Abstract:
The invention relates to a hydraulic piston pump, particularly for a slip-controllable vehicle braking system. A piston pump according to the invention includes a hydraulically permanently permeable stopper, the throughflow of which is carried out as a function of the pressure in an outflow channel of the piston pump. A pressure medium flow only takes place if the pressure level in the outflow channel has exceeded a threshold value. The latter takes place, for example, if the kinematic viscosity of the flowing pressure medium decreases due to low ambient temperatures, or if a throughflow of the outflow channel is obstructed. By means of the proposed solution, excess pressure increases in the interior of the piston pump can be avoided, and the resulting loads for the pressurized pump components and the drive can be reduced. Otherwise, the operating behavior of the piston pump according to the invention corresponds to that of a known piston pump.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a 35 USC 371 application of PCT/EP2008/052506 filed on Sep. 19, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is based on a hydraulic piston pump, in particular for a vehicle brake system with electronic traction control. 
     2. Description of the Prior Art 
     Brake systems with electronic traction control are also known as ABS/TC/ESP brake systems. These brake systems, for regulating the brake pressure at the individual wheel brakes as a function of any wheel slip that might occur, have a hydraulic unit that is triggerable by an electronic control unit. This hydraulic unit includes a metal housing block, with hydraulic components secured to the block. The piston pumps on which the invention is based form a substantial part of these hydraulic components. They are needed for supplying pressure fluid inside the hydraulic system of a vehicle brake system. 
     From German Patent Disclosure DE 199 28 913 A1, a piston pump is already known. This known piston pump comprises a piston, a piston bush embodied as a cylinder, inlet and outlet valves, and sealing elements. The valves control the flow direction of pressure fluid through the piston pump. The inlet valve serves to cause the pressure fluid not to flow back to the intake side during a working stroke of the compression, while the outlet valve prevents a return flow of the pumped pressure fluid into the pump interior. Typically, the valves are embodied as spring-loaded ball valves. An outflow conduit of the piston pump is embodied between a closure stopper and the bottom of the piston bush. 
     The closure stopper closes a bore, which receives the piston pump, off from the environment. It is produced by either metal-cutting or non-metal-cutting shaping techniques; from an economic standpoint, non-metal-cutting shaping is attractive for high-quantity production. 
     The noise behavior can be varied by way of the geometry of the outflow conduit of the piston, pump. Typically, the outflow conduit therefore has a suitable taper, in order to establish a throttling action. By means of this throttling action, a hydraulic low-pass filter is created, which has a favorable effect on the noise. The behavior of the kinematic viscosity of the brake fluid in the range between 0° C. and 120° C. can be considered virtually constant, and the optimal throttling action is defined for that temperature range. 
     However, the kinematic viscosity of the pressure fluid changes sharply in the low-temperature range (−40° C. to 0° C.), which leads to a pressure increase in the interior of the piston pump and thus to an increased load on the pressure-impinged components of the piston pumps and on the entire drive of the piston pump. Dirt particles in the pressure fluid can also prevent an outflow of pressure fluid and cause a pressure increase. 
     ADVANTAGES AND SUMMARY OF THE INVENTION 
     A piston pump as defined by the invention has the advantage over the prior art that excessive pressure increases in the pump interior, with the attendant loads on the pressure-impinged components, are reduced. The mechanism proposed for this develops an action only as needed, or in other words when the pressure level in the interior of the piston pump has exceeded a threshold value. Below that threshold value, the proposed means are inactive, since because of their greater flow resistance, pressure fluid does not flow through them. The proposed mechanism requires no additional components, since it can be provided solely by means of a structural design of the components that are already present. This geometric design of the mechanism is variable on a use-specific basis and can advantageously not be defined until in the course of installation of the components. 
     Because of the mechanism provided according to the invention, the drive power required for driving the piston pumps is reduced. This in turn makes it possible to optimize the construction of these components. 
     It is also advantageous that the mechanism of the invention do not affect the flow conditions, particularly at the closing member of the outlet valve, of a known piston pump. This means that the closing member of a piston pump according to the invention opens in an unchanged manner in a preferred direction, and that a piston pump of the invention has unchanged good noise properties. 
     Further advantages or advantageous refinements of the invention will become apparent from the dependent claims or the ensuing description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are shown in the drawings and described in further detail in the ensuing description in conjunction with the accompanying drawings, in which: 
         FIG. 1 , in a longitudinal section, shows the outlet of a known piston pump; 
         FIG. 2  shows a closure stopper of a piston pump of the invention in a top view; 
         FIG. 3  shows a second exemplary embodiment of the invention; 
         FIG. 4  shows a third exemplary embodiment; and 
         FIG. 5  shows a fourth exemplary embodiment, in each case in a top view on the face end, toward a bush of the piston pump, of a closure stopper. 
     
    
    
     Components corresponding to one another in the various exemplary embodiments are identified by the same reference numerals in the various drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows the outlet region of a piston pump known from the prior art (see for instance DE 199 28 913 A1). The portion of a bush  10  can be seen as well as a closure stopper  12  on one face end of which the bush  10  rests. The closure stopper  12  and the bush  10  are inserted in a bore  14 , shown in suggested fashion, of a pump housing  11 , and the closure stopper seals off this bore from the environment. 
     The bush  10  is embodied hollow-cylindrically and has a bush bottom  16  on its end toward the closure stopper  12 . A through bore  18  is disposed centrally in the bush bottom  16 . The through bore discharges into a valve chamber  20 , which is embodied in the closure stopper  10 . A valve closing body  22  in the form of a ball is received movably in this valve chamber  20 . The valve closing body  22  is pressed by a valve spring  24  against a valve seat  26  on the face end of the bush  10 . The valve spring  24  is braced for that purpose, by its end remote from the valve closing body  22 , on the bottom of the valve chamber  20 . For centering the valve spring  24 , a projecting peglike protrusion  28  is integrally formed onto the bottom of the valve chamber  20 . An additional function of this protrusion  28  is to limit an opening stroke of the valve closing body  22 . 
     In the interior of the bush  10 , a piston, not visible in  FIG. 1 , is axially movably guided in the known way. This piston is driven to a reciprocating stroke motion counter to the force of a piston restoring spring  30 , also disposed in the interior of the bush  10  and braced on the bush bottom  16 . In the process, the volume of a work chamber  32 , defined between the piston and the valve closing body  22 , varies. 
     In this work chamber  32 , during the working stroke of the piston, the pressure rises until a pressure force caused by this pressure, which acts in an opening manner on the valve closing body  22 , is greater than the oppositely oriented force, acting in the closing direction, of the valve spring  24 . As soon as that is the case, the valve closing body  22  lifts from the valve seat  26 , and pressure fluid flows out of the work chamber  32  into an outflow conduit  34 . 
     The outflow conduit  34  is formed by a groove  35 , which in the exemplary embodiment is embodied as an example on the face end toward the bush of the closure stopper  12 . This groove  35  extends transversely to a longitudinal axis  36  of the closure stopper  12  and has its beginning in the valve chamber  20 . The face end of the closure stopper  12  is embodied by a flat countersunk region  38 . Because of this flat countersunk region  38 , the closure stopper  12  has an encompassing collar  40 , which surrounds the end of the bush  10  circumferentially. On the inside of this collar  40 , there is a recess  42 , oriented parallel to the longitudinal axis  36  of the bush  10 . This recess opens into the groove  35  and together with this groove  35  forms the outflow conduit  34 . The outflow conduit  34  is deflected once at a right angle at the transition from the groove  35  to the recess  42 . 
     The face end of the closure stopper  12  is provided with only a single groove  35 . The cross section of the groove is smaller than the cross section of the through bore  18  of the bush bottom  16 . These proportions result in a throttling action in the outflowing pressure fluid and thus an intended pressure rise in the valve chamber  20 . The pressure rise determines the flow conditions at the valve closing body  22 , in such a way that on lifting from its valve seat  26 , the valve closing body  22  executes a deflection motion oriented counter to the direction of the groove  35 . This deflection motion, because of the radial orientation of the groove  35 , always takes place in the same direction in space and thus defines a preferential position for the valve closing body  22  in the open state. Because of this preferential position, the pressure conditions of a piston pump can be better mastered. Moreover, a defined preferential position of the valve closing body  22  has a favorable effect on the noise behavior of the piston pump. 
       FIG. 2  shows a closure stopper  121 , embodied according to the invention, in a top view. Unlike the closure stopper  12  of  FIG. 1 , this closure stopper  121  has a first outflow conduit  34  and an additional, second outflow conduit  341 . The outflow conduits  34  and  341  extend to both sides of the valve chamber  20  and are opposite one another in aligned fashion. The two outflow conduits  34 ,  341  have flow cross sections of different sizes; the first outflow conduit  34 , which points upward in  FIG. 2 , has a cross section that is multiple times greater than the cross section of the second outflow conduit  341  that in  FIG. 2  points downward. Because of the law of least resistance, the second outflow conduit  341  having the smaller cross section does not have a flow through it until the pressure level in the first outflow conduit  34  has risen and exceeded a threshold value. This threshold value can be determined structurally by the ratio of the cross-sectional areas and by the choice of the cross-sectional shape of the outflow conduits  34 ,  341 . Below the threshold value, the pressure fluid flows virtually solely to the first outflow conduit  34 . Thus the second outflow conduit  341  forms a means for limiting the pressure, since it is permanently hydraulically passable, and the flow through it takes place as a function of the prevailing pressure level in the outflow conduit  34 . 
     A rise in this pressure level occurs for instance when because of falling ambient temperatures the viscosity of the pressure fluid and thus the flow resistance increase. A pressure increase would also be conceivable if dirt particles in the interior of the piston pump prevent a flow through the first outflow conduit  34 . With a flow through both outflow conduits  34 ,  341 , the pressure level in the interior of the piston pump, and thus the hydraulic load on the components subjected to pressure, are limited. Another reason why the two outflow conduits  34 ,  341  have different cross sections is so that the valve closing body  22  ( FIG. 1 ) will without change assume a preferential position, regardless of the pressure conditions upon lifting from the valve seat  26  ( FIG. 1 ), as has already been explained in conjunction with the description of  FIG. 1 . 
     The two outflow conduits  34 ,  341  are each embodied such that beginning at the valve chamber  20 , there is first a first throttling portion  44  with parallel groove flanks. This throttling portion  44  is adjoined radially outward by a respective second groove portion  46  that widens the cross section of the outflow conduits  34 ,  341 . The second groove portions  46  merge with recesses  42  on the inside of the encompassing collar  40  of the closure stopper  121 . This collar  40  may have a plurality of such recesses  42  distributed over, its circumference, in order to simplify the orientation of the bush  10  relative to the closure stopper  12  upon assembly of the piston pump. The protruding portions of the collar  40  that are located between the recesses  42  bring about centering of the two components relative to one another. 
       FIG. 3  shows a second exemplary embodiment of a second outflow conduit  342  of a closure stopper  122  of a piston pump. In this special embodiment, the flow cross section of the second outflow conduit  342  is blocked off by a crossing rib  482 . Before the assembly of the piston pump, this rib  482  can be shaped without metal cutting or removed with a suitable tool; such as a punch or a grinding tool. By way of a recess created in this way, the flow cross section of the second outflow conduit  342  can be adapted in shape and dimensions even during the production of the piston pump to the later conditions of use of a piston pump, and the pump characteristics can thus be determined. 
       FIG. 4 , in a third exemplary embodiment, shows a further possibility of an embodiment of the outflow conduit  343  on a closure stopper  123 . In this exemplary embodiment, the second outflow conduit  343 , in a distinction from the examples of  FIGS. 2 and 3 , is not connected to the valve chamber  20  via its own throttling portion; instead, it is connected to the throttling portion  44  of the first outflow conduit  34  via semicircular first and second curved portions  503  and  523 . The two curved portions  503  and  523  extend with radial spacing around the valve chamber  20  and together form a closed ring. The curved portions  503 ,  523 , for determining the pump characteristics, may have the same conduit cross section or may be embodied with different cross sections. The provision of only a single curved portion may even suffice under some circumstances. The curved portions  503  and  523  are especially simple to produce by non-metal-cutting shaping technology. 
     In the exemplary embodiment of  FIG. 5 , a bursting throttle restriction  544  is provided in the second outflow conduit  344 , between the valve chamber  20  and the recess  42  in the collar  40  (see  FIG. 1  also) of the closure stopper  124 . This bursting throttle restriction  544  is formed by two vanes  564  and  584 , which are integrally formed in one piece onto the bottom of the flat countersunk region  38  of this closure stopper  124 . The ends of the two vanes  564 ,  584  are opposite one another and by way of their spacing determine the flow cross section of the second outflow conduit  344 . The vanes  564 ,  584  of the bursting throttle restriction  544  are embodied with comparatively thin walls and are plastically deformable as soon as the pressure level in the outflow conduit  344  exceeds a threshold value. Upon a deformation of the vanes  564 ,  584 , the flow cross section of the second outflow conduit  344  increases. In this way, the bursting throttle restriction  544  of  FIG. 5  is capable of preventing dirt particles, blocking the flow cross section, from causing a pressure rise in the interior of the piston pump in which the components subjected to pressure might suffer damage. 
     It is understood that modifications or additions to the exemplary embodiments described are possible without departing from the fundamental concept of the invention. In this respect, it should be noted that a subject according to the invention may also have more than one second outflow conduit. Moreover, the invention is not limited to an even number of outflow conduits. The outflow conduits may, as described, be embodied entirely on the face end of the closure stopper located in the interior of the piston pump, or partly or solely on the face end of the bush bottom oriented toward the closure stopper.