RIGID PISTON-ACTUATOR-ASSEMBLY SUPPORTED FOR PERFORMING A PENDULUM-TYPE TOLERANCE COMPENSATION MOTION

Abstract

A pump for pumping fluid, wherein the pump includes a working chamber, a piston assembly configured for reciprocating within the working chamber to thereby displace fluid, a piston actuator being rigidly assembled with the piston assembly at least in a working mode of the pump to thereby transmit drive energy to the piston assembly to reciprocate along a common rigid axis of the piston-actuator-assembly, and a bearing arrangement bearing the piston assembly and the piston actuator in the pump so that the piston-actuator-assembly provided by the piston assembly and the piston actuator is capable of performing a pendulum-type compensation motion around a pendulum point at the piston actuator on the common rigid axis.

Claims

1. A pump for pumping fluid, the pump comprising: a working chamber; a piston assembly configured for reciprocating within the working chamber to thereby displace fluid; a piston actuator being rigidly assembled with the piston assembly at least in a working mode of the pump to thereby transmit drive energy to the piston assembly to reciprocate along a common rigid axis of the piston-actuator-assembly provided by the piston assembly and the piston actuator; and a bearing arrangement bearing the piston assembly and the piston actuator in the pump so that the piston-actuator-assembly is capable of performing a pendulum-type compensation motion around a pendulum point at the piston actuator on the common rigid axis.

2. The pump according to claim 1, wherein the bearing arrangement comprises an actuator bearing supporting the piston actuator at the pendulum point.

3. The pump according to claim 2, wherein the actuator bearing has a configuration selected from the group consisting of: the actuator bearing is configured for supporting the piston actuator at the pendulum point while allowing for a rotation of the piston-actuator-assembly around the pendulum point; the actuator bearing is the only bearing which bears the piston actuator; the actuator bearing is configured as a spherical bearing having a plurality of bearing balls all located on a surface of a sphere around the pendulum point; the actuator bearing is configured as a pair of spherical bearings each having a respective plurality of bearing balls all located on a respective surface of a respective sphere, wherein the spheres both have the pendulum point as common center but have different radii; the actuator bearing is configured as a groove ball bearing having a plurality of bearing balls all located in a circular ring space around the pendulum point; and the actuator bearing is configured as a groove ball bearing having a plurality of bearing balls all located in a circular ring space around the pendulum point, and a cross-sectional area of the circular ring space is larger than a cross-sectional area of the balls to thereby enable a compensation motion of the balls perpendicular to their motion around the circular ring space.

4.-8. (canceled)

9. The pump according to claim 1, wherein the bearing arrangement comprises a piston bearing supporting the piston assembly at a piston bearing location with a volume-type positioning allowance to spatially limit the pendulum motion at the piston assembly.

10. The pump according to claim 9, wherein the piston bearing is the only bearing which bears the piston assembly.

11. The pump according to claim 9, wherein the piston bearing comprises: a fixed hollow abutment structure delimiting the piston bearing location and having a through hole through which the piston assembly extends; and at least one bearing ring surrounding an exterior surface of the piston assembly and being located within the fixed hollow abutment structure.

12. The pump according to claim 11, wherein the at least one bearing ring is configured as one of the group consisting of a piston sealing, a pair of axially spaced bearing rings, and at least one bearing ring in addition to a piston sealing.

13. The pump according to claim 9, wherein an axial distance between the pendulum point and a center of the piston bearing is larger than an axial range of the piston bearing location.

14. The pump according to claim 1, wherein the bearing arrangement comprises a plurality of bearings, wherein at least one of the piston assembly and the piston actuator is supported by only a single one of the plurality of bearings.

15. The pump according to claim 1, comprising a piston actuator rotation inhibitor configured for cooperating with the piston actuator so as to inhibit rotation of the piston actuator around the common rigid axis.

16. The pump according to claim 15, wherein the piston actuator rotation inhibitor has a configuration selected from the group consisting of: the piston actuator rotation inhibitor s configured for performing a rotation inhibiting interaction with the piston actuator by taking up moments from the piston actuator in a plane including the pendulum point and being perpendicular to the common rigid axis; at least part of the piston actuator rotation inhibitor is arranged at a position selected from a group consisting of a position in an interior of the piston actuator, and a position around the piston actuator; the piston actuator rotation inhibitor has a first end which is fixedly connected to the piston actuator and comprises a free second end located within a hollow body so as to enable a limited compensation motion of the second end; the piston actuator rotation inhibitor has a first end which is fixedly connected to the piston actuator and comprises a free second end located within a hollow body so as to enable a limited compensation motion of the second end, and the second end is arranged to interact with the hollow body in a plane including the pendulum point and being perpendicular to the common rigid axis.

17.-20. (canceled)

21. The pump according to claim 1, wherein the pump comprises a drive unit for generating the drive energy for driving the piston actuator and, in turn, the piston assembly.

22. The pump according to claim 21, wherein the drive unit is configured for generating rotational drive energy at a drive shaft of the drive unit.

23. The pump according to claim 22, comprising a gear mechanism configured for converting the rotational drive energy into a linear motion of the piston actuator.

24. The pump according to claim 23, wherein the gear mechanism in cooperation with the piston actuator are configured as a ball screw comprising a mandrel as the piston actuator and a nut cooperating with the mandrel and being rotatable by the drive unit.

25. The pump according to claim 24, comprising a piston actuator rotation inhibitor member configured for cooperating with the mandrel so as to inhibit rotation of the mandrel when the nut rotates.

26. The pump according to claim 24, comprising a shaft tooth wheel mounted for rotating with the drive shaft and comprising a nut tooth wheel mounted for rotating with the nut, wherein the shaft tooth wheel and the nut tooth wheel are arranged to engage one another so as to transfer drive energy from the drive shaft via the nut to the mandrel.

27. The pump according to claim 26, comprising at least one of the following features: the shaft tooth wheel and the nut tooth wheel are located so as to engage one another in a plane including the pendulum point and being perpendicular to the common rigid axis; the shaft tooth wheel and the nut tooth wheel are a pair of straight toothed spur wheels; the shaft tooth wheel and the nut tooth wheel are a pair of helical toothed spur wheels.

28. The pump according to claim 26, wherein the shaft tooth wheel and the nut tooth wheel are located so as to engage one another outside of a plane including the pendulum point and being perpendicular to the common rigid axis, and the second end of the piston actuator rotation inhibitor is arranged to interact with the hollow body at an opposite side of this plane, and wherein a contact area of the shaft tooth wheel and the nut tooth wheel on the one hand and a contact area of the second end on the other hand are located relatively to one another so that residual moments acting on the piston actuator are less than 20% of the moments generated by the drive unit.

29.-36. (canceled)

37. A method of operating a pump for pumping fluid, the method comprising: rigidly assembling a piston actuator with a piston assembly which is located at least partially within a working chamber of the pump; moving the piston actuator to thereby transmit drive energy to the piston assembly to reciprocate along a common rigid axis of the piston-actuator-assembly to thereby displace fluid within the working chamber; and bearing the piston assembly and the piston actuator in the pump by a bearing arrangement so that the piston-actuator-assembly provided by the piston assembly and the piston actuator is capable of performing a pendulum-type compensation motion around a pendulum point at the piston actuator on the common rigid axis.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0060] Other objects and many of the attendant advantages of embodiments of the present invention will be readily appreciated and become better understood by reference to the following more detailed description of embodiments in connection with the accompanied drawings. Features that are substantially or functionally equal or similar will be referred to by the same reference signs.

[0061] FIG. 1 shows a liquid separation device in accordance with embodiments of the present invention, particularly used in high performance liquid chromatography (HPLC).

[0062] FIG. 2 illustrates a cross-sectional view of a pump according to an exemplary embodiment of the invention in a working mode.

[0063] FIG. 3 to FIG. 6 illustrate schematic views of pumps according to exemplary embodiments of the invention.

[0064] The illustration in the drawing is schematically.

[0065] Before, referring to the drawings, exemplary embodiments will be described in further detail, some basic considerations will be summarized based on which exemplary embodiments of the invention have been developed.

[0066] An exemplary embodiment of the invention provides a pendulum drive for a piston and valve based High Pressure Solvent Delivery System (SDS) for a High Performance Liquid Chromatography (HPLC) apparatus.

[0067] Conventional high pressure fluid pumps divide the functionality of force generation and piston movement on the one hand and the functionality of solvent delivery on the other hand in two separate mechanical assemblies. Such an approach generates a critical interface between these assemblies which needs to offer all necessary degrees of freedom, in terms of a consequently very complex bearing arrangement, to prevent redundant guidance or forces. Within the past view years, the pressure requirements for SDS increased dramatically (400 bar, 600 bar, 1200 bar), and to fulfill the above described approach additional expensive parts and tight tolerance work are required in terms of the design of a bearing arrangement.

[0068] Exemplary embodiments of the invention allow for an increase in reliability of a pump at lower manufacturing costs. Such an approach provides a generic concept as a building set for a broad application range. Furthermore, a broad tolerance acceptance for critical interfacing parts is immanent. This leads to cost savings within the mechanical arrangement according to exemplary embodiments. In such an embodiment, the electrical driving moment can be indirectly coupled by a pair of toothed spur wheels (in particular a drive tooth wheel and a nut tooth wheel) which allows gear reduction and two additional degrees of freedom in rotation if arranged according to embodiments of the invention. The gear reduction leads to a further simplification of the drive unit such as an electric motor (which may be configured with more speed while allowing to downsize the requirements in terms of torque demand) and of an incremental encoder (which may operate with higher resolution by gear reduction while at the same time allowing for a lower resolution demand for the encoder).

[0069] A common rigid axis or axle for the functionality of force generation and piston movement on the one hand and the functionality of solvent delivery on the other hand is a feature according to an exemplary embodiment of the invention which provides significant advantages. Another advantageous feature implementable in a pump according to an exemplary embodiment of the invention is the reduction, as compared to conventional approaches, of the complexity and hence space requirements of a bearing arrangement to two axle bearing points or sections. A first axle bearing point (which can also be denoted as actuator bearing) of the force generation and piston movement limits two translational degrees of freedom but keeps the full freedom in rotation (like a ball joint or three-dimensional pendulum bearing). A second axle bearing point or section (which may also be denoted as piston bearing) can be located close to a piston sealing of the solvent delivery part and may be configured to limit two of rotatory degrees of freedom. The third translational degree of freedom may be controlled by an axial ball screw movement of either a mandrel or a nut of a ball screw assembly and may deliver the motion for the pump function. The third rotatory degree of freedom may be limited close to a swivel area of the first axle bearing point of the force generation and piston movement. This is one part of a further feature of a pump according to an exemplary embodiment of the invention since this arrangement keeps the second axle bearing point almost free of additional load while compensating for driving torque. Another part of the mentioned feature is the way how the driving torque is coupled into the ball screw assembly. If the ball screw nut is driven, a toothed spur wheel can be fixed to the ball screw nut aligned with the first axle bearing point of the force generation and piston movement. While a pinion of the motor can be fixed to a drive cabinet or casing of the pump, the spur wheel of the ball screw may move along the allowance of the freedom in rotation. It should be clear that two of these directions of freedom in rotation may be limited (in particular may be minimized) to compensate for part and interface tolerances and while mounting or maintaining the pump drive. According to such an embodiment, it is possible to keep all driving forces and moments very close to the neutral area of the first axle bearing point of the force generation and piston movement resulting in marginal additional load for the fixed bearing or the second axle bearing point located close to the piston sealing of the solvent delivery part.

[0070] Referring now in greater detail to the drawings, FIG. 1 depicts a general schematic of a liquid separation system 10. A pump 20 receives a mobile phase from a solvent supply 25, typically via a degasser 27, which degases and thus reduces the amount of dissolved gases in the mobile phase. The pump 20as a mobile phase drivedrives the mobile phase through a separating unit 30 (such as a chromatographic column) comprising a stationary phase. A sampling unit 40 can be provided between the pump 20 and the separating unit 30 in order to subject or add (often referred to as sample introduction) a sample fluid into the mobile phase. The stationary phase of the separating unit 30 is configured for separating compounds of the sample liquid. A detector 50 is provided for detecting separated compounds of the sample fluid. A fractionating unit 60 can be provided for outputting separated compounds of sample fluid.

[0071] While the mobile phase can be comprised of one solvent only, it may also be mixed from plural solvents. Such mixing might be a low pressure mixing and provided upstream of the pump 20, so that the pump 20 already receives and pumps the mixed solvents as the mobile phase. Alternatively, the pump 20 might be comprised of plural individual pumping units, with plural of the pumping units each receiving and pumping a different solvent or mixture, so that the mixing of the mobile phase (as received by the separating unit 30) occurs at high pressure and downstream of the pump 20 (or as part thereof). The composition (mixture) of the mobile phase may be kept constant over time, the so called isocratic mode, or varied over time, the so called gradient mode.

[0072] A data processing unit 70, which can be a conventional PC or workstation, might be coupled (as indicated by the dotted arrows) to one or more of the devices in the liquid separation system 10 in order to receive information and/or control operation. For example, the data processing unit 70 might control operation of the pump 20 (e.g. setting control parameters) and receive therefrom information regarding the actual working conditions (such as output pressure, flow rate, etc. at an outlet of the pump). The data processing unit 70 might also control operation of the solvent supply 25 (e.g. setting the solvent/s or solvent mixture to be supplied) and/or the degasser 27 (e.g. setting control parameters such as vacuum level) and might receive therefrom information regarding the actual working conditions (such as solvent composition supplied over time, flow rate, vacuum level, etc.). The data processing unit 70 might further control operation of the sampling unit 40 (e.g. controlling sample injection or synchronization sample injection with operating conditions of the pump 20). The separating unit 30 might also be controlled by the data processing unit 70 (e.g. selecting a specific flow path or column, setting operation temperature, etc.), and sendin returninformation (e.g. operating conditions) to the data processing unit 70. Accordingly, the detector 50 might be controlled by the data processing unit 70 (e.g. with respect to spectral or wavelength settings, setting time constants, start/stop data acquisition), and send information (e.g. about the detected sample compounds) to the data processing unit 70. The data processing unit 70 might also control operation of the fractionating unit 60 (e.g. in conjunction with data received from the detector 50) and provides data back.

[0073] FIG. 2 illustrates a cross-sectional view of a pump 20 according to an exemplary embodiment of the invention in a working mode (i.e. when the pump 20 is assembled and therefore ready to displace fluid such as a liquid).

[0074] The pump 20 comprises a working chamber 200 and a piston assembly 202 configured for reciprocating within the working chamber 200 to thereby displace fluid. By a reciprocation of the piston assembly 202 along a horizontal direction of FIG. 2, the fluid may be displaced between an inlet valve 277 and an outlet valve 279 (their function may also be interexchanged). The piston assembly 202 is constituted by a cylindrical piston 226 rigidly mounted on a piston base 228 (which may also be denoted as piston foot). A linear piston actuator 204 is also foreseen and rigidly assembled with the piston assembly 202 in the illustrated working mode of the pump 20, so that the piston actuator 204 and the piston assembly 202 are fully disabled to perform any motion or rotation independently from one another. In other words, when rigidly coupled to one another as shown in FIG. 2, the piston actuator 204 and the piston assembly 202 perform any translational and/or rotational motion always and only together as a whole. In the embodiment of FIG. 2, a pair of engagement levers 275 mounted on the piston actuator 204 engage with the piston assembly 202 and therefore ensure the rigid coupling between piston assembly 202 and piston actuator 204 in the shown working mode of the pump 20. For maintenance purposes, i.e. in a maintenance mode (not shown), it is however possible to temporarily disassemble the piston assembly 202 from the piston actuator 204, for instance for maintenance, repair or substitution of a (for instance damaged or worn out) piston assembly 202. A corresponding separation between piston assembly 202 and piston actuator 204 may be achieved by forcing the engagement levers 275 to rotate (for instance triggered by disassembling a pump head on the right-hand side of FIG. 2 from a pump base on the left-hand side of FIG. 2) so that the engagement levers 275 disengage the piston assembly 202. Such a maintenance mode is however different from the above-described working mode of the pump 20 in which the piston assembly 202 and the piston actuator 204 remain rigidly coupled.

[0075] A drive unit 216, such as an electric motor, generates rotational energy used for driving the piston actuator 204 and, in turn, the piston assembly 202, for pumping the fluid. The drive unit 216 rotates a drive shaft 218 for providing kinetic energy which is to be transferred to the piston assembly 202. For the purpose of this energy transfer, the pump 20 comprises a gear mechanism which is configured for converting the rotational drive energy of the drive shaft 218 into a linear motion of the piston actuator 204 along a horizontal axis of FIG. 2. Via a shaft tooth wheel 220 constituted by a plurality of teeth circumferentially surrounding the drive shaft 218, the rotational energy is transferred to a nut tooth wheel 222 which is rigidly coupled to and arranged around a nut 224 of a ball screw assembly, thereby rotating the nut 224. The teeth of the shaft tooth wheel 218 and the teeth of the nut tooth wheel 220 mesh or engage one another with some degree of clearance so as to enable the shaft tooth wheel 218 and the nut tooth wheel 222 to perform some independent compensation motion, for instance in the event of part and/or interface tolerances and while mounting or maintaining parts of the pump 20. The shaft tooth wheel 218 and the nut tooth wheel 220 are dimensioned to provide a gear reduction of about 1:5 from the drive unit 216 to the nut 224, which reduces the demand concerning required torque to be provided by the drive unit 216. This allows not only to implement a very simple drive unit 216, but also to implement a very simple incremental encoder located at the drive shaft 218 of the drive unit 216 (not shown). Via a pair of threads (not shown in FIG. 2), the rotating nut 224 cooperates with a tubular mandrel constituting the piston actuator 204. The nut 224 and the mandrel-type piston actuator 204 thereby constitute a ball screw assembly.

[0076] A piston actuator rotation inhibitor 214 impacts the piston actuator 204 in such a manner that the piston actuator 204 can only be moved in a translatory manner along a common rigid axis 206 of the rigidly coupled piston-actuator-assembly without rotating around the common rigid axis 206. Thereby, it is possible to transmit exclusively longitudinal drive energy to the piston assembly 202 to reciprocate along the common rigid axis 206 of the piston-actuator-assembly.

[0077] The pump 20 furthermore comprises a bearing arrangement 208, 210 for bearing the piston assembly 202 and the piston actuator 204 within the pump 20. The bearing arrangement 208, 210 reduces the free movability of the rigid pendulum like piston-actuator-assembly in the interior of the pump 20 in a defined manner. More specifically, the bearing arrangement 208, 210 supports the piston actuator 204 and the piston assembly 202 so that the piston-actuator-assembly is capable of performing a collective tolerance compensating pendulum motion around a pendulum point 212 located in the piston actuator 204 on the common rigid axis 206. The bearing arrangement 208, 210 is hereby formed by two bearings only, i.e. an actuator bearing 208 and a piston bearing 210.

[0078] The actuator bearing 208 (here configured as a groove ball bearing, compare FIG. 5 and FIG. 6 for details) is the only bearing assigned to the piston actuator 204 and supports the piston actuator 204 at the pendulum point 212 while allowing for a rotation with exactly three rotational degrees of freedom of the piston-actuator-assembly around the pendulum point 212. Rotation axes corresponding to two rotational degrees of freedom are both oriented perpendicular to the common rigid axis 206 and limited by the piston bearing 210. The piston actuator rotation inhibitor 214 is here configured to limit the third rotational degree of freedom performing a rotation inhibiting interaction with the piston actuator 204 by taking up moments from the piston actuator 204 which prevents rotation around the common rigid axis 206.

[0079] The piston bearing 210 is the only bearing which bears the piston assembly 202. Advantageously, an axial distance L (for instance in a range between 30 mm and 200 mm, for example approximately 100 mm), between the actuator bearing 208 and the piston bearing 210 is significantly larger than an intrinsic volume of the actuator bearing 208 and an intrinsic volume of the piston bearing 210. Therefore, the actuator bearing 208 and the piston bearing 210 may effectively behave as point-like bearings (although in particular the piston bearing 210 may have some not neglectable intrinsic axial extension, as shown in FIG. 3 in more detail. The actuator bearing 208 is configured as a groove ball bearing with preferred intentionally increased clearance (between groove delimiting walls and balls running along the groove), to thereby enable the actuator bearing 208 to tolerate equilibration or compensation motions within a certain defined range (of for instance 3 mrad).

[0080] As a result of its simplicity and the low number of bearing positions along the extension of the piston-actuator-assembly in the direction of the common rigid axis 206, the bearing arrangement 208, 210 does not involve any undesired overdetermination into the bearing architecture and is therefore compatible with relaxed demands in terms of the accuracy of mechanical fits used for bearing and supporting the piston-actuator-assembly.

[0081] FIG. 3 illustrates a schematic view of a pump 20 according to another exemplary embodiment of the invention.

[0082] FIG. 3 shows that the pump 20 comprises a pump head 314 (for solvent delivery, i.e. the hydraulic part of the pump 20) which comprising the working chamber 200 and the piston assembly 202. Furthermore, the pump 20 comprises a pump base 316 (for force generation and piston movement) accommodating the piston actuator 204. The drive unit 216 is located within the pump base 316 as well. The pump head 314 and the pump base 316 are configured to be fastenable to one another or unfastenable from one another by actuating a fastener (not shown).

[0083] Some details illustrated in FIG. 3 shall be described in the following: Reference numeral 330 in FIG. 3 indicates individual sections or portions of a drive cabinet or housing of the pump 20 via which the corresponding members of the pump 20 are rigidly supported. A rigid coupler rigidly coupling the piston assembly 202 with the piston actuator 204 is denoted with reference numeral 332. It is however also possible to directly rigidly couple the piston assembly 202 with the piston actuator 204. Friction reduction balls 334 may run along grooves between the piston actuator 204 and the nut 224.

[0084] As in FIG. 2, also the pump 20 shown in FIG. 3 comprises the shaft tooth wheel 220 mounted for rotating with the drive shaft 218 and the nut tooth wheel 222 mounted for rotating with the nut 224. The shaft tooth wheel 220 and the nut tooth wheel 222 are arranged so that their respective teeth engage one another so as to transfer drive energy from the drive shaft 218 via the shaft tooth wheel 220, the nut tooth wheel 222, and the nut 224 to the mandrel constituting the piston actuator 204. As can be taken from FIG. 3, the shaft tooth wheel 220 and the nut tooth wheel 222 are located so that their teeth engage one another in a plane 333 including the pendulum point 212 and being perpendicular to the common rigid axis 206, i.e. at or very close to a neutral axis. In the scenario of a tilting of the piston-actuator-assembly, the teeth of the tooth wheels 222 follow this tilting motion along a neutral axial distance using the engaging clearance between the shaft tooth wheel 220 and the nut tooth wheel 222. Hence, an undesired transmission of lateral forces between the tooth wheels 220, 222 and an undesired axial displacement is efficiently suppressed. In other words, with the shown gear mechanism, an effective direction of force is guided close to the swivel area of the ball screw.

[0085] In the embodiment of FIG. 3, the piston actuator rotation inhibitor 214 is arranged in an interior through hole of the tubular piston actuator 204 which results in a compact construction. As can be taken from FIG. 3, also the piston actuator rotation inhibitor 214 operates (i.e. contacts an interior surface of the piston actuator 204 for force transmission) at or very close to the plane 333 and symmetrical to the common rigid axis 206 (pendulum point 212 or center of sphere 302).

[0086] The bearing arrangement 208, 210 in the embodiment of FIG. 3 comprises the actuator bearing 208 (which may also be denoted as first axle bearing point or bearing section) in the configuration of a spherical bearing (or ball nut bearing) having a plurality of bearing balls 300 all located on a surface of a (virtual) sphere 302 around the pendulum point 212. This symmetric arrangement has a very high tolerance with regard to external distortions and therefore allows for proper balancing or equilibration motions of the system.

[0087] The bearing arrangement 208, 210 in the embodiment of FIG. 3 comprises the piston bearing 210 (which may also be denoted as second axle bearing point or bearing section). Before continuing to explain FIG. 3 in further detail, a general remark concerning the embodiments of FIG. 3 to FIG. 6 should be made. In all of the schematic illustrations of FIG. 3 to FIG. 6, the piston bearing 210 is shown with a piston bearing location 304 which appears in the drawing to have a certain interior volume. However, it should be clarified that this interior volume only schematically illustrates a virtual volume (i.e. for a volume-type positioning allowance) within which the bearing arrangement 208, 210 is capable of compensating parts and/or interface tolerances. In this active working mode, the piston-actuator-assembly is fixedly supported within the bearing arrangement 208, 210.

[0088] The piston bearing 210 is configured for supporting the piston assembly 202 at piston bearing location 304 to spatially limit the tolerance compensating pendulum motion of the pendulum-type piston-actuator assembly at the position of the piston assembly 202. More specifically, the piston bearing 210 comprises a fixed hollow abutment structure 306 defining the volume-type positioning allowance and the piston bearing location 304 and having a through hole 308 through which the piston assembly 202 extends in an axial direction. Two axially spaced bearing rings 310 of the piston bearing 210 surround an exterior surface of the piston assembly 202 circumferentially and are supported by the fixed hollow abutment structure 306 fixedly mounted at the drive cabinet 330. In addition, a piston sealing 312 is located at a front flange face of one of the bearing rings 210 and seals the piston assembly 202.

[0089] As can be taken from FIG. 3, an axial distance between the pendulum point 212 and a center of the piston bearing 210 is many times larger than an axial range of the piston bearing location 304 or an axial range of the intrinsic extension of the piston bearing 210. Thus, although the piston bearing 210 has some internal composition, it acts efficiently as one single bearing.

[0090] In the embodiment of FIG. 3, all driving forces and moments act very close to the neutral area (see plane 333) resulting in only marginal additional load for the fixed bearing. Two allowed rotational degrees of freedom are labeled 1 and 2 in FIG. 3, whereas a third rotational degree of freedom limited or disabled by the piston actuator rotation inhibitor 214 is labeled 3 in FIG. 3. An axial motion corresponding to a reciprocation is indicated by reference numeral 335 in FIG. 3.

[0091] FIG. 4 illustrates a schematic view of a pump 20 according to another exemplary embodiment of the invention. In the following, basically the differences with regard to the embodiment of FIG. 3 will be explained.

[0092] According to FIG. 4, the actuator bearing 208 is configured asymmetrically as a pair of spherical bearings each having a respective set of bearing balls 300, wherein all bearing balls 300 of a set are located on a respective surface of a respective sphere 400, 402. A first set of bearing balls 300 is configured for running along a first trajectory 431 assigned to the sphere 400. A second set of bearing balls 300 is configured for running along a second trajectory 433 assigned to the sphere 402. The spheres 400, 402 both have the pendulum point 212 as common center but have different radii. Sphere 402 has a larger radius than sphere 400. Three bearing shells 410, 412 and 414 are shown which form part of the pair of spherical bearings. The bearing shells 410, 414 are arranged at the nut 224, whereas the central bearing shell 412 is attached to the drive cabinet 330. A rotation direction of the drive shaft 218 is indicated by reference numeral 447.

[0093] The embodiment of FIG. 4 has the advantage of a compact design since the drive unit 216 can be arranged on the right-hand side of the pendulum point 212. Further on the gearing interface is located outside of the bearing arrangement 208, 210 which increases the flexibility in coupling the drive unit 216. The spatial distance between the piston bearing 210 and the actuator bearing 208 is further increased advantageously by locating the tooth wheels 220, 222 as well as the functional part of the piston actuator rotation inhibitor 214 far on the left-hand side of FIG. 4. Hence, in contrast to FIG. 3, the drive unit 216 is arranged, with regard to the direction of the common rigid axis 206, between the actuator bearing 208 and the piston bearing 210.

[0094] FIG. 5 illustrates a schematic view of a pump 20 according to still another exemplary embodiment of the invention. In the following, basically the differences with regard to the embodiment of FIG. 3 will be explained.

[0095] According to FIG. 5, the piston actuator rotation inhibitor 214 (being of a cantilever type and being basically L-shaped in the cross-sectional view of FIG. 5) is arranged around and fixedly connected to (for instance welded to) the piston actuator 204. More specifically, the piston actuator rotation inhibitor 214 has a first end 502 which is fixedly connected (for instance welded) to the piston actuator 204 and comprises a free second end 504 located within a hollow body 506 (being fixed at the drive cabinet 330 and having a constriction 510) so as to define a narrow contact area close to the plane 333 including the pendulum point 212 restricting the axial rotational degree of freedom only. The second end 504 is arranged to interact with the hollow body 506 in the plane 333 including the pendulum point 212 and being perpendicular to the common rigid axis 206, i.e. in the piston actuator rotation inhibition operates very close to or even at the neutral point.

[0096] By the implementation of a cup shaped member 520 being fixedly connected to the nut 224 and carrying the nut tooth wheel 222 at an exterior surface, it can be ensured that the force transmission between the tooth wheels 220, 222 as well as the piston actuator rotation inhibition occur within actuator bearing plane 333 which is oriented perpendicular to the common rigid axis 206 and comprises the pendulum point 212. This arrangement as well as the described principles of FIG. 3 and FIG. 4 keep the piston bearing 210 in principle free of any additional driving load beside of the concentrically load due to the piston movement and the pump pressure. As can be taken from FIG. 5, the cup shaped member 520 is axially guided around the actuator bearing 208 so as to bring the nut tooth wheel 222 in functionally cooperating position within plane 333.

[0097] According to FIG. 5, the actuator bearing 208 is configured as a compact and simple groove ball bearing having a plurality of bearing balls 300 all located in a circular ring space 500, which may be denoted as a groove, around the pendulum point 212. A cross-sectional area of the circular ring space 500 is larger than a cross-sectional area of the bearing balls 300. Therefore, also a ring space diameter H is larger than a bearing ball diameter h, so that the condition h<H is fulfilled. By loosely locating the bearing balls 300 in the circular ring space 500, it is possible to enable a tolerance equilibration or compensation motion of the bearing balls 300 perpendicular to their motion around the circular ring space 500.

[0098] FIG. 6 illustrates a schematic view of a pump 20 according to yet another exemplary embodiment of the invention. In the following, basically the differences with regard to the embodiment of FIG. 5 will be explained.

[0099] According to FIG. 6, the shaft tooth wheel 220 and the nut tooth wheel 222 are located so as to engage one another outside (according to FIG. 6 on the left-hand side) of plane 333 including the pendulum point 212 and being perpendicular to the common rigid axis 206. Furthermore, the second end 504 of the piston actuator rotation inhibitor 214 is arranged to interact with the hollow body 506 outside (according to FIG. 6 on the right-hand side, i.e. on an opposite side) of the plane 333. In other words, tooth wheel force transmission on the one hand and piston actuator rotation inhibition on the other hand occur on opposing sides of plane 333. However, compensation of part and/or interface tolerances and/or disturbing forces and loads is nevertheless possible also according to FIG. 6, since the shaft tooth wheel 218 and the nut tooth wheel 220 on the one hand and the second end 504 of the piston actuator rotation inhibitor 214 on the other hand are located relatively to one another so that effectively acting residual moments of the drive unit 216 (which would be needed to be compensated by the piston bearing 210) are substantially zero or at least small. This can achieved by arranging the mentioned components in accordance with a geometric condition balancing the leverages A with B and C with D and the angular condition between the contact area of the tooth wheels 220, 222 and the contact area of the piston actuator rotation inhibitor 214 with the constriction 510 of the hollow body 506. This angular condition could not be shown within the cross section of FIG. 6 since it is perpendicular to the common rigid axis 206 or parallel to the plane 333. In this condition, A denotes a radial distance between the common rigid axis 206 and the teeth engagement position between the tooth wheels 220, 222. B denotes a radial distance between the common rigid axis 206 and a radial center of the second end 504. C denotes an axial distance between the pendulum point 212 and the teeth engagement position between the tooth wheels 220, 222. D denotes an axial distance between the pendulum point 212 and the piston actuator rotation inhibition position at constriction 510. The advantage of FIG. 6 is an extremely simple and cost efficient design which nevertheless ensures a high lifetime of the components of the pump 20.

[0100] It should be noted that the term comprising does not exclude other elements or features and the a or an does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.