Vacuum pump screw rotors with symmetrical profiles on low pitch sections

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 at least two 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 the 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 at least two displacer elements are of a one-pieced design.

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

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

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

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

10. The vacuum pump screw rotor according to claim 9, wherein the aluminum or aluminum alloy has an expansion coefficient of less than 18*10-6/K, and has a silicon percentage of at least 15%.

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

12. The screw vacuum pump according to claim 11, comprising an internal compression of at least 2.

13. The screw vacuum pump according to claim 11, wherein the two vacuum pump screw rotors have a lower expansion coefficient than the housing, wherein the expansion coefficient of the housing is 5% larger than that of the screw rotors.

14. The screw vacuum pump according to claim 11, wherein the housing is made of aluminum or an aluminum alloy.

15. The screw vacuum pump according to claim 11, 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.

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

17. A vacuum pump screw rotor, comprising: a rotor shaft; a suction-side displacer element arranged on the rotor shaft, wherein the suction-side displacer element has an asymmetric contour and has a first helical recess with a uniform contour over an entire length of the suction-side displacer element, and wherein the suction-side displacer element has an asymmetric contour; a pressure-side displacer element arranged on the rotor shaft, wherein the pressure-side displacer element has a symmetric contour and has a second helical recess with a uniform contour over an entire length of the pressure-side displacer element, and wherein the pressure-side displacer element has a symmetric contour, wherein the suction-side displacer element and the pressure-side displacer element have constant pitches that differ from each other; and a ring-shaped cylindrical recess between the suction-side displacer element and the pressure-side displacer element, wherein the ring-shaped cylindrical recess is sized and configured as a tool run-out zone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The following is shown:

(2) FIG. 1 shows a schematic plan view of a first preferred embodiment of a vacuum pump screw rotor,

(3) FIG. 2 shows a schematic plan view of a second preferred embodiment of a vacuum pump screw rotor,

(4) FIG. 3 shows a schematic sectional view of displacer elements with asymmetric profile,

(5) FIG. 4 shows a schematic sectional view of displacer elements with symmetric profile, and

(6) FIG. 5 shows a schematic sectional view of a screw vacuum pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(7) 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 or void 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.

(8) 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.

(9) 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.

(10) 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 19 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.

(11) Such a symmetric profile is preferably provided in the suction-side displacer element 10.

(12) 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.

(13) A symmetric profile as shown in FIG. 4 is preferably provided in the pressure-side displacer elements 12.

(14) 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.

(15) 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.

(16) 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 22 via bearings 28 with a gap 30 defined between the housing 22 and the second, pressure-side displacer element 12. The gap 30 has a height in a range of 0.05 mm to 0.5 mm. 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.

(17) 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.