FIELD OF THE INVENTION

The field of the invention relates to a Holweck drag pump and to a method of manufacturing such a pump.

BACKGROUND

Drag pumps operate by imparting momentum to molecules in a fluid within the pump in a direction from an inlet towards an outlet. Contours on one surface of the pump form channels that run from the inlet towards the outlet. There is a corresponding surface facing and close to the contoured surface. Relative rotation of the two surfaces pushes gas molecules along the channels. Drag pumps may operate in both the molecular flow region and the continuous flow regions.

A Holweck drag pump comprises cooperating cylindrical surfaces, one of which comprises one or more helical channels along which the fluid is driven. The component providing this channel, is required to be both structurally stiff, not unduly bulky and resistant to high temperatures and some corrosive substances. These are complex requirements that have conventionally been met by machining a metal such as aluminium to provide the contoured surface. This process can be expensive and the machined metal structure is relatively thick.

It would be desirable to provide a compact, robust Holweck drag pump that is relatively inexpensive to manufacture.

SUMMARY

A first aspect provides a Holweck drag pump comprising: at least one hydroformed cylinder comprising a contoured surface formed by hydroforming a metal cylinder, said contoured surface providing helical paths for fluid flowing from an inlet end of said hydroformed cylinder to an outlet end; said at least one hydroformed cylinder being mounted such that said contoured surface faces a plane cylindrical surface; said at least one hydroformed cylinder and said plane cylindrical surface being mounted for relative rotational movement, such that rotation of one surface relative to the other pushes gas molecules along said helical paths from said inlet to said outlet end of said at least one hydroformed cylinder.

A Holweck drag pump provides a helical channel along which a fluid flows. The component providing this channel, is required to be stiff, relatively compact and resistant to high temperatures and some corrosive substances. These requirements have conventionally been met by machining a metal such as aluminium to provide the contoured surface providing these channels. This process can be expensive and the structure is relatively thick. It was recognised that an alternative way of manufacturing such a component might be to generate the contoured surface through hydroforming.

Hydroforming is a cost-effective way of shaping ductile metals into lightweight, structurally stiff and strong pieces with often complex shapes that has conventionally been used in the automotive and bicycle industry. It was recognised that the properties of a hydroformed component were generally the properties required of the contoured component of the drag pump. Furthermore, the cylindrical shape of such a component lends itself particularly well to hydroforming. The use of hydroforming to produce the contoured surface required for the channels, provides for a component that is cheap and energy efficient to manufacture, requires less material than conventional manufacturing methods and provides extremely good repeatability.

Although the helical paths may run between either ends of said hydroformed cylinder, in some embodiments, they may not extend to the very end, but may run through a substantial middle portion of the cylinder.

In some embodiment, a base of said helical path at a furthest distance from said plane cylindrical surface comprises a curved cross section. A further advantage of using hydroforming relates to the preferred shape of the channels in the Holweck pump. In this regard, curved channels with no sharp corners have been found to be more effective at pumping a fluid and hydroforming as a method of manufacture lends itself naturally to generating shapes without sharp corners.

In some embodiments, said at least one hydroformed cylinder comprises a thickness of between 0.8 and 1.3 mm, preferably between 0.9mm and 1.1 mm.

A further advantage of a Holweck drag pump where the cylinder having the contoured surface is formed by hydroforming is that this cylinder may be relatively thin and yet still provide the structural stiffness required. This allows a more compact mechanism to be provided and indeed may allow additional stages to be included in the same volume. In this regard the metal cylinder may have a thickness close to 1 mm.

Although, the cylinder may be formed of any type of ductile metal in some embodiments said metal cylinder is formed of aluminium or stainless steel.

Aluminium and stainless steel both have many of the properties required for a Holweck pump and may be formed into a shape by hydroforming.

In some embodiments the hydroformed cylinder comprises end surfaces which each extend radially beyond said contoured surface in either an inner radial or outer radial direction.

Having end surfaces which extend radially beyond the channels formed by the contoured surface allows the cylinder to be mounted to the pump housing or to another cylinder, with each end portion providing an attachment surface. ln some embodiments, the Holweck pump comprises two hydroformed cylinders each comprising a contoured surface comprising said helical paths, said two hydroformed cylinders being mounted one within the other, an outer one of said two hydroformed cylinders being larger than an inner one of said two hydroformed cylinders, said inner hydroformed cylinder comprising an enlarged diameter portion which extends radially beyond said contoured surface at each end, and said outer hydroformed cylinder comprising a reduced diameter portion at each end, said inner and outer hydroformed cylinders being joined at said respective reduced and enlarged diameter portions.

Holweck drag pumps may be configured to have multiple surfaces each having helical flow paths, the surfaces being mounted in parallel such that the fluid is pumped along one, and back along the other, allowing the fluid to be further compressed as it is pumped along a longer path with reducing channel cross section. In effect each cylinder with a contoured surface forms a stage of the pump and each stage may have a different channel shape, subsequent stages in the direction of flow comprising narrower channels to create more compression. Hydroformed cylinders lend themselves particularly well to providing such contoured surfaces, with the hydroforming also being used to generate end portions on the cylinders that extend beyond the channels allowing the cylinders to be mounted to each other in a back to back arrangement or be mounted to the pump housing.

The contact points between the back to back stages or the contact point between the back of a stage and the pump housing is at either end of the cylindrical surface and advantageously extend only slightly beyond the furthest extent of a channel, the additional extent being less than 10% of the depth of the channel, such an arrangement providing a particularly compact configuration.

In some embodiments, said inner and outer hydroformed cylinders are joined by welding or by gluing said reduced and enlarged diameter end portions together. It is advantageous if the inner and outer cylinders are sealed together to avoid any gas between them leaking into the fluid flow. They may be joined by welding or in some cases or by gluing. Either ends are attached together and in preferred embodiments the reduced and enlarged diameter end portions have substantially the same size such so that they fit together well.

In some embodiments, said Holweck drag pump comprises a further plane cylindrical surface, said two joined cylinders being mounted such that an inner face of said inner cylinder and an outer face of said outer cylinder face respective plane cylindrical surfaces, said plane cylindrical surfaces and said two joined cylinders being mounted for relative rotation.

There may be several stages within the drag pump with the gas being pumped along one spiral channel and then in the other direction along a further spiral channel with the compression increasing by decreasing the cross-sectional area of the channels. Given the thinness of the hydroformed contoured cylinders, there may be enough space within the pump for several of these stages thereby improving performance.

In some embodiments, said at least one hydroformed cylinder comprises at least a portion of a stator of said drag pump, said drag pump further comprising a rotor, said rotor comprising at least one rotationally mounted cylinder comprising an inner and outer plane cylindrical surface each plane surface being configured to face a contoured surface of said stator.

Although, the contoured surface may rotate to provide the relative motion between the contoured surface and the plane surface, in some cases it is the plane surface that rotates as it is easier to balance a component with a plane surface than one with a contoured surface. In other embodiments, both the contoured and the plane surface may rotate in opposite directions thereby providing increased pumping speeds. ln some embodiments, at least one of said at least one hydroformed cylinder is mounted to a cylindrical housing of said pump via an adhesive or via welding.

Again, in order to avoid or at least inhibit trapped gas leaking into the vacuum the hydroformed cylinder may be sealed to the housing at the point that it is mounted to the housing. To do this welding or an adhesive may be used. Where it is an adhesive an epoxy adhesive in some embodiments a two-part epoxy adhesive may be used.

In some embodiments, said stator comprises an outer hydroformed cylinder and an inner hydroformed cylinder each mounted to a respective portion of a housing of said pump, said two joined cylinders being mounted therebetween; and said rotor comprises two rotationally mounted cylinders each comprising an inner and outer plane cylindrical surface an outer rotationally mounted cylinder being mounted between said outer hydroformed cylinder and said two joined cylinders and an inner rotationally mounted cylinder between mounted between said two joined cylinders and said inner hydroformed cylinder.

A second aspect provides a method of manufacturing a Holweck pump according to any preceding claim, comprising: hydroforming at least one metallic cylinder to form a corresponding at least one hydroformed cylinder having a contoured surface providing helical paths for directing fluid flowing from an inlet end of said cylinder to an outlet end; mounting said at least one hydroformed cylinder within a pump housing such that said contoured surface faces a plane cylindrical surface, said mounting being such that relative rotational movement is provided between said surfaces, such that rotation of one surface relative to the other pushes gas molecules along said helical paths from said inlet to said outlet end of said at least one cylinder.

In some embodiments, said method comprises: hydroforming two cylinders one being smaller than the other; mounting said two hydroformed cylinders one within the other, an inner one of said two hydroformed cylinders comprising an enlarged diameter portion which extends radially beyond said contoured surface at each end, and said outer hydroformed cylinder comprising a reduced diameter portion at each end; joining said inner and outer hydroformed cylinders by welding said respective reduced and enlarged diameter portions together.

It should be noted that the step of joining the inner and outer cylinder by welding may be done in situ using a robotic automation manufacturing technique.

In some embodiments, said method comprises hydroforming a further two cylinders, said further two hydroformed cylinders being respectively larger and smaller than said two hydroformed cylinders; mounting said largest and said smallest hydroformed cylinder to respective portions of a pump housing using an epoxy; mounting said two joined hydroformed cylinders between said largest and smallest cylinders. rotationally mounting a double rotor comprising two cylinders, such that an outer rotor cylinder is between said largest hydroformed cylinder and said two joined hydroformed cylinders and an inner rotor cylinder is between said smallest hydroformed cylinder and said two joined hydroformed cylinders.

A further aspect provides a Holweck pump manufactured according to a method of a second aspect.

Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims. Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 schematically shows how hydroforming may be used to generate a contoured surface of a Flolweck drag pump;

Figure 2 schematically shows a section through the two end stages of a stator of a Flolweck drag pump;

Figure 3 schematically shows section through the two middle stages of a Flolweck drag pump according to an embodiment;

Figure 4 shows a section through the assembled drag pump; and

Figure 5 shows a flow diagram illustrating steps in a method manufacturing a

Flolweck drag pump according to an embodiment.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided.

Contoured surfaces are formed by hydroforming a tube having a thickness of 1 mm or more. The hydroforming is performed to provide contours of a helical shape, providing a spiral channel from an inlet end to an outlet end of the tube. This contoured tube is mounted for relative rotational movement close to and facing a plane cylindrical surface which together provide a stage of a Flolweck pump. Rotation of the plane surface relative to the contoured surface driving gas molecules along the helical path from the inlet end to the outlet end.

The manufacturing process uses hydroforming to generate the contoured surface. The hydroforming process improves the repeatability, reduces the manufacturing costs, the energy spent during the manufacturing process, and increases the manufacturing speed, and reduces the manufacturing errors.

Where multiple stages are manufactured the middle stages are mounted placed back to back, and in this case a portion of the tube that extends beyond the channels is provided at both ends and these portions may be welded together. A robotic automated process may be used for the welding process, which may allow it to occur in -situ when building the pump.

For the end stages that are in contact with a surface of the pump or motor housing the Holweck stage may be bonded using perhaps a 2-part epoxy adhesive (epoxy + hardener).

A hydroformed cylinder with a contoured surface can be distinguished by a skilled person from other cylinders with contoured surfaces by one or more of the shape, thickness and surface finish. A cylinder with a machined contoured surface, for example, will carry the marks of the machining and will have a less brilliant surface than a hydroformed one, it will also generally be thicker. A rolled contoured cylinder, will also be distinguishable by the form of the contours and in particular, will not have end surfaces extending beyond the channel heights which portions allow the cylinder to be mounted to a housing or back to back with another contoured cylinder.

Figure 1 schematically shows the hydroforming process performed on a cylindrical tube 8 of about 1 mm in thickness to generate a contoured surface stator stage 10. Flydroforming is a process that is performed on a ductile metal where a metal tube is placed in proximity to and within a mould, the mould having the desired shape, in this case contours that form helical channels running from close to one end to close to the other. The tube is filled with a liquid under pressure such that the tube deforms against the surface of the mould and take the shape of the mould. In this way, a contoured surface may be generated in an energy efficient, low cost and repeatable manner. Furthermore, the process allows the contoured surface to be formed on a relatively thin tube that nevertheless is comparatively stiff and forms a robust surface for the drag pump. By suitable choice of the metal of the tubing, a contoured surface that is resistant to relatively high temperatures and resistant to some corrosive substances may be provided. In some embodiments, aluminium is used and in others stainless steel,

Figure 2 schematically shows a section through two stator stages of a Holweck drag pump without the rotor in place. In this embodiment, pump housing 20 which includes the outer pump housing and the inner pump housing which surrounds the motor have respective hydroformed cylinders 10A and 10D attached to them at points 22. In this embodiment, they are attached using an adhesive, while in other embodiments the hydroformed cylinders may be welded to the pump housing. The outer hydroformed cylinder 10A forms a first stage of the Holweck drag pump and the channels are relatively wide. A rotor with a plane surface is mounted close to this hydroformed cylinder and rotation of the rotor will cause gas to flow along the cylinder from the top towards the bottom. Hydroformed cylinder 10D forms the final stage of the Holweck drag pump and as can be seen the cross section of the helical channels formed by the contoured surface are significantly smaller which provides the compression of the gas as it passes through the Holweck drag pump.

Figure 3 shows the two intermediate stages of the Holweck drag pump 10B and 10C. They are formed from hydroformed cylinders in a similar way to stages 10A and 10B. These two cylinders are formed to have a similar size to each other the inner cylinder 10C being slightly smaller than the outer cylinder 10B. The inner cylinder 10C has end sections with an enlarged diameter which extends slightly beyond the furthest extent of the channels. Outer cylinder 10B has end portions with a smaller diameter that extends slightly beyond the channels on this cylinder. The two cylinders are configured such that the smaller end diameters of the outer cylinder 10B has a diameter that is similar to the largest diameter of the outer end portion of the inner cylinder 10C such that the two may be joined together. In this embodiment they are joined together by welding at points 24.

Figure 4 shows a cross section through the assembled Holweck drag pump where the first stage 10a is mounted to the outer housing 20 and the inlet to the drag pump is at opening 30. There are cylindrical rotors 42 mounted on shaft 40 and configured to rotate during operation. These rotors have plane cylindrical surfaces that face the respective contoured surfaces of the stator stages of the Holweck drag pump. Thus, on rotation of the shaft 40 the two cylindrical rotors rotate and gas molecules are pushed along the channels from inlet 30 down the first stage 10A around the bottom of the outer rotor 42 and then up the channels in stator 10B. As can be seen the stator channels in 10B are smaller than those in 10A and this produces compression in the gas. The gas travels along stator 10B from the lower side to the upper side round the top of the two joined stator stages and then down the stator stage 10C in this case being driven by the inner cylinder of the rotor 42. When the gas reaches the end of stage 10C it will pass under the rotor and then up the final stage 10D to exhaust 32.

As can be seen, this arrangement with the relatively thin contoured cylinders provides a compact Holweck drag pump. Furthermore, the hydroforming process allows good repeatability and a low cost manufacturing process.

Figure 5 shows a flow diagram illustrating steps in a method of manufacturing a Holweck drag pump according to an embodiment. In a first step S10 four metal cylinders of different diameters are hydroformed such that each has a contoured surface with helical paths running from close to one end to close to the other. At step S20 the largest hydroformed cylinder is mounted to the inner surface of an outer portion of the pump housing. It may be mounted using an adhesive such as an epoxy adhesive. At step S30 the two intermediate sized cylinders are mounted together so that they are back to back, the end portions of these two cylinders being welded together.

At step S40 the smallest cylinder is mounted to an inner portion of the pump housing, this may be the housing around the motor.

At step S50 the two joined hydroformed cylinders of an intermediate size are mounted between the largest and smallest cylinders.

At step S60 a rotor is rotationally mounted within the pump. The rotor is a double rotor having two plane surface cylinders, the outer one being placed between the largest hydroformed cylinder and the two joined hydroformed cylinders while the smaller of the two plane surface cylinders is placed between the smallest hydroformed cylinder and the two joined hydroformed cylinders. In this way a Holweck pump with several stages is manufactured and fluid may be pumped from an inlet through each of the stages to an outlet. The channels though the stages become progressively smaller resulting in compression of the pumped fluid.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents. REFERENCE SIGNS

8 blank cylinder

10 hydroformed cylinder

10A, B, C, D stator stages 10 pump housing

22 attachment points to pump housing 24 joined cylinder attachment points 30 inlet 32 exhaust 40 shaft

42 rotor