The invention relates to a piston pump as generically defined by the preamble to the main claim, which is intended in particular for a hydraulic vehicle brake system with slip control.
- ADVANTAGES OF THE INVENTION
One such piston pump is known for instance from German Patent Disclosure DE 44 07 978 A1. The known piston pump has a piston that can be driven to execute a reciprocating stroke motion. In its stroke motion, the piston decreases and increases a volume of a positive-displacement chamber of the piston pump. When the volume of the positive-displacement chamber is decreased, fluid is positively displaced; this is a pumping stroke of the piston pump. When the volume of the positive-displacement chamber is increased, fluid is aspirated, which is an intake stroke of the piston pump.
The piston pump of the invention having the characteristics of the main claim has a positive-displacement body, which is disposed in the positive-displacement chamber. This positive-displacement body decreases an available volume of the positive-displacement chamber and is embodied in particular such that a residual or idle volume remaining at the end of the pumping stroke is as small as possible.
This improves the efficiency of the piston pump of the invention.
The piston pump of the invention is intended in particular as a pump in a brake system of a vehicle and is used to control the pressure in 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 a wheel brake cylinder or a plurality of wheel brake cylinders to a master cylinder (ABS) and/or to pump brake fluid out of a supply container into a wheel brake cylinder or a plurality of 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 the wheel slip control (ABS or ASR), 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 for instance 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 breaking out of the track 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.
Advantageous features and refinements of the invention defined by the main claim are the subject of the dependent claims.
DESCRIPTION OF THE FIRST EXEMPLARY EMBODIMENT
The invention will be described in further detail below in terms of two exemplary embodiments shown in the drawing. FIGS. 1 and 2 shows axial sections through two exemplary embodiments of piston pumps of the invention; FIG. 3 is an end view of a retaining part of the piston pump shown in FIG. 2; and FIG. 4 shows a modification of the piston pump shown in FIG. 2.
The piston pump of the invention shown in FIG. 1 and identified overall by reference numeral 10 has a boltlike piston 12, which is axially displaceable guided in a plastic bush 14. The bush 14 is inserted into a hydraulic block of a hydraulic vehicle brake system, not otherwise shown, that forms a pump housing 16. Other hydraulic components, such as magnet valves, not shown, are inserted and hydraulically connected to one another and to the piston pump 10 in the hydraulic block, of which the drawing shows only a fraction surrounding the piston pump 10.
A face end of the bush 14 is closed with a disklike closure part 18, which is fixed by a caulking 20 of the pump housing 16. The bush 14, piston 12 and closure part 18 enclose a positive-displacement chamber 22 of the piston pump 10. A helical compression spring is disposed as a piston restoring spring 24 in the positive-displacement chamber 22; it is supported on the closure part 18 and presses the piston 12 against a circumferential face of an eccentric element 26, which can be driven to rotate by an electric motor and by means of which the piston 12 can be driven in a manner known per se to execute a reciprocating stroke motion in the bush 14.
As the inlet and outlet valves, two spring-loaded check valves 28 are disposed outside the bush 14, in the hydraulic block forming the pump housing 16, and they are shown symbolically in FIG. 1. The check valves 28 communicate with the positive-displacement chamber via a radial bore 30 in the bush 14.
- DESCRIPTION OF THE SECOND EXEMPLARY EMBODIMENT
A positive-displacement body 32 is located in the interior of the piston restoring spring 24 and largely fills up the interior of the piston restoring spring 24. The positive-displacement body 32 is like a tang and protrudes integrally from the closure part 18. The positive-displacement body 32 decreases the available volume in the positive-displacement chamber 22 for fluid that can be pumped by the piston pump 10, and in particular reduces the residual volume or so-called idle volume available in the positive-displacement chamber 22 at the end of a pumping stroke, when the piston 22 assumes its position thrust farthest into the bush 14, to a minimum. This improves the efficiency of the piston pump 10. The fact that the wire comprising the piston restoring spring has a rectangular coil cross section also contributes to minimizing the idle volume. Instead of a rectangular coil cross section, other cross sections may also be chosen, such as parallelogram- or gable-shaped coil cross sections, whose coil faces oriented toward one another are virtually complementary, in order to reduce a gap width between spring coils and thus to reduce the idle volume when the piston restoring spring 24 is compressed. The piston restoring spring 24 fills up an annular chamber between the positive-displacement body 32 and the bush 14 extensively. The positive-displacement body 32 can also be provided as a tanglike extension on the piston 12 and can extend (not shown) in the interior of the piston restoring spring 24.
The piston pump 34 of the invention, shown in FIG. 2, is inserted into a pump housing 36, which is formed by a hydraulic block of a hydraulic vehicle brake system not otherwise shown. Other hydraulic components, such as magnet valves and the like, are inserted into the hydraulic block, of which the drawing for the sake of simplicity shows only a fragment surrounding the piston pump 10, and are hydraulically connected to one another and to the piston pump 34 therein. The piston pump 34 has a bush 38 with a bush bottom 40 integral with it that is press-fitted into a cylinder bore 42. By means of a crimp 44, a cylindrical closure plug 46 is mounted on the bush bottom 40. The closure plug 46 is fixed in the pump housing 36 by a caulking 48 and closes off one end of the cylinder bore 42 in pressure-tight fashion. A check valve as an outlet valve 52 is inserted into a blind bore 50 of the closure plug 46; as its valve closing body, this valve has a valve ball 54, which is pressed by a helical compression spring as the valve closing spring 56 against a conical valve seat 58, which is embodied at an orifice of an axial through hole 60 in the bush bottom 40. An outlet is effected through a radial outlet bore 62 in the closure plug 46, which communicates with an outlet bore 64 in the pump housing 36.
A piston 66 of the piston pump 34 of the invention is embodied as a composite part, with a steel core 68 whose circumference has a lubricant jacket 70 of plastic spray-coated onto it. As the plastic for the lubricant jacket, a fiber-reinforced plastic with approximately 15% carbon fibers and with teflon components that lend the lubricant jacket 70 good sliding properties is for instance suitable. With its lubricant jacket 70, the piston 66 is guided slidingly displaceably in the axial direction in the cylinder bore 42 or in the bush 40.
An end face of the steel core 68 of the piston 66 that is remote from the bush bottom 40 and protrudes from the bush 38 is bare; that is, it is not covered by the lubricant jacket 70. This end face of the steel core 68 forms a slide face 72, with which the piston 66 is pressed by a piston restoring spring 74 against a circumference of an eccentric element 76 which can be driven to rotate by an electric motor, and with which the piston can be driven to execute an axially reciprocating stroke motion. The slide face 72 is embodied in low-wearing fashion by the use of a low-wear material for the steel core 68, or by hardening the steel core 68.
The steel core 68 is in terms of its shape a very easily made, substantially cylindrical part. The lubricant jacket 70 surrounds the circumference of the steel core 68 with a hollow-cylindrical portion 78; it continues, lengthening the steel core 68 or the piston 66, on into the bush 38 on a face end of the steel core 68 remote from the eccentric element 76. The portion 80 of the lubricant jacket 70 that lengthens the piston 66 has a transverse hole 82, which passes through an axial blind bore 84 that is also made in the portion 80 of the lubricant jacket 70 that lengthens the piston 66. The blind bore 84 widens, forming a conical valve seat 86, and discharges at a face end of the piston 66 toward the bush bottom 40. A check valve in the form of an inlet valve 88 is inserted into the widened part of the blind bore 84 and has a valve ball 90 as its valve closing body, which is pressed against the valve seat 86 by a helical compression spring as the valve closing spring 92. The transverse hole 82 in the piston 66 communicates through a cylindrical filter screen 94, which is mounted on a open face end of the bush 38, with an inlet conduit 96, which is made radially to the piston pump 34 in the pump housing 36. The valve closing spring 92 is supported on a retaining part 98, which is mounted on the face end of the piston 66 toward the bush bottom 40. The retaining part 98 has a rim 100 in the form of a perforated disk, which is pressed by the piston restoring spring 74 against the face end, toward this rim, of the piston 66, and as a result the retaining part 98 is held on the piston 66. The retaining part 98 is shown in an end view, seen from the direction of the piston 66, in FIG. 3. Four angled retaining ribs 102 protrude from a side remote from the piston 66 and are integral with the rim 100 in the form of the perforated disk. The retaining ribs 102 retain a cylindrical positive-displacement body 104, which is integral with them and protrudes coaxially through the perforated-disk rim 100 of the retaining part 98. An annular interstice exists between the positive-displacement body 104 and the perforated-disk rim 100, so that fluid flowing through the inlet valve 88 into the piston pump 34 can flow through the perforated-disk rim 100 and between the retaining ribs 102 on into a positive-displacement chamber 106 of the piston pump 34.
The valve closing spring 92, which is supported on the retaining ribs 102 of the retaining part 98, like the piston restoring spring 74 has a rectangular coil cross section. The rectangular coil cross section of these two springs 74, 92, like the positive-displacement body 104, serves to reduce a volume of the positive-displacement chamber 106 of the piston pump 34 that is available to a fluid that can be pumped by the piston pump 34. In particular, the idle volume of the positive-displacement chamber 106, which remains when the piston 66 has been thrust maximally inward into the bush 38, is reduced by the positive-displacement body 104 and the springs 74, 92 with their rectangular coil cross sections, thus increasing the efficiency of the piston pump 34 of the invention. The positive-displacement body 104 of the piston pump 34 shown in FIG. 2, like the positive-displacement body 32 of the piston pump 10 shown in FIG. 1, can be embodied such that it maximally fills up the idle volume in the positive-displacement chamber 106 and thus reduces the idle volume to a minimum and increases the efficiency of the piston pump 34.
In the extension of the retaining ribs 102, the retaining part 98 has four centering prongs 106, integral with it and protruding inward into the blind bore 84 from its perforated-disk rim 100, which hold the retaining part 98 coaxially on the piston 66. In the exemplary embodiment shown, the centering prongs 108 have a press fit in the blind bore 84; that is, the retaining part 98 is joined by frictional engagement to the piston 66 by means of a clamping connection. The connection of the retaining part 98 with the piston 66 serves to keep the retaining part 98 on the piston 66, counter to the force of the valve closing spring 92, until the piston has been inserted into the bush 38 and the retaining part 98 is held on the piston 66 by the piston restoring spring 74. Joining the retaining part 98 to the piston 66 can for instance also be done by positive engagement, by means of a snap or detent connection, known per se, or by material engagement, such as adhesive bonding.
The positive-displacement body 104 defines an opening stroke of the valve ball 90 of the inlet valve 88 and forms a valve stroke limiter. This valve stroke limiter reduces a force with which the valve ball 90 strikes the valve seat 86 upon closure of the inlet valve 88 and thereby reduces wear to the valve seat 86. This is especially important whenever the valve seat 86 is shaped from a soft material. As a result, in a valve seat 86 that is shaped from plastic as in the exemplary embodiment shown, a long service life is attained. Since the positive-displacement body 104 forming the valve stroke limiter is of plastic, the valve stroke limiter can be made to more precise size than a valve stroke limiter made for instance as a deep-drawn sheet-metal part.
Since the centering prong 108 aligns the retaining part 98 in exact alignment with the blind bore 84 and the valve seat 86 in the piston 66, and the retaining part 98 centers the valve closing spring 92, the valve ball 90 is also guided in alignment with the valve seat 86 when the inlet valve 88 is opened.
The retaining part 98 at the same time forms a guide and sealing element for the piston 66 of the piston pump 34 of the invention: The retaining part 98 has a low, hollow-cylindrical rim 110, which protrudes a short distance into an annular interstice between the piston 66 and the bush 38 and is integral with the perforated-disk rim 100 on the circumference of the piston 66. This hollow-cylindrical rim 110 and a circumferential edge of the perforated-disk rim 100 of the retaining part 98 guide the piston 66 in the bush 38. Under the other side from the hollow-cylindrical rim 110, that its, away from the piston 66, an encompassing sealing lip 112 protrudes integrally from the perforated-disk rim 100 of the retaining part 98. The sealing lip 112 is also integral with the retaining part 98. A spreader ring 114 with a conical spreader face 116 on its circumference is located between the piston restoring spring 74 and the sealing lip 112 of the retaining part 98. The spreader ring 114 is pressed axially by the piston restoring spring 74 against an approximately hollow-conical inside face 118 of the sealing lip 112 and in this way spreads the sealing lip 112 radially open and presses it into sealing contact against an inner wall of the bush 38. Via the spreader ring 114, the piston restoring spring 74 assures a durable elastic spreading behavior of the sealing lip 112 and thus a durably reliable sealing of the piston 66 in the bush 38. Via the cone angles of the spreader face 116 of the spreader ring 114 and of the inside face 118 of the sealing lip 112, the spreading force with which the sealing lip 112 is pressed against the inner wall of the bush 38 can be adjusted. The retaining part 98 comprises the same plastic as the lubricant jacket 70; it is low-wear and has good sliding properties. The plastic retaining part 98 can be made inexpensively.
Another advantage of the retaining part 98 mounted on one face end of the piston 66 is that it protects this face end of the piston against damage before and during the insertion of the piston 66 into the bush 38. The piston pump 120 shown in FIG. 4 is a modification according to the invention of the piston pump 34 shown in FIG. 2. The retaining part 98 of the piston pump 120 shown in FIG. 4 has no sealing lip and no hollow-cylindrical rim for guiding the piston. The guidance of the piston 66 in the bush 38 is effected directly at the outer circumference 122 of the piston 66. For sealing off the piston 66 in the bush 68, a rubber sealing ring 124 is placed on an annular shoulder 126 of the piston 66, where the piston 66 narrows toward its end located inside the bush 38. The perforated-disk rim 100 of the retaining part 98 holds the sealing ring 124 axially on the piston 66. The sealing ring 124 can be slipped on to the annular shoulder 126 before the retaining part 98 is joined to the piston 66. This makes it substantially easier to join the sealing ring 124 to the piston 66. Otherwise, the piston pump 120 shown in FIG. 4 is embodied identically and functions in the same way as the piston pump 34 shown in FIG. 2. To avoid repetition, reference is made to the description of FIG. 2. Identical components are identified by the same reference numerals.