Patent Publication Number: US-6338612-B1

Title: Method for limiting the pressure provided by a hydraulic pump

Description:
The present invention relates to a process to control the hydraulic pressure provided by a hydraulic pump and a hydraulic pump with pressure control. 
     Processes and devices for controlling the hydraulic pressure provided by a hydraulic pump are known in the art. Typically, modern hydraulic pumps, particularly for power assisted steering applications, are assembled from modular subassemblies or pump components that are arranged relative to each other. The pumps are designed to produce a hydraulic pressure matching the system requirements and, in the line coming from the pump, or in a pump area subject to operating pressure, a pressure control valve is arranged. The design of this pressure control valve can vary. Typical are springloaded pressure control valves, which open in case of overpressure and may also carry oil in a bypass leading back to the reservoir. 
     Despite increasing miniaturization and a production and materials engineering related decrease in the costs of the corresponding components, particularly pressure control valves, said components continue to be a major cost factor in the production of hydraulic systems using hydraulic pumps. They are separate units that must be separately produced and separately installed. This substantially increases production costs, installation costs and overall system production costs. 
     On the other hand, such pressure control valves cannot currently be dispensed with since an undesirable overpressure in the hydraulic system may damage other components, which for cost reasons are all designed for minimum pressure ratios, or may produce substantial malfunctions. 
     Based on the described state of the art, the object of the present invention is to define a process and a hydraulic pump permitting pressure control on the one hand and elimination of separate and/or additional pressure control valves on the other hand. 
     To attain this object on the process side, the invention proposes a process to control the hydraulic pressure provided by a hydraulic pump that has at least two pump components that are connected in pressure tight manner by fastening means in order to form a hydraulically pressurized volume, characterized in that at least one of the pump components is arranged on another such that the former can be moved relative to the other pump component above a given hydraulic pressure within the pump interior to open the pressurized volume at least slightly. 
     The process according to the invention has the significant advantage of component reduction, which is expressed, in particular, by the elimination of conventional pressure control valves. The manufacturing method, however, hardly differs from the previous manufacturing method so that pure production cost savings result. 
     An exemplary design for a power assisted steering application of a hydraulic pump, which forms part of prior art, comprises a valve assembly, a pump body, a resonator chamber and a resonator cover. Together, these components are disposed within a housing providing an oil reservoir from which practically only the valve assembly protrudes for connecting the hydraulic intake and return lines. 
     According to the invention, the valve assembly, pump body, resonator and resonator cover are now fastened by means of screws for which force F is applied. The resonator cover is typically pressurized with the operating pressure. Starting from a certain pressure, the hydraulic force acting on the resonator cover becomes greater than the prestressing force of the bolts, which is adjusted to this pressure, so that gaps may form between valve assembly, pump body, resonator and resonator cover. A small volumetric oil flow passes through these gaps into the surrounding reservoir area, which creates the desired pressure control. 
     This object is attained in that the screws or bolts, the mechanical properties of which are all known and predefined, are tightened with a given torque. The elastic properties of the screw material together with the given torque makes it possible positively to predetermine the liftoff pressure or the pressure at which gaps are formed. 
     Until now, torques were selected to be high enough so that no gaps could form in the pressure areas where the pressure control valve had not already previously responded. For this reason static seals are inserted between the individual pump components. This practically prevents a volumetric oil flow. The pressure is controlled by means of conventional pressure control valves. 
     In contrast, the process according to the invention represents a significant improvement since, through the type of mounting provided, the conventionally assembled pump now becomes a self-contained pressure control system in which the holding forces are placed in direct relation to the internal pressure produced. 
     Independent of the pure fastening force, the type of the fastening elements can also exercise the same process function. It is quite feasible, for example, to assemble the individual pump components by means of clamp-type mechanical fastening elements, which for their part have sufficient elastic properties to exercise a pressure control function in accordance with the process in that they permit the formation of gaps starting from a specific hydraulic pressure within the interior of the pump components. Besides, gaps may form between two or also between several pump components. 
     The invention provides a completely novel hydraulic pump, which itself has means to control the hydraulic pressure provided, namely fastening means that permit relative motion of pump components above a given hydraulic pressure within the pump interior to open the pressurized volume at least slightly. 
     Advantageously, these means are the fastening bolts for the resonator cover, which are tightened with a given torque, whereby the mechanical properties of the bolts are given and defined. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the invention will become evident from the discussion below by means of the figures. The following show: 
     FIG. 1 a schematic partial section of an exemplary embodiment of a hydraulic pump; 
     FIG. 2 a section along line II—II according to FIG. 1; 
     FIG. 3 a top view onto the resonator cover; 
     FIG. 4 a sectional view along line IV—IV in FIG. 3; 
     FIG. 5 a top view onto a resonator chamber; 
     FIG. 6 a sectional view along line VI—VI in FIG. 5; 
     FIG. 7 a top view onto a pump chamber and 
     FIG. 8 a view of a valve assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The figures show an exemplary embodiment of a hydraulic pump. The entire hydraulic pump unit  1  comprises a valve assembly  2 , a pump housing  3 , a resonator  4  and a resonator cover  5 . Said components are enclosed by a reservoir housing  6  that is mounted to a corresponding cylindrical shoulder of the valve assembly using seals  10 . Reservoir housing  6  is filled with hydraulic fluid via cover  7  and simultaneously represents the hydraulic reservoir. 
     The valves  8 ,  9  serve for sucking in and pressing out the hydraulic fluid. The pump chamber  20  contains mechanical components wherein the hydraulic fluid is put under pressure. After leaving the pump chamber  20 , the hydraulic fluid passes through the resonator  4 , in order obtain an even flow. 
     FIG. 2 shows bolts  11 ,  12 ,  13  and  14  used to fasten resonator cover  5  to the resonator (not depicted), which is located underneath. 
     FIGS. 3 to  8  show individual details of said components. 
     The resonator cover, according to FIGS. 3 and 4, is provided with bores  16 ,  17 ,  18  and  19  into which said bolts  11 ,  12 ,  13 ,  14  are inserted. The resonator cover has a resonator chamber  15 , which is known per se. 
     The resonator itself guides the oil from the pump to the chambers in the cover and finally to the valve assembly. Resonator  4 , according to FIGS. 5 and 6, is provided with blind holes into which bolts  11 ,  12 ,  13 ,  14  are inserted. After assembly, blind holes  16 ′,  17 ′,  18 ′,  19 ′ are aligned with bores  16 ,  17 ,  18  and  19  of the resonator cover. 
     For fastening with the pump, bores are provided on the underside of the resonator, which align with bores  21 ,  22 ,  23 ,  24  of pump housing  3  when the units are assembled. In addition, the bores also align with bores  21 ′,  22 ′,  23 ′ and  24 ′ in valve assembly  2  as shown in FIG. 8 on a slightly reduced scale. 
     Thus pump housing  3  can be mounted on valve assembly  2  and thereupon resonator  4  and interconnected with bolts and screws (not depicted). In addition, the resonator cover can be fastened to the resonator by means of screws or bolts  11 ,  12 ,  13 ,  14 . 
     To implement the invention, it is advantageous to use said screws  11 ,  12 ,  13 ,  14 , which are fixedly defined with respect to their material properties, to fasten the resonator cover to the resonator using a given calculated torque. The calculation is based on the elastic and other material properties of the screws and takes into account the pressure ratios occurring in the lid. Since here the hydraulic medium is redirected from a straight flow, a corresponding internal pressure builds up, which subjects said screws  11 ,  12 ,  13 ,  14  to tensile stress. Due to the given torque and the elastic properties of the material, the resonator cover is lifted from the resonator starting from a certain pressure to form a gap, so that automatic pressure control is provided. When the pressure falls below that point, the resonator cover makes contact again and forms an adequate seal. 
     The same teaching can also be applied to the connecting screws of the lower components. 
     The exemplary embodiments described serve for illustration and shall not be construed as a limitation. In particular, other types of fastening elements or quite differently designed opening areas may be provided. For example, radial indentations may be formed as a function of the pressure, elastic clamps may be used, or even drive units for the relative displacement of housing parts, for example in the sense of a rocker or the like.