The Chailey Harness with Carbon-Fibre Reinforced Plastic
NIGEL D. RING
When externally powered arms were first fitted to thalidomide-damaged children, a plastic waistcoat was used as a platform on which the arms were mounted. The problem of heat dissipation with this waistcoat was acute and the Oxford jacket was developed. It consisted of a metal framework which approximated the outline of the rib cage. In recent years, this jacket has been widely used in many centres and has made a valuable contribution to the comfort of the children.
In 1966 the development of a new composite, Carbon Fibre Reinforced Plastic (cfrp) at the R.A.E. Farnborough was announced. This material is approximately four times as stiff as metal per unit weight, although its strength is lower than that of metal. Since stiffness for stability is of prime importance in the Oxford jacket, and much of the weight of the jacket is the penalty paid for this stiffness, it seemed that cfrp might be a suitable material with which to replace the metal. The result was the "Chailey Harness." The process by which it is manufactured is described briefly below.
For a device to be a worthwhile replacement for the Oxford jacket, it had to satisfy certain criteria:
- The fabrication technique had to be simple, and preferably not require special apparatus.
- A significant weight saving had to be achieved, with controlled stiffness of the framework.
- The time needed to manufacture the item had to be no longer than that required for the metal framework (about one man-week).
- The framework had to be comfortable for the child, and simple to repair if damaged.
- Suitable means of suspension for the arms, attachment of the valves, and so on, had to be readily available.
- The device had to be cosmetically acceptable.
Method of Fabrication
In order to achieve a suitable framework the provision of a mould for the resin-impregnated fibre during fabrication was necessary. Plaster-of-Paris casts were already used widely in the prosthetics field, and it was decided that a plastic foam channel attached to a cast of this type would provide a suitable mould, and would also provide padding for the finished article. Accordingly, Plastazote channels were mounted on the plaster-of-Paris model in the configuration required for the final harness ( Fig. 1 , Fig. 2 , and Fig. 3 ). The channels were attached to the plaster with small nails and supported by fillets of plaster of Paris.
The carbon-fibre tows are impregnated with polyester resin using one of the standard methods, and are laid in the channels in such a way that a uniform number of tows appears at any section of the harness. Care is taken to ensure that a uniform distribution of fibre exists at all junctions between struts. During this laying-up procedure suitable inserts are incorporated as necessary for attachment of valve mounts, shoulder pivots, and clasps.
Once the lay-up has been completed a standard vacuum-bag technique is used to compress the fibre, and to remove any entrapped air from the composite. The complete fabrication takes less than one man-day, which represents a considerable saving in time.
When the resin had been completely cured, various tests were performed. Loads were applied to a series of half-frames (i.e., one side only of the harness) between the "sternum" and the "spine," and the load/deflection curves were compared. From Figure 4 it can be seen that the stiffness of the cfrp harness was between 1 1/2 and 2 1/2 times as great as that of the metal equivalent for the samples tested. In these samples the weights of the cfrp half-frames were 30 to 40 per cent of the metal half-frames, which gave a weight saving in the final harness of about 350 grams. However, if the stiffness of the cfrp harness was designed to approximate that of the metal frame, an even greater saving in weight would be realised.
Due to the properties of cfrp, there is no plastic region in the stress/strain curve. Accordingly, comparison in terms of strength can be made with the metal jacket only by comparing the ultimate strength of the cfrp (12-15 Kg) with the yield point of the metal (about 9 Kg). Thus it can be seen that permanent damage to the harness is less likely to occur with the cfrp than with the metal.
The cfrp harness has one further advantage related to its stiffness and strength. Once deformed, the metal framework is virtually useless?it is extremely difficult to achieve a good fit with a repaired frame. However, since the cfrp takes on no permanent deformation a break may be repaired, and the original fit is maintained. In addition, the strength and stiffness of the repaired article are quite acceptable ( Fig. 4 ).
In order to check the resin content of the cfrp, the polyester was burned out of the composite at different sections of the framework. It was found that the proportion by volume of fibre within the composite lay between 44.5 per cent and 51.5 per cent in the six samples considered. The recommended content is 40-60 per cent.
The possibilities for attachment of valve mounts and polypropylene hinges have not yet been fully explored. Attempts have been made to use adhesives for the purpose, but these have proved to be unsatisfactory in use, and at present self-tapping screws are used. However, no other trouble has been experienced during the preliminary user-trials which, so far, have lasted fourteen months. A boy wearing the completed harness with prosthetic arms attached is shown in Fig. 5 .
I should like to acknowledge with thanks the co-operation of Mr. L. N. Phillips of the R.A.E. Farnborough, Mr. E. J. Cook of Courtaulds Ltd., and Mr. J. A. Raymond of Scott-Bader, as well as the many others who have offered advice and support to this particular project.
* Reprinted from the Annual Report of The Experimental Workshop, Chailey Heritage (Craft School and Hospital), Chailey, Lewes, Sussex, England, No. 1, July 1970.
Research Engineer, Chailey Heritage (Craft School and Hospital) Chailey, Lewes, Sussex, England