SCREW-TYPE VACUUM PUMP

Abstract

A screw vacuum pump comprises a housing forming a pumping chamber, wherein the housing is made of aluminum or an aluminum alloy. Further provided are two screw rotors arranged in the pumping chamber, each screw rotor comprising at least one displacer element having a helical recess for forming a plurality of windings, wherein the at least one displacer element is made of aluminum or an aluminum alloy. Between the region in which prevail 5% to 30% of the outlet pressure and a pressure-side end of the rotor (pump outlet), at least six, particularly at least eight, and with particular preference at least ten windings are provided.

Claims

1. A screw vacuum pump comprising a housing defining a pumping chamber, wherein the housing is made of aluminum or an aluminum alloy, two screw rotors arranged in the pumping chamber, each screw rotor comprising at least one displacer element having a helical recess for defining a plurality of windings, wherein the at least one displacer element is made of aluminum or an aluminum alloy, wherein, between the region in which prevail 5% to 20% of the outlet pressure and a pressure-side end of the rotor (pump outlet), at least six, particularly at least eight, and with particular preference at least ten windings are provided.

2. The screw vacuum pump according to claim 1, wherein the one displacer element is designed as pressure-side displacer element and, for each screw rotor, at least one further displacer element is provided.

3. The screw vacuum pump according to claim 2, wherein the pressure-side displacer element causes a pressure ratio of less than 20.

4. The screw vacuum pump according to claim 2, wherein the pressure-side displacer element has an average working pressure of more than 50 mbar at least in 6 windings.

5. The screw vacuum pump according to claim 1, wherein, between a surface of the displacer element and an inner surface of the pumping chamber, a gap having a height in the range from 0.05 mm to 0.3 mm is formed.

6. The screw vacuum pump according to claim 2, wherein the pressure-side displacer elements have a constant pitch over their entire length.

7. The screw vacuum pump according to claim 2, wherein the recesses of the pressure-side displacer elements have a uniform, in particular symmetrical contour over their entire length.

8. The screw vacuum pump according to claim 2, wherein the pressure-side displacer elements are single-threaded.

9. The screw vacuum pump according to claim 1, wherein each screw rotor comprises a rotor shaft supporting the at least one displacer element.

10. The screw vacuum pump according to claim 1, wherein the displacer elements of a screw rotor are formed in one piece.

11. The screw vacuum pump according to claim 1, wherein a screw rotor made of aluminum or an aluminum alloy has a low expansion coefficient and particularly an expansion coefficient of less than 22*10.sup.6 1/K.

12. The screw vacuum pump according to claim 1, wherein the screw rotors, and particularly the at least one displacer element have, for each screw rotor, a lower expansion coefficient than the housing, wherein the expansion coefficient of the housing particularly is at least larger than that of the screw rotors and respectively of the at least one displacer element.

13. The screw vacuum pump according to claim 1, wherein the screw rotors do not have a rotor interior cooling.

14. The screw vacuum pump according to claim 1, wherein the screw rotors do not comprise channels having coolant flowing through them.

15. The screw vacuum pump according to claim 1, wherein the screw rotors are solid.

16. The screw vacuum pump according to claim 2, wherein a temperature difference in the region of the pressure-side displacer elements between these and the housing in normal operation is less than 50K.

17. The screw vacuum pump according to claim 1, wherein, in the region of the pressure side displacer elements, the mean heat flow density is less than 20000 W/m.sup.2.

18. The screw vacuum pump according to claim 1, wherein the distance between the region in which prevail 5% to 20% of the outlet pressure, up to the last winding of the pressure-side displacer element is at least in the range from 20% to 30% of the rotor length.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The following is shown:

[0033] FIG. 1 shows a schematic plan view of a first preferred embodiment of a screw rotor of the screw vacuum pump of the disclosure,

[0034] FIG. 2 shows a schematic plan view of a second preferred embodiment of a screw rotor of the screw vacuum pump of the disclosure,

[0035] FIG. 3 shows a schematic sectional view of displacer elements with asymmetric profile,

[0036] FIG. 4 shows a schematic sectional view of displacer elements with symmetric profile, and

[0037] FIG. 5 shows a schematic sectional view of a screw vacuum pump.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0038] The screw rotors shown in FIGS. 1 and 2 can be used in a screw vacuum pump as shown in FIG. 5.

[0039] 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.

[0040] 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.

[0041] 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 shafts 20. Alternatively, a continuous shaft 20 can also be produced from another material that is different from the displacer elements 10, 12.

[0042] 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.

[0043] Such a symmetric profile is preferably provided in the suction-side displacer element 10.

[0044] 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.

[0045] A symmetric profile as shown in FIG. 4 is preferably provided in the pressure-side displacer elements 12.

[0046] 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.

[0047] In the schematic view of FIG. 5, showing a preferred embodiment of a screw vacuum pump of the disclosure, two screw rotors as shown in FIG. 1 are arranged in a housing 26. The vacuum pump housing 26 comprises an inlet 28 through which gas is sucked in the direction of arrow 30. The inlet 28 is connected e.g. to a chamber which is to be evacuated. Pump housing 26 further comprises a pressure-side outlet 32 through which gas is discharged in the direction of arrow 38. Preferably, the screw vacuum pump of the disclosure will pump immediately against atmosphere so that no pre-vacuum pump is connected to the outlet 32 anymore, while this would also be possible.

[0048] In the illustrated exemplary embodiment, the two pressure-side displacer elements 12 comprise 10 windings per screw rotor. Particularly, in a region 40, i.e. in a region of the first winding of the pressure-side displacer element 12 as viewed in the conveying direction, there prevails a pressure of 5%-20% of the pressure prevailing at the outlet 32.

[0049] Between the surfaces 42 of the two pressure-side displacer elements 12 and an inner surface 44 of a pumping chamber 46 defined by the pump housing 26, a gap is formed whose height is preferably in the range from 0.05 mm-0.3 mm and particularly in the range from 0.1 mm-0.2 mm.

[0050] In the illustrated exemplary embodiment, the vacuum pump housing 26 is closed by two housing covers 47. The left housing cover 47 in FIG. 4 comprises two bearing seats in which respectively one ball bearing 48 arranged for support of the two rotor shafts. On the right-hand side in FIG. 4, the shaft journals 50 of the two screw rotor shafts extend through the covers 47. On the outer side, the two shaft journals 50 have a respective toothed wheel 52 arranged on them. In the illustrated exemplary embodiment, the toothed wheels 52 mesh with each other for mutual synchronization of the two screw rotors. Further, also in the right-hand cover 47 as viewed in FIG. 4, two bearings 48 are arranged for support of the screw rotors.

[0051] The lower shaft in FIG. 5 is the drive shaft, which is connected to a drive motor, not shown.

[0052] Particularly good results according to the disclosure can obtained by the following specification which therefore is especially preferred:

TABLE-US-00001 material of housing AlSi7Mg (cast, expansion coefficient 22 * 10.sup.6K.sup.1 or AlMg0.7Si (extrusion, expansion coefficient 23 * 10.sup.6K.sup.1) material of rotor AlSi9Mg (cast, expansion coefficient 21 * 10.sup.6K.sup.1) or AlSi17Cu4Mg (cast, expansion coefficient 18 * 10.sup.6K.sup.1) Silicon percentage at least 9%, particularly preferred more than 15% of rotor thermal expansion at least 5% larger, particularly preferred 10% larger coefficient of housing/rotor

[0053] Intermediate Pressure Between the Suction-Side and the Pressure-Side Displacer Element:

[0054] Pressure Ratio

[0055] Outlet Pressure/Intermediate Pressure

[0056] Particularly Preferred Less than:

[00001]$\frac{1000.Math..Math.\mathrm{mbar}}{200.Math..Math.\mathrm{mbar}}=5.Math..Math.\mathrm{intermediate}.Math..Math.\mathrm{pressure}=20.Math.\%.Math..Math.\mathrm{outlet}.Math..Math.\mathrm{pressure}$

[0057] Particularly less than

[00002]$\frac{1000.Math..Math.\mathrm{mbar}}{100.Math..Math.\mathrm{mbar}}=10.Math..Math.\mathrm{intermediate}.Math..Math.\mathrm{pressure}=10.Math.\%.Math..Math.\mathrm{outlet}.Math..Math.\mathrm{pressure}$

[0058] Less than

[00003]$\frac{1000.Math..Math.\mathrm{mbar}}{50.Math..Math.\mathrm{mbar}}=20.Math..Math.\mathrm{intermediate}.Math..Math.\mathrm{pressure}=5.Math.\%.Math..Math.\mathrm{outlet}.Math..Math.\mathrm{pressure}$

[0059] height of cold gap 0.05 mm-0.3 mm [0060] Particularly preferred 0.1 mm-0.2 mm