THINGS are looking up for the designer of a screw that could save aerospace and engineering companies a fortune in maintenance costs. Tests of the design show that it outperforms the three most common types of screw, and British aerospace companies have agreed to examine the new findings later this month.
John Galbraith, an independent inventor and freelance aircraft engineer based in London, has been trying for years to interest companies in his patented 鈥淕albraith recess鈥� (Technology, 10 August 1991). The screwhead has a triangular recess that can be turned using a screwdriver with a prismshaped head. Galbraith says that his screws can be fastened between 30 and 50 per cent tighter than other designs, because the turning force or torque is spread equally between the three surfaces of the recess.
The screwheads are also less likely to be damaged when the screwdriver tilts and slips out. Galbraith says that the untapered sides of the Galbraith recess makes it virtually impossible for a screwdriver to slip out through tilting.
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Aerospace companies demanded independent validation of the screw鈥檚 performance before they would consider it. Galbraith believes he now has enough theoretical and practical evidence to convince doubters. 鈥淎t last, it鈥檚 scientific confirmation of the advantages,鈥� he says.
Galbraith commissioned comparisons of his design with three different types of screwhead: a single slot, a crosshead and a hexagon. Jingzhe Pan of the Department of Engineering at the University of Cambridge made a theoretical comparison using the computerised modelling technique called finite element analysis.
Pan found the Galbraith design was better than the others at withstanding twisting forces without deforming. The Galbraith recess was four times as durable as the crosshead design, and 1.2 times as durable as the other two. 鈥淭his means I can apply more load without damage,鈥� says Galbraith. 鈥淭he implication is that you can save money by not having to drill out failed screws.鈥�
Physical tests on prototype screws reflected the theoretical predictions, though they were not quite as impressive. Mike Arnott, a physicist who has been helping Galbraith in his spare time, found that the triangular recess withstood torques 63 per cent higher than the crosshead and around 20 per cent higher than the other two head designs.
Arnott says that the triangular design would have performed better if the prototype screws had been of better quality. 鈥淭he ones we received for torsion testing were dreadful,鈥� he says. 鈥淭he estimates I gave were probably extremely conservative, and the performance could be better with proper samples of the screw.鈥�
But it will still be difficult to persuade large engineering companies to change. For example, aircraft fly all over the world and operators must be sure that wherever they land, local engineers will have screwdrivers to fit the screwheads (see Diagram).
