The present invention relates to a vacuum pump impeller. In its preferred embodiment, the invention relates to an impeller for a turbomolecular vacuum pump.

A standard turbomolecular pump includes an impeller comprising arrays of angled blades and arranged for rotation at high speed, for example up to ninety thousand revolutions per minute between alternately arranged arrays of stationary blades of a stator to convey gas from one end of the shaft to the other.

Typically, the arrays of impeller blades are formed from aluminium or an alloy thereof, and are mounted on an aluminium or stainless steel impeller shaft. The shaft typically carries a set of magnetic bearing rings located in a bore co-axial with one or more of the arrays of impeller blades at the low-pressure end of the shaft. Each array of impeller blades may be located on a discrete disc mounted on the shaft using a shrink-fitting technique. Alternatively, or additionally, a number of arrays of impeller blades may be located on a single block or sleeve similarly mounted on the shaft.

A problem encountered with this design is that during use of the pump, expansion of the aluminium discs or sleeves under a combination of centrifugal and thermal loads can increase the tendency for the bearing rings to become placed under tension. This can increase the likelihood of the relatively brittle rings cracking during use. Furthermore, the loads exerted on an aluminium shaft during use can cause the shaft to bend, which can result in increased noise and bearing damage during use of the pump.

In one aspect, the present invention provides an impeller for a vacuum pump, the impeller comprising a shaft having mounted thereon magnetic bearing means, wherein at least the part of the shaft which carries the bearing means is formed from titanium or an alloy thereof. Forming at least the part of shaft from titanium or one of its alloys, for example Ti- 6AI-4V, having a relatively high stiffness and relatively low thermal expansion coefficient in comparison to aluminium and its alloys, can assist in maintaining the magnetic bearing means, for example magnetic rings carried by the shaft, under a state of compression during use of the pump, thereby inhibiting cracking of the rings. Titanium and its alloys have comparable specific stiffness and strength as high tensile steels such as EN24T (AISI 4340), but are significantly lighter, thereby offering a reduction in impeller inertia in comparison to pumps having steel shafts.

In the preferred embodiment, the bearing rings are located within a bore of the shaft, the shaft extending around the bearing rings. Only the part of the shaft that extends around the rings may be formed from titanium or titanium alloy, or alternatively the majority, or all, of the shaft may be formed from titanium or titanium alloy.

Arrays of blades may be mounted on and/or integral with the shaft. In the preferred embodiment at least one of the arrays is integral with the titanium shaft, with at least one other of the arrays being mounted on the shaft and formed from material having a lower stiffness than said at least part of the shaft, such as aluminium, an aluminium alloy, for example Al 7075, 7175, 7475, 2024 or 2618, magnesium or a magnesium alloy. Alternatively, all of the arrays of blades may be integral with the shaft, such that the entire impeller is formed from titanium or an alloy thereof.

In a second aspect, the present invention provides an impeller for a vacuum pump, the impeller comprising a shaft, at least part of the shaft being formed from titanium or an alloy thereof, the shaft having mounted thereon one or more arrays of blades formed from material having a lower stiffness than said at least part of the shaft.

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which: Figure 1 illustrates a cross-section through a first embodiment of an impeller for a turbomolecular vacuum pump; and

Figure 2 illustrates a cross-section through a second embodiment of an impeller for a turbomolecular vacuum pump.

With reference first to Figure 1 , impeller 10 comprises a shaft 12. The shaft 12 has integral therewith a number of arrays or rows of impeller blades 14, three rows in the embodiment shown in Figure 1 although any number could be provided. Alternatively, there may be no rows of blades integral with the shaft 12. In this embodiment, there are also a number of rows of impeller blades 16 which are mounted on the shaft 12, five rows in the embodiment shown in Figure 1 although again any number could be provided. As illustrated, these rows of impeller blades 16 may be integral with a sleeve or block 18 shrink-fitted on to the shaft 12. Alternatively, each of these rows 16 may be located on a respective disc mounted on the shaft 12, again using a shrink-fitting technique.

The impeller 10 includes concentric magnetic bearing rings 20 located in a bore 22 formed at one end of the shaft 12. These rings 20 are retained in position by a retaining ring 24, which exerts a compressive force on the rings 20, and are arranged to co-operate with corresponding concentric rings provided on a stator of the pump to regulate the radial position of the impeller 10 relative to the stator. An armature 26 of a motor for rotating the impeller 10 is also mounted on the shaft 12, a plastics sleeve 28, formed for example from carbon fibre reinforced plastic, being located around the armature 26.

In this embodiment, the shaft 12 and rows 14 of impeller blades are formed from titanium or a titanium alloy, such as TΪ-6AI-4V. Alternatively, only that part 30 of the shaft 12 that surrounds the bearing rings 20 may be formed from titanium or a titanium alloy, with the remainder of the shaft 12 being formed from other material, such as steel or aluminium. The rows 16 of impeller blades and sleeve 18 - A -

mounted on the shaft 12 are formed from material that has a lower stiffness than part 30 of the shaft 12, for example aluminium or an aluminium alloy. The higher stiffness (E = 125 GPa) and lower thermal expansion coefficient (α = 8.5x10'6 μm/m/°C) of titanium in comparison to aluminium (E = 72 GPa; α = 5 24x106 μm/m/°C) will tend to maintain the rings 20 in a state of compression during use of the pump, thereby inhibiting cracking of the bearing rings 20.

In the second embodiment illustrated in Figure 2, all of the rows 14, 16 of impeller blades are integral with the shaft 12, which is formed from titanium or a titanium 10. alloy, such as TΪ-6AI-4V.

In summary, an impeller for a turbomolecular vacuum pump comprises a shaft having mounted thereon a plurality of rotor elements. Part of the shaft extends around magnetic bearing means located within a bore of the shaft. At least this 15 part of the shaft is formed from titanium or an alloy thereof so that the bearing means can remain in compression during use of the impeller, thereby inhibiting cracking of the bearing means.