are only partially satisfied. To improve the impermeability of the rubber-like membrane in the known accumulator, annular bead-like elevations project out of the main separating plane in tight succession. Because the elevations increase the average wall density, diffusion tightness is in fact improved. However the significant increase of wall thickness leads to considerable stiffening and accordingly to a deterioration of mobility. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a pressure tank, in particular a hydraulic accumulator, having improved operating behavior. 
     According to the invention, this object is basically achieved by a pressure tank having a separating element produced from a substance having a fluoroplastic material or formed preferably entirely of fluoroplastic material. Outstanding diffusion tightness is ensured, while a separating element is provided having mechanical properties that are optimum for use as a membrane in hydraulic accumulators, such as extreme cyclic bending strength. Therefore, very small wall thicknesses can be used leading to the desired ease of movement of the membrane. Based on the resulting good response behavior, the pressure tank is therefore especially well-suited for use as a pulsation damper. 
     Polytetrafluoroethylene has been found to be an especially suitable material. 
     Polytetrafluoroethylene (PTFE) due to its very high melt viscosity cannot be plastically molded. The desired molded article from this material is cold pressed from powdered raw material with 200 to 400 bar and is sintered unpressurized at 370° to 380°. If films are to be obtained, they are generally peeled off solid cylindrical blocks. Polytetrafluoroethylene therefore is commercially available in general in the form of rigid solid bodies such as slabs, rods, tubes, etc. One with average skill in the art in the field of membrane technology would find it surprising to obtain separating elements produced in whole or in part from polytetrafluoroethylene material and having high mobility such that they can even assume the function of a flexible rolling membrane. 
     Since PTFE materials can moreover have especially good chemical resistance, the pressure tank according to the invention is also suitable for use in the presence of chemically corrosive media. 
     In advantageous embodiments, the separating element defines a domed main separating plane on whose side lying inside relative to the dome, annular bead-like elevations project. By using a membrane domed in this way in the pressure tank, a separating wall with a comparatively large area is available that is easy to deform and can effect a comparatively large change of volume of the bordering working spaces in the pressure tank. 
     In preferred embodiments, succeeding elevations are separated from one another by flat wall sections extending along the main separating plane. Between adjacent elevations, one free space at a time is available for relative movements of adjacent elevations so that without annular beads bordering one another, mutually supporting one another and stiffening the structure, the separating element can undergo deformation as a rolling membrane. 
     Preferably, the peaks of the annular bead-like elevations have a round dome so that notch effects are avoided. 
     In especially advantageous embodiments, the annular bead-like elevations are formed by folds open on the outer side and forming annular groove-like depressions in the main separating plane. According to the height of the folds in a membrane made in this way, similarly to a bellows, an especially great length of the movable material strip is available to roll up or pull out the membrane. 
     Preferably, the arrangement is made such that the height of at least one fold measured from the open end to the peak of the folds along its vertical axis is different relative to the height of other folds. 
     Especially good mechanical properties are obtained when the first fold nearest the annular edge has a smaller height than the other adjoining folds. 
     In this respect, it is also advantageous if the wall section extending from the annular edge to the nearest first fold has a wall thickness at the annular edge that has the largest value. The wall thickness then decreases toward the first fold to the value of the wall thickness of the wall sections between the folds. The edge thickening formed in this way, without adversely affecting the resilience of the remaining membrane, enhances the clamping of the membrane on the housing element of the pressure tank and the formation of a seal connection at the clamping site. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings which form a part of this disclosure: 
         FIG. 1  is a partial side elevational view, schematically simplified, in section of a pressure tank according to one exemplary embodiment of the invention in the form of a hydraulic accumulator, with only the region of the bottom part of the housing and the bordering part of the top part of the housing being shown; 
         FIG. 2  is a partial side elevational view in section of only the separating element of the exemplary embodiment from  FIG. 1 , which element is made as a rolling membrane, and which section is shown as one half side and enlarged compared to  FIG. 1 ; and 
         FIG. 3  is a partial side elevational view of the region identified with III in  FIG. 2  which has been further enlarged compared to  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The exemplary embodiment of the pressure tank according to the invention is shown in the form of a hydraulic accumulator.  FIG. 1  shows merely the bottom part  3  of the housing with a bottom-side fluid connection  9  concentric to the longitudinal axis  7  of the housing. A piece of the top part  5  of the housing borders the bottom part  3  of the housing. At the connection site between the bottom part  3  of the housing and the top part  5  of the housing the open, annular edge  13  of the separating element is clamped tight in the form of a rolling membrane  1 . The thickened edge  21  of the rolling membrane  1  is supported on an annular surface  22  of the bottom part  3  of the housing and adjoins an O-ring  15  sitting in an annular groove  20  on an axially projecting annular body  14  of the top part  5  of the housing. 
       FIGS. 1 and 2  show the roll membrane  1  in the completely unrolled or extended state, in which the space  11  located above the membrane  1  in  FIG. 1 , the gas side of the hydraulic accumulator, has the largest volume. No fluid pressure is at the fluid connection  9  allowing the membrane  1  to lie against the inside wall of the bottom part  3  of the housing. A central reinforcing bead  29  of the membrane  1  overlaps the edge of the fluid connection  9  to form a mechanical safeguard against the membrane  1  being pressed into the fluid connection  9  when fluid pressure is absent. 
       FIGS. 2 and 3  illustrate more details of the rolling membrane  1  produced from PTFE material. Due to the very good diffusion tightness of the PTFE material and especially good strength properties, the rolling membrane  1  merely needs a small wall thickness of the membrane as it emerges from the annular edge  13 . This membrane  1  defines the domed main separating plane. Successive annular bead-like elevations project to the inside from this main separating plane and are formed in the illustrated example, not by beads in the form of solid bodies, but by folds including the first fold  17  nearest the edge  13  and adjoining folds  19 . As is apparent from  FIG. 1 , proceeding from the thickened wall  21  on the annular edge  13 , the membrane wall thickness changes such that the wall thickness decreases as far as the first fold  17  to the thickness value of flat wall sections  23  each located between the folds  17  and  19 . In a practical embodiment, the wall thickness decreases from the thickening  21  to the first fold  17  from a value of 1.2 mm to a value of 0.5 mm of the succeeding wall section  23  between the folds  17  and  19 . As  FIG. 2  likewise shows, the thickened edge  21  on the inside forms a type of shell shape forming a partial enclosure of the O-ring  15  (not shown in  FIG. 2 ). 
     As can likewise be recognized from  FIG. 2 , the fold height measured along the vertical axis  25  for the first fold  17  is smaller than for the succeeding folds  19 . Each fold  19  has the same height. All folds  17  and  19  are domed at their peak. The folds  17  and  19  are open on the side that is the outer side relative to the dome. Annular groove-like depressions  27  (see in particular  FIG. 3 ) are formed, with each forming interruptions in the course of the dome of the main separating plane between the wall sections  23 . As can be recognized especially from  FIG. 3 , the inside width of the annular groove-like depressions  27  on the open end of the folds  17 ,  19  is much smaller than the fold height measured along the vertical axis  25 . In this example, the height of the folds  19  is larger approximately by a factor of 4. The fold heights are less than the width of wall sections  23  between adjacent folds such that the folds do not border one another, support one another or stiffen the separating element. 
     As is likewise apparent from  FIG. 3 , the insides of the depressions  27  of the folds  17 ,  19  extend slightly diverging toward the open end so that the open end of the depressions  27  has a greater width than the base of the depressions  27  on the peak region of the folds. As  FIG. 2  shows, the vertical axes  25  of the folds  19  each extend in roughly the vertical direction to the tangential plane relative to the adjacent wall sections  23 . The vertical axis  25  of the first fold  17  extends slightly tilted to this tangential plane, with the vertical axis  25  of the first fold  17  enclosing an angle of approximately 10° with the plane of the annular edge  13 . For the succeeding folds  19 , the vertical axes  25  from fold to fold are tilted increasingly more steeply to the plane of the edge  13 . 
     In this example, the annular bead-like elevations projecting on the inside of the membrane  1  are formed by folds  17  and  19 , as a result of which especially easy mobility for rolling up the membrane results. Also, annular bead-shaped elevations can be made as solid bodies. Unfilled PTFE materials can be used, or those with a filler and/or filler combinations as can be provided conventionally for PTFE materials, for example, when extreme temperature resistance or other special properties are desirable. Glass fiber materials, carbon, or metallic fillers can be considered, among other materials. 
     Semifinished articles of PTFE materials are available in many forms, for example, films peeled off blocks, solid bars, round blanks, and the like. Based on the mechanical properties, finished products, such as the rolling membrane used in the pressure tank according to the invention, can be produced by cutting from molded bodies. These bodies are pressed and sintered from powdered raw material. In particular, for thin-walled articles, however, shaping by blow molding of a PTFE dispersion before sintering is possible. If the spherical membrane shape shown in  FIG. 1  is obtained from a solid polytetrafluoroethylene body, it can then be brought into the illustrated shape of the separating membrane by cutting of the raw body. To minimize the polytetrafluoroethylene scrap forming in the cutting process, preferably a preform body as the blank can be produced in a half shell shape as a mold. 
     The polytetrafluoroethylene material as a fluoroplastic material can comprise both pure PTFE and modified PTFE, and can include both unfilled PTFE and PTFE compounds. For a modified PTFE material, fillers such as bronze, carbon dust, MoS 2 , as well as glass fiber and carbon fiber materials are possible. In addition to PTFE, as other fluoroplastic materials the following can be used: ethylene tetrafluoroethylene (ETFE), ethylene chlorotrifluoroethylene copolymer (ECTFE), polychlorotrifluoroethylene copolymer (PCTFE), perfluoroalkoxy copolymer (PFA), polyvinylidene fluoride (PVDF) and tetrafluoroethylene perfluoropropylene (FEP). 
     While one embodiment has been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the appended claims.