The present invention relates to scroll-type apparatus and, more particularly, to scroll-type apparatus for use in vacuum pumps.

Scroll-type apparatus is known for operating compressors or vacuum pumps. In both cases the apparatus comprises two scroll members each comprising a flat end plate on which are upstanding strips defining a spiral (or scroll) type structure which interact by placing the scroll members together with the respective spiral type structures inter-engaging and allowing one scroll member to "orbit" relative to the other. In this way it is possible to trap a volume of gas between the scroll members thereby to urge the gas from one end of the respective spirals to the other. In a vacuum pump in particular the gas enters via a pump inlet at the periphery of the scroll members and exits via a pump outlet at the centre of the scroll members.

Figure 1 illustrates one known arrangement of such scroll-type apparatus in the form of a vacuum pump 2, which includes a stator 4 comprising a fixed scroll member 6. Complimenting the fixed scroll member 6 and intermeshing therewith in a manner known per se is an orbital scroll member 8. The orbital member 8 is mounted on bearings 10, which are supported on a crank end 12 of a drive shaft 14 extending from a motor 16. Bearings 17 serve to support the drive shaft 14 relative to the housing 4. Each scroll member 6, 8 includes an end plate to which is attached and from which extends outwardly therefrom a spiral wrap 18, 20. The height of each wrap 18, 20 is substantially the same as the distance between the opposed end plates so that the free edge of each wrap 18, 20 forms a seal against the face of the complimentary scroll member end plate. To assist in the sealing process, complaint tip seals (not shown) are commonly provided at the free edge of each wrap 18, 20 to take up the axial clearance between the edge of each wrap 18, 20 and the end plate of the opposing scroll member.

In use, a pumped gas is drawn into the pump 2 from inlet 22 and exits via outlet 24 and non-return valve 25. In order to seal the vacuum space in the housing 4 from ambient atmosphere and the pump drive system, a metal bellows 26 is disposed substantially co-axially with the drive shaft 14. The metal bellows is attached at one end by bolts 28 to the orbital scroll member 8 and at its opposite end to a stationary part 30 of the pump by bolts 32. O-rings 29, 31 serve to seal the attachment of the metal bellows 26.

By virtue of the torsional stiffness of the metal bellows 26, the bellows 26 also serves to limit undesirable travel in the circular translation of the orbital scroll member 26 and thus maintain the correct angular position between the fixed scroll member 6 and the orbital scroll member 8. However, the use of such bellows restricts the design of any vacuum pump incorporating such bellows to the dimensions of adequately sized bellows having the required torsional stiffness and sufficient radial freedom for required crank throw to achieve in practice an acceptable life.

Furthermore, the use of compliant tip seals to provide axial sealing between the scroll members 6, 8 can. generate the dust due to erosion of tip seals during.use of the pump. Whilst this dust is generally conveyed through the outlet 24 together with the exhaust gas, when the pump is switched off this dust could migrate through the inlet 22 towards the evacuated apparatus, leading to unwanted contamination of the evacuated apparatus. The tip seals therefore require regular replacement, and their use further restricts the design of any vacuum pump incorporating such seals to the dimensions of adequately sized tip seals.

It is an aim of at least the preferred embodiment of the present invention to solve these and other problems.

The present invention provides scroll-type pumping apparatus comprising a stator comprising a fixed scroll member having an end plate with a first spiral wrap extending therefrom, an orbital scroll member having an end plate with a second

spiral wrap extending therefrom to intermesh with the first spiral wrap, means for driving the orbital scroll member to orbit relative to the fixed scroll member, and a plurality of flexible rods each having one end connected to the orbital scroll member and another end connected to the stator to support the orbital scroll member relative to the stator.

The use of such rods can provide support of the orbital scroll member relative to the stator with a high degree of axial accuracy, thus enabling very close running clearances between the edge of each spiral wrap and the end plate of the opposite scroll member without the need for tip seals, and so the generation of dust associated with the use of tip seals is avoided.

The rods may also serve to inhibit rotation of the orbital scroll member relative to the fixed scroll member, so that there is no requirement for a separate device for inhibiting rotation of the orbital scroll member. Therefore, the present invention also provides use of a plurality of rods in a scroll-type apparatus having a fixed scroll member and a orbital scroll member, to inhibit rotation of the orbital scroll member relative to the fixed scroll member. Whilst a bellows may be provided to separate a vacuum space from the ambient atmosphere, there is no requirement- for the bellows to have a particular torsional stiffness. As a result, there is a greater freedom of design of a vacuum pump incorporating the apparatus of the present invention, enabling a relatively small, low cost pump to be provided which has good axial sealing between the scroll members.. However, an additional device such as an Oldham coupling may be provided between the orbital scroll member and the stator for preventing rotation of the orbital scroll member relative to the fixed scroll member.

An abrasive coating may be provided on a surface of at least one of the scroll members. The coating may extend over the surface of the end plate and the spiral wrap of the, or each, scroll member.

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The driving means preferably comprises a drive shaft having a crank connected to the orbital scroll member and positioned such that rotation of the drive shaft about a longitudinal axis thereof causes the orbital scroll member to orbit about the longitudinal axis. The flexible rods preferably extend about a longitudinal axis of the drive shaft.

As mentioned above, a flexible member such as a bellows may be provided for separating a vacuum space in the stator from ambient atmosphere. The flexible member may be substantially co-axial with, and/or arranged about, the flexible rods.

The rods may be formed from a metallic or composite material.

The scroll-type apparatus may be used as either a compressor or as an expander, and so the present invention further provides scroll-type apparatus comprising a stator comprising a fixed scroll member having an end plate with a first spiral wrap extending therefrom, an orbital scroll member having an end plate with a second spiral wrap extending therefrom to intermesh with the first spiral wrap, means for driving the orbital scroll member to orbit relative to the fixed scroll member, and a plurality of flexible rods each having one end connected to the orbital scroll member and another end connected to the stator to support the orbital scroll member relative to the stator.

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

Figure 1 is a cross-section through a known scroll-type vacuum pump;

Figure 2 is a simplified cross-section through a first embodiment of a scroll-type vacuum pump;

Figure 3 is a simplified cross-section through a second embodiment of a scroll- type vacuum pump; and

Figure 4 is a cross-section through the vacuum pump of Figure 1 modified according to the present invention.

With reference to Figure 2, a first embodiment of a scroll-type apparatus is in the form of a vacuum pump 102, which includes a stator 104 comprising a fixed scroll member 106. Complimenting the fixed scroll member 106 and intermeshing therewith in a manner known per se is an orbital scroll member 108. The orbital member 108 is attached to a crank provided at one end of the drive shaft 112, which extends from a motor 114. Bearings 116 serve to radially support the drive shaft 112 relative to the stator 104. Each scroll member 106, 108 includes an end plate to which is attached and from which extends outwardly therefrom a spiral wrap 118, 120. The height of each wrap 118, 120 is substantially the same as the distance between the opposed end plates so that the free edge of each wrap 118, 120 forms a seal against the face of the complimentary scroll member end plate. An abrasive coating may be provided on the end plate and wrap of at least one of the scroll members 106, 108. ,«

The orbital scroll member 108 is supported axially relative to the fixed scroll member 106 by a plurality of flexible rods 122 provided about the longitudinal axis of the drive shaft 112. One end of each rod 122 is secured to the stator 104 by any suitable means, for example, adhesive or bolts, and the other end of each rod is similarly secured to the end plate of the orbital scroll member 108. At least three rods 122 are generally required, although any number may be provided. The rods 122 may be formed from any suitable material having a stiffness that allows the rods 122 to flex to allow the orbital scroll member 108 to orbit about the longitudinal axis of the drive shaft 112 relative to the fixed scroll member 106. For example, the rods 122 may be formed from metallic material, such as steel, or a composite material, such as an organic matrix composite material comprising a resin filled with reinforcing particles and/or fibres. This material is preferably a

thermosetting resin, such as epoxy, filled with reinforcing carbon fibres. In this embodiment, the rods 122 also serve to inhibit rotation of the orbital scroll member 108 relative to the fixed scroll member 106. Alternatively, an Oldham coupling may be provided between the stator 102 and the orbital member 108 to inhibit rotation of the orbital scroll member 108 relative to the fixed scroll member 106.

In the embodiment shown in Figure 2, the end plate of the fixed scroll member 106 is integral with the stator 104. Alternatively, the end plate of the fixed scroll member 106 may be provided in the form of a plate mounted on the stator by any convenient method. For example, in the second embodiment shown in Figure 3, the fixed scroll member 106 is mounted on, or integral with, an enclosure 124 for the motor 114, the drive shaft 112 passing through an aperture provided in the fixed scroll member 106 and supported therein by bearings 116.

As shown in Figure 4, the rods 122 may be provided in combination with a flexible member 126 for sealing a vacuum space of the pump from the ambient atmosphere, as previously illustrated in Figure 1 , and where like reference numerals indicate the same features as shown in Figure 1. In the example shown in Figure 4, the rods 122 are external to the flexible member, preferably provided in the form of a bellows 126, although, as an alternative, the bellows 126 may surround the rods 122. In this example, unlike the bellows 26 of the prior art shown in Figure 1 , the bellows 126 are not required to have a torsional stiffness necessary to inhibit rotation of the orbital scroll member 8, as this functionality is provided by the rods 122, and so the bellows 126 may be formed from less rigid, and therefore less expensive, material.

In a further refinement of the design, the stiffness of the rods 122 is such that the natural frequency of the orbital scroll member 8, 108 in a translational (x or y, or combination of both, i.e. circular) mode is matched to the operating speed of the pump 2, 102. In this case radial forces on the drive crank are greatly reduced,

allowing a smaller bearing to be used. Damping devices may also be used to give a flatter response curve and make tuning of the pump more tolerant.