GB2537635A

GB2537635A - Pump

Info

Publication number: GB2537635A
Application number: GB1506773.9A
Authority: GB
Inventors: Andrew Galtry Michael; Turner Neil
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2015-04-21
Filing date: 2015-04-21
Publication date: 2016-10-26
Also published as: GB201506773D0

Abstract

Vacuum screw pump with male 12 and female 14 rotors the male rotors lobes 16 which interlock with the grooves 18 on the female rotor, the lobes and grooves have symmetrical cross sections, with each helix extending through at least 360 degrees about their respective centres of rotation, where the male rotor has a plural number of n lobes balanced about a male centre of rotation A1, where n is at least 2 and the female rotor has n+1 grooves balanced about a female centre of rotation A2. The screw pump may be a dry Quimby mechanism, where the lobe curvature is equal to the curvature of the stator and may extend through approximately 20 degrees around the centre of rotation. A separate apparatus is also claimed where the male rotor has a plural number of n lobes, where n is at least 3 and the female rotor has n-1 grooves, a Lysholm mechanism.

Description

PUMP

The invention relates to a vacuum pump, in particular a dry, non-lubricated, vacuum pump.

Screw pumps are well known and generally comprise one of two types of pumping mechanisms. The first screw pump mechanism, known as a Quimby mechanism, has a pair of intermeshing rotors having respective helical lobes which on rotation compress gas conveyed between an inlet and an outlet. Typically, there is a single lobe on each of the rotors which can be shaped to mesh with a small clearance within the groove on the other rotor and with the stator, making the mechanism suitable for vacuum pumping in a dry environment, in which lubricant does not provide a sealing effect. There is, however, a problem that the rotors can distort under the influence of internal, centrifugally-generated stresses caused by the cross-sectional asymmetry of the lobe profile. Such distortion can result in vibration during operation of the machine, particularly when operated at high frequency. It is not possible to make a symmetrical rotor profile that works well in a Quimby mechanism, as disclosed in EP0736667B1 A second type of screw pump mechanism, often known as a Lysholm mechanism, comprises a pumping mechanism having a pair of intermeshing male and female rotors having respective helical lobes and grooves which, on rotation, convey gas between an inlet and an outlet. Lysholm-type rotors are symmetrical in cross-section and therefore balanced about their centre of rotation and will not distort at high speed. However, conventional Lysholm rotors have very little sealing between the rotor tips on the male rotor and either the female rotor grooves or the stator. This makes most conventional Lysholm mechanisms unsuitable for use in a dry-running vacuum pump device. Instead, Lysholm pumping mechanisms are typically used for oil-sealed positive pressure pumping such as air compressors, where the sealing is adequate. Lysholm air compressor rotors are manufactured in configurations which are single turn or less. That is, the lobes and grooves extend axially through less than 360° about respective central axes of the rotors between the inlet and the outlet of the compressor. A typical configuration comprises four lobes on the male rotor and six grooves on the female rotor. This configuration is adequate for positive pressure, compressor pumping in which gas is urged from inlet to outlet by rotation of the rotors. This configuration is to be contrasted with the configuration of Quimby type vacuum pump mechanisms in which the lobes extend axially through more than 360° about respective central axes of the rotors from the inlet to the outlet. Normally the lobes extend around several full turns of the axis. This latter configuration allows gas to be trapped in isolated pockets which are conveyed from the inlet to the outlet by rotation of the rotors. It is the trapping of multiple gas pockets, creating multiple seals between the inlet and outlet, which is essential to pumping at pressures less than atmosphere.

The typical Lysholm configuration does not make a good vacuum pump because, with less than one turn about the rotation axis, there are no gas pockets formed. Even if multiple turns are executed, the pockets so formed are not isolated and combine one with another when conveyed from the inlet to the outlet.

The present invention provides a screw pump suitable for pumping at pressures less than atmosphere, having rotors which are of a symmetrical design and dynamically balanced.

The present invention provides a vacuum pump comprising a screw pumping mechanism, the pumping mechanism comprising a pair of intermeshing male and female rotors having respective helical lobes and grooves which, on rotation, compress gas conveyed between an inlet and an outlet of said vacuum pump, said male rotor lobes being of symmetrical cross section and balanced about a male centre of rotation, and said female rotor grooves being of symmetrical cross section and balanced about a female centre of rotation; wherein said respective lobes and said grooves extend helically through at least 360° about their respective centres of rotation between the inlet and the outlet and wherein the male rotor has a plural number of 'n' said lobes balanced about a male centre of rotation, where n is at least 2, and the female rotor has n+1 grooves balanced about a female centre of rotation; or where n is at least 3 and the female rotor has n-1 grooves balanced about a female centre of rotation Therefore it is possible to provide a screw type vacuum pump with minimal vibration, in particular a dry screw vacuum pump.

In order that the invention may be well understood, some embodiments thereof, which are given by way of example only, will now be described with reference to the drawings in which: Figure 1 is a perspective view of two rotors of a pumping mechanism matching the description of the new invention; Figure 2 is a cross section taken along line II-II in Figure 1 showing the two rotors in a stator of a dry vacuum pump; and Figure 3 shows a prior art Lysholm compressor type pumping mechanism.

Referring to Figures 1 and 2, a pumping mechanism 10 is shown. The mechanism is a dry pump in which lubricant is not used in the pumping chamber for lubrication or sealing. Lubrication may be used externally of the pumping chamber for example to lubricant the bearings or gears. Although Lysholm pumping mechanisms are typically used for positive pressure pumping applications at pressures above atmosphere, the present mechanism provides vacuum pumping capability by compression of gas at pressures down to about 0.1 mbar or below.

The pumping mechanism 10 comprises two rotors 12, 14. The first rotor 12 is a male rotor having a plurality of helical lobes 16. Two such lobes are shown in Figures 1 and 2, but more than two lobes may be provided depending on pumping requirements. The second rotor 14 is a female rotor having a plurality of helical grooves. Three such grooves are shown in Figures 1 and 2 but more than three grooves may be provided depending on pumping requirements and on the number of lobes on the male rotor. On rotation of the rotors, gas is compressed and conveyed between an inlet 20 and an outlet 22.

The rotors 12, 14 are located in a stator 24 which defines two pumping chamber portions 26, 28 corresponding respectively to the male 12 and female 14 rotors. The pumping chamber portions 26, 28 are generally circular in cross-section. The chamber 26 has a larger radius corresponding to the radial extent of the male rotor and the chamber 28 has a smaller cross-section corresponding to the radial extent of the female rotor.

In use the rotors are rotated by a motor (not shown) about respective rotational axes Al, A2. The rotors are balanced about their respective axes because they have rotational/cross sectional symmetry about the respective axes. This symmetry is to be contrasted with other types of screw pump such as Quimby mechanisms which have rotational/cross sectional asymmetry about their respective axes. The symmetry of the rotors reduces rotor vibration during use (rotation). Therefore, the clearances can be reduced between the male rotor 12 and the female rotor 14 and between the rotors and the stator 24 which in turn reduces leakage, thus providing a more effective pump. Prior art Lysholm pumping mechanisms are used for positive pressure pumping, such as for a supercharger in a car. The helical configurations of the rotors in these types of compressors extend axially through less than 360° about the respective rotational axes Al, A2. For example, in known arrangements, the helical configurations may extend axially through only 270°. At positive pressures the rotors simply urge the gas from the inlet to the outlet. Additionally, these positive pressure pumps are reliant on end face sealing between the axial ends of the rotors and head plates located at the inlet side and the outlet side of the stator.

In order to achieve appreciable vacuum pressures it is a requirement that the helical configurations of the rotor lobes and grooves extend axially through at least 360° thereby forming isolated gas pockets and multiple sealing barriers between the inlet 20 and the outlet 22. A suction phase causes gas at the inlet to be conveyed into the stator where rotation of the rotors traps the gas in a pocket. The gas pockets may then be compressed as they are conveyed towards the outlet by narrowing of the pitch of the rotor lobes and grooves and/or tapering of the stator and rotors. Gas is discharged through the outlet at approximately atmospheric pressure. Since gas pockets are defined between upstream and downstream portions of the rotors, the requirement for end face sealing is reduced, unlike positive pressure pumps where end face sealing is crucial to a pump's effectiveness.

Rotation of the rotors causes the gas pockets to be conveyed towards the outlet allowing pressures less than atmosphere to be achieved. Accordingly, in Figures 1 and 2, the helical lobes 16 of the male rotor 12 and the helical grooves 18 of the female rotor 14 extend through at least 360° about their respective centres of rotation between the inlet and the outlet. It is preferable for achieving high vacuum pressures that the helical configurations of the rotors extend through substantially more than 360° and as shown in Figure 1 each of the helical lobes extends through approximately four full 360° turns about the axis Al and each of the helical grooves extend approximately three full turns about axis A2 to enable a plurality of gas pockets to be formed which are gradually compressed between the inlet and outlet. Although three or four turns are shown in Figure 1, any number of turns can be provided as required.

The number of male lobes and female grooves in Lysholm pumping mechanisms known hereto is not compatible with a helical configuration which extends through at least 360°. For example, there are typically four helical lobes and six helical grooves. With the known number of lobes and grooves, if the helical configurations are extended through more than 360° about the rotational axis the pockets so formed are not isolated one from another, instead each pocket is connected to a pocket closer to the outlet of the pump. Therefore a leakage passage is created through which gas can flow between gas pockets and ultimately between the inlet and the outlet. This leakage passage or duct reduces the compression of gas from one pocket to the next and between the inlet and the outlet.

Accordingly, in Figures 1 and 2, the male rotor has 'n' helical lobes and the female rotor has 'n+l'helical grooves. In the illustrated example there are two lobes and three grooves, but there may be three lobes and four grooves or four lobes and five grooves and so on. The arrangement of n lobes and 'n+1' (where n is 2 or more) or 'n1' (where n is 3 or more) grooves permits helical configurations which extend axially through more than 360° about the rotational axis without creating a leakage passage between gas pockets or between the inlet and the outlet. Arrangements with 'n' lobes on the male rotor and 'n+1' or n-I' grooves on the female rotor also result in isolated pockets being generated within the pumping mechanism. Therefore in the illustrated example, the pumping mechanism is capable of compressing gas in isolated pockets between the inlet and outlet without leakage of gas and can provide a pumping capacity at relatively low pressures in the region of 0.1 mbar.

In the example of a dry pumping mechanism shown in Figures 1 and 2 the shape of at least the male lobes 16 is different from the shape of the male lobes of known Lysholm-type compressor pump mechanisms. A known mechanism is shown in Figure 3. Figure 3 shows partial rotation of a male rotor 56 and a female rotor 58 progressing in time from the left to the right in the drawing to illustrate meshing of a male lobe 50 and a female groove 52. The lobes 50 taper to a point in the illustrated cross-section at their outermost radial extent. When such a rotor is extended in a helical fashion along the rotor, the feature generated by the tip of the male lobe is exceptionally narrow, and does not isolate adjacent pockets sufficiently to sustain adequate compression. As shown, particularly in Figure 1, the shape of the male and female rotors 12, 14 according to the present invention is arranged to reduce leakage between the rotors and between the male rotor 12 and the stator 24. In this regard, the outer radial extent, or tip, of the lobes 16 is curved. Preferably, the tip of the lobes describes in cross-section a circular arc about the rotational axis Al extending through at least 5°, preferably around 10° and more preferably around 20°. This configuration extends the sealing surface in the circumferential direction and reduces leakage. In particular, it will be seen that the circular arc of the lobes corresponds with the circular shape of the pumping chamber portion 26 which is itself circular in cross-section. The surface formed by the lobe tip in the helical extrusion of this cross-section has a substantial width (as seen in Figure 1) and the isolation between adjacent pockets is achieved. Additionally, the shape of the grooves 18 minimises leakage with the lobes. Preferably, as shown in cross-section, the grooves are curved having a radius of curvature which corresponds generally with the radius of curvature of the tip of the lobes.

Claims

CLAIMS1. A vacuum pump comprising a screw pumping mechanism, the pumping mechanism comprising a pair of intermeshing male and female rotors having respective helical lobes and grooves which, on rotation, compress gas conveyed between an inlet and an outlet of said vacuum pump, said male rotor lobes being of symmetrical cross section and balanced about a male centre of rotation, and said female rotor grooves being of symmetrical cross section and balanced about a female centre of rotation; wherein said respective lobes and said grooves extend helically through at least 360° about their respective centres of rotation between the inlet and the outlet and wherein the male rotor has a plural number of 'n' said lobes balanced about a male centre of rotation, where n is at least 2, and the female rotor has n+1 grooves balanced about a female centre of rotation.
2. A vacuum pump comprising a screw pumping mechanism, the pumping mechanism comprising a pair of intermeshing male and female rotors having respective helical lobes and grooves which, on rotation, compress gas conveyed between an inlet and an outlet of said vacuum pump, said male rotor lobes being of symmetrical cross section and balanced about a male centre of rotation, and said female rotor grooves being of symmetrical cross section and balanced about a female centre of rotation; wherein said respective lobes and said grooves extend helically through at least 360° about their respective centres of rotation between the inlet and the outlet and wherein the male rotor has a plural number of 'n' said lobes balanced about a male centre of rotation, where n is at least 3 and the female rotor has n-1 grooves balanced about a female centre of rotation.
3. A vacuum pump as claimed in any of the preceding claims, wherein vacuum pump is a dry pump and the tip of the lobes is curved to correspond with the internal curvature of the stator.A dry vacuum pump as claimed in claim 3, wherein the tip of the lobes has a circular arc in cross-section about the centre of rotation.A dry vacuum pump as claimed in claim 3 or 4, wherein the tip of the lobes extends through approximately 20° around the centre of rotation.