Patent Publication Number: US-2012036996-A1

Title: Piston Pumps for a Hydraulic Vehicle Brake System

Description:
This application claims priority under 35 U.S.C. §119 to German patent application no. DE 10 2010 039 269.3, filed Aug. 12, 2010 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The disclosure relates to a piston pump for a hydraulic vehicle brake system. Such piston pumps are used in motor vehicles for generating a hydraulic brake pressure for brake actuation in traction control and/or power-assisted brake systems. 
     A piston pump of this type is known from laid-open publication DE 195 03 621 A1. The known piston pump has two pump pistons which are arranged in an opposed arrangement, that is to say so as to lie opposite one another. Between the two pump pistons is arranged a rotationally drivable cylindrical eccentric with a rolling bearing, against the bearing ring of which mutually confronting end faces of the pump pistons bear. The rotary drive of the eccentric usually takes place by means of an electric motor. The bearing ring is an outer ring of the rolling bearing, and an inner ring may be present, for example pressed onto the eccentric. An inner ring is not necessary, however, and in the known piston pumps rolling bodies of the rolling bearing roll on a circumferential face of the eccentric. During a rotary drive of the eccentric, the rolling bodies of the rolling bearing roll on the circumferential face of the eccentric and on the inside of the bearing ring and the rolling bodies orbit around the eccentric. The bearing ring executes a movement on a circular track, having the eccentricity of the eccentric, about its axis of rotation. The circular movement of the bearing ring drives the pump pistons in a lifting movement which in a way known per se causes the suction intake and displacement, that is to say conveyance, of fluid. In a hydraulic vehicle braking system, the fluid conveyed is brake fluid. As a result of pump pistons coming to bear against the bearing ring, the latter does not corotate with the eccentric, but is held essentially fixedly in terms of rotation, although it can rotate slightly. 
     When the two pump pistons lying opposite one another press with the same force against the bearing ring from outside, their forces are compensated, so that the bearing ring does not press from outside against the rolling bodies and does not press these against the eccentric. Consequently, the rolling bodies bear loosely against the eccentric and are not pressed against it. 
     SUMMARY 
     The piston pump according to certain aspects of the disclosure has at least two pump pistons which are arranged so as to be offset with respect to one another in the longitudinal direction of a pump shaft. The pump pistons pressing from outside against the bearing ring of the rolling bearing cause a moment about an imaginary radial axis with respect to the pump shaft of the piston pump. As a result of the moment, the bearing ring presses at ends or in any event eccentrically from outside against some of the rolling bodies of the rolling bearing and presses these rolling bodies against the pump shaft. The bearing ring presses at one end or eccentrically against one or more adjacent rolling bodies and, on the opposite side, at the other end or eccentrically in the opposite direction against one or more rolling bodies. The moment which the pump pistons bearing against the bearing ring on the outside exert upon the bearing ring on account of their offset has the effect that at least two opposite rolling bodies are pressed against the pump shaft even when the forces with which the pump pistons press against the bearing ring from outside are compensated. This ensures that the rolling bodies roll on the rotating pump shaft. A further advantage of the disclosure is freedom from play of the rolling bodies due to the moment exerted upon the bearing ring by the offset pump pistons, and, consequently, a quiet running of the rolling bearing. 
     Advantageous refinements and developments of the disclosure are set forth further below. 
     The moment which the pump pistons exert upon the bearing ring should be such that they ensure that the rolling bodies roll reliably on the rotating pump shaft when the forces with which the pump pistons press against the bearing ring from outside are compensated. In order to keep wear of the rolling bearing low, the moment of the bearing ring should be low. The offset of the pump pistons in the longitudinal direction of the pump shaft is therefore small, providing an offset which is smaller than a diameter of the pump pistons. 
     The disclosure provides a rolling body cage which ensures that all the rolling bodies orbit around the rotating pump shaft, even when, because of the moment of the bearing ring, only two rolling bodies lying opposite one another are pressed against the pump shaft. 
     According to the disclosure, each pump piston has a return spring which acts upon it from outside against the bearing ring and thereby, because of its offset, gives rise to the desired moment on the bearing ring about the imaginary radial axis with respect to the pump shaft. 
     The features of the disclosure can basically be used for various arrangements of the pump pistons around the pump shaft, such as, for example, a V-arrangement of two pump pistons or a star arrangement of more than two pump pistons, although the pump pistons do not have to be arranged so as to be distributed uniformly around the pump shaft. However, it is preferable to use the features of the disclosure for an opposed arrangement of two pump pistons lying opposite one another, with the pump shaft and the rolling bearing between the two pump pistons. 
     The rolling bearing is preferably not a ball bearing, but a roller or needle bearing with cylindrical or else conical rolling bodies. The disclosure provides, in general, bodies of revolution as rolling bodies. 
     To convert the rotational movement of the pump shaft into a lifting movement of the pump pistons, according to the disclosure the bearing ring is arranged eccentrically with respect to the pump shaft, and the rolling bodies have different diameters correspondingly to a different gap width between the pump shaft and the bearing ring. When the rolling bodies roll on the circumference of the pump shaft when the latter is driven in rotation and at the same time orbit around the rotating pump shaft, the largest, smallest and each gap width between the bearing ring and the pump shaft also orbit around the pump shaft together with the rolling bodies. The bearing ring executes a circular movement about the axis of rotation of the pump shaft even when the pump shaft has no eccentricity. The circular movement of the bearing ring drives the pump pistons in their lifting movement. In this refinement of the disclosure, the pump shaft may have eccentricity, that is to say, when driven in rotation, may move on a circular path about its axis of rotation. Preferably, in this refinement of the disclosure, however, the pump shaft has no eccentricity, but, instead, a geometric axis of the pump shaft is at the same time its axis of rotation. The disclosure therefore speaks of a pump shaft instead of an eccentric. However, the disclosure can also be used for piston pumps with an eccentric which is surrounded concentrically by the bearing ring, the rolling bodies having identical diameters. In this case, the pump shaft is to be interpreted as an eccentric. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The disclosure is explained in more detail below by means of an embodiment illustrated in the drawing in which: 
         FIG. 1  shows an axial section and 
         FIG. 2  a cross section of a piston pump according to the disclosure. 
     
    
    
     The drawing is to be understood as being a diagrammatic and simplified illustration for understanding and for explaining the disclosure. 
     DETAILED DESCRIPTION 
     The piston pump  1  according to the disclosure, illustrated in the drawing, has a pump shaft  2  which can be driven in rotation by means of an electric motor, not illustrated. The piston pump  1  has two pump pistons  3  which are arranged in an opposed arrangement, that is to say lying opposite one another, with the pump shaft  2  between mutually confronting end faces  4  of the pump pistons  3 . The end faces  4  of the pump pistons  3  bear on the outside against a bearing ring  5  of a rolling bearing  6 . The bearing ring  5  may be interpreted as the outer ring of the rolling bearing  6 . The rolling bearing  6  is arranged on the pump shaft  2 . It has bodies of revolution, to be precise cylindrical rollers as rolling bodies  7 , this form of construction not being absolutely necessary for the rolling bearing  6  of the piston pump  1 . The rolling bodies may also be needles or tapers, and a ball bearing is not in principle ruled out by the disclosure, although a rolling bearing with bodies of revolution as rolling bodies is nevertheless preferred. The rolling bodies  7  roll on the pump shaft  2 . The rolling bodies  7  are received rotatably in a rolling body cage  8  which keeps them at a distance from one another in the circumferential direction. 
     The two pump pistons  3  are located in an axial place of the pump shaft  2  and have an offset a in the axially parallel direction, that is to say in the longitudinal direction of the pump shaft  2 . The offset a of the pump pistons  3  is small, so that wear of the rolling bearing  6  on account of the moment exerted upon the bearing ring  5  is not increased or, at most, is increased negligibly. In the exemplary embodiment, the offset a amounts to about half a diameter of the pump pistons  3 . 
     Piston return springs, not illustrated in the drawing, act upon the pump pistons  3  from outside against the bearing ring  5 . The piston return springs are helical compression springs which are arranged in pump bores  9  on a side of the pump pistons  3  which is remote from the bearing ring  5 , are supported on a bottom of the pump bores  9  and press against end faces, facing away from the bearing ring  5 , on the pump pistons  3 . The pump pistons  3  are received axially displaceably in the pump bores  9 . The pump bores  9  are arranged radially with respect to the pump shaft  2 , that is to say the pump pistons are displaceable radially with respect to the pump shaft  2 . The pump bores  9  have the offset with respect to one another in the longitudinal direction of the pump shaft  2 . 
     As a result of the offset a of the pump pistons  3 , the forces with which the pump pistons  3  press from outside against the bearing ring  5  give rise to a moment on the bearing ring  5  about an imaginary axis radially with respect to the pump shaft  2 . In  FIG. 1 , the moment acts counterclockwise upon the bearing ring  5 . The moment exerted upon the bearing ring  5  causes the rolling bodies  7 , which are located near the pump pistons  3 , to press against the pump shaft  2 . Specifically, the rolling bodies  7  are pressed with their end regions or, in any event, eccentrically against the pump shaft  2 . In  FIG. 1 , the upper end or upper region of the right rolling bodies  7  and the lower end or lower region of the left rolling bodies  7  are pressed against the pump shaft  2 . By the rolling bodies  7  of the rolling bearing  6  being pressed against the pump shaft  2  on account of the moment exerted upon the bearing ring  5  by the pump pistons  3 , the rolling bodies  7  roll on the pump shaft  2  and orbit around it when this is driven in rotation, even when the forces with which the two pump pistons  3  press from outside against the bearing ring  5  are identical, that is to say cancel one another. Slip of the rolling bodies  7  is avoided. 
     The rolling body cage  8  ensures that all the rolling bodies  7  orbit around the rotationally driven pump shaft  2 , that is to say even the rolling bodies  7  which are not pressed or are pressed only slightly against the pump shaft  2 . 
     The pressing of the rolling bodies  7  by the moment which the pump pistons  3  exert upon the bearing ring  5  avoids or reduces running or rattling noises of the rolling bodies  7  and counteracts the generation of noise. Rattling noises may occur when the force with which one pump piston  3  presses against the bearing ring  5  rises and the force with which the other pump piston  3  presses from the other side against the bearing ring  5  decreases, the forces with which the two pump pistons  3  press against the bearing ring  5  meanwhile being compensated. Then, without the moment on the bearing ring  5  caused by the offset or of the pump pistons  3 , the rolling bodies  7  would be relieved, and a change in direction of the resultant force which the two pump pistons  3  exert upon the bearing ring  5  would occur. This may lead to a rattling. 
     The illustrated embodiment of a piston pump  1  according to the disclosure has no eccentric, and its pump shaft  2  rotates about its geometric axis which is at the same time its axis of rotation  10 . The bearing ring  5  of the rolling bearing  6  is arranged eccentrically with respect to the pump shaft  2 , and the gap width between the bearing ring  5  and pump shaft  2  changes in its circumferential direction. The rolling bodies  7  have different diameters correspondingly to the gap width between the bearing ring  5  and pump shaft  2  at the point where the respective rolling body  7  is located. When the pump shaft  2  is driven in rotation, the rolling bodies  7  roll on the pump shaft  2  and orbit around the pump shaft  2 . Also, as it were, the gap width between the bearing ring  5  and the pump shaft  2  orbits together with the rolling bodies  7 , the largest gap width orbits with the rolling body  7  having the largest diameter, the smallest gap width orbits with the rolling body  7  having the smallest diameter, and every other gap width likewise orbits about the shaft  2  when the latter is driven in rotation. As a result, the bearing ring  5  moves on a circular path about the axis and axis of rotation  10  of the pump shaft  2 , without corotating with the pump shaft  2 . The movement of the bearing ring  5  on the circular path causes a lifting drive of the pump pistons  3  bearing against it on the outside and, consequently, the conveyance of fluid, that is to say of brake fluid, in a way known with regard to piston pumps. 
     The pump bores  9  are formed in a pump casing  11  of the piston pump  1  and issue radially into a cylindrical eccentric space  12  which is pierced axially by the pump shaft  2  and in which the rolling bearing  6  is located. The pump casing  11  is an integral part of what is known as a hydraulic block in which, in addition to the pump pistons  3 , further hydraulic structural elements, not illustrated, such as solenoid valves of a traction control device for a hydraulic brake system of a motor vehicle are arranged and are connected hydraulically to one another. Such hydraulic blocks are known per se and will not be explained in any more detail here. 
     The disclosure can in principle also be used for piston pumps which in a known way have an eccentric instead of the pump shaft  2 , the eccentric being, in particular, a cylindrical body which can be driven (not illustrated) in rotation about an eccentric axis of rotation. In this case, the rolling bearing is preferably a conventional rolling bearing, the rolling bodies of which have identical diameters and the bearing ring of which surrounds the eccentric concentrically, that is to say is eccentric with respect to the axis of rotation of the eccentric, as is also the case with regard to the bearing ring  5  of the piston pump  1  depicted. In this case, the pump shaft  2  is to be equated with an eccentric.