VACUUM PUMP SCREW ROTOR

Abstract

A vacuum pump screw rotor, comprising at least two helical displacer elements on a rotor shaft. The at least two displacer elements have different pitches, but the pitches of each displacer element are constant. Furthermore, the displacer elements each have a helical recess, each having a contour that remains the same over its entire length. Hereby, a suction-side displacer element has a recess having an asymmetric contour, and a pressure-side displacer element has a recess having a symmetrical contour.

Claims

1. A vacuum pump screw rotor, comprising at least two helical displacer elements arranged on a rotor shaft, wherein the at least two displacer elements have pitches differing from each other but being constant for each displacer element, and wherein the displacer elements each comprise at least one helical recess, each recess having a uniform contour over its entire length, wherein a suction-side displacer element has an asymmetric contour, and wherein a pressure-side displacer element has a symmetric contour.

2. The vacuum pump screw rotor according to claim 1, wherein at least two rotor elements comprising respective helical displacer elements are provided, wherein the displacer elements have pitches differing from each other but being constant for each displacer element.

3. The vacuum pump screw rotor according to claim 1, wherein the pressure-side displacer element comprises more than 8 windings.

4. The vacuum pump screw rotor according to claim 1, wherein a pressure-side displacer element is of the single-threaded type.

5. The vacuum pump screw rotor according to claim 1, wherein the rotor shaft and the displacer elements are of a one-pieced design.

6. The vacuum pump screw rotor according to claim 1, wherein the at least one change of pitch between two adjacent displacer elements is non-uniform (abrupt).

7. The vacuum pump screw rotor according to claim 1, wherein the profile of the suction-side displacer element is free of blowholes at least on one of the flanks.

8. The vacuum pump screw rotor according to claim 1, wherein, between two displacer elements, a tool run-out zone is provided at the change of pitch.

9. (canceled)

10. The vacuum pump screw rotor according to claim 1, wherein the entire vacuum pump screw rotor is made of aluminum or an aluminum alloy.

11. The vacuum pump screw rotor according to claim 1, wherein the aluminum has a lower expansion coefficient, particularly less than 18*10.sup.?6/K, and that particularly a high silicon percentage of at least 15% is provided.

12. A screw vacuum pump, comprising two mutually meshing screw rotors according to claim 1, a housing enclosing the screw rotors, and a drive means connected to the two screw rotors.

13. The screw vacuum pump according to claim 12, wherein the internal compression of the screw vacuum pump is at least 2.

14. The screw vacuum pump according to claim 12, wherein the screw rotors have a lower expansion coefficient than the housing, wherein the expansion coefficient of the housing are about 5% larger than that of the screw rotors.

15. The screw vacuum pump according to claim 12, wherein the housing is made of aluminum or an aluminum alloy.

16. The screw vacuum pump according to claim 12, wherein, between the pressure-side displacer elements and the housing, a gap is arranged, said gap having a height in the range of 0.05 mm to 0.5 mm.

17. A method for producing a screw rotor according to claim 1, comprising the steps of: providing a base body of the screw rotor, generating a helical recess of a first displacer element by use of a form cutter or a grinding screw, and generating a further helical recess of a further displacer element by use of a further form cutter or grinding screw.

18. The method according to claim 17, wherein the manufacturing of displacer elements with symmetric profile is performed by use of a single tool, particularly in one working step.

19. The method according to claim 17, wherein, between adjacent displacer elements, prior to generating the helical recesses, a particularly circular cylindrical recess is generated as a tool run-out zone.

20. The method according to claim 17, wherein, between two adjacent displacer elements, a recess is generated in at least one flank for withdrawal of the tool.

21. The method according to claim 17, wherein, to generate the helical recesses, use is made, for each displacer element, of a tool reproducing the outer contour of the helical recess.

22. The method according to claim 17, wherein the base body is cylindrical.

23. The method according to claim 17, wherein the base body is formed as a semi-finished product with already preformed recess and/or bearing pin.

24. The vacuum pump screw rotor according to claim 1, wherein, between two displacer elements, a void is provided at the change of pitch in at least one of the flanks of the displacer elements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The following is shown:

[0027] FIG. 1 shows a schematic plan view of a first preferred embodiment of a vacuum pump screw rotor,

[0028] FIG. 2 shows a schematic plan view of a second preferred embodiment of a vacuum pump screw rotor,

[0029] FIG. 3 shows a schematic sectional view of displacer elements with asymmetric profile,

[0030] FIG. 4 shows a schematic sectional view of displacer elements with symmetric profile, and

[0031] FIG. 5 shows a schematic sectional view of a screw vacuum pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0032] According to the first preferred embodiment of the vacuum pump screw rotor, the rotor comprises two displacer elements 10, 12. A first, suction-side displacer element 10 has a large pitch of about 10-150 mm/revolution. The pitch is constant along the entire displacer element 10. Also the contour of the helical recess is constant. The second, pressure-side displacer element 12 again has, along its length, a constant pitch and a constant contour of the recess. The pitch of the pressure-side displacer element 12 is preferably in the range of 10-30 mm/revolution. Between the two displacer elements, a ring-shaped cylindrical recess 14 is provided. Said recess has the purpose of realizing a tool run-out zone in view of the one-pieced design of the screw rotor shown in FIG. 1.

[0033] Further, the one-pieced screw rotor comprises two bearing seats 16 and shaft end 18. To the shaft end 18, there is connected e.g. a toothed wheel for driving.

[0034] In the second preferred embodiment shown in FIG. 2, the two displacer elements 10, 12 are produced separately and will then be fixed on a rotor shaft 20 e.g. by pressing them on. This production method may be somewhat more complex but there is obviated the need for the cylindrical distance 14 between two adjacent displacer elements 10, 12 for tool run-out. The bearing seats 16 and the shaft ends 18 can be integral components of the displacer elements. Alternatively, a continuous shaft 20 can also be produced from another material that is different from the displacer elements 10, 12.

[0035] FIG. 3 shows a schematic lateral view of an asymmetric profile (e.g. a Quimby profile). The asymmetric profile shown is a so-called Quimby profile. The sectional view shows two screw rotors which mesh with each other and whose longitudinal direction extends vertically to the plane of the drawing. The rotation of the rotors in opposite senses in indicated by the two arrows 15. With respect to a plane 17 extending vertically to the longitudinal axis of the displacer elements, the profiles of the two flanks 10 and 21 are different in each rotor. Thus, the mutually opposite flanks 19, 21 have to be produced independently from each other. However, in the manufacture which for this reason is somewhat more complex and difficult, an advantage resides in that there does not exist a throughgoing blowhole but only a short circuit between two adjacent chambers.

[0036] Such a symmetric profile is preferably provided in the suction-side displacer element 10.

[0037] The schematic lateral view in FIG. 4, in turn, shows a sectional view of two displacer elements and respectively two screw rotors which again rotate in opposite senses (arrows 15). With respect to the axis of symmetry 17, the flanks 23 have a symmetric design in each displacer element. In the preferred embodiment of a symmetrically designed contour shown in FIG. 4, a cycloidal profile is used.

[0038] A symmetric profile as shown in FIG. 4 is preferably provided in the pressure-side displacer elements 12.

[0039] The further embodiment, shown in FIG. 5, is again of a one-pieced design. For withdrawal of the tool, such as e.g. an end mill, the flank of the displacer element 12 is provided with a recess or void.

[0040] Further, it is possible to provide more than two displacer elements. These can optionally have different head diameters and corresponding foot diameters. Herein, it is preferred that a displacer element with larger head diameter is arranged at the inlet, i.e. on the suction side, so as to realize a larger suctional capacity in this region and/or to increase the volume ratio. Also combinations of the above described embodiments are possible. For instance, two or more displacer elements can be produced in one piece with the shaft, or an additional displacer element can be produced independently from the shaft and then be mounted on the shaft.

[0041] A schematic sectional view of a vacuum pump (FIG. 5) shows, within a housing 22, two vacuum pump screw rotors 26 arranged in a pumping chamber 24. The two rotors are supported in the housing via bearings 28. Connected to two shaft ends 18 are respective toothed wheels 32. The latter mesh with each other, thus ensuring a synchronization of the two shafts. One of the two toothed wheels 32 is coupled to a drive means such as e.g. an electric motor.

[0042] As can be seen in FIG. 5, the suctional intake of the gas occurs in the region of the suction-side displacer elements 10, as indicated by arrow 34. Discharge of the gas occurs, correspondingly, at the end of the second, pressure-side displacer element 12, as indicated by arrow 36.