ROTOR FOR AN ECCENTRIC SCREW PUMP AND METHOD FOR THE MANUFACTURE THEREOF

Abstract

A method of manufacturing a metallic rotor of an eccentric screw pump, comprising clamping a workpiece extending along a central longitudinal axis in a workpiece clamping device and removing material from the workpiece by cutting with a cutting tool. The invention further comprises not producing the surface of the rotor in a three-axis whirling process, using the cutting tool to produce the outer surface geometry of the rotor, advancing the cutting tool along an axis of advance that is parallel to the longitudinal axis of the rotor, and rotating the cutting tool about an axis of tool rotation that is parallel to the longitudinal axis of the rotor.

Claims

1. A rotor for an eccentric screw pump, the rotor having a helical geometry with a central worm axis that winds helically around a central longitudinal axis of the rotor, wherein the rotor is produced in a milling process in which a milling tool rotating about a tool rotation axis which is not parallel to a central longitudinal axis of the rotor is used for the machining production of the rotor from a workpiece.

2. The rotor according to claim 1, wherein the rotor extends from a first end along a central longitudinal axis thereof to a second end, and a geometry of the rotor generated in the milling process is a helical geometry defined by at least one geometric parameter including: an eccentricity of a worm thread, defined as the distance between a central worm axis running in the center of a worm cross-sectional area and the center longitudinal axis; a surrounding outer diameter of the helical geometry in relation to the central longitudinal axis; an enveloping inner diameter of the helical geometry in relation to the central longitudinal axis; an outer diameter of the helical geometry in relation to the central worm axis; and a pitch of the central worm axis; wherein the at least one of the geometric parameters changes in the axial direction along the central longitudinal axis of the rotor, wherein the first end of the rotor is greater than at the second end of the rotor or the geometric parameter is different at one end of the rotor from a cross section of the rotor lying between the first and second ends in an axial direction.

3. The rotor according to claim 2, wherein the at least one geometric parameter is changed continuously towards the second end of the rotor, starting from the first end of the rotor.

4. The rotor according to claim 2, wherein the at least one geometric parameter starting from the first end of the rotor towards the second end of the rotor towards the second end of the rotor is first enlarged and then reduced in size, or is first reduced and then enlarged.

5. The rotor according to claim 2, wherein the at least one geometric parameter is varied from the first end of the rotor to the second end of the rotor along the entire length of the rotor or along an axial section of the rotor with a single, double, or triple exponential dependence on the axial feed of the milling tool along the central longitudinal axis.

6. The rotor according to claim 2, wherein: the helical geometry of a manufactured rotor has a non-circular cross section with respect to the central worm axis, the helical geometry of the manufactured rotor has a non-point symmetrical cross section with respect to a point of symmetry located in the central worm axis; at least one lubrication pocket is formed in the surface of the helical geometry of the manufactured rotor; or an outer geometry of the rotor has at least one wear allowance section in which the outer geometry has axially protruding sections deviating from a continuous worm outer geometry.

7. The rotor according to claim 2, wherein at least two of the geometric parameters are changed along the axial feed path of the milling tool such that: a first geometric parameter is increased and a second geometric parameter is increased or decreased in a proportional or exponential ratio to the increase of the first parameter; or a first geometric parameter is increased and a second geometric parameter is increased or decreased in a non-correlated ratio to the increase of the first parameter.

8. A stator for an eccentric screw pump comprising a stator cavity having a geometry according to the rotor according to claim 2.

9. The use of a rotor according to claim 1 as a stator core for manufacturing a stator in a master moulding process.

10. An eccentric screw pump comprising a rotor according to claim 1.

11. An eccentric screw pump comprising a stator according to claim 8.

12. An eccentric screw pump of comprising: a rotor having a helical geometry with a central worm axis that winds helically around a central longitudinal axis of the rotor, wherein the rotor is produced in a milling process in which a milling tool rotating about a tool rotation axis which is not parallel to a central longitudinal axis of the rotor is used for the machining production of the rotor from a workpiece, or the rotor is manufactured in an additive manufacturing process comprising a laser sintering process or a laser melting process, in which a selective material application process or a selective material curing process is controlled on the basis of the geometric data of the rotor; and a stator according to claim 8; wherein at least one geometric parameter of the rotor varying in a direction along the central longitudinal axis of the rotor and the at least one geometric parameter of the stator is varying along a central longitudinal axis of the stator are coincident geometric parameters.

13. The eccentric screw pump according to claim 12, wherein the at least one geometric parameter of the rotor and the at least one geometric parameter of the stator change synchronously in a direction along the central longitudinal axis.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Preferred embodiments of the invention are explained with reference to the accompanying Figures, wherein:

[0073] FIG. 1 shows a schematic cross-sectional view of the geometric conditions of a reference profile of a rotor of a previously known eccentric screw pump;

[0074] FIG. 2 shows a schematic perspective view of a rotor manufactured according to the method of the invention or of a rotor according to the invention;

[0075] FIG. 3 shows a schematic representation of the manufacturing process in two variants, as shown in FIG. 1;

[0076] FIG. 4 shows a schematic representation of three rotors A, B, C, which are designed or manufactured according to the invention;

[0077] FIG. 5 shows a cross-sectional view of a rotor with geometric features formed or produced thereon according to the invention;

[0078] FIG. 6 shows a side view of a rotor according to an embodiment according to the invention or according to the manufacturing process according to the invention; and

[0079] FIG. 7 shows a schematic view of an additive manufacturing process of a rotor according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0080] Referring first to FIG. 1, the eccentric rotary motion of a rotor in a stator is subject to geometric boundary conditions determined by the diameter of the rotor profile and the eccentricity between the central longitudinal axis MS of the stator and the central longitudinal axis MR of the rotor. As designated in the Figure: [0081] D=Diameter of the rotor profile [0082] e=eccentricity [0083] MS=centre of the central axis of the stator (central axis is perpendicular to drawing plane) [0084] MR=centre of the central axis of the rotor (central axis is perpendicular to drawing plane) [0085] MP=centre of the rotor profile [0086] A=Driven direction of rotation of the rotor [0087] B=Resulting direction of rotation of the rotor centre due to worm shape

[0088] It is to be understood that the rotor is driven around the axis MR by the wobble shaft and that this causes a rotation around the axis MP.

[0089] FIG. 2 shows a rotor 10 for an eccentric screw pump. The rotor extends along a central longitudinal axis 15 from a first end 11 to a second end 12. The rotor 10 is designed as a worm with a worm central axis 14 winding helically around the central longitudinal axis 15. The worm central axis 14 is offset from the central longitudinal axis 15 by an eccentricity 18 in the front cross section at the first end 11. In the rotor 10 shown, this eccentricity remains constant over the entire axial length from the first end 11 to the second end 12.

[0090] The rotor 10 has an elliptical cross-sectional geometry 13 at the first end 11 and is characterised by an outer envelope 16 which is elliptical in cross section. The outer envelope 16 is an elliptical body with the central longitudinal axis 15. The rotor has a pitch 17.

[0091] With reference to FIG. 3, the rotor according to the invention is shown with the milling tools and movements of the milling tools necessary for its manufacture. As can be seen, according to a first variant of manufacture, the rotor 10 can be manufactured with a milling tool 20a, which rotates about a tool rotation axis 21a and thereby executes the cutting movement. The tool rotation axis 21a is perpendicular to the central longitudinal axis 15. The tool rotation axis 21a is radial to the central longitudinal axis 15 and intersects this central longitudinal axis 15 at an intersection point 25a. The tool 20a can be infed in a first infeed motion in a direction 24a that is parallel to a radial direction with respect to the centre longitudinal axis 15. Further, the milling tool 20a can be advanced in an axial direction 23a that is parallel to the longitudinal centre axis 15. Finally, the milling tool 20a can also be infed in a direction 22a that is parallel to a radial direction and perpendicular to the direction 24a. An infeed in the direction of the axes 22a and 24a causes a machining of the rotor 10, which causes a change in the extension in the radial direction of the rotor 10. An advancing along the advancing axis 23a produces a constant radius along the direction of the central longitudinal axis 15.

[0092] Alternatively, the rotor 10 can be machined with a milling tool 20b that rotates about a tool rotation axis 21b to produce the cutting action. In this case, the tool rotation axis 21b is also parallel to a radial direction with respect to the central longitudinal axis 15, but runs offset to the central longitudinal axis 15 and does not intersect it.

[0093] In such a variant, the peripheral cutting edges, preferably only the peripheral cutting edges, of the milling tool 20b can be used. The milling tool 20b is preferably guided tangentially to the desired contour.

[0094] During the manufacturing process, the rotor 10 can be rotated around the central longitudinal axis 15 in order to achieve machining on all sides. This rotation around the central longitudinal axis 15 is synchronised with an infeed movement of the milling tool 20a or the milling tool 20b along the infeed axes 22a, 24a and the advancing axis 23a.

[0095] By superimposing the movements in this way, it is possible to create the shape only by superimposing the movements, but without specifying individual surface points, line, or surface elements for this in a CAD programme. Instead, direct production is possible. Neither the use of further software, such as CAM programmes, 3D models, or individual milling programmes, are necessary. The system limit of the machine with control is not exceeded by any data format.

[0096] The geometry of the rotor 10 preferably has a tangentially continuous, preferably curvilinear, surface. By laterally traversing this surface tangentially, the pitch component of the spiral can be compensated for, such that a milling tool effectively cuts only in the plane (only lateral milling and no plunging or climbing). It is thus possible to cut only with the peripheral cutting edges of the cylindrical milling tool (and not with the face). This means that both the widely used face milling cutters and circumferential end milling cutters can be used. Since circumferential end milling cutters only have cutting edges on the circumference, these tools are more cost-effective than face milling cutters. Furthermore, tool wear is lower on the periphery than on the face, as the cutting edges can be designed more firmly (due to larger installation space). Furthermore, greater feed rates can be realised on the circumference than on the face, as the cutting speed on the lateral surface of a cylinder is constant (with face milling cutters, the cutting speed approaches zero at constant speed in the centre of the tool). The minimisation of tool costs and the use of larger possible feeds are advantages of the method described here compared to conventional CAM methods.

[0097] In contrast to conventional CAM/3D-CAM methods, in which the surface of a 3D body is scanned in the mould by a CAM programme and a milling path is calculated from the points found, which approximates the surface with a defined error, a different strategy is followed in the method described here. The movement path of the milling tool is described by formula and/or parametrically. This allows the milling tool to be moved specifically (tangentially) past the workpiece so that the desired surface results.

[0098] Preferably, all corrections and/or the path distribution are calculated currently in the process and during machining. This makes it possible to use different cylindrical tools for machining. After a tool breakage, for example, machining can be continued with a tool of the same or different type (for example, continuing with a different tool diameter is possible).

[0099] With reference to FIG. 4, three rotors A, B, C manufactured by the method according to the invention or three embodiments of the rotor according to the invention are illustrated. In the rotor according to rotor A of FIG. 4, the cross-sectional area 119 is circular in shape and this circular cross section produced thereby extends from the first end 111 to the second end 112 of the rotor A.

[0100] The rotor B shown in FIG. 4 is a milled rotor with an elliptical geometry, in which the rotor has an elliptical cross section that remains geometrically constant from the front end 211 to the rear end 212 and has a uniform eccentricity to the central longitudinal axis.

[0101] The rotor C shown in FIG. 4 has a circular cross-sectional surface contour that has three geometric features. Firstly, a rounded elevation 319a is formed on one side of the cross-sectional surface, which extends along the entire rotor around the circumference of the worm. Offset by approximately 90 degrees from this protrusion 319a, three smaller rounded protrusions 319b are formed side by side on the outer circumference, also extending helically along the entire worm. Finally, approximately opposite to the elevation 319a, a rounded recess 319c is formed on the cross section, which also extends helically along the entire worm. The characteristics of the features are shown to be constant, but can be varied as desired.

[0102] FIG. 5 shows the cross-sectional geometry of the rotor according to rotor C shown in FIG. 4 in greater detail. As can be seen, the elevations 319a, the triple elevation 319b and the recess 319c lie at defined angles 319a, 319b and 319c to the cross-sectional axes of the cross-sectional area. These angles are predetermined and defined for optimal wear prevention.

[0103] FIG. 6 shows another embodiment of a rotor 410 extending from a first end 411 to a second end 412 along a central longitudinal axis 415. As can be seen, at the first end 411 the rotor has an eccentricity 418a of the central worm axis 414 relative to the central longitudinal axis 415, which decreases in an axial direction 415 along the central longitudinal axis 415 to a smaller eccentricity 418b at the second end 412.

[0104] As can further be seen, the rotor 410 has a cross-sectional diameter 413a at the first end 411 which increases in axial direction 415 along the central longitudinal axis 415 to a cross-sectional diameter 413b at the second end 412.

[0105] The reduction in eccentricity 418a, 418b along the central longitudinal axis 415 and the increase in cross-sectional diameter 413a, 413b along the central longitudinal axis 415 are constant in the axial direction and in a simple potential dependence on the axial position along the central longitudinal axis 415. The reduction in eccentricity and the increase in cross-sectional diameter are inversely related. The reduction and enlargement are chosen such that an outer envelope 430 of the rotor results in a cylindrical body around the central longitudinal axis 415.

[0106] In contrast, a virtual envelope 431 of the central worm axis 414 extends conically from the first end 411 to the second end 412. A virtual envelope 432 of the cross-sectional edge of the worm facing the central longitudinal axis 415 initially decreases to a radius=0 starting from the first end 411 to approximately the centre of the rotor between the first end 411 and the second end 412 and increases again starting from this centre to the second end 412 of the rotor.

[0107] In principle, it should be understood that a rotor illustrated in FIGS. 1 to 6 can be used as a rotor for an eccentric screw pump in all embodiments. However, the body illustrated in FIGS. 1 to 6 can as well be used as a stator core for manufacturing a stator, in particular, for manufacturing a stator using this stator core in a moulding process. It is to be understood that such a stator manufactured with the body according to FIGS. 1 to 6 cannot be operated with a rotor according to FIGS. 1 to 6, since the number of threads of the stator in an eccentric screw pump is greater than that of the rotor.

[0108] FIG. 7 shows a functional principle of manufacturing a rotor in an SLS/SLM process. The rotor 510 is built up in layers on a vertically displaceable building platform 520, whereby the layer plane is perpendicular to the central longitudinal axis of the rotor. From a powder reservoir 530, a thin layer of a powder material is repeatedly applied over a previously selectively cured layer by means of a doctor blade 540. This newly applied layer is then selectively cured in those areas corresponding to the cross-sectional geometry of the rotor 510 in that layer by exposure to a laser beam 550. For this purpose, the laser beam 550 is directed by means of a controllably adjustable deflection device 551 onto these areas of the layer to be selectively cured.

[0109] Curing is done by melting the powder material and simultaneously bonding the powder material of the layer to the previously selectively cured areas of the underlying layer. After this is done, the build platform is lowered by a layer thickness, a new layer is applied, and this layer is again selectively cured as previously described. The thickness of a single layer can be in the range of 50-200 m. The powder material can be a metallic alloy with a grain size in the range between 5-100 m.

[0110] The rotor produced in the additive manufacturing process regularly has the outer geometries and inner microstructures produced by the layer-by-layer manufacturing process. These are often sufficient for the tolerances and sealing requirements of an eccentric screw pump. However, the surface can be smoothed with a geometrically defined or geometrically undefined machining process (e.g., by means of electropolishing) if necessary.

[0111] Preferably, the manufactured rotor is hollow inside or at least has an inner cavity. For rotors for small eccentric screw pumps, an additive manufacturing process is preferable in which a plastic material is processed, for example a 3D printing process or a stereolithography process.