GB2449652A

GB2449652A - Rod tensioning mechanism

Info

Publication number: GB2449652A
Application number: GB0710216A
Authority: GB
Inventors: Nicholas Mann; Jennifer Ridley
Original assignee: Siemens Magnet Technology Ltd
Current assignee: Siemens Magnet Technology Ltd
Priority date: 2007-05-30
Filing date: 2007-05-30
Publication date: 2008-12-03
Anticipated expiration: 2027-05-30
Also published as: GB0710216D0; US8393163B2; GB2449652B; JP5294712B2; US20080295528A1; JP2008302218A

Abstract

The mechanism is used for tensioning a rod 10 which may be provided with a head 12. A suspension bracket (32 Fig.2) having a hole or a slot is used as an anchoring point. The mechanism consists of a tubular rod carrier or sleeve 20 which has a rod bearing surface 18 and an external thread 42 for receiving a nut 22 and a locking nut 24. A rod end piece 14 is provided with a bore so as to allow the rod to be passed there through. The rod, together with the end piece, is passed through the sleeve until the end piece abuts against the rod bearing surface. The mechanism, including the threaded nuts and a washer 40 is passed through the hole or slot of the bracket and, in use, either the nut or the washer rest against the surface of the bracket. When the nut is adjusted the sleeve moves upward from the surface of the bracket and the bearing surface tensions the rod. The end piece can have a spherical surface 16 matching a complementary bearing surface and inside the sleeve may be tapered or stepped at 38 to allow for angular movement of the rod.

Description

SUSPENSION ROD TENSIONING ARRANGEMENTS

In order to improve access for clinicians and improve patient comfort, it is advantageous to make magnets used in MRI scanners as compact as possible. The superconducting magnet typically used must be held at cryogenic temperatures, and this requires that superconducting magnets must be suspended by a suspension system, and heat conduction through the suspension system must be minimised.

Suspension rods of high strength metal such as austenitic stainless steel are an established way of supporting superconducting magnets. In conventional designs thermal load is minimised by the use of long rods, typically with threaded ends to facilitate assembly and tensioning.

However, these designs result in relatively large cryostats to provide space for assembly and suspension rod termination systems. In order to reduce the required length, high strength materials with lower conductivity than metals may be used, for example glass or carbon fibre composites, but these are inherently expensive to manufacture.

The present invention aims to provide a suspension system which provides low heat load within a compact cryostat without the use of expensive materials.

Compact suspension arrangements utilising composite bands and facilitating easy (hook-on") assembly are described in United Kingdom patent application GB2426545 of Siemens Magnet Technology Ltd. The present invention addresses a similar problem and provides lower cost, typically metal, rod suspension.

Known metal suspension rods as used to support superconducting magnets for MRI imaging equipment have long been restrained using a threaded end with a co-operating nut. However, to achieve acceptable fatigue strength the threads must be manufactured to a high standard. In addition, to minimise thermal load, the diameter of the non-threaded part of the rods should be reduced below the diameter required for thread forming, which is costly in manufacture. Also, features such as flats must be provided on the rod to prevent rotation during tensioning of the nut; these are expensive to manufacture and difficult to use, for example several spanners may be required simultaneously in order to tension nuts on a suspension rod.

Alternative arrangements for terminating rods are known, for example using cold-formed heads to overcome the disadvantages of threads, and are used, for example, in the field of marine rigging. However, this does not inherently provide a method of length adjustment, and the known provisions to achieve length adjustment are bulky and compromise thermal length in a cryogenically cooled superconducting magnet support application.

Fig. 3 shows a known arrangement using threaded fasteners, typical of the systems used in MRI magnet systems. Use of such threaded fasteners limits fatigue life of the support system and does not optimise the thermal length.

Rod 10 has a widened, threaded portion 38 which passes through a hole or slot in outer suspension bracket 32. A washer 40 is placed over the threaded portion to abut an outer surface of the outer suspension bracket 32. In order to eliminate bending loads in the rod, this single washer may be replaced by a pair of washers with complimentary male and female spherical contacting surfaces thus providing angular movement. A tensioning nut 22 is threaded onto the threaded portion 38 of the rod, and is tightened to apply the required tension to the rod 10. A locking nut 24 is preferably added, tightened into the tensioning nut 22 to prevent loosening of the tensioning nut 22. The end of the rod may be provided with a slotted head 14 or equivalent, so that the rod may be held by a suitable tool, so as to prevent of rotation of the rod while the nuts 22, 24 are tightened.

The present invention accordingly provides an improved rod suspension arrangement, in which: -a thermal length is maximised; -the threading of the rod is avoided, so avoiding the need for different rod thicknesses at different positions on the rod, and minimising required rod thickness over the main length; -the operation of applying preload tension to the support rod is simplified; -angular misalignment of the rod is allowed for; and -fatigue life is optimised by avoiding deformation or thinning of the rod.

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

The above, and further, objects, characteristics and advantages of the present invention will become more apparent from consideration of the following description of certain embodiments thereof, in conjunction with the accompanying drawings, wherein: 2 -4-Fig. I illustrates a rod mounting and tensioning assembly according to an aspect of the present invention; Fig. 2 illustrates a complete suspension rod assembly according to an embodiment of the present invention; Fig. 3 illustrates known arrangements which use threaded fasteners; and Figs. 4A-4C show a hook-on arrangement, suitable for attaching an inner end of a suspension rod to an inner end suspension bracket.

The tension rod support arrangement of the present invention is required 1.0 provide at least some of the following features: -means are provided for achieving a consistent application of preload; -angular misalignment of the rod must be tolerated within a defined range; ability to take up variations in actual and required length of the rod; -the thermal length of the rods to be maximised between the supported article, such as a superconducting magnet, and a support surface; -a locking arrangement should be provided to prevent loosening of the suspension rod, and so to maintaining an applied preload; -an assembly process which allows the tensioning end of the rod to be fitted after location of the opposite end of the rod; -optimised fatigue life; and -twisting of the suspension rod during length adjustment is prevented.

Fig. 2 shows an entire suspension unit according to an embodiment of the present invention, including the inner end suspension bracket on to which the rod preferably hooks. Typically, at least eight such suspension units would be employed within a single system to provide support of the suspended vessel in all degrees of freedom.

Referring first to Fig. 2, a suspension rod assembly is shown, in which a suspension rod 10 is provided, between two articles. One is referred to as an "inner end suspension bracket" 30, while the other is referred to as an "outer end suspension bracket" 32. Such terms are particularly applicable to the suspension of a cryogenically cooled superconducting magnet, wherein the inner end suspension bracket will be closer to the cryogenically cooled magnet, while the outer end suspension bracket will be closer to the outer vacuum container conventionally provided [or thermal insulation of a cryogenically cooled superconducting magnet. The suspension rod assembly of the present invention may, however, find application in other equipment, and terms such as "outer" and "inner" should be taken as convenient labels, rather than limiting the invention in any way.

The suspension rod 10 preferably attaches to the inner end suspension bracket 30 by a hook-on process, such that it is simple to hook the "inner" end of rod 10 with a suitable end arrangement onto a co-operating feature of the inner end bracket, by moving the rod at the "outer" end, without needing direct access to the inner end suspension bracket. A more complete description of the features of rod 10 and inner end bracket 30 which allow such hook-on operation follows.

Figs. 4A, 4B and 4C illustrate a hook-on arrangement which may be used to attach an inner end of suspension rod 10 to an inner end suspension bracket 30. Other arrangements may of course be used, but the inventors regard such hook-on arrangements as useful in combination with the tensioning and retaining arrangement of the present invention in providing a simple, easily-adjustable suspension arrangement. For ease of description, Figs. 4A, 4B, 4C will be described together.

A particular form of inner end suspension bracket 30 is illustrated. It includes a slot 82 of sufficient width to allow the body of rod 10 to pass into the slot. Al an inner end of the slot, a cavity 84 is formed. The rod 10 is provided with an enlarged head 86 at its inner end, by any suitable method such as be mechanical upsetting. A retaining piece 88 is provided, having a through hole through which the body of the rod 10 passes, and a receiving cavity which accommodates and preferably also retains the enlarged head 86 of the rod. Features 89 such as a swaged feature may be provided for holding the retaining piece and the enlarged head together.

Alternatively, the rod and the retaining piece may be held together by a circlip. The cavity 84 in the inner end suspension bracket 30 is dimensioned to accommodate the retaining piece 88 and the enlarged head 86. During assembly of the suspension arrangement, retaining piece 88 is placed over the enlarged head. This may be done by threading the retaining piece onto the rod from the other end, or by threading the retaining piece onto the rod before formation of the enlarged head, or by assembling a retaining piece 88, from two or more pieces, around the body of the rod 10. The body of rod 10 is passed into slot 82, ensuring that the retaining piece 88 protrudes beyond the inner end of the bracket 30. The rod is then pulled in the outwards direction, to cause the retaining piece 88 to enter the cavity 84, as shown in Fig. 4C. The contacting surfaces of the cavity 84 and the retaining piece 88 are preferably approximately spherical, to allow some degree of alignment adjustment between the rod and the bracket. If misalignment is not present, it may be found that the retaining piece 88 may be dispensed with, the rod being retained within cavity 84 directly upon its enlarged head 86.

Referring again to Fig. 2, it can be seen that the rod 10 passes through outer end suspension bracket 32, and is tensioned by a mechanism 34 which will be described in more detail with reference to Fig. 1. Of particular interest with reference to the outer end suspension bracket is the provision of slot 36, into which the rod 10 and mechanism 34 may be brought. Use of such slot 36 means that the rod 10 and mechanism assembly 34 do not need to be disassembled during attachment and tensioning.

The overall method for fitting and tensioning a rod suspension assembly according to the invention and as illustrated in Fig. 2 is preferably: -the inner end is hooked on the inner end suspension bracket 30; -the outer end and the mechanism 34 are swung into the slot 36 of the outer end suspension bracket 32; and -pretension is applied to the rod by operation of the mechanism 34, as discussed below with reference to Fig. 1.

Fig. 1 illustrates an annotated cross-section of the tensioning and retaining mechanism 34 with the details as described previously.

Fig. 1 shows the outer end of rod 10 and the tensioning and retaining mechanism 34. The rod 10 passes through a tubular rod carrier 20 and is retained against a surface of the rod carrier by a rod end piece 14. The end of rod 10 is formed into an enlarged head 12, preferably by an upsetting operation. Tension will be applied to the rod through this enlarged head, so avoiding the stress concentration of the former threaded rods. Rod end piece 14 has a through-hole through which the rod 10 passes, and an inner retaining cavity to retain the enlarged head 12 of the rod 10. The inner retaining cavity preferably conforms closely to the shape of the enlarged head 12, so providing an effective mechanical contact over a relatively large surface area, increasing the fatigue life of the rod. The rod end piece 14 is provided with a spherical surface 16 abutting a complementary seat 18 on the end of the rod carrier 20. The arrangement of spherical surface 16 abutting complementary seal 18 allows operation of the tensioning and retaining mechanism 34 despite misalignment of respective axes of the rod 10 and the rod carrier 20 within a predetermined range of acceptable angular misalignment, for example 2 in any direction. The bore of the rod carrier 20 is preferably tapered, either in the form of a truncated conical bore, or a bore having a number of differing diameters in a step arrangement 38 as illustrated in Fig. 1, to allow clearance to the rod 10 over the predetermined range of acceptable angular misalignment.

The rod carrier 20 is arranged to locate within the slot 36 of the outer end suspension bracket 32 (Fig. 2). Preferably, the rod carrier 20 has a shaped inner end 26, for example having opposed flat surfaces, which contact sides of the slot 36 and prevent rotation of the rod carrier 20. The rod carrier is threaded 42 on its outer surface along at least part of its length between the rod end piece 14 and the shaped inner end 26. A tensioning nut 22 is provided, threaded onto the thread 42 of the rod carrier 20. A washer 40 is preferably provided, over the rod carrier 20 to contact the surface of the outer end suspension bracket 32. As is conventional in itself, the washer provides a suitable bearing surface for tensioning nut 22.

The tensioning nut 22 provides tensioning of the suspension rod over the length of the thread 42, for example 20mm, to allow adjustment to compensate for manufacturing tolerances of the components, and to allow pretensioning of the suspension rod. As the rod carrier is prevented from rotating by interaction of the shaped inner end 26 and the slot in the outer end suspension bracket, tensioning of the suspension rod 10 may simply be achieved by the use of a single spanner on tensioning nut 22.

A locking nut 24 is preferably provided, and is tightened onto tensioning nut 22 once the required tension has been established in the rod 10. This locking nut 24 prevents tensioning nut 22 from loosening after assembly.

The tensioning nut 22 and the locking nut 24 may be assembled by passing over the rod end piece 14. The shaped inner end 26 may preclude nuts from being attached over that end.

Assembly of the mechanism 34 to the rod 10 may proceed by any one of numerous routes. For example, rod end 14 may be placed on the enlarged head 12 of the rod 10, with the nuts 22 and 24 being threaded onto rod carrier 20, which is then slid onto the other end of the rod. Alternatively, the rod carrier 20 with its nuts 22, 24 may be slid over the enlarged head 12 of the rod, with the rod end piece 14 being a two-part assembly, later positioned around the enlarged head.

Particular advantages of the suspension assembly of the present in'vention include the following. The rod 10 is restrained near its end 12, beyond the tensioning nut 22 and the outer end suspension bracket 32. This lengthens the thermal path between the inner end of the suspension rod and the outer end suspension bracket, as compared to former arrangements in which the rod was threaded and passed through a tensioning nut at the outer end suspension bracket. Tensioning of the rod is simplified to a single-spanner operation, by providing a shaped inner end 26 of the rod carrier, which prevents rotation of the rod carrier within the outer end suspension bracket.

The present invention accordingly provides a rod suspension arrangement including a tensioning assembly enabling simple installation, by hooking an inner end into an inner end suspension bracket, slotting the tensioning mechanism into a slot. 26 in the outer end suspension bracket and tightening the tensioning and lock nuts 22, 24 to apply a preload to the suspension rod.

The present invention accordingly provides a compact, low cost, low thermal load suspension system for a superconducting magnet used in an MRI scanner. Key features include metal rods with headed ends in place of threads, and a termination system that maximises thermal length and facilitates easy assembly and tensioning during manufacture.

Claims

1. A tensioning and retaining mechanism (34) for holding a suspension rod (10) in tension braced against a suspension bracket (32), said tensioning and retaining mechanism comprising: a tubular rod carrier (20) arranged, in use, to pass through a hole or slot in the suspension bracket, the rod carrier having: -an axial bore through which, in use, the rod may pass; -a thread (42) on its outer surface along at least part of its length; and -a rod bearing surface (18) for, in use, transmitting force to the rod; and -a tensioning nut (22) threaded onto the thread (42) so as to, in use, brace against the suspension bracket to urge the tubular rod carrier in the direction of the rod bearing surface (18) so as to apply tension to the rod.

2. A tensioning and retaining mechanism (34) according to claim 1, further comprising a locking nut (24) threaded onto the thread at a position between the tensioning nut (22) and the rod bearing surface.

3. A tensioning and retaining mechanism (34) according to claim I or claim 2, further comprising a washer (40) which, in use, is positioned between the tensioning nut (22) and suspension bracket (32).

4. A tensioning and retaining mechanism (34) according to any preceding claim, further comprising a rod end piece (14) having a through-hole through which, in use, the rod (10) may be arranged to pass, and an inner retaining cavity wherein, in use, an end of the rod (10) may be accommodated, said rod end piece having an end piece bearing surface (16) arranged to bear upon the rod bearing surface (16) of the tubular rod carrier.

5. A tensioning and retaining mechanism (34) according to claim 4, wherein the rod end piece (14) is provided with a spherical surface (16) abutting a complementary seat (18) on the end of the rod carrier (20).

6. A tensioning and retaining mechanism (34) according to any preceding claim, wherein the bore of the rod carrier (20) is tapered to allow clearance for the rod over the predetermined range of acceptable angular misalignment.

7. A tensioning and retaining mechanism (34) according to any preceding claim, wherein the rod carrier (20) has a shaped inner end (26), which, in use, contacts sides of a slot (36) in the suspension bracket (32) thereby to prevent rotation of the rod carrier.

8. An MRI system substantially as described.

8. A tensioning and retaining mechanism substantially as described and/or as illustrated in Figs 1, 2 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. An MRI system comprising a cryogenically cooled superconducting magnet suspended within an outer vacuum container, the suspension system comprising a number of suspension rods each tensioned between a respective inner end suspension bracket and a respective outer end suspension bracket, the inner end suspension bracket being closer to the cryogenically cooled superconducting magnet than the outer end suspension bracket, at least one of the suspension rods being provided with a tensioning and retaining mechanism (34) for holding a suspension rod (10) in tension braced against a suspension bracket (32), said tensioning and retaining mechanism comprising: -a tubular rod carrier (20) arranged, in use, to pass through a hole or slot in the suspension bracket, the rod carrier having: -an axial bore through which, in use, the rod may pass; -a thread (42) on its outer surface along at least part of its length; and -a rod bearing surface (18) for, in use, transmitting force to the rod; and -a tensioning nut (22) threaded onto the thread (42) so as to, in use, brace against the suspension bracket to urge the tubular rod carrier in the direction of the rod bearing surface (18) so as to apply tension to the rod.

2. An MRI system according to claim 1, wherein the tensioning and retaining mechanism (34) further comprises a locking nut (24) threaded onto the thread at a position between the tensioning nut (22) and the rod bearing surface.

3. An Mill system according to claim 1 or claim 2, wherein the tensioning and retaining mechanism (34) further comprises a washer (40) which, in use, is positioned between the tensioning nut (22) and suspension bracket (32).

4. An MRI system according to any preceding claim, wherein the tensioning and retaining mechanism (34) further comprises a rod end piece (14) having a through-hole through which, in use, the rod (10) may be arranged to pass, and an inner retaining cavity wherein, in use, an end of the rod (10) may be accommodated, said rod end piece having an end piece bearing surface (16) arranged to bear upon the rod bearing surface (16) of the tubular rod carrier.

5. An MRI system according to claim 4, wherein the rod end piece (14) is provided with a spherical surface (16) abutting a complementary seat (18) on the end of the rod carrier (20). c'J

6. An MRI system according to any preceding claim, wherein the bore of the rod carrier (20) is tapered to allow clearance for the rod over the predetermined range of acceptable angular misalignment.

7. An MRI system according to any preceding claim, wherein the rod carrier (20) has a shaped inner end (26), which, in use, contacts sides of a slot (36) in the suspension bracket (32) thereby to prevent rotation of the rod carrier.