Tension bands and methods for their manufacture

A tension band comprises a number of layers of resin bonded fibre rovings, the rovings in each layer being laterally offset from the rovings in adjacent layers. Methods and apparatus for manufacturing such tension bands are also described.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to tension bands and methods and apparatus for 
manufacturing such tension bands. The invention is concerned in particular 
with tension bands for use as supports in cryogenic applications. 
2. Description of the Related Art 
Superconducting magnets are commonly used in magnetic resonance imaging 
(MRI) systems and are essentially solenoids which are cooled to 
operational temperatures in a bath of liquid helium provided in a 
cryostat. 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 built 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 refrigerators 
that use external power. Others use as an intermediate shield a vessel 
which contains liquid nitrogen which boils at 77 K. 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 NMR 
imaging magnet cryostats. Examples of these struts are described in USSN 
912,246. Unique combinations of strength and thermal isolation are 
required for NMR magnets. 
The weight of the imaging magnet and vessel range from 1500 to 4000 
kilograms dependent upon magnet field strength and vessel configuration. 
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 a support member capable 
of withstanding a sustained fatigue loading of 15 to 30 KN, with an 
ultimate tensile strength of greater than 75 KN. This represents 2 g 
fatigue and 5 g shock loading, for the heaviest magnet considered, when 
distributed across 4 suspension members. 
A paper entitled "Fatigue Resistance of a Uniaxial S-Glass/Epoxy Composite 
at Room and Liquid Helium Temperatures" by Tobler and Read in J. Composite 
Materials, Vol. 10 (January 1976), p.32 and a paper entitled "Filament 
Wound Composite Thermal Isolator Structures for Cryogenic Dewars and 
Instruments" by Morris in Composites for Extreme Environments, ASTM SDP768 
N.R.A. Adsit, Ed, American Society for Testing Materials 1982, p. 95-109 
describe composite materials for manufacturing tension bands which have 
been proposed in the past for satisfying the above requirements. These 
papers describe a variety of uses for such tension bands including their 
use as thermal isolator straps for cyogenic dewars and instruments. 
Although these tension bands have a good thermal/tensile strength, they 
have to be produced individually at very high cost if they are to have 
sufficient strength for use in heavy duty applications such as cryostats 
for NMR imaging magnets. 
SUMMARY OF THE INVENTION 
In accordance with one aspect of the present invention, a tension band 
comprises a plurality of layers of resin bonded fibre rovings, the rovings 
of the plurality of layers extending in the same direction, and the 
rovings in each layer being laterally offset from the rovings in adjacent 
layers in a direction transverse the direction in which the rovings 
extend. 
In the invention, the rovings in each layer are laterally offset in a 
regular manner from the rovings in adjacent layers by a predetermined 
amount preferably the degree of overlap is substantially 50%. This has 
resulted in a significant increase in tensile strength compared with known 
tension bands of similar dimensions. This means that it is possible to 
achieve a high tensile and fatigue strength tension band while minimizing 
the cross-sectional width of the band and thus minimizing the conducted 
heat load within a cryostat or other cryogenic environment in which the 
band is used. In addition, this new tension band is particularly suited to 
mass production at a much lower cost than previously. 
In this context, a "roving" is defined as comprising a bundle of individual 
fibres and will typically have a rectangular cross-section. 
The rovings may be woven or, preferably, non-woven (i.e., unidirectional). 
The resin is preferably an epoxy resin. 
Although the rovings in each layer may be of the same material, it is 
preferred to include one or more partial layers of rovings of a different 
material from the remainder of the layers. These additional partial layers 
may be in the form of pre-impregnated fibre sheets of unidirectional 
fibres. 
The use of these additional layers is particularly advantageous when the 
tension band is in the form of an elongated, endless loop, in which case 
the inserts are provided at each end of the loop. The advantage of this is 
that any failure in the tension band will occur within the length of the 
band rather than at one of the ends which results in an increase in 
fatigue strength. 
The concept of additional layers may be applied to conventional tension 
bands so that, in accordance with a second aspect of the present 
invention, a tension band comprises a plurality of layers of first resin 
bonded fibre rovings, and a plurality of additional, partial layers of 
second resin bonded fibre rovings interleaved between the layers of first 
fibre rovings. 
The first and second fibre rovings may be the same or different materials. 
The second fibre rovings are preferably unidirectional. 
The fibres preferably comprise glass fibres such as S-glass fibres, 
although other fibres such as carbon fibres or Kevlar fibres could be 
used. 
The advantage of these new tension bands over conventional tension rods 
which have previously been used can be seen by comparing the 
cross-sectional area of a tension band with the cross-sectional area of a 
tension rod providing the same tensile strength. Thus, a tension band 
having a cross-section of about 60 mm.sup.2 is equivalent to a tension rod 
of the type described in U.S. Ser. No. 912,246 having a cross-section of 
about 78 mm.sup.2. This reduction in cross-section leads to a reduction in 
the heat conduction capability of the band which is very important when 
the band is used in a cryogenic application and in the cost of the band. 
In accordance with a third aspect of the present invention, a method of 
manufacturing a tension band comprises laying down on a mandrel a number 
of layers of resin impregnated fibre rovings, the rovings in each layer 
being laid down so that they are laterally offset from the rovings in 
adjacent layers; and curing the fibre rovings. 
Preferably, during the laying down step, one or more fibre sheets are 
periodically laid down on the most recently laid layer. These sheets may 
be made from the same or different fibres from the other layers. 
The third aspect of the invention is particularly suitable for 
manufacturing a number of tension bands in which a composite, uncured 
structure is manufactured by laying down on a mandrel a number of layers 
of resin impregnated fibre rovings, the rovings in each layer being laid 
down laterally offset from the rovings in adjacent layers; curing the 
composite structure; and cutting the cured composite structure into two or 
more tension bands. 
Preferably, the laid down, uncured rovings are compacted prior to curing. 
In accordance with a fourth aspect of the present invention, an apparatus 
for manufacturing an uncured structure which after curing comprises a 
tension band includes a first support to which is rotatably mounted a 
mandrel; a second support for a resin bondable fibre roving store; 
indexing means to which rovings from the store are fed, the indexing means 
and the mandrel being relatively movable, whereby in use layers of rovings 
are laid down on the mandrel, the rovings in each layer being laterally 
offset from the rovings in adjacent layers; and drive means for causing 
relative movement between the indexing means and the mandrel. 
Preferably, the apparatus further comprises a resin bath through which the 
rovings pass prior to being laid down on the mandrel. Alternatively, the 
rovings could be pre-impregnated with resin before being positioned in the 
store or impregnated after laying down on the mandrel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The tension band shown in FIGS. 1 and 2 has an elongate portion 1 with a 
length of about 350 mm and a pair of semi-circular end portions 2 with 
inner radii of about 12.5 mm and outer radii of about 15.3 mm. The band is 
built up from a number of first layers of rovings of glass fibre, four of 
the layers 3-6 being shown in FIG. 2. The rovings shown in FIG. 2 are 
formed by bundles of S-glass fibres impregnated with an epoxy resin. As 
can be seen in FIG. 2, the rovings in each layer 3-6 are laterally offset 
from the rovings in the adjacent layers. In practice, the rovings in one 
layer are offset by about 50% from the rovings in each adjacent layer. 
This regular offset, which is applied throughout the thickness of the 
band, leads to a high tensile strength band being produced. 
It will be seen in FIG. 1 that the ends of the elongate section 1 taper 
outwardly towards the semi-circular portions 2. This is due to the 
interleaving of additional short second layers of unidirectional R glass 
pre-impregnated rovings in sheets at each end during manufacture of the 
band. These additional partial layers may be laid between each of the main 
layers extending throughout the length of the tension band (such as layers 
3-6) or between only some of these layers. 
Preferably, each interleaved, partial layer is shorter than a radially 
inner interleaved, partial layer. This leads to a smooth taper without 
high stress positions and a strong band. 
The purpose of the interleaved layers is to move the region of failure from 
the end portions 2 to the elongate portion 1. 
An example of a typical laminate is given in the following Table with the 
innermost layers listed first. 
TABLE 
______________________________________ 
No of Layers Fibre Type 
Length (mm) 
______________________________________ 
2 S Glass 
1 R Glass 200 
2 S Glass 
1 R Glass 171 
2 S Glass 
1 R Glass 142 
2 S Glass 
1 R Glass 112 
2 S Glass 
1 R Glass 83 
3 S Glass 
______________________________________ 
The S glass layers extend fully around the tension band. 
FIGS. 3 to 5 illustrate an example of an apparatus for manufacturing the 
tension band shown in FIGS. 1 and 2. The apparatus comprises a support 
structure 7 on which is rotatably mounted a spindle 8 carrying a platen or 
mandrel 9. The support structure 7 also supports a platform 10 on which is 
mounted at one end a store support 11. Rotatably mounted in the store 
support 11 is a cylinder 12 around which is wound a length of S glass 
fibre rovings. Below the store support 11 is positioned a heater control 
13 to control a heater (now shown) in a resin bath 14 through which the 
rovings pass after being unwound from the store 12. As the rovings 30 pass 
through the resin bath 14 they are impregnated with epoxy resin. They then 
pass to an indexing unit 16. The roving passes from the indexing unit 16 
to the mandrel 9 onto which it is wound. 
Movement of the indexing unit 16 is controlled by a drive motor 17 via a 
drive chain 31 and drive gear 15. The drive motor 17 also causes rotation 
of the mandrel 9. In addition, a tensioning element (not shown) is 
provided to tension the roving drawn off the store 12. 
The indexing unit 16 comprises a platform 32 on which are mounted a pair of 
half nuts 33, 34. The platform 32 can take up one of two lateral positions 
under the control of a pneumatic actuator 35 having an actuating element 
36 which engages a depending flange 37 mounted underneath the platform 32. 
In the position shown in FIGS. 4 and 5, the half-nut 34 engages the thread 
of a lead screw 38 rotatably mounted between bearings 39, 40 supported in 
side walls (not shown) of the apparatus. In the other position of the 
platform 32, in which the platform is moved to the right, as seen in FIG. 
4, the half-nut 33 engages the thread of a lead screw 41 rotatably mounted 
between bearings 42, 43 also mounted to the side walls of the apparatus. 
Gears 44, 45 are non-rotatably mounted to the lead screws 41, 38, 
respectively, axially outwardly of the bearings 43, 40 while a further 
drive gear 46 is non-rotatably mounted to the lead screw 38. The drive 
chain 31 is entrained around the drive gear 46. 
The drive chain 31 is also entrained around a gear 47 non-rotatably mounted 
to the shaft 8 on which the spindle 9 is mounted. Thus, the drive motor 17 
causes rotation of the lead screws 38, 41 and the spindle 8. 
The half-nuts 33, 34 are relatively off-set in the axial direction of the 
lead screws 38, 41 by one half pitch of the lead screw threads which are 
the same. The reason for this will be described in more detail below. 
The roving 30 passes from the resin bath 14 through an aperture 48 in a 
plate 49 mounted to the platform 32 before extending to the mandrel 9. 
A pair of detectors 50, 51, such as proximity switches are mounted in 
alignment with the flange 37 and on opposite sides of the platform 32 so 
that as the platform 32 approaches each end of its traverse, the 
corresponding proximity switch will be actuated and a control circuit (not 
shown) will cause the pneumatic actuator to move the platform 32 from its 
current position to its other position. 
In operation, the glass fibre roving 30 is withdrawn from the store 12 by 
actuating the drive motor 17 and is drawn along the path shown in FIG. 3 
onto the mandrel 9 as the mandrel is rotated. During this rotation, the 
indexing unit 16 moves parallel with the mandrel 9 due to the engagement 
of one of the half nuts 33, 34 with the corresponding lead screw 41, 38 so 
that a layer of rovings, such as the layer 3 in FIG. 2, is built up. Once 
the indexing unit 16 reaches the end of its path, the corresponding 
proximity switch 50,51 is actuated causing the control cicruit, such as 
flip-flop, to actuate the penumatic actuator 35. This will cause the other 
half-nut 33, 34 to engage its lead screws 38, 41. The lead screws 38, 41 
rotate in opposite directions due to the gears 44, 45 so that the platform 
32 will start to return to its initial position. Since the half-nuts 33, 
34 are offset by half a thread pitch, the first roving of the next layer 
will overlap by equal amounts the last two rovings of the preceding layer. 
Thereafter, the indexing unit 16 moves at the original speed so that the 
next layer is fully laid down. This is repeated as necessary with the 
rovings of each layer overlapping the rovings of the preceding layer. At 
intervals, the drive motor 17 is stopped and fibre sheets of a different 
material are manually laid onto the mandrel 9 at its ends, corresponding 
to the curved portions 2 of the tension band. Typically, these sheets will 
be made from R glass fibres. 
After winding, the mandrel 9 is dismounted from the support 7 and 
positioned between a pair of compacting plates (not shown) which squeeze 
the laminated rovings to a pre-determined thickness to generate a 
composite laminate 52 (FIG. 6). The compacted, composite laminate is then 
cured in a conventional manner and finally split in the elongate direction 
into a number of tension bands 53-56. Typically, a composite laminate is 
formed having a width of 200 mm and this is divided into 10 tension bands, 
each having a width of 20 mm. The endmost bands 57, 58 are usually 
discarded since these may have an over supply of resin. 
FIG. 7 illustrates an alternative form of tension band 59 which is 
constructed at one end 60 in a similar manner to the example shown in FIG. 
1 but at its other end 61 the arms of the band are drawn together and 
cured inside a metal ferrule 62 which is externally screw threaded. The 
rovings are secured in the ferrule 62 using the method described in U.S. 
Ser. No. 912,246, the disclosure of which is incorporated herein by 
reference. This allows the tension band to be connected at the end 61 in a 
manner similar to a conventional tension rod. 
FIG. 8 illustrates an example of the use of a tension band as illustrated 
at 18. FIG. 8 illustrates part of a cryostat having a liquid helium vessel 
19 including a rod 20 around which the band 18 is mounted. The band 18 
extends through an aperture 21 in a radiation shield 22 and through an 
aperture 23 of part of a liquid nitrogen vessel 24. The other end of the 
tension band 18 is mounted about a rod 25 supported in a titanium alloy 
clevis 26 anchored to a bracket 27 mounted within part of a vacuum chamber 
28.