Abstract:
A hydraulic device includes a housing and a closure element arranged thereon. The closure element is connected to the housing by torsional welding.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/068163, filed on Oct. 18, 2011, which claims the benefit of priority to Serial No. DE 10 2010 063 318.6, filed on Dec. 17, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The disclosure relates to a hydraulic device, in particular a low-pressure accumulator, having a closure element, and to a method for producing a hydraulic device, in which method a connection of a closure element to a housing of the hydraulic device is realized. 
     Known hydraulic devices have cavities in which hydraulic fluids are situated during the operation of the hydraulic device and which must be closed off with respect to the surroundings of the hydraulic device. Such cavities often serve as pressure accumulators, in particular low-pressure accumulators, on a hydraulic device. Furthermore, piston pumps as hydraulic devices have a piston which is guided in an axially movable manner in a cylinder liner inserted into a cylinder bore of a pump housing. A closure element in the form of a cover is arranged on the cylinder liner of the cylinder bore and closes off the latter in a pressure-tight manner. The closure element is often also a constituent part of other components of a hydraulic device, in particular of an accumulator, of a valve, of a hydraulic connector and/or of a motor. 
     The closure element is normally inserted into the housing of the hydraulic device using known connecting techniques. Here, the closure element is calked or is arranged in the housing by means of an additional element, for example a circlip, or by way of connecting means such as screws. It is also known for the closure element to be attached by means of the so-called “self clinch” connecting technique. 
     Here, known connecting techniques are relatively expensive and may be difficult to seal off over the service life of the hydraulic device. Finally, said connecting techniques have a certain space requirement, which increases the overall dimensions of the hydraulic device. 
     SUMMARY 
     According to the disclosure, a closure element is connected to the housing of a hydraulic device, in particular of a low-pressure accumulator on a piston pump used for a vehicle brake system of a motor vehicle, by torsional welding or torsional friction welding by means of a sonotrode. Here, by means of plastic deformation of the material of the two components closure element and housing, a fluid-tight, in particular gas-tight cohesive connection of the closure element to the housing is realized. It is thus achieved that the closure element is non-detachably connected to the housing in an extremely small space. In the case of torsional welding, it is in particular the case that the sonotrode penetrates plastically into the closure element and sets the latter in high-frequency vibration (in particular by ultrasound). 
     With the method of torsional welding used according to the disclosure, the leak-tightness of the connection is ensured over the entire service life of the hydraulic device. Owing to the non-detachable connection, it is for example also the case that the problem of the loosening of a screw connection as a result of vibration loading during operation can no longer arise. Furthermore, the method according to the disclosure results in a space saving, because for example O-rings for sealing and also support rings and threads are no longer required, and/or no material need be made available for a calking process involving deformation. As a result, the production costs for the production process according to the disclosure can be reduced, because components and material can be saved and because lower demands can be placed on the tolerances of the components used, in particular the closure element. Overall, it is possible to save on material for the housing because the axial forces during torsional welding (for example 1 kN to 10 kN) are lower than those that arise in particular during calking. 
     According to the disclosure, an interface is provided in the housing by virtue of a cylindrical bore being formed by means of a chip-removing process, which cylindrical bore may then in particular also serve as a piston running surface for an accumulator piston. Said bore has a first chamfer, that is to say a beveled surface at its edge for transition to an abutment shoulder, which merges into a second chamfer. Said second chamfer in turn merges into a transition radius of the outer surface of the housing. Here, the depth of the axial abutment surface is preferably between 0.5 mm and 1.0 mm, and is in particular 0.7 mm. The first chamfer has an angle of approximately 30° with respect to the longitudinal axis of the closed bore, and the second chamfer has an angle of approximately 40° to 45°. 
     The centering of the closure element with respect to the housing is realized by means of the second chamfer in interaction with in particular the outer edge of the closure element. The second chamfer thus also serves as an assembly aid if, for the connection of the closure element to the housing, the closure element is mounted onto the interface on the housing. Here, the second chamfer need not extend over the entire circumference of the closure element, but rather may be omitted in regions, in particular in two opposite side regions of a cuboidal housing with the opposite side surfaces thereof. It is sufficient for the closure element to be centered in the remaining region of its contact surface against the housing. The outer edge of the closure element may thereby nevertheless be centered over the region of the thickness or plate thickness of the housing. It is important merely that the closure element makes contact over its entire circumference with the housing, in order to be torsion-welded over said circumference. 
     After the mounting of the closure element, the connection to the housing is produced by means of the stated torsional welding technique using a sonotrode of a torsional welding plant. 
     Here, the sonotrode is designed such that, through the axial exertion of pressure on the closure element, positive and non-positive locking is generated between the geometry of the sonotrode and the closure element. The surface of the closure element thus takes on the negative form of the punch, because the material of the closure element is plastically deformed by the action of the sonotrode. Here, the sonotrode is particularly advantageously of corrugated design, such that, through plastic deformation, the closure element exhibits positive and non-positive locking with the housing. 
     The connection of the material of the closure element to the base material of the housing is then realized by virtue of the sonotrode being excited at high frequency (approximately 10 kHz to 20 kHz) and thus, in the natural frequency range of the sonotrode, a suitable amplitude (approximately 30 μm to 50 μm) being generated which leads to a connection of said type. 
     In the case of existing housings of hydraulic assemblies for vehicle brake systems, the accumulator diameter of an associated accumulator, in particular of a low-pressure accumulator, is a size determinative of the plate thickness of the housing. It was thus hitherto necessary for the plate thickness to make available the material that was necessary for previously conventional calking processes for the fastening of a closure element. For example, in a conventional calking process, it was necessary for enough material to be applied to the closure element in order to satisfy the static and dynamic pressure demands of up to 500 bar. 
     In the case of the plate thickness of the housing according to the disclosure, it is no longer necessary for said material for deformation, which was conventionally required, to be made available. 
     In the case of the connection according to the disclosure, only one step, in particular with the stated second chamfer and the transition radius, is required at the interface. The ratio between the plate thickness and the effective diameter of the closure element and of an associated component, such as in particular an accumulator, can thus advantageously be reduced. Furthermore, the housing can be designed so as to be of smaller height, because no material needs to be made available for calking or for the arrangement of a circlip. 
     Furthermore, the axial loading and thus undesired deformations of other components or the housing interfaces thereof are reduced. Here, it is particularly advantageous that good functionality with regard to leak-tightness and stability is attained with the connection according to the disclosure, wherein the housing simultaneously has a lower operating weight requirement (preferably predominantly aluminum), and thus material is also saved. Furthermore, as a result of the cohesive connection according to the disclosure, a noise reduction in particular is attained in the case of a damper or accumulator. 
     The connection of a closure element to a housing by means of the method according to the disclosure may be used for a multiplicity of components, in particular for a piston chamber of a low-pressure accumulator. It is also preferable according to the disclosure for these to be components of a high-pressure damper chamber. 
     The connection according to the disclosure is preferably of gas-tight, in particular helium-tight form. 
     Furthermore, the strength of the connection preferably corresponds to at least the minimum tensile strength of the two materials of the parts to be connected, which simplifies a configuration on the basis of the requirements. In particular, the load capacity is defined by the connecting surface area multiplied by the minimum strength of the material. 
     For the connection according to the disclosure, use may preferably be made of a known welding plant which is suitable for torsional welding. Here, said welding plant has a converter with opposed piezo actuators and a vibration converter which imparts the force via a booster to the sonotrode and thus to the part to be welded, which is clamped into the lower part of the welding plant. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the solution according to the disclosure will be explained in more detail below on the basis of the appended schematic drawings, in which: 
         FIG. 1  shows an axial longitudinal section of a pump housing during the step of the pressing-on of a closure element, as per the prior art, 
         FIG. 2  shows a plan view of a pump housing of a piston pump according to the disclosure, 
         FIG. 3  shows a view from below of a sonotrode for torsional welding on the pump housing as per  FIG. 2 , 
         FIG. 4  shows the section IV-IV in  FIG. 2  in a production step “chip-removing drilling”, 
         FIG. 5  shows the section V-V in a production step “chip-removing drilling”, 
         FIG. 6  shows the view as per  FIG. 4  in a production step “mounting of the cover”, 
         FIG. 7  shows the view as per  FIG. 4  in a production step “welding of the cover”, 
         FIG. 8  shows the view as per  FIG. 5  in the production step “welding of the cover”, 
         FIG. 9  shows the detail IX in  FIG. 8 , and 
         FIG. 10  shows a variant of the detail X in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates the connection of a closure element  7  in the form of a cover to a pump housing  10  (in the present case a housing of a hydraulic device in the form of a piston pump) by pressing by means of a press plunger  22 , as per the prior art. 
     The figure shows the pump housing  10 , a cylindrical bore  12  formed therein, a helical spring  14  arranged in the bore  12 , and an accumulator piston  16  resiliently preloaded by the helical spring  14 . The accumulator piston  16  forms, together with the helical spring  14 , an accumulator or an accumulator arrangement for the piston pump which is formed by the pump housing  10  and which is not illustrated in any more detail. 
     The closure element  7  is held, by means of a holding plunger  24 , on a stepped abutment  26  which is formed in the interior of the cylindrical bore  12 . The press plunger  22  is designed such that, through the exertion of axial pressure on the edge of the cylindrical bore  12 , material of the pump housing  10  is displaced over the outer edge region of the closure element  7 . 
     Here, in order to be able to attain fluid-tight sealing over the entire circumference of the closure element  7 , it is important that material is displaced over the edge region of the closure element  7  likewise over the entire circumference thereof. The pump housing  10  is thus designed to be so thick or broad with regard to its thickness  18  that sufficient material for deformation is available even on side surfaces  20  of the pump housing  10 . 
     The piston pump designed in this way with a pump housing  10  and a closure element  7  on the associated accumulator is designed for delivering fluids in particular for a hydraulic motor vehicle brake system in motor vehicles. Said piston pump serves in particular for selectively decreasing or increasing the brake pressure in the wheel brake cylinders within the context of a traction control system. 
     The piston pump according to the disclosure may however also be used, for example, as a high-pressure fuel delivery pump for an internal combustion engine, wherein the fuel enclosed in a delivery chamber is compressed. 
       FIG. 2  shows a plan view of a pump housing  10 , according to the disclosure, of a piston pump  34 . On the pump housing  10  there is formed an interface for the mounting and attachment of a closure element  7 , said interface being shown in more detail in sectional views in  FIGS. 4 and 5 . The interface forms a support for the closure element  7  for the subsequent connection of said closure element in a cohesive and in particular also leak-tight manner to the pump housing  10  by means of a torsional welding process. 
       FIG. 3  shows a view from below of a corrugated sonotrode  28  of a torsional welding plant (not illustrated in any more detail). Said sonotrode  28  generates the cohesive connection between the closure element  7  and pump housing  10  by means of plastic deformation. 
       FIG. 4  illustrates a first step of the method according to the disclosure for the production of a connection between a closure element  7  and a pump housing  10 . For this purpose, there is likewise formed in the pump housing  10  a cylindrical bore  12  which then serves as a piston surface for an accumulator piston  16 . On the outer edge of the cylindrical bore  12  there is formed a first chamfer  2 , that is to say a beveled surface for a transition to an abutment shoulder  3 , which merges into a second chamfer  4 . The second chamfer  4  merges into a transition radius  5  on the top side  21  of the pump housing  10 . The depth of the abutment shoulder  3  relative to the top side  21  is approximately 0.7 mm. 
       FIG. 5  illustrates the form of said edge configuration at the side surfaces  20  of the pump housing  10 . In said lateral region of the pump housing  18 , the latter is formed so as to be so thin or narrow that only the first chamfer  2  and the abutment shoulder  3 , but not the second chamfer  4 , exist over the entire circumference of the cylindrical bore  12 . As can be seen in particular in  FIG. 4 , the pump housing  10  may thus be formed with a particularly thin or narrow thickness  8 . 
       FIG. 6  shows, in an axial section, the next step for the production of the connection of pump housing  10  and closure element  7 . Here, the closure element  7  is placed onto the abutment shoulder  3  and, in the process, is centered with the outer radius  6  of the closure element  7  on the transition radius  5  and the second chamfer  4 . Said centering by means of the transition radius  5  and the second chamfer  4  is preferably not realized at the edge regions at the side surfaces  20 , because no second chamfer  4  is provided there. 
       FIGS. 7, 8 and 9  show the positively locking connection by means of torsional welding in a third step in which the welding itself takes place. For this purpose, a pressure force is applied axially to the closure element  7  by means of the sonotrode  28 , and the sonotrode  28  is simultaneously excited at high frequency by means of a torsional welding plant which is not illustrated in any more detail. In this way, the material at the lower edge region of the closure element  7  connects to the material on the abutment shoulder  3  of the pump housing  10 . The material which fuses on or melts on in the process is indicated in  FIG. 7  and  FIG. 9  by reference numeral  30 . 
     At the same time, the negative form of the sonotrode  28  is impressed on the top side of the closure element  7 , because said sonotrode is in particular of corrugated form on its contact surface facing the closure element  7 . 
       FIG. 10  shows a variant of the connection of the closure element  7  to the pump housing  10  and the cylindrical bore  12  thereof. The figure shows the chamfer  2  at the top edge of the cylindrical bore  12 , and a step  32  which is formed on the closure element  7  and which projects into the cylinder bore  12  and, in so doing, covers the chamfer  2  in the axial direction of the bore  12 . With said step  32  on the closure element  7 , it is achieved that chips formed at the edge of the bore  12  during the formation of the torsional weld are retained in the chamfer  2  and cannot enter into the bore  12  and thus into the associated hydraulic system. 
     All of the features presented in the description, in the following claims and in the drawings may be essential to the disclosure both individually and also in any desired combination with one another.