MOLECULAR DRAG PUMPING MECHANISM

The present invention relates to a molecular drag pumping mechanism, and in particular to a Siegbahn pumping mechanism.

Molecular drag pumping mechanisms operate on the general principle that, at low pressures, gas molecules striking a fast moving surface can be given a velocity component from the moving surface. As a result, the molecules tend to take up the same direction of motion as the surface against which they strike, which urges the molecules through the pump and produces a relatively higher pressure in the vicinity of the pump exhaust.

These pumping mechanisms generally comprise a rotor and a stator provided with one or more helical or spiral channels opposing the rotor. Types of molecular drag pumping mechanisms include a Holweck pumping mechanism comprising two co-axial cylinders of different diameters defining a helical gas path therebetween by means of a helical thread located on either the inner surface of the outer cylinder or on the outer surface of the inner cylinder, and a Siegbahn pumping mechanism comprising a rotating disk opposing a disk- like stator defining spiral channels that extend from the outer periphery of the stator towards the centre of the stator. Another example of a molecular drag pumping mechanism is a Gaede mechanism, whereby gas is pumped around concentric channels arranged in either a radial or axial plane. In this case, gas is transferred from stage to stage by means of crossing points between the channels and tight clearance 'stripper' segments between the adjacent inlet and outlet of each stage. Siegbahn and Holweck pumping mechanisms do not require crossing points or tight clearance 'stripper' segments because their inlets and outlets are disposed along the channel length.

An example of a Siegbahn pumping mechanism is illustrated in Figures 1 to 3. The pumping mechanism comprises a rotor in the form of a disk 10 mounted on, or integral with a drive shaft 12 rotatable about axis 14 by a motor (not

shown). The stator 16 comprises a plurality of walls 18 defining a plurality of spiral flow channels 20 within the stator 16 that generate a gas flow from the outer periphery 22 of the stator 16 towards the inner portion 24 of the stator 16, from which gas is exhausted through a central outlet 26. Conversely, the spiral flow channels 20 may be designed such that the pumping action is from the inner portion 24 towards the outer periphery 22 by reversing the relative angle of the channels 20 or the rotation direction of the shaft 12. It is also possible to reverse the rotating and stationary features, such that the plain disc is stationary and the spiral flow channels 20 form part of the rotating component.

For manufacturing purposes the Siegbahn pumping mechanism may be preferred to the Holweck and Gaede pumping mechanisms. However, in the application of molecular drag mechanisms to a vacuum pump, the Holweck pumping mechanism is often considered as providing a higher level of performance at low power.

For a given rotor-stator clearance, the Siegbahn pumping mechanism typically requires more pumping stages to achieve the same levels of compression and pumping speed as the Holweck pumping mechanism. In addition, vacuum pumps which traditionally employ such pumping mechanisms are often able to control tighter clearances in a radial direction (preferential to a Holweck pumping mechanism) than in an axial direction (preferential to a Siegbahn pumping mechanism), further enforcing the need for more pumping stages to achieve the same level of performance. The addition of pumping stages leads to higher levels of power consumption. It is for this reason that turbomolecular pump manufacturers have tended towards the use of Holweck pumping mechanisms in preference to Siegbahn pumping mechanisms (although not exclusively). It is an aim of at least the preferred embodiments of the present invention to reduce the power consumption of a Siegbahn pumping mechanism so that it can provide a viable alternative to a Holweck pumping mechanism.

With reference to Figure 3, each wall 18 of the stator 16 of a Siegbahn mechanism comprises a base 30, a top surface 32 located adjacent the rotating disk 10 and side surfaces 34, 36 extending between the base 30 and the top surface 32. The stator 16 of a Siegbahn mechanism may be produced using a casting process, and a requirement of this casting process is that the side surfaces 34, 36 slightly taper towards the top surface 32 to enable the stator 16 to be ejected from the casting tool. This requirement is generally expressed as a minimum taper angle of around 1.5°, but taper angles of up to 4° may be common in practice.

The power consumed by a Siegbahn pumping mechanism is dominated by the drag between the rotating disk 10 and the top surfaces 32 of the walls 18 of the stator 16. In view of this, the walls 18 of the stator 16 tend to be machined or cast as thinly as possible to reduce the surface area of the top surfaces 32 of the walls 18. However, there is a limit to how thinly the walls 18 may be made before the mechanical strength of the walls 18 is reduced below a threshold value (due to the reduced surface area of the bases 30 which connect the walls 18 to the bulk of the stator 16) and/or before the integrity of the casting process is compromised.

In a first aspect, the present invention provides a Siegbahn pumping mechanism comprising a rotor and a stator located proximate the rotor, one of the rotor and the stator comprising a plurality of walls having side surfaces extending towards a substantially planar surface of the other of the rotor and the stator and defining a plurality of spiral channels, at least one of the side surfaces of each wall tapering towards the other of the rotor and the stator with an angle of taper greater than or equal to 10°.

The angle of taper is thus significantly greater than that which would be required to produce, for example, the stator using a casting process. Of course, taper angles between, say, 4° and 10° may be used, but taper angles of at least 10° are preferred as these can enable a sufficient mechanical

strength to be provided at the base of the walls whilst enabling a relatively small seal size to be achieved between the rotor and the stator, which can significantly reduce the power consumption of the pumping mechanism.

Said at least one side surface may taper in any manner towards the top surface of the wall. In the preferred embodiments, one or both of the side surfaces have a regular taper, but one or both of the side surfaces may be curved or stepped. For any manner of taper of the side surfaces, the area of the base of the wall is preferably at least 100% larger than the area of the top surface, most preferably as much as 300% larger than the area of the top surface. Therefore, in a second aspect, the present invention provides a Siegbahn pumping mechanism comprising a rotor and a stator, one of the rotor and the stator comprising a plurality of walls extending towards a substantially planar surface of the other of the rotor and the stator, each wall having a base, a top surface located proximate the planar surface of the other of the rotor and the stator, and side surfaces extending from the base to the top surface, the walls defining a plurality of spiral channels, at least one of the side surfaces of each wall tapering towards said other of the rotor and the stator so that the area of the base is at least 300% larger than the area of the top surface.

An alternative way of expressing the difference between the base and the top surface of the wall is that the width of the base of the wall is preferably larger than the width of the top surface of the wall by at least 0.35h, where h is the height of the wall. Therefore, in a third aspect the present invention provides a Siegbahn pumping mechanism comprising a rotor and a stator, one of the rotor and the stator comprising a plurality of walls extending towards a substantially planar surface of the other of the rotor and the stator, each wall having a base, a top surface located proximate the other of the rotor and the stator, and side surfaces extending from the base to the top surface, the walls defining a plurality of spiral channels, at least one of the side surfaces of each wall tapering towards said other of the rotor and the stator so that the width of

the base is larger than the width of the top surface by at least 0.35h, where h is the height of the wall. The width of the top surface is preferably between 0.5 and 1.5 mm, whilst the width of the base may be between 1.0 and 4.5 mm depending on the required depth of the spiral channels.

Said one of the rotor and the stator may be produced by casting and/or by machining. The plurality of walls are preferably formed in the stator, although, as discussed above, the plurality of walls may be formed in the rotor.

The present invention also provides a vacuum pump comprising at least one Siegbahn pumping mechanism according to any of the aforementioned aspects. This vacuum pump may comprises at least one turbomolecular pumping stage upstream from said at least one Siegbahn pumping mechanism. The vacuum pump may also comprise additional molecular drag and/or fluid dynamic stages downstream of the Siegbahn pumping mechanism. Examples of these downstream stages include Holweck, Gaede and/or regenerative pumping mechanisms.

Preferred features of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a cross-sectional view of a known Siegbahn pumping mechanism;

Figure 2 is a perspective view of a stator of the mechanism of Figure 1 ;

Figure 3 is a cross-sectional view of a wall of the mechanism of Figure 1 ;

Figure 4 is a cross-sectional view illustrating a first embodiment of a tapered wall of a stator of a Siegbahn pumping mechanism;

Figure 5 is a cross-sectional view illustrating a second embodiment of a tapered wall of a stator of a Siegbahn pumping mechanism; and

Figure 6 is a cross-sectional view of the tapered wall of Figure 5 which indicates the angle of taper of the side surfaces of the wall.

The Siegbahn pumping mechanism has the same configuration as that illustrated in Figures 1 and 2, in that the rotor comprises a disk 10 mounted on, or integral with a drive shaft rotatable about axis by a motor. The disk 10 has a substantially planar surface facing the stator 16. The stator 16 comprises a plurality of walls 18 extending towards the planar surface of the disk 10 defining a plurality of spiral flow channels 20 within the stator 16 that generate a gas flow in a chosen direction between the outer periphery of the stator 16 and the inner portion of the stator 16.

In the first embodiment illustrated in Figure 4, one of the side walls 34 of each wall 18 of the stator 16 tapers towards the top surface 32 of the wall 18 with a taper angle of at least 10°. In the second embodiment illustrated in Figure 5, both of the side walls 34, 36 of each wall 18 of the stator 16 taper towards the top surface 32 of the wall 18 with a taper angle of at least 10°.

Consequently, the area of the base may be as much as 300% larger than the area of the top surface. This can enable the base 30 of the wall 18 to retain a sufficient mechanical strength whilst enabling a relatively small seal size to be achieved between the rotor and the stator, thereby reducing the power consumption of the pumping mechanism. With reference to Figure 6, for the second embodiment of Figure 5 the width, T _mot , of the base 30 may be expressed as:

^τ _rθo< = ^τ _lψ + 2htana

where T _t j _P is the width of the top surface 32, h is the height of the wall 18 and α is the taper angle. For a taper angle of at least 10°, the width of the base of

the wall is thus preferably larger than the width of the top surface of the wall by at least 0.35λ. For example, for a wall of height 1.75 mm having a top surface of width 1 mm, the width of the base is preferably around, or greater than, 1.6 mm, whilst for a wall of height 7 mm having a top surface of width 1 mm, the width of the base is preferably around, or greater than, 3.5 mm.

In summary, a Siegbahn pumping mechanism comprises a rotor and a stator. The stator comprises a plurality of walls, each having a base, a top surface located proximate the rotor, and side surfaces extending from the base to the top surface. The rotor has a substantially planar surface facing the stator. The walls define a plurality of spiral channels within the stator. In order to reduce the power consumed during operation of the pumping mechanism, at least one of the side surfaces of each wall tapers towards the rotor so that the area of the top surface is smaller than the area of the base.