GB2509087A

GB2509087A - Sealed rotary drive arrangement, providing drive into a high-pressure gas vessel

Info

Publication number: GB2509087A
Application number: GB1222969.6A
Authority: GB
Inventors: Philip Alan Charles Walton; Trevor Bryan Husband; Marcel Jan Marie Kruip
Original assignee: Siemens PLC
Current assignee: Siemens PLC
Priority date: 2012-12-19
Filing date: 2012-12-19
Publication date: 2014-06-25
Also published as: WO2014096194A1; GB201222969D0

Abstract

A mechanical arrangement for providing rotary drive into a vessel containing high-pressure gas, preferably a cryogenic refrigerator associated with a superconducting magnet in a MRI system, from a motive source, preferably an electric motor 10, which is exposed to atmospheric pressure, wherein a housing of the motive source is sealed against one end of an elongate outer part 34 of a drive arrangement and the vessel is sealed against another end of the elongate outer part such that a high-pressure enclosure is provided, itself comprising the vessel housing, the housing of the motive source 10 and the outer part 34 of the drive arrangement 30. The drive arrangement may be a drive shaft or a drive belt. When the drive arrangement is a drive shaft, the drive shaft and outer part may be flexible.

Description

A MECHANICAL ARRANGEMENT FOR PROVIDING ROTARY DRIVE

The present invention relates to mechanical arrangements for providing rotary drive into a vessel containing high-pressure gas from a motive source such as an electric motor which is exposed to atmospheric pressure.

In such cases, it is difficult tc provide the required high-pressure sealing around a rotary drive shaft linking the motive source into the high-pressure vessel.

The present invention particularly relates to arrangements for providing motive power to a cryogenic refrigerator, commonly and hereafter referred to as a coldhead', including helium gas at a high pressure such as 28bar. It is well known that heliun gas is difficult to seal. Such arrangements are currently employed in cooling arrangements for supercocducting magnets, for example superconducting magnets as employed in magnetic resonance imaging (MRI) systems.

MRI systems require a strong background magnetic field which is very homogeneous in an imaging region. It is the task of the superconducting magnet to provide this magnetic field, and most other considerations are secondary to that. The coldhead operates to provide cryogenic cooling, typically to around 4K, the boiling point of helium. Such cooling is required to enable the superconducting magnet to operate in its superconducting state. The coldhead is typically operated by an electric motor which provides motive force to the coldhead itself. Electric motors and the magnetic field generated by the superoonduoting magnet are largely inoorrpatible. Despite significact efforts by designers, a certain amount of stray field escapes from the superconducting magnet, and regions outside of the magnet but in close proximity to it can experience an appreciable magnetic field. This can interfere with the operation of an electric motor in such regions.

Conversely, electric motors tend to generate significant electromacnetic interference, which can disrupt the homogeneous magnetic field required for MRI imaging.

Accordingly, the location of the coidhead in MRI systeir.s is very carefully considered to ensure that an associated electric motor is positioned in a low magnetic field to reduce interference with operation of the motor. This becomes harder to achieve as the field strength of the magnet increases,

especially if the stray field expands.

Fig. 1 illustrates a conventional arrangement for linking a motor 10 to a coldhead 12. A rotary drive shaft 14 transfers motive power from motor 10 to a component 16 of the coldhead.

A key-way 17 or similar arrangement such as a splined shaft is provided for mechanical engagement between the rotary drive shaft 14 and the component 16. The coldhead will include a gas such as helium at a relatively high pressure, such as 28 bar.

The outside of the coldhead, and the motor, are at atmospheric pressure. It has been considered difficult to directly seal the rotary shaft 14 to prevent escape of gas, as the seal would need to bear against the surface of the rotary shaft and so would be liable to wear. The conventional arrangement of Fig. 1 allows a certain leakage of gas from the coidhead into a volume 18 located between a casing of the coldhead 12 and a casing of the motor 10. Similarly, as the casing of the motor will rot be sealed against the rotary shaft 14, gas is allowed into the casing of the motor, and a gas pressure within the casing of the motor may build up to the same value as the gas pressure within the coldhead. The casing of the motor 10 may accordingly be sealed against gas leaks between the high pressure within the ccldhead, such as 28 bar, and atmospheric pressure. A volume 18 between the housing of the coldhead and the housing of the motor is closed by a seal 20. Yleohanical means, not illustrated, serve to compress the seal between the housings and seal high pressure gas within the housings and the volume 18 from the atmosphere. As the seal bears only against stationary surfacas, no wear is expected and the required seal is easy to achieve.

The arrangement of Fig. 1 is only possible where the motor 10 is directly mounted onto the coidhead 12. Such arrangement does nct address the problems of electromagnetic interference from the motor 10 interfering with an imaging process, or the problem of a stray mannetic field from the superconducting magnet ioterferiog with operation of the motor.

The present invention accordingly aims to provide effective sealing for a rotary drive into a vessei containing high-pressure gas from a motive soirce which is exposed to atmospheric pressure. Such arrangements are required to permit the motive source, such as ao electric motor, to be placed at an arbitrary position, remote from the coidhead and the superconducting magnet. This reduces or eliminates electromagnetic interference in the imaging procedure as generated by a motor, and reduces or eliminates effects of

stray magnetic field on the motive source.

The motive sourca may accordingly be positioned in a low stray fieid position, even outside of the environment of The MRI system altogether. Furthermore, the present invention permits the motive source to be located in an unusual position, for example as may be required where there is insufficient space for the notive source to be positioned close to the coldhead.

The present invention accordingly provides apparatus and methods as defined in the appended claims.

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from the following description of certain embodiments thereof, given by way of non-limiting example only, wherein: Fig. 1 schematically illustrates a conventional arrangement for sealing against leaks of high-pressure gas from a coldhead at its interface with a source of motive power; and Fig. 2 schematically illustrates a partially exploded view of an arrangement for sealing against leaks of high-pressure gas from a coldhead at its interface with a source of motive power according to an embodiment of the invention.

The present invention provides an arrangement which enables the source ot motive power to a remote position, while stiii providing effective sealing of the high-pressure gas within the coidhead and effective mechanical linkage between the source of motive power and the coidhead.

The source of motive power is typically an electric motor; however other sources may be used suoh as a pneumatic or hydraulic motor. However, for brevity and simplicity, a general source of motive power will be referred to below as a "motor".

A feature of the present invention is that the motor 10 is linked to the ooldhead 16 using a flexible drive 30, as illustrated in Fig. 2, where features corresponding to features of Fig. 1 carry corresponding reference numerals.

The flexible drive comprises an inner part 32 which is a flexible rotary shaft of conventional type, such as flexible drives known for use with electric drills. The flexible drive also comprises an outer part 34 which does not rotate, but is affixed co the housing of the coidhead 12, and the housing of the motor 10 in a gas-tight manner to the pressure encountered within the coldhead.

The flexible rotary shaft 32 carries splined ends 36 in the illustrated embodiment. The splined ends are not in themselves flexible, and are preferably provided with a locking device to ensure reliable engagement. Respective splined ends engage with complementary fittings attached to component 16 within the coldhead, and a rotor on motor 10, to provide the required motive power. Either or both splined ends may be replaced by an alternative engagement means, such as a hexagonal protrusion or recess, a screwdriver-type end or similar arrangement, provided that a respectively compatible complementary fitting is provided on component 16 and the rotor of motor 10.

A technical difficulty which the present invention aims to address is the requirement to seal the rotating drive shaft to helium gas at a high pressure such as 28 bar. As with the conventional arrangement of Fig. 1, no attempt is made to form a leak-tight seal directly against the surface of the rotating shaft, but rather provides a high-pressure enclosure comprising the coldhead housing, the motor housing and the outer part 34 of the flexible drive 30. These components do not rotate with respect to one another, and a gas-tight seal to the required quality may be achieved with conventional means.

The cuter part 34 comprises a high-pressure hose of suitable internal diameter. A braiding of stainless steel or similar may be provided over a flexible hose, which may itself be of cnnvoluted stainless steel tubing. Such hoses are conventionally available and capable of withstanding gases such as helium at the pressures under consideration. The hose must be chosen such that it has an internal diameter large enough to allow rotation of the flexible drive shaft 32 with a suitable olearance to ensure a minimal contact between the flexible drive shaft and the interior surface of the hose, to mininise wear on both parts. The flexible drive shaft 32 is fed through the flexible hose and engaged with the component 16 of the coidhead 12, and the motor, using suitable mechanical couplings as described above.

The flexible hose is provided with a high-pressure seal arrangement 38 at each end. :n the illustrated embodiment, these are shown disassembled, but when in use serve to seal the hose against the housing of the coldhead 12 and against the housing of the motor 10. The high-pressure seal arrangements 38 could for example be Garbo]?' NW40 seals.

Once assembled, and prior to use, the high-pressure enclosure comprising the coldhead housing, the motor housing and the outer part 34 of the flexible drive 30 is evacuated, purged, and charged with helium gas through a suitable vacuum port (not illustrated) The surface of the flexible drive shaft 32 and/or an inner surface of the house of outer part 34 may be coated with a friction-limiting material, to reduce wear and to reduce a frictional load on the motor. A liguid such as an oil or a grease may be provided between the inner surface of the outer part 34 and the flexible drive shaft to reduce friction; alternatively a solid lubricant such as graphite powder may be used, but care must be taken to ensure That such lubricants do not contaminate the motor or the ooldhead.

The flexible drive 30 and hose 34 may be or arbitrary length, within reason, and their flexibility allows a great range of relative positions of onidhead 12 and motor 10 to be employed.

An alternative embodiment of the invention employs a rigid drive shaft 32 located within a pressurised chamber such as that provided by outer part 34 in the embodiment of Fig. 2.

This would allow less freedom in positioning of the motor, but would still provide advantages in reducing magnetic interaction between the motor and the superconducting magnet by significantly increasing the distance between motor and magnet. Alternatively, a belt drive may be provided to transmit the rotary drive from the motor to the ooldhead, A rather differently-shaped chamber may be required than the outer part 34 shown in Fig. 2 if a belt drive is to be accommodated, but the design and manufacture of such a chamber is within the ability of those skilled in the art. The shape of the pressurised chamber could be designed to suit the type of drive selected.

In another embodinent, a flexible drive shalt is provided which comprises two or more rigid shaft parts linked together by flexible joints, such as universal joints or relatively short lengths of flexible drive shaft material. The outer part may include a flexible hose, flexible over its entire length as discussed above with reference to Fig. 2, or may include a number of rigid rubular sections, each corresponding to a rigid drive shaft part, the rigid tubular sections being joined to one another by flexible sections, each corresponding to a flexible joint in the drive shaft.

While the present invention has been described with reference to a limited number of exemplary embodiments, those skilled in the art will be able to derive numerous modifications and variants without departing from the scope of the present invention, which is defined by the appended claims.

Claims

BCLAIMS: 1. A mechanical arrangement for providing rotary drive into a vessel containing high-pressure gas from a motive source (10) which is exposed to atmospheric pressure, wherein a housing of the motive source is sealed against one end of an elongate outer part (34) of a drive arrangement and the vessel is sealed against another end of the elongate outer part such that a high-pressure enclosure is provided, itself comprising the vessel housing, the housing of the motive source (10) and the outer part (34) of the drive arrangement (30), the drive arrangement (30) further comprising an elongate drive shaft (32) substantially located within the outer part (34) and entirely located within the high-pressure enclosure, said drive shaft being mechanically linked to a component (16) within the vessel and to a rotor of the motive source, so as to be able ro rotate within the outer part (34), thereby to provide motive force from the motive source (13) into the vessel.
2. A mechanical arrangement according to claim 1 wherein the vessel is a housing of a cryogenic refrigerator associated with a superconducting magnet.
3. A mechanical arrangement according to claim 1 or claim 2 wherein the motive source is an electric motor.
4. A mechanical arrangement according to any preceding claim wherein the rotary shaft (32) is flexible, and the outer part (34) is flexible.
5. A mechanical arrangement according to any preceding clam, wherein the rotary shaft (32) carries an engagement means at each end, with complementary fittings provided on the component (16) and on the rotor of the motive source (10)
6. A mechanical arrangement according to ciaim 5 wherern one or more of the engagement means is provided with a looking device.
7. A mechanical arrangement according to any preceding claim wherein the outer part (34) comprises a high-pressure hose of convoluted stainless steel tubing.
8. A mechanical arrangement according tc any preceding claim wherein the outer pert (34) comprises breiding of stainless steel or similar over a flexible hose.
9. A mechanical arrangement according to claim 1 wherein the drive shaft (32) is rigid.
10. A mechanical arrangement according to any of claims 1-9 wherein the drive shaft (32) is a flexible drive shaft which comprises two or more rigid shaft parts linked toge:her by flexIble joints.
11. A mechanical arrangement according to claim 10 wherein the outer part (34) includes a flexible hose, flexible ova: its entire length.
12. A mechanical arrangement according to claim 10 wherein the cuter part (34) includes a number of rigid tubular secticns, each corresponding to a rigid drive shaft pert, the rigid tubular sections being joined to one another by flexible sections, each corresponding to a flexible joint in the drive shaft.
13. A mechanical arrangement for providing rotary drive into a vessel containing high-pressure gas from a motive source (10) which is exposed to atmospheric pressure, wherein a housing of the motive source is sealed against one end of an elongate outer part (34) of a drive arrangement and the vessel is seated against another end of the elongate outer part suon that a high-pressure enclosure is provided, itself comprising the vessel housing, the housing of the motive source (10) and the outer part (34) of the drive arrangement (30), the drive arrangement (30) further comprising an elongate belt drive substantially located wi:hin the cuter part (34) and entirely located within the high-pressure enclosure, said belt drive being mechanically linked to a component (16) within the vessel and to a rotor of the mctive source, thereby to provide motive force from the motive source (10) into the vessel.
14. A method for providing motive force to a cryogenic refrigerator associated with a superconducting magnet comprising providing a mechanical arrangement according to any preceding clam, and operating the motive source to provide motive power to the cryogenic refrigerator.
15. A method according to claim 14, wherein, once the mechanical arrangement is assembled, and before use, the high-pressure enclosure is evacuated, purged, and charged with helium gas.Amendments to the claims have been filed as follows CLAIMS: 1. A mechanical arrangement, comprising a drive arrangement (30), a motive source (10) which is exposed to atmospheric pressure, and a vessel (12) containing high-pressure gas, wherein a housing of the motive source is sealed against one end of an elongate outer part (34) of the drive arrangement and the vessel is sealed against another end of the elongate outer part such that a high-pressure enclosure is provided, itself comprising the vessel housing, the housinc of the motive source (10) and the outer part (34) of the drive arrangement (30) the drive arrangement (30) further comprising an elongate drive shaft (32) substantially located within the outer part (34) and entirely located within the high-pressure enclosure, said drive shaft being mechanically linked to a component (16) within the vessel and to a rotor of the motive source, so as to be able to rotate within the outer part (34) , thereby to o provide motive force from the motive source (10) into the vessel. a) r2. A mechanical arrangement according to claim 1 wherein the vessel is a housing of a cryogenic refrigerator associated with a superconducting magnet.3. A mechanical arrangement according to claim 1 or claim 2 wherein the motive source is an electric motor.4. A mechanical arrangement according to any preceding claim wherein the drive shaft (32) is flexible, and the outer part (34) is flexible.5. A mechanical arrangement according to any preceding claim, wherein the drive shaft (32) carries an engagement means at each end, with complementary fittings provided on the component (16) and on the rotor of the motive source (10) 6. A mechanical arrangement according to claim 5 wherein one or more of the engagement means is provided with a locking device.7. A mechanical arrangement according to any preceding claim wherein the outer part (34) comprises a high-pressure hose of convoluted stainless steel tubing.8. A mechanical arrangement according to any preceding claim wherein the outer part (34) comprises braiding of stainless steel or similar over a flexible hose.9. A mechanical arrangement according to claim 1 wherein the drive shaft (32) is rigid.10. A mechanical arrangement according to any of claims 1-7 or 9 wherein the drive shaft (32) is a flexible drive shaft o which comprises two or more rigid shaft parts linked together by flexible joints. a) r11. A mechanical arrangement according to claim 13 wherein the outer part (34) includes a flexible hose, flexible over its entire length.12. A mechanical arrangement according to claim 13 wherein the outer part (34) includes a number of rigid tubular sections, each corresponding tc a rigid drive shaft part, the rigid tubular sections being joined to one another by flexible sections, each corresponding to a flexible joint in the drive shaft.13. A mechanical arrangement, comprising a drive arrangement (30), a motive source (10) which is exposed to atmospheric pressure, and a vessel (12) containing high-pressure gas, wherein a housing of the motive source is sealed against one end of an elongate outer part (34) of a drive arrangement and the vessel is sealed against another end of the elongate outer part such that a high-pressure enclosure is provided, itself comprising the vessel housing, the housing of the motive source (10) and the outer part (34) of the drive arrangement (30), the drive arrangement (30) further comprising an elongate belt drive substantially located within the outer part (34) and entirely located within the high-pressure enclosure, said belt drive being mechanically linked to a component (16) within the vessel and to a rotor of the motive source, thereby to provide motive force from the motive source (10) into the vessel - 14. A method for providing motive force to a cryogenic refrigerator associated with a superconducting magnet comprising providing a mechanical arrangement according to any preceding claim, and operating the motive source to provide o motive power to the cryogenic refrigerator.15. A method according to claim 14, wherein, once the r mechanical arrangement is assembled, and before use, the high-pressure enclosure is evacuated, purged, and charged with helium gas.