Sandwiches for Bodybuilding
Page 49
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THE title of this resstme of a paper read by Mr. F. C. Lynam, F.R.Ac.S., technical director, the Airscrew Company and Jicwood, Ltd., before the Institute of Road Transport Engineers on March 15, does not refer to the proverbial railway sandwich, but to light-weight materials which are, or could be, employed in the construction of commercial vehicles.
Characteristics of their construction, said the author, were their light weight, great stiffness, relatiyc strength, and that they could be formed into reasonably large components quickly and in quantity. A sandwich panel, either flat or curved, comprised mainly two materials —the skins, with high strength and relatively high density, and the thick. light but comparatively weak core. An essential was a bonding medium, probably a synthetic resin adhesive.
Stiffness in Large Panels
Metal-faced plywood should be regarded as a sandwich material, and its use was well known. The plywood core enabled the metal sheets to give their full strength in a large, stiff, dimensionally stable panel. Such material had been used for bodywork, particularly panels, bulkheads, etc.. because designers realized that they could obtain stiffness and strength with less framing than when plain sheet-metal panels were used. In such cases, the exposed edges should be sealed and, normally, the metal flanged over and soldered together; but this material is comparatively heavy, owing to the high density of the core.
Lighter sandwiches were developed by licwood, Ltd., following its wartime work on aircraft materials, and an interesting sandwich type bus roof was being supplied to London Transport. This showed a weight saving of 15-20 per cent, over orthodox methods. The one-piece roof had smooth surfaces, and there were fewer parts—in the case of this particular roof, 52 less than with the orthodox fabricated type.
The roof comprised an outer section of 22 s.w.g. aluminium and an inner section of 1/16-1n. plywood, whilst Onazote was the core material. In moulding, light pressure only was provided by inflated rubber bags, and bonding by a cold-setting synthetic adhesive, but for large production heating could be introduced. Cleaning up CoultLbe carried out in a few hours.
Precision Limits With this material and process, a finished roof could be produced within close limits, such as, length, plus 0 minus 1 /16 in.; width, plus or minus 1/32 in.; contour, plus or minus 1/64 in.
The curved sandwich gave a finished weight equal to 1.34 lb. per sq. ft. of projected area, and a roof 17 ft. long and 7 fi wide would be expected to weigh 170-200 lb.: this depended upon additional functional members, such as lighting rails and inserts for fittings, Additionally, an excellent thermal insulation performance was given, and condensation was practically eliminated, because the core permitted little difference between skin and air temperature. Vibration from the power unit was damped down, whilst drumming and panting were largely avoided, being absorbed by the core.
The sandwich roof was more expensive in first cost, but there was a saving on erection, there being only one part to handle.
Food-carrying vehicles up to 5-ton capacity had been built with 18 sandwich panels 1 7/16 in. thick, the body and underframe, complete, weighing 13 cwt. The outer and inner skins were 22 and 24 s.w,g. Alclad respectively, and the core was in. Onazote, the weight per sq ft. being 3.24 lb., compared with 5-6 lb. per sq. ft. when using an in-filling of slab cork. The author stated the advantages of sandwich construction to be as follows: (I) minimum structure weight and less expensive material; (2) simplicity of construction, eliminating internal stiffeners and requiring the minimum of components; (3) lower cost in quantity production and more rapid turnover; (4) ease of handling in large panel sizes; (5) skilled labour could be practically eliminated; (6) case of producing moulded units; (7) better insulation; (8) smooth and rigid contours.
Cores could be of expanded rubber or ebonite, foamed plastics, cotton or glass fabric, and paper, aluminium or steel foil formed as a honeycomb. Paper was usually Kraft impregnated with a phenolic resin. A paper' honeycomb sandwich with plywood skins had a strength-to-weight ratio two and a half times that of fin. plywood.