Flat disks make ultimate spring

Titanium disk springs offer exceptional performance when suitably mounted and heat treated. Tom Shelley reports

A pair of titanium disks, pressed together by shallow domes, could be the ultimate spring for heavy duty clutches, brakes and mechanical products for aerospace and other applications where power to weight ratios are critical. Developed for use in pistons with sprung tops, the disk circumferences sit in special rocking rings to ensure stability at high reciprocating speed and to avoid scuffing. The idea is the latest to come from Bill Galvin, whose sprung piston featured on the cover of the June 1999 edition of Eureka. In his original concept, there were titanium Belville washers between the crown and body of the piston. While this worked well enough to demonstrate the energy saving abilities of the device, he found that because there has to be room for them to extend laterally when compressed, they had room to move around and scuff their edges. He has now come up with a design in which the Belville washers are replaced by flat titanium disks, the edges of which are held in steel rings with rounded undersides. This allows the edges to rock in grooves in spacer rings, when the centres of the disks are pressed towards each other by domes on the insides of the piston crown and piston body. Fatigue life has also been greatly enhanced, thanks to advice from David Peacock, former head of research at Timet UK. He suggested a revised heat treatment regime aimed at maximising fatigue life, at the expense of a slightly lower yield stress. Details are confidential, although we can reveal that the revised regime require no very special measures, as long as it is followed exactly. The fatigue life can now accurately predicted using software developed by Professor John Draper of Safe Technology. The alloy used is Timet's 10-2-3 high strength forging alloy. It contains 10% vanadium, 2% iron and 3% aluminium and is usually employed in airframes and aero engines. The disks are 1.5mm thick, and take 6,300N to deflect them through 2.7mm. The pistons are designed to produce maximum engine efficiency at about 2,000 rpm. The jury is still out as to whether the sprung pistons are a viable commercial proposition for cars and motorbikes. Galvin is the process of negotiating grants to undertook further development and experiments. But for applications such as brakes, clutches and mechanical mechanisms in aerospace, the revised spring designs look to be able to offer exceptional performance per unit weight. All aspects of the development are covered by patents and patent applications. The development received a DTI SMART Award in 2001. Bill Galvin Pointers * Springs are made in the form of a pair of titanium alloy disks, whose edges are held in steels rings, able to rock in grooves in a spacer ring * Prototype spring consists of two disks 1.5mm thick, able to be deflected 2.7mm by 6300N force.