IMPROVEMENTS RELATING TO EPOXY BONDED GLASS FIBRE RODS

Abstract: A method and apparatus for manufacturing a rod of epoxy bonded glass fibre having at least one end fitting. The method comprises threading a number of loops of uncured glass fibre (31) through a sleeve (5) of substantially rigid material such as austenitic stainless steel. A rigid insert (17) is positioned in the loops of glass fibre and the loops of glass fibre and at least part of the insert (17) are drawn into the sleeve (5). The internal surface (5A) of the sleeve (5) cooperates with the insert (17) so that an interference fit occurs between them. Finally, the glass fibre (31) is cured.

Description: IMPROVEMENTS RELATING TO EPOXY BONDED GLASS FIBRE RODS The invention relates to methods and apparatus for manufacturing epoxy bonded glass rods particularly for use in supporting superconducting magnets in cryostats. Superconducting magnets are commonly used in magnetic resonance (MR) imaging systems and are essentially solenoids which are cooled to operational temperatures in a bath of liquid helium. Liquid helium boils at a temperature of 4.2 K and to maintain this helium bath it is necessary to design a cryostat that reduces to a minimum the transfer of heat from the environment. Three methods of heat transfer are usually considered: conduction plus convection through gas (air) between the helium vessel and its environment, radiation from the environment onto the helium vessel, and conduction through the physical support structure of the helium vessel. To deal with the problems of heat transfer, cryostats to contain imaging magnets are built with a series of radiation shields between the surface of the outer vessel and the helium vessel. In some configurations of imaging cryostats the intermediate shields are cooled by refridgerators that use external power. Others use as an intermediate shield a vessel which contains liquid nitrogen which boils at 77K at normal atmospheric pressure. In this case, the shield is maintained at an intermediate temperature by the boiling nitrogen and energy from the environment is absorbed by the latent heat of boiling. It is necessary to design a support system for the elements of the cryostat which is strong enough to support the vessels and shields and which does not allow heat to be conducted directly from the outer vessel to the helium can. The forces which the elements of the cryostat undergo include gravity, acceleration during movement of the cryostat, and magnetic interactions with the structural iron. Conventionally, a system of struts operating in three dimensions and supporting the vessels while acting in tension has been used for MR imaging magnet cryostats. Unique combinations of strength and thermal isolation are required for MR magnets. It is typically required that the helium boil off from an NMR imaging cryostat due to all heat sources is between 0.3 and 0.4 Lhr . This is equivalent to 780 to 1300 Jhr~ energy ie. 0.21 to 0.36 watts. The weight of the imaging magnet and vessel range from 2.000 kilograms for a 0.5T coil assembly to 4000 kilograms for a 1.5T coil assembly. Until recently, support systems were designed solely to deal with gravitational forces. Relatively recently, it has become an operational requirement that imaging magnets are built into mobile scanners housed in custom built bodies on air ride trucks. The vibration and impact accelerations experienced by the magnet can occur in any of three dimensions. These requirements have resulted in the need for support struts having tensile strength ranging between 3000 kilograms and 10000 kilograms. The lower strength struts are used for limited mobility systems and the higher strength for fully mobile systems. By fully mobile we mean systems moved once per day at normally prevailing road speeds. it will be appreciated from this discussion that the struts must have a high tensile strength, and low heat conductivity. Suitable struts based on glass fibre epoxy bonded composites have been manufactured up to now in the manner described in GB-A-902197. The rod so formed has a yield tensile strength of 1020 MPa. To form a practical strut, a means of attaching the rod to the metal vessels is required and it is the solution to this problem that critically affects the performance of the strut. Struts used prior to this invention have utilised attachments formed by clamping a steel sleeve around the glass rod at each of its ends. See GB-A-902197. Several major difficulties have been found using this method of fabricated struts when the struts are used in cryostatic assemblies. (a) Control of clamping pressure when attaching the steel sleeve. If the clamping dies exert too much pressure, the glass rod is crushed within the sleeve and will shatter on tensile loading. If the clamping pressure does not properly close the sleeve, insufficient friction exists to transmit tensile loads from the attachment point to the rod. The sleeves will just draw off the rod under tension. (b) To provide enough tolerance of practical die clamping variations, long metal sleeves have to be used. These shorten the thermal isolation length of the strut which is critical as explained above. (c) The glass rod is prone to failure at the point where the rod enters the sleeve due to stress concentrations. Such failures are particularly prevalent under impact or fatigue loading constrictions and always arise due to thermal strains when the rod is cooled. With the advent of mobile magnets such loading conditions are encountered and the present invention is essential to this operational requirement. In accordance with one aspect of the present invention, a method of manufacturing a rod of epoxy bonded glass fibre having at least one end fitting comprises (a) threading a plurality of loops of uncured glass fibre through a sleeve of substantially rigid material; (b) " positioning a substantially rigid insert in the loops of glass fibre; (c) drawing the loops of glass fibre and at least part of the insert into the sleeve, the internal surface of the sleeve cooperating with the insert so that an interference fit occurs between them; and (d) curing the glass fibre. With this invention, instead of securing the end fitting after the rod has been formed, as in the known methods, the end fitting is secured simultaneously with the rod and this is achieved by positioning an insert in the glass fibre loops and then drawing the insert into the sleeve to achieve an interference fit. It has been found that this method has a number of important advantages: (a) Very reproducible mechanical assembly which cannot damage the rod structure. The attachment point is incorporated in the rod as part of the composite structure during the manufacturing process. (b) Much greater rigidity of connection between rod and sleeve which permits struts to operate to the intrinsic tensile strength of the rod. (c) No stress concentration points to reduce fatigue and impact strength of the strut. (d) Progressive strain after the tensile load on a strut has been exceeded. In this condition the rod is permanently damaged but has not fractured. This is of vital importance to maintaining the integrity of the cryostat components when the cryostat is overloaded mechanically. With progressive strut failure, serious damage to the magnet under mechanical overload is avoided. Preferably, the internal surface of the sleeve and the external surface of the insert taper relatively to one another in opposite directions to achieve the interference fit. Conveniently, the insert has a morse taper. The insert may be made of E glass pultruded rod while the sleeve may be made from austentitic stainless steel. A further advantage of the method is that the glass fibres may be S 2 glass fibres which have a lower thermal conductivity and better tensile strength relative to the conventional E glass fibres. It is particularly convenient, where the loops of uncured glass fibre are formed from parts of a continuous winding of glass fibre and define one end of a plurality of elongate turns, if the method further comprises repeating steps a - c with the loops forming the other end of the elongate turns and then carrying out step d on both ends. This produces a rod having end fittings at each end of the rod. Curing may be achieved in any conventional manner. In accordance with a second aspect of the present invention, an epoxy bonded glass fibre rod has at least one end fitting comprising a sleeve of substantially rigid material surrounding an insert of substantially rigid material positioned within loops of glass fibre whereby the sleeve is an interference fit on the insert. In accordance with a third aspect of the present invention, apparatus for use in the manufacture of a rod of epoxy bonded glass fibre having at least one end fitting comprises a mounting for securing a sleeve of substantially rigid material; and moving means for causing relative movement between the mounting and loops of uncured glass fibre whereby the loops of uncured glass fibre may be drawn through the sleeve in both directions. Conveniently, the moving means comprises a fluid operated piston/cylinder arrangement which is adapted to engage at least the loops of uncured glass fibre. In one arrangement, the mounting is adapted to support coaxially two sleeves of substantially rigid material, one sleeve being fixed relatively to the mounting and the other being movable relatively to the mounting when attached to the moving means whereby a rod with two end fittings may be manufactured. An example of a method and apparatus for manufacturing an epoxy bonded glass fibre rod in accordance with the invention will now be described with reference to the accompanying drawings, in which:- Figure 1 is a schematic view of the apparatus; Figure 2 is a side view of part of the winding fork shown in Figure 1; Figure 3 illustrates the ferrule mounting in more detail; Figure 4 illustrates the curing process; Figure 5 is a part sectional view of an end of the completed rod; Figure 6 illustrates the insert in more detail; Figure 7 illustrates schematically the use of rods according to the invention in a MR imaging magnet cryostat; and, Figure 8 is a partial perspective view of an end of the cryostat shown in Figure 7. The apparatus comprises a pneumatic cylinder 1 having a rod 2 connected to a piston within the cylinder. A connector 3 is attached to the protruding end of the rod 2. The rod 2 may be moved along a line which forms the axis of two austentitic stainless steel, externally screw-threaded ferrules 5, 6 supported in a mounting 7. This assembly is positioned within a .housing 4. The mounting 7 is shown in more detail in Figure 3. The mounting 7 has a pair of coaxial, circular cross-section bores 8 , 9 connected by a reduced diameter bore 10. The mounting 7 is fixed relatively to the cylinder 1 by means omitted for clarity in Figure 1. Positioned beneath the mounting 7 (Figure 1) is a winding jig 12 non-rotatably mounted at its centre to a drive shaft 13 rotatably driven by a motor 13'. A glass fibre roving spool 14 is rotatably mounted in alignment with adjacent the winding jig 12 on a support arm 12' extending from adjacent the jig. Glass fibre from the roving spool 14 is wound onto a pair of winding forks 15 at each end of the jig 12 (one shown in Figure 2) on rotation of the winding jig. The glass fibre is preferably Owens Corning S2 glass roving HTS 449AA-750, 9.0 μm per fibre, process impregnated by passage through a bath (not shown) with heated Ciba Geigy MY 750 epoxy resin, HT 917 hardener and 063 accelerator. Initially, 95 turns of glass fibre roving (190 toes) are wound onto the winding jig 12. In addition, the two ferrules 5, 6 are positioned in the mounting 7, the ferrule 6 being secured in the bore 9 by means of a gauge screw 16. The rod 2 is moved to its extended position shown in phantom in Figure 1. It will be seen in Figure 5 that the internal surface 5B of the ferrule 5 has a morse taper 5A which is about 3.35°. The ferrule 6 has a similar taper and the ferrules are positioned back to back in the mounting 7 with the narrower ends of their bores adjacent. The ferrules 5, 6 are then warmed to about 60°C by the heaters 11. With the winding jig 12 vertically aligned (as in Figure 1) with the ferrules 5, 6, a single toe of Kevlar roving 30 is attached to the top loop of windings, and passes through the ferrules 5, 6 to the connector 3 which is in the extended position. The windings are removed from the jig 12 and the cylinder 1 is then activated and the windings pulled through the top ferrule 5 far enough to allow a taper pin 17 to be inserted into their centre ensuring that the radius of the thick end of the pin follows that of the loop in the windings. The taper pin 17 is shown in more detail in Figure 6. The taper pin 17 is made from commercial E glass pultruded rod and has dimensions as shown in Figure 6. The Kevlar toe is then discarded and the connector 3 of the cylinder rod 2 is attached to the top ferrule 5. When activated, the cylinder 1 causes the rod 2 to retract thus pulling the top ferrule 5 away from the mounting 7 taking the rovings with it. The stroke of the cylinder is stopped short to allow another taper pin to be inserted into the lower winding loops in a similar manner to that at the top. Subsequently .the cylinder is reactivated to draw the second taper pin and loops towards the ferrule 6. The cylinder pulls the rod assembly to its completed length of 86 cms and simultaneously locks both ferrules 5,6 in position under a load of 1000 kilograms by virtue of the interference fit between the taper pins and the ferrules. In this position the pins are positioned fully within the ferrules (Figure 5) . The uncured rod is then removed from the piston and heat shrink tape 18 is spirally wound around the exposed glass fibre 31. The wound rods are then placed in a split mould having two mould halves 19, 19' The rods are prestressed by tightening nuts 20 on each ferrule 5, 6 against respective ends of the mould. This reduces any tendency of the glass fibre to twist or bow in the heat. The assembly is then cured for about eight hours at a temperature of 165°C. After the cure cycle, the heat shrink sleeve 18 is stripped from the rods and the composite surface polished. Figure 7 illustrates how some of the finished rods 34 are positioned to support the elements of a cryostat 33 for mounting an NMR imaging magnet. The rods 34 support a helium vessel 35 under tension. The cryostat shown in Figure 7 is otherwise of conventional form having a radiation shield 36 surrounding the helium vessel, and a nitrogen vessel 37 surrounding the radiation shield 36. Other radiation shields and evacuated cavities are provided in a conventional manner. The magnet has, however, been omitted for clarity. Figure 8 illustrates the mounting arrangement in more detail in which can be seen that the tension rod 34 extends through a slot 38 in the radiation shield 36 and is connected at one end to the helium vessel 35 and at the other end to the nitrogen vessel 37. A second tension rod 39 is also illustrated in Figure 8 for supporting other parts of the cryostat radially outwardly of the helium vessel. Since the constraints on strength and heat conduction are of less importance in this position, the rod 39 may be of a conventional form or be constructed in accordance with the invention.

Claims: CLAIMS 1. A method of manufacturing a rod of epoxy bonded glass fibre having at least one end fitting, the method comprising (a) threading a plurality of loops of uncured glass fibre through a sleeve of substantially rigid material; (b) positioning a substantially rigid insert in the loops of glass fibre; (c) drawing the loops of glass fibre and at least part of the insert into the sleeve, the internal surface of the sleeve cooperating with the insert so that an interference fit occurs between them; and (d) curing the glass fibre. 2. A method according to claim 1, wherein the internal surface of the sleeve and the external surface of the insert taper relatively to one another in opposite directions to achieve the interference fit. 3. A method according to claim 1, in which the loops of uncured glass fibre are formed from parts of a continuous winding of glass fibre and define one end of a plurality of elongate turns, the method further comprising repeating steps a - c with the loops forming the * other end of the elongate turns and then carrying out step d on both ends. 4. An epoxy bonded glass fibre rod having at least one end fitting comprising a sleeve of substantially rigid material surrounding an insert of substantially rigid material positioned within loops of glass fibre whereby the sleeve is an interference fit on the insert. 5. A rod according to claim 4, wherein the glass fibres comprise S 2 glass fibres. 6. A rod according to claim 4, wherein the insert is made of E glass pultruded rod, and the sleeve is made from austentitic stainless steel. 7. Apparatus for use in the manufacture of a rod of epoxy bonded glass fibre having at least one end fitting, the apparatus comprising a mounting for securing a sleeve of substantially rigid material; and moving means for causing relative movement between the mounting and loops of uncured glass fibre whereby the loops of " uncured glass fibre may be drawn through the sleeve in both directions. 8. Apparatus according to claim 7, wherein the moving means comprises a fluid operated piston/cylinder arrangement which is adapted to engage at least the loops of uncured glass fibre. 9. Apparatus according to claim 7, wherein the mounting is adapted to support coaxially two sleeves of substantially rigid material, one sleeve being fixed relatively to the mounting and .the other being movable relatively to the mounting when attached to the moving means whereby a • rod with two end fittings may be manufactured.

Inventors: GILGRASS GRAHAM, GILGRASS, GRAHAM
Applicants: OXFORD MAGNET TECH, OXFORD MAGNET TECHNOLOGY LIMITED
Application year: 1986
Main classification: B29C67
IPC codes: B29C70, B29L31