Patent Description:
A turbomolecular pump is a type of vacuum pump which operates by pushing gas molecules in a desired pumping direction using rotating blades in one or more bladed pumping stages.

One type of turbomolecular pump includes, in addition to one or more bladed turbomolecular pumping stages, a drag pumping mechanism (e.g. one or more Holweck and/or Siegbahn drag pumping stages). This type of turbomolecular pump is sometimes referred to as a compound turbomolecular pump. In this type of turbomolecular pump, it is generally desirable to increase the drag generated by the drag pumping mechanism in order to improve pumping efficiency.

<CIT> is an example of such a compound turbomolecular pump.

According to a first aspect of the invention, there is provided a drag pumping mechanism for a turbomolecular pump according to claim <NUM>.

The drag generation structure may be formed from the same type of material as the part of the motor with which it is integrally formed. The type of material may be epoxy.

The plurality of walls may all extend by substantially the same distance from the frustoconical surface.

The drag pumping mechanism may further comprise a stator, one or more Holweck and/or Siegbahn drag pumping stages formed by the impeller and the stator.

The drag generation structure may be located downstream of the one or more Holweck and/or Siegbahn drag pumping stages.

The drag pumping mechanism may further comprise one or more Holweck and/or Siegbahn drag pumping stages located downstream of the drag generation structure.

The annular body of the drag generation structure may have a generally triangular cross-section.

The plurality of channels of the drag generation structure may be curved.

The plurality of walls of the drag generation structure may be integrally formed with the annular body of the drag generation structure.

The bottoms of the plurality channels may be flat.

The bottoms of the plurality of channels may be curved.

According to a second aspect of the invention, there is provided a turbomolecular pump comprising the drag pumping mechanism of the first aspect.

<FIG> is a schematic illustration (not to scale) showing a turbomolecular pump <NUM>.

The turbomolecular pump <NUM> comprises a bladed turbomolecular pumping stage <NUM> and a drag pumping mechanism <NUM>. The bladed turbomolecular pumping stage <NUM> is configured to receive gas from a location external to the turbomolecular pump <NUM> (e.g. a chamber from which it is desired to pump gas), pump the received gas therethrough, and output the pumped gas to the drag pumping mechanism <NUM>. The drag pumping mechanism <NUM> is configured to receive the pumped gas from the bladed turbomolecular pumping stage <NUM>, pump the received gas therethrough and output the pumped gas out of the turbomolecular pump <NUM> (e.g. for disposal or for conveyance to another location).

The operation of the bladed turbomolecular pumping stage <NUM> of the turbomolecular pump <NUM> is well understood and will not be described here in detail. However, briefly, the bladed turbomolecular pumping stage <NUM> comprises a plurality of rotor blades and a plurality of stator blades intermeshed with the rotor blades. The rotor blades are angled relative to the stator blades such that rotation of the rotor blades pushes gas through the spaces between the rotor and stator blades in a desired pumping direction to pump gas through the bladed turbomolecular pumping stage <NUM>.

The turbomolecular pump <NUM> will now be described in more detail with reference to <FIG>.

<FIG> is a schematic illustration (not to scale) showing a cross-sectional view of the turbomolecular pump <NUM>.

The turbomolecular pump <NUM> comprises an impeller <NUM>, a stator <NUM>, a motor <NUM>, a drag generation structure <NUM>, an inlet <NUM> and an outlet (not depicted). The impeller <NUM> is configured to rotate relative to the stator <NUM> and the drag generation structure <NUM> to pump gas from the inlet <NUM> to the outlet. To achieve this, the impeller <NUM>, stator <NUM> and drag generation structure <NUM> together form the turbomolecular pumping stage <NUM> and a plurality of drag pumping stages, which will be described below in more detail. The motor <NUM> is configured to drive the rotation of the impeller <NUM>. The inlet <NUM> is configured to receive gas from an entity from which gas is to be pumped (e.g. a chamber connected to the turbomolecular pump <NUM>). The inlet <NUM> comprises one or more openings at the top of the turbomolecular pump <NUM> which are fluidly connected to the bladed turbomolecular pumping stage <NUM>. The outlet is configured to output gas which has been pumped through the turbomolecular pump <NUM> out of the turbomolecular pump <NUM> entirely. The direction to the location of the outlet is depicted by arrow <NUM>.

In more detail, the impeller <NUM> comprises an impeller shaft <NUM>, a plurality of turbomolecular rotor elements <NUM>, a plurality of Holweck rotor elements <NUM> and a connecting member <NUM>. In this embodiment, the plurality of turbomolecular rotor elements <NUM> comprises a first turbomolecular rotor element 214a and a second turbomolecular rotor element 214b. In this embodiment, the plurality of Holweck rotor elements <NUM> comprises a first Holweck rotor element 216a and a second Holweck rotor element 216b. The plurality of turbomolecular rotor elements <NUM> and the plurality of Holweck rotor elements <NUM> are attached to the impeller shaft <NUM> via the connecting member <NUM>. The impeller shaft <NUM> is generally cylindrical and defines a longitudinal direction, a radial direction and a circumferential direction. The impeller shaft <NUM> also defines a rotation axis of the impeller <NUM> about which the impeller <NUM> is configured to rotate when driven by the motor <NUM>. The rotation axis of the impeller <NUM> extends in the longitudinal direction. Each of the plurality of turbomolecular rotor elements <NUM> is an annular blade extending in the radial direction outwards from the connecting member <NUM>. Each of the plurality of Holweck rotor elements is a cylindrical wall (also known as a "skirt") extending circumferentially around the impeller shaft <NUM> and extending in the longitudinal direction from the connecting member <NUM>.

The stator <NUM> comprises a plurality of turbomolecular stator elements <NUM> and a plurality of Holweck stator elements <NUM>. In this embodiment, the plurality of turbomolecular stator elements <NUM> comprises a first turbomolecular stator element 222a and a second turbomolecular stator element 222b. In this embodiment, the plurality of Holweck stator elements <NUM> comprises a first Holweck stator element 224a, a second Holweck stator element 224b and a third Holweck stator element 224c. Each of the plurality of turbomolecular stator elements <NUM> is an annular blade extending in the radial direction inwards towards a centre line of the impeller shaft <NUM>. Each of the plurality of Holweck stator elements <NUM> is a cylindrical wall comprising helical channels (or grooves) in its surface for generating drag for drag pumping.

The plurality of turbomolecular rotor elements <NUM> are intermeshed with the plurality of turbomolecular stator elements <NUM> to form the bladed turbomolecular pumping stage <NUM>. The physical mechanism behind how the bladed turbomolecular pumping stage <NUM> works is well known and has been briefly described above with reference to <FIG>.

The plurality of Holweck rotor elements <NUM> are intermeshed with the plurality of Holweck stator elements <NUM> to form a plurality of Holweck drag pumping stages (also known as a "Holweck pack"). Specifically, the first Holweck rotor element 216a and the first Holweck stator element 224a together form a first Holweck drag pumping stage. The first Holweck rotor element 216a and a first side of the second Holweck stator element 224b together form a second Holweck drag pumping stage. The second Holweck rotor element 216b and a second side of the second Holweck stator element 224b opposite the first side together form a third Holweck drag pumping stage. The second Holweck rotor element 216b and the third Holweck stator element 224c together form a fourth Holweck drag pumping stage. The physical mechanism behind how Holweck drag pumping stages work is well understood and will not be described here for brevity.

The drag generation structure <NUM> is the inventive additional drag generating structure which is separate to the Holweck drag pumping stages described above. The drag generation structure <NUM> is configured to generate additional drag to supplement the drag pumping performed by the Holweck drag pumping stages.

According to the invention, this drag generation structure <NUM> is disposed on the motor <NUM> (specifically on an axial end of the motor <NUM>) and is located in the space between the motor <NUM> and the connecting member <NUM> of the impeller <NUM>. The drag generation structure <NUM> is integrally formed with the part of the motor <NUM> on which it is disposed. The drag generation structure can be formed from the same type of material (e. g epoxy) as the part of the motor <NUM> with which it is integrally formed. Integrally forming the drag generation structure <NUM> with part of the motor <NUM> using the same type of material tends to enable the drag generation structure <NUM> to be manufactured more easily, since the drag generation structure <NUM> can simply be formed out of the same block of material as the part of the motor <NUM> on which it is disposed.

When the impeller <NUM> rotates, gas is pumped into the space between the drag generation structure <NUM> and the connecting member <NUM>, where the gas interacts with the drag generation structure <NUM> to generate drag for drag pumping. Thus, the drag generation structure <NUM> and the connecting member <NUM> together form an additional drag pumping stage separate to the Holweck drag pumping stages. Specifically, the drag generation structure <NUM> is a stator element of the additional drag pumping stage and the connecting member <NUM> is a rotor element of the additional drag pumping stage. The Holweck drag pumping stages and this additional drag pumping stage together constitute the drag pumping mechanism <NUM> described above with reference to <FIG>. The precise structure and operation of the drag generation structure <NUM> will now be described in more detail below with reference to <FIG>.

<FIG> is a schematic illustration (not to scale) showing the flow path of gas being pumped through the turbomolecular pump <NUM> of <FIG>.

Specifically, arrows <NUM> in <FIG> show the direction of travel of the gas pumped through the turbomolecular pump <NUM>. In more detail, referring back to the elements described with reference to <FIG>, during operation, the turbomolecular pump <NUM> receives gas at the inlet <NUM> and conveys the received gas to the bladed turbomolecular pumping stage <NUM>, where the gas is pumped by the bladed turbomolecular pumping stage <NUM> towards the first Holweck drag pumping stage. The first Holweck drag pumping stage receives the pumped gas from the bladed turbomolecular pumping stage <NUM> and drag pumps the received gas towards the second Holweck drag pumping stage. The second Holweck drag pumping stage receives the pumped gas from the first Holweck drag pumping stage and drag pumps the received gas towards the third Holweck drag pumping stage. The third Holweck drag pumping stage receives the pumped gas from the second Holweck drag pumping stage and drag pumps the received gas towards the fourth Holweck drag pumping stage. The fourth Holweck drag pumping stage receives the pumped gas from the third Holweck drag pumping stage and drag pumps the received gas towards the additional drag pumping stage (i.e. the one formed by the drag generation structure <NUM> and the connecting member <NUM>). The additional drag pumping stage receives the pumped gas from the fourth Holweck drag pumping stage and drag pumps the received gas towards the outlet. The outlet receives the pumped gas from the additional drag pumping stage and outputs the pumped gas out of the turbomolecular pump <NUM>.

Thus, in this embodiment, the bladed turbomolecular pumping stage <NUM> is located downstream of the inlet <NUM>, the first Holweck drag pumping stage is located downstream of the bladed turbomolecular pumping stage <NUM>, the second Holweck drag pumping stage is located downstream of the first Holweck drag pumping stage, the third Holweck drag pumping stage is located downstream of the second Holweck drag pumping stage, the fourth Holweck drag pumping stage is located downstream of the third Holweck drag pumping stage, the additional drag pumping stage is located downstream of the fourth Holweck drag pumping stage, and the outlet of the turbomolecular pump <NUM> is located downstream of the additional drag pumping stage.

<FIG> is a schematic illustration (not to scale) showing a perspective view of a section of the drag generation structure <NUM>.

In this embodiment, the drag generation structure <NUM> comprises an annular body <NUM> comprising a frustoconical surface <NUM>, and a plurality of walls or ridges <NUM> (also known as "seals") extending from the frustoconical surface <NUM>. In this embodiment, the plurality of walls <NUM> all extend by substantially the same distance from the frustoconical surface <NUM> (i.e. the plurality of walls <NUM> are all substantially the same height). The plurality of walls <NUM> define therebetween a plurality of channels or grooves <NUM>. In this embodiment, the bottoms of the plurality of channels <NUM> are flat. However, in other embodiments, the bottoms of the plurality of channels <NUM> are curved instead. In this embodiment, the annular body <NUM> and the plurality of walls <NUM> are integrally formed with each other and are formed from the same material. Also, in this embodiment, each of the plurality of walls <NUM> is curved along its length and thus each of the plurality of channels <NUM> defined between the plurality of walls <NUM> is also curved along its length. Specifically, each channel <NUM> is spirally shaped along its length and extends from an outer periphery of the annular body <NUM> to a central aperture defined by the annular body <NUM>. In this embodiment, each of the plurality of walls <NUM> extends perpendicularly from the frustoconical surface <NUM>.

In this embodiment, the annular body <NUM> has a generally triangular cross-section. Specifically, the cross-section is generally that of a right-angled triangle with the hypotenuse of the triangle defining the frustoconical surface <NUM>.

Referring back to elements described above with reference to <FIG>, the drag generation structure <NUM> is located between the motor <NUM> and a surface of the impeller <NUM> which is facing the frustoconical surface <NUM>. Specifically, the surface which is facing the frustoconical surface <NUM> is a surface of the connecting member <NUM> of the impeller <NUM>. The surface of the connecting member <NUM> facing the frustoconical surface <NUM> of drag generation structure <NUM> is also a frustoconical surface. Both the frustoconical surface <NUM> and the surface of the connecting member <NUM> are inclined relative to the longitudinal direction defined by the impeller shaft <NUM> centre line. The annular body <NUM> of the drag generation structure <NUM> extends around the impeller shaft <NUM>. Specifically, the annular body <NUM> extends all the way around the impeller shaft <NUM> in the circumferential direction such that the impeller shaft <NUM> passes through the central aperture defined by the annular body <NUM>. Thus, the frustoconical surface <NUM> extends all the way around the impeller shaft <NUM> in the circumferential direction.

During operation of the drag pumping mechanism <NUM>, rotation of the impeller <NUM> relative to the drag generation structure <NUM> forces gas through the plurality of channels <NUM> of the drag generation structure <NUM> in order to generate drag for pumping. Specifically, the gas is forced into the space between the frustoconical surface <NUM> and the parallel facing surface of the connecting member <NUM>, which in turn forces the gas into the plurality of channels <NUM> at an outer periphery of the annular body <NUM>. The gas then travels in a curved path radially inwards and longitudinally downwards along the channels <NUM> to the central aperture defined by the annular body <NUM>, where the gas leaves the channels <NUM>.

Thus, a drag pumping mechanism for a turbomolecular pump is provided.

Advantageously, by including the above-described additional drag pumping stage, the drag pumping mechanism tends to be able to generate additional drag for drag pumping on top of the drag already generated by the Holweck drag pumping stages. Thus, the drag pumping mechanism tends to be able to perform drag pumping more effectively.

The provision of a drag generating structure with a frustoconical surface in the location described above tends to provide a drag generating shape which is half-way between a Siegbahn type shape and a Holweck type shape (i.e. inclined with respect to the longitudinal direction, rather than perpendicular or parallel to the longitudinal direction). For example, the shape of the drag generating structure can be said to be a conical Siegbahn shape. This shape tends help to enable as much of the available space within the turbomolecular pump to be used for drag generation as possible. This shape also tends to provide a balance between the advantages and disadvantages between Siegbahn type shapes and Holweck type shapes.

Advantageously, the provision of a drag generating structure with a frustoconical surface tends to enable a better fit for the drag generating structure within the drag pumping mechanism when used with impeller structures with a surface which is inclined with respect to the longitudinal direction, since the inclination of the frustoconical surface may be matched with the inclination of the impeller surface.

In the above embodiments, the drag generation structure is integrally formed with the part of the motor on which it is disposed.

In the above embodiments, the drag generation structure is formed from the same type of material as the part of the motor on which it is disposed. However, in other embodiments, the drag generation structure is formed from a different type of material to the part of the motor on which it is disposed.

In the above embodiments, the drag generation structure is formed from epoxy. However, in other embodiments, the drag generation structure is formed from a different type of material, e.g. a metal or a different type of polymer material.

In the above embodiments, the plurality of walls extend perpendicularly from the frustoconical surface. However, in other embodiments, one or more (or all) of the plurality of walls are inclined relative to the frustoconical surface.

In the above embodiments, the drag pumping mechanism comprises four Holweck drag pumping stages. However, in general, the drag pumping mechanism may comprise any number of any appropriate type of drag pumping stages. For example, in other embodiments, the drag pumping mechanism comprises a different number of Holweck drag pumping stages, e.g. only one or more than two. In yet other embodiments, the drag pumping mechanism comprises one or more Siegbahn drag pumping stages and no Holweck drag pumping stages. In yet other embodiments, the drag pumping mechanism comprises one or more Holweck drag pumping stages and one or more Siegbahn drag pumping stages.

Claim 1:
A drag pumping mechanism (<NUM>) for a turbomolecular pump (<NUM>), the drag pumping mechanism comprising:
an impeller (<NUM>) comprising an impeller shaft (<NUM>) defining a rotation axis of the impeller;
a motor (<NUM>) configured to drive the rotation of the impeller; and
a drag generation structure (<NUM>), the drag generation structure comprising:
an annular body (<NUM>) extending around the impeller shaft, the annular body comprising a frustoconical surface (<NUM>),
and
a plurality of walls (<NUM>) extending from the frustoconical surface the plurality of walls defining a plurality of channels (<NUM>) therebetween wherein the impeller is configured to rotate relative to the drag generation structure about the rotation axis to pump gas through the plurality of channels of the drag generation structure,
characterised in that
the drag generation structure is disposed on the motor, and in that the drag generation structure is integrally formed with a part of the motor.