Abstract:
A pump for a brake system in which the pump includes two pump pistons driven by a motor. Each pump piston feeds the brake fluid both during a retraction stroke and during an extension stroke, and a relatively uniform feed flow is thereby attainable by a fluid output from the piston being driven in both directions. The piston pump is intended for slip-controlled vehicle brake systems in motor vehicles.

Description:
BACKGROUND OF THE INVENTION 
     The invention is based on a piston pump and a brake system having a piston pump. 
     The piston pump is intended in particular as a pump in a brake system of a vehicle and is used for controlling the pressure in the wheel brake cylinders. Depending on the type of brake system, the abbreviations ABS, ASR, FDR and EHB are used for such brake systems. In the brake system, the pump serves for instance to return brake fluid from one or more wheel brake cylinders to a master cylinder (ABS) and/or to pump brake fluid out of a supply container into one or more wheel brake cylinders (ASR or FDR or EHB). In a brake system with wheel slip control (ABS or ASR) and/or a brake system serving as a steering aid (FDR) and/or an electrohydraulic brake system (EHB), the pump is needed. With wheel slip control (ABS or ASR), for instance, locking of the wheels of the vehicle during a braking event involving strong pressure on the brake pedal (ABS) and/or spinning of the driven wheels of the vehicle in the event of strong pressure on the gas pedal (ASR) can be prevented. In a brake system serving as a steering aid (FDR), a brake pressure is built up in one or more wheel brake cylinders independently of an actuation of the brake pedal or gas pedal, for instance to prevent the vehicle from shifting out of the lane desired by the driver. The pump can also be used in an electrohydraulic brake system (EHB), in which the pump pumps the brake fluid into the wheel brake cylinder or wheel brake cylinders if an electric brake pedal sensor detects an actuation of the brake pedal, or in which the pump is used to fill a reservoir of the brake system. 
     What is important is that in the brake system, the pressure in the individual brake cylinders can be controlled independently of the pressure in the other brake cylinders. Therefore in a vehicle with four brake cylinders, typically piston pumps are provided in which each of the brake cylinders is assigned its own pump element each with one pump piston. Along with this, U.S. Pat. No 4,875,741 shows a brake system in which two brake cylinders are connected to one pump element. However, so that the pressure medium cannot overflow unintentionally from one of the brake cylinders into the other brake cylinder in this version, a pump element with two work pressure chambers is used here. One of the two work pressure chambers is located on the face end on the pump piston, and the other work pressure chamber is formed as an annular chamber by providing the pump piston with a shoulder. In the brake system shown in U.S. Pat. No. 4,875,741, one inlet-side check valve and one outlet-side check valve is provided in the piston pump for each of the brake cylinders. While the pump piston is forced into the housing of the piston pump by the eccentric element, the pressure medium is forced out of the face-end work pressure chamber and at the same time out of the annular work pressure chamber into an outlet connection, each via a respective outlet valve. When the pump piston then moves out of the housing again, both work pressure chambers increase in size, and the pump element aspirates the pressure medium into both work pressure chambers. As a result, the pump operates very unevenly, creating major pressure pulsations and hence is very noisy. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The piston pump of the invention and the brake system set forth have the advantage over the prior art that upon motion of the pump piston, the feeding of the pressure medium from the two work pressure chambers is done in phase-offset fashion, thus making the pumping of pressure medium substantially more uniform, which leads to a considerable reduction in noise. Because feeding is more uniform, the maximum flow speed of the pressure medium is also less, and as a result the efficiency of the piston pump is improved, or else smaller line cross sections can be selected. 
     For safety reasons, two brake circuits are often provided in brake systems. For safety reasons, it is important that the two brake circuits be cleanly separated from one another. One substantial advantage of the piston pump proposed is that the individual brake circuits of the vehicle brake system can be cleanly separated from one another. 
     One particular advantage is that the feed flow can be made more uniform despite low effort and expense for drilling inside the housing of the piston pump. 
     Advantageous refinements of and improvements to the piston pump and the brake system are possible with the provisions recited hereinafter. 
     If the transmission mechanism or the eccentric element between the drive motor and the pump piston is embodied such that compressive and tensile forces can be transmitted by the drive motor to the outlet connection, this offers the advantage of being able to dispense with a large-sized spring to restore the pump piston. 
    
    
     The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of a preferred embodiment taken in conjunction with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a brake system with a piston pump embodied according to the invention; and 
     FIGS. 2 and 3 show different views and details of the exemplary embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 symbolically and as an example shows a preferably selected vehicle brake system of a motor vehicle with four wheels, assuming that the two front wheels, for instance, can be driven by a driving engine, not shown, and the two rear wheels can be non-driven. In symbolic form, the drawing shows a right rear wheel with a brake cylinder RR represented by a rectangle, a left front wheel with a brake cylinder FL, a right front wheel with a brake cylinder FR, and a left rear wheel with a brake cylinder RL. Also shown symbolically are a brake pedal  2 , a master cylinder  4 , a tank  6  containing a pressure medium, preferably brake fluid, a drive motor  8 , a piston pump  9 , a first blocking valve  11 , a second blocking valve  12 , a third blocking valve  13 , a fourth blocking valve  14 , a fifth blocking valve  15 , and a sixth blocking valve  16 . The blocking valves  11 ,  12 ,  13 ,  14 ,  15 ,  16  are electrically controllable. The drive motor  8  is an electric motor. The electric lines leading to the blocking valves  11 ,  12 ,  13 ,  14 ,  15 ,  16  and to the drive motor  8  have not been shown in the drawing, for the sake of simplicity. The piston pump  9  includes a first pump element  10  and a second pump element  10 ′. With the aid of a mechanical transmission mechanism  18 , symbolically represented by a double line in FIG. 1, the drive output generated by the electric drive motor  8  can be transmitted by the pump elements  10 ,  10 ′; in the exemplary embodiment selected, the mechanical transmission mechanism  18  is an eccentric element  18   e  that can be driven to rotate by the drive motor  8 . 
     The first pump element  10  has a first inlet connection  21 , a second inlet connection  22 , and an outlet connection  24 . The second pump element  10 ′ likewise has a first inlet connection  21 ′, a second inlet connection  22 ′, and an outlet connection  24 ′. The preferably selected piston pump  9  thus has a total of four inlet connections  21 ,  22 ,  21 ′,  22 ′, and two outlet connections  24 ,  24 ′. 
     One line  26  connects the master cylinder  4  to the blocking valve  11  and the blocking valve  15 ; one line  28  connects the outlet connection  24  of the pump element  10  to the two blocking valves  12  and  15 ; one line  31  connects the first inlet connection  21  of the piston pump element  10  to the blocking valve  11  and to the brake cylinder RR of the right rear wheel; one line  32  connects the second inlet connection  22  of the piston pump element  10  to the blocking valve  12  and to the brake cylinder FL of the left front wheel; one line  33  connects the second inlet connection  21 ′ to the pump element  10 ′ and to the blocking valve  13  and to the brake cylinder FR of the right front wheel; one line  34  connects the second inlet connection  22 ′ to the pump element  10 ′ and to the blocking valve  14  and to the brake cylinder RL of the left rear wheel; one line  36  connects the master cylinder  4  to the  25  blocking valve  14  and to the blocking valve  16 ; and one line  38  connects the outlet connection  24 ′ of the piston pump element  10 ′ to the blocking valve  13  and the blocking valve  16 . The lines  26 ,  28 ,  31 ,  32 ,  33 ,  34 ,  36 ,  38  are hydraulic pipes or hoses that contain the pressure medium, such as brake fluid. 
     Via the brake pedal  2 , brake pressure can be built up in the brake cylinders RR, FL, FR, RL. In a departure from the brake pressure specified via the brake pedal  2 , the pressure in the brake cylinders RR, FL, FR, RL can be reduced or built up with the aid of the piston pump  9 . For instance, if the brake pedal  2  is actuated too forcefully, then with the aid of a controller, not shown, the blocking valves  11 ,  12  are controlled all the way or partway into their blocking position, and the piston pump  9  pumps pressure medium out of the lines  31 ,  32 , through the line  28 , and through the opened blocking valve  15  into the tank  6 . As a result, the pressure in the brake cylinders RR, FL drops. By opening the blocking valves  11  and/or  12  to a variable width, the brake pressure in each of the brake cylinders RR, FL can be lowered independently of the brake pressure in the other brake cylinder RR or FL. It is important in this respect that the pressure medium cannot overflow uncontrolled out of one of the brake cylinders RR, FL into the respective other brake cylinder RR or FL, not even via the lines  31  and  32 . 
     If a brake pressure is to be built up in the brake cylinder FL of the left front wheel while the brake pedal  2  is not actuated, the control unit, not shown, then moves the blocking valve  15  into the blocking position, and the pump element  10  of the piston pump  9  pumps pressure medium out of the tank  6 , through the line  26 , through the opened blocking valve  11 , through the line  31 , via the inlet connection  21 , through the pump element  10 , via the outlet connection  24 , through the line  28 , and through the opened blocking valve  12  to the brake cylinder FL of the left front wheel. The pump element  10  in the process also pumps pressure medium out of the line  32 , via the inlet connection  22 , via the outlet connection  24  and through the line  28  back into the line  32 , so that this short-circuited flow course does not change the pressure. So that the pressure medium cannot flow unintended out of the brake cylinder FL of the left front wheel, which cylinder is at high pressure, into the brake cylinder RR of the right rear wheel, which cylinder is at low pressure, it is important that no pressure medium can flow out of the line  32  into the line  31 , and this includes that it cannot flow inside the pump element  10 . The piston pump  9  must be constructed accordingly. 
     The piston pump  9  selected as an example includes the pump element  10  and the pump element  10 ′. The pump element  10  is connected to the brake cylinders RR and FL. Correspondingly, the brake cylinders FR and RL are connected to the pump element  10 ′. The pump element  10 ′ is substantially identically embodied to the pump element  10 , and the ensuing explanation will therefore be limited primarily to the pump element  10 . 
     So that the first brake circuit, including the brake cylinders RR and FL and the first pump element  10 , will be cleanly separated from the second brake circuit that includes the brake cylinders FR and RL and the second pump element  10 ′, care must be taken that the two brake circuits are securely separated from one another under all circumstances, including inside the piston pump  9 . The piston pump  9  must therefore be constructed such that even if a seal inside the piston pump  9  might eventually fail, the pressure medium cannot be exchanged between the two pump elements  10 ,  10 ′. 
     FIGS. 2 and 3 by way of example show a preferably selected version of the piston pump  9 . FIG. 2 shows a section through the piston pump  9  in the plane in which the pump pistons  40 ,  40 ′ and the drive shaft  58  are located. FIG. 3 shows a fragmentary section through the piston pump  9  in the direction marked III in FIG.  2 . 
     In all the drawings, elements that are the same or function the same are provided with the same reference numerals. 
     The pump element  10  has a pump piston  40 , and the pump element  10 ′ has a pump piston  40 ′. The pump piston  40  is in the extended position in FIGS. 2 and 3, and the pump piston  40 ′ is in the retracted position in FIGS. 2 and 3. In FIGS. 2 and 3, a detail of a hydraulic block acting as a housing  42  is shown in section. Both pump pistons  40 ,  40 ′ and the blocking valves  11 ,  12 ,  13 ,  14 ,  15 ,  16  can all be integrated in the hydraulic block or housing  42 . The drive motor  8  (FIG. 1) is preferably flanged to the housing  42 . The two pump pistons  40  and  40 ′ are supported displaceably in the housing  42 . The pump piston  40  has an end  40   a  toward the eccentric element  18   e , and also has a guide region  40   c , a largest-diameter region  40   d , and a face end  40   f  remote from the eccentric element  18   e . There is a constriction  40   b  between the end  40   a , toward the eccentric element, and the guide region  40   c.    
     Because the diameter of the guide region  40   c  is less than the hydraulically effective diameter of the region  40   d , a hydraulically effective annular face  44  is formed on the face end, at the transition from the guide region  40   c  to the region  40   d . The largest-diameter region  40   d  forms a hydraulically effective end face  46  on the face end  40   f . A guide bush  48  closed off from the outside is press-fitted tightly and firmly into the housing  42 . A first work pressure chamber  50  is formed between the end face  46  and the guide bush  48 ; a second work pressure chamber  52  is formed between the annular face  44  and the guide bush  48 . The largest-diameter region  40   d  of the pump piston  40  is located between the first work pressure chamber  50  and the second work pressure chamber  52 . A sealing and guide ring  54  comprising a plurality of individual rings are at the circumference of the largest-diameter region  40   d . The sealing and guide ring  54  serves the purpose of sliding guidance of the largest-diameter region  40   d  of the pump piston  40  in the guide bush  48  and of sealing off the two work pressure chambers  50  and  52  from one another. A further sealing and guide ring  56  is provided in the housing  42 . The sealing and guide ring  56  serves the purpose of sliding guidance of the guide region  40   c  of the pump piston  40  and of sealing off the second work pressure chamber  52  from a hollow chamber  59  in the housing  42 . The eccentric element  18   e  is located in the hollow chamber  59 . 
     From the drive motor  8  (FIG.  1 ), a drive shaft  58  leads into the housing  42  (FIG.  2 ). The drive shaft  58  is rotatably supported in the housing  42  via a bearing  60 . A journal provided eccentrically to the pivot axis of the drive shaft  58  is located on the drive shaft  58 . The journal forms the eccentric element  18   e . An eccentric bearing  62  is located on the journal. The eccentric bearing  62  is a roller bearing, with an outer ring  62   a  and with roller bodies that roll between the outer ring  62   a  and the eccentric journal. By way of example, the eccentric bearing  62  is a needle bearing with needlelike cylindrical pins as roller bodies. 
     The outer ring  62   a  of the eccentric bearing  62  is in engagement with a clamp  66 . By way of example, the clamp  66  is formed by bending a stable steel band multiple times. The clamp  66  has a middle region  66   a , an axial region  66   b , an axial region  66   b ′, two radial regions  66   c  and  66   c ′, and two retaining regions  66   d  and  66   d ′. With respect to the longitudinal axis of the drive shaft  58 , the axial regions  66   b ,  66   b ′ and the retaining regions  66   d ,  66   d ′ all extend axially, while the middle region  66   a  and the radial regions  66   c ,  66   c ′ extend radially. The axial region  66   b , the radial region  66   c  and the retaining region  66   d  are located on the side of the eccentric bearing  62  toward the pump piston  40 ; the axial region  66   b ′, the radial region  66   c ′ and the retaining region  66   d ′ are located on the side of the eccentric bearing  62  toward the pump piston  40 ′. 
     An inlet valve  71  allows a flow of pressure medium out of the inlet connection  22  into the first work pressure chamber  50 ; an outlet valve  73  allows a flow of pressure medium out of the first work pressure chamber  50  into the outlet connection  24 ; an inlet valve  72  allows a flow of pressure medium from the inlet connection  21  into the second work pressure chamber is  52 ; an outlet valve  74  allows a flow of pressure medium out of the second work pressure chamber  52  into the outlet connection  24 . The valves  71 ,  72 ,  73 ,  74 , embodied as check valves, prevent a flow in the reverse direction. 
     The drive motor  8  can drive the pump piston  40  into the housing  42  (downward motion, in terms of FIGS. 2 and 3) and retrieve it (upward motion, in terms of FIGS.  2  and  3 ), by rotation of the drive shaft  58  via the eccentric element  18   e , the eccentric bearing  62  and the clamp  66 . When the pump piston  40  is driven into the housing  42 , the pump piston  40  moves in the first direction of motion  76 , marked by an arrow  76  in FIGS. 2 and 3. The direction opposite the first direction of motion  76  will be referred to hereinafter as the second direction of motion  78 . 
     Upon motion of the pump piston  40  in the first direction of motion  76 , the first work pressure chamber  50  decreases in size, and at the same time the second work pressure chamber  52  increases in size. In the process, pressure medium is forced out of the first work pressure chamber  50  through the outlet valve  73  into the outlet connection  24 , and at the same time pressure medium is aspirated from the inlet connection  21  into the work pressure chamber  52  through the inlet valve  72 . Upon an opposite motion, that is, when the pump piston  40  is driven in the second direction of motion  78 , the first work pressure chamber  50  increases in size, and at the same time the second work pressure chamber  52  is reduced in size. In the process, the pressure medium is aspirated from the inlet connection  22  into the work pressure chamber  50  by the inlet valve  71 , and at the same time pressure medium is forced out of the second work pressure chamber  52  into the outlet connection  24  by the outlet valve  74 . 
     It is accordingly apparent that when the pump piston  40  is actuated in the first direction of motion  76  and when the pump piston  40  is actuated in the second direction of motion  78 , pressure medium is aspirated and pumped into the outlet connection  24 . Because as a result pumping is done relatively uniformly with one pump piston  40 , the speed of change in the flow velocity of the pressure medium pumped into the outlet connection  24  is relatively slight, which is advantageously expressed in only relatively slight noise development. 
     In the exemplary embodiment shown, the eccentric element  18   e , eccentric bearing  62  and clamp  66  are part of the transmission mechanism  18 , with which both compressive forces and tensile forces can be transmitted from the drive shaft  58 , driven by the drive motor  8 , to the pump piston  40  and the pump piston  40 ′. Upon a rotation of the drive shaft  58  in a particular first rotational angle range, pressure is exerted on the end  40   a  toward the eccentric element of the pump piston  40  by the drive shaft  58 , via the eccentric element  18   e , eccentric bearing  62 , outer ring  62   a , and axial region  66   b , and the pump piston  40  is driven in the first direction of motion  76 ; at the same time, the retaining region  66   d ′ of the clamp  66  on the pump piston  40 ′ engages the end  40   a ′ of the second pump piston  40 ′ toward the eccentric element. Upon further rotation of the drive shaft  58  over a second rotational angle range, the clamp  66  is driven in the second direction of motion  78  by the drive shaft  58 , via the eccentric element  18   e , eccentric bearing  62 , and outer ring  62   a , which is transmitted for the clamp  66  as tensile force to the pump piston  40  by the axial region  66   b ′ and via the middle region  66   a , axial region  66   b , radial region  66   c  and retaining region  66   d . As a result, compressive forces and tensile forces can be transmitted from the drive shaft  58  to the pump piston  40  via the eccentric element  18   e.    
     As explained in terms of FIG. 1, the inlet connection  22  is connected to the brake cylinder FL of the left front wheel, and the inlet connection  21  is connected to the brake cylinder RR of the right rear wheel. Typically, more pressure medium must be fed from or to the brake cylinder of a front wheel than from or to the brake cylinder of a rear wheel. For this reason, the front wheel is connected to the first work pressure chamber  50 , which has the larger cross-sectional area, and the rear wheel is connected to the second work pressure chamber  52 , which has the smaller cross-sectional area. As a result, it is desirably obtained that upon each stroke of the pump piston  40 , more pressure medium is aspirated from the inlet connection  22  connected to the front wheel than from the inlet connection  21  connected to the rear wheel. 
     The foregoing relates to a preferred exemplary embodiment of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.