TECHNICAL FIELD

This invention relates to a sealing arrangement for an ultra high vacuum (UHV) pump, and to a UHV pump incorporating such a sealing arrangement.

BACKGROUND OF THE INVENTION

A UHV pump is essentially a pump that can operate with inlet pressures within the UHV range, that is to say less than lxlO ^"9 mbar. To achieve UHV pressures, baking is usually required so the pump is typically expensive as a stainless steel envelope is used to house a pumping mechanism, the envelope containing knife-edged seals for copper gaskets.

It is known to use an aluminium envelope instead of a stainless steel envelope in an attempt to reduce costs. Unfortunately, an aluminium envelope can only be used to give UHV performance if expensive and complex seals (such as Helicoflex ^® rings coated in a soft metal such as gold or silver) are used. Alternatively, the envelope can contain a knife-edged seal which deforms to create a UHV seal. This is a cheap UHV alternative, but requires the entire envelope to be replaced if the seal is broken.

A UHV pump such as a turbomolecular pump requires its main inlet to be sealed to UHV levels so that an ancillary blanking plate forming part of such a pump needs to have the same requirement. Unfortunately, due to the chosen envelope material, conventional copper seals cannot be used, and a fluoroelastomer ring has a leak rate which is too high for a UHV application. A conventional metal ring also has too high a leak rate.

The main aim of the invention is to provide a sea ling arrangement for a UHV pump which permits a cheaper pump envelope to be utilised. The invention also aims to provide a UHV pump of the type having a blanking plate that has a reduced leak rate, so that UHV performance can be achieved. SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a sealing arrangement for a UHV pump having first and second housing members, the first housing member housing a pumping mechanism, the sealing arrangement being positioned to provide a seal between the first and second housing members, wherein the sealing arrangement comprises inner and outer seals extending around the periphery of the first housing member and a plenum positioned between the inner seal and the outer seal, and wherein pumping means are provided for pumping the plenum to a sub-atmospheric pressure.

The first and second housing members may be made of aluminium, the inner seal may be a simple O-ring or metal seal such as a C-section, a Wills ring or a fluoroelastomer ring, and the outer seal may be a fluoroelastomer ring.

This sealing arrangement defines a self-contained differentially-pumped seal that enables a UHV pump to be made that is much cheaper than more conventional UHV pumps. Moreover, this sealing arrangement has a reduced leakage rate. I n one preferred em bodi ment, the pum pi ng mea ns is constituted by the pum ping mechanism. In this case, the pumping mechanism may be connected to the plenum by a passage formed in the first housing member. Alternatively, the pumping mechanism is connected to the plenum by external pipework and first and second vacuum connections, the first vacuum connection being connected to the plenum, and the second vacuum connection being connected to the pumping mechanism.

In another embodiment, the pumping means is constituted by a secondary pump connected to the plenum by a vacuum connection.

The first housing member may be an envelope, and the second housing member may be an end cap, an envelope and vacuum chamber or a vacuum vessel. I n a further aspect, the i nvention provides a U HV pum p com prising a first housi ng member housing a pum ping mechanism a nd a second housing member, the sea ling arrangement being positioned to provide a seal between the first and second housing members, wherein the sealing arrangement is as defined above..

I n yet a further aspect, the invention provides a UHV pump comprising a first housing member housing a pumping mechanism, a second housing member and a blanking plate, the sealing arrangement being positioned to p rovide a sea l betwee n the fi rst housing member and the bla nking plate, wherein the sealing arrangement is as defined above, and the pum ping mechanism is in pa rt a turbomolecular pumping mechanism.

The provision of this differentially-pumped sealing arrangement ensures a reduced leak rate and good UHV performance without the use of expensive seals or an expensive steel envelope.

Advantageously, the blanking plate is made of aluminium. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described i n greate r deta i l, by way of exa m ple on ly, with reference to the drawings, in which:-

Figure 1 is a sectiona l view illustrating the principle of the sealing arrangement of the invention;

Figure 2 is a sectional view illustrating a first form of sealing arrangement constructed in accorda nce with the invention;

Figure 3 is a sectional view illustrating a second form of sealing arra ngement constructed in accorda nce with the invention;

Figure 4 is a sectional view illustrating a third form of sealing arra ngement constructed in accorda nce with the invention; and

Figure 5 is a schematic representation of a UHV pump incorporating the third form of sealing arra ngement. DETAILED DESCRIPTION OF AN EMBODIMENT

Referring to the drawings, Figure 1 illustrates the principle underlying the invention. A UHV pump includes an aluminium envelope 1 (only part of which can be seen) and a vacuum sealing end cap, chamber or vacuum vessel 2 made of aluminium. The envelope 1 houses a conventional pumping mechanism (not shown), and is sealed with respect to the end cap 2 by a sealing arrangement constituted by a pair of seals 3 and 4. The inner seal 3 could be a simple metal seal such as a C-section ring, a Wills ring or a fluoroelastomer ring, and the outer seal 4 is a fluoroelastomer ring made of, for example, Viton. A plenum 5 is positioned between the inner and outer seals 3 and 4. The plenum 5 is pumped to below atmospheric pressure via a vacuum connection 6. The resulting reduction in pressure within the plenum 5 reduces the leak rate of the inner seal 3 as that seal no longer needs to seal to atmosphere, thereby enabling lower pressures to be achieved at the main pumping inlet (not shown).

Pumping of the plenum to a sub-atmospheric pressure can be effected by using the pumping mechanism of the UHV pump and external pipework (see Figure 2), by using a secondary vacuum pump (see Figure 3), or by using the pumping mechanism and internal cross drilling within the pump envelope (see Figure 4).

Figure 2 shows a first practical form of sealing arrangement and uses the same reference numerals for the same parts. The pumping mechanism is designated by the reference numeral 7. As this mechanism is conventional it will not be described in any detail, though the low pressure pump inlet 8 is shown adjacent to the inner seal 3. The plenum 5 is pumped to below atmospheric pressure by the pumping mechanism 7 via the vacuum connection 6, external pipework (not shown), a further vacuum connection 9 and an inlet 10 to the pump. The arrow A shows the direction of the pumped gases. Figure 3 shows a second practica l form of sea li ng a rrangement a nd uses the sa me reference numerals for the same parts. The pumping mechanism is again designated by the reference numeral 7. As this pumping mechanism is conventional it will not be described in any detail, though the low pressure pump inlet 8 is shown adjacent to the inner seal 3. The plenum 5 is pumped to below atmospheric pressure by a secondary pump (not shown) via the vacuum connection 6. The arrow B shows the direction of the pumped gases.

Figure 4 shows a third practical form of sealing arrangement and uses the same reference numerals for the same parts. The pumping mechanism is again designated by the reference numeral 7. As this mechanism is conventional it will not be described in any detail, though the low pressure pump inlet 8 is shown adjacent to the inner seal 3. The plenum 5 is pumped to below atmospheric pressure by the pumping mechanism 7 via a cross drilling 5a formed within the envelope 1. The arrow C shows the direction of the pumped gases.

Each of the sealing arrangements described above has its inner seal differentially pumped so that the pressure difference between the inner and outer diameters of the inner seal is less than if that seal was sealing directly from atmosphere. This differentially-pumped seal can be adapted to lower the leak rate of a UHV pump utilising a sealed blanking plate. Such a UHV pump is typified by a turbomolecular (TMP) pump, and Figure 5 shows such a pump provided with a differentially-pumped sealing arrangement.

Figure 5 shows a TMP indicated generally by the reference numeral 20. The pump 20 has an aluminium envelope 21 which is sealed with respect to a vacuum sealing end cap 22 by means of a differentially-pumped sealing arrangement acting against a blanking plate 22a. Both the end cap 22 and the blanking plate 22a are made of aluminium. As with the embodiments of Figures 2 to 4, the sealing arrangement is constituted by a pair of of seals 23 and 24. The inner seal 23 could be a simple metal seal such as a C-section ring, a Wills ring or a fluoroelastomer ring, and the outer seal 24 is a fluoroelastomer ring made of, for example Viton. A plenum 25 is positioned between the inner and outer seals 23 and 24. The plenum 25 is pumped to below atmospheric pressure by the TMP 20 via a cross drilling 27. This sealing arrangement is, therefore similar to that of Figure 4. The TMP includes a UHV region 31 in the region of the end cap 22 and a sub- atmospheric region 32 positioned between two turbomolecular pump stages 33 and 34 and adjacent to a port 35 contiguous with the cross drilling 27. The TMP has a port 36 for exhausting pumped gases. As a result of the differentially-pumped sealing a rrangement, UHV pressures can be achieved. Moreover, as the differential pumping is totally integrated into the envelope 21, it does not rely on any additional mechanisms or interfaces to enable it to function. This pump has similar advantages to pumps incorporating the sealing arrangements of Figures 2 to 4, namely the use of cheap aluminium for the main parts of the pump, and the avoidance of having to use copper gaskets/expensive metal seals.

It will be appreciated that various modifications can be made to the sealing arrangement described herein without departing from the scope of the present invention.