8
INTERVIEW
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a leaf spring on which the contacts were mounted; if the motor stalled, heat would cause the thermostat to trip before the windings burnt out. With thinner windings, however, there was insufficient time for the heat soak to trip the thermostatic switch before the windings failed. John Taylor therefore redesigned the switch such that the current passed directly through the bimetallic component. Because heating is proportional to the square of the current (i2R), a motor experiencing even a small rise in current would benefit from fast-acting protection. While at Otter Controls and, later, at Strix on the
Isle of Man, John Taylor mainly designed thermostatic controls. With the trend towards miniaturisation, 12.5 mm diameter became a common size for bimetallic discs, giving a typical snap-action movement of 0.3 mm. However, because the bimetallic disc also has to be assembled with other components – usually an insulated pusher, leaf spring and contacts – the tolerance stacks mean that often the only way to achieve a complete assembly that will function correctly and give adequate life is to selectively assemble components. Clearly this is expensive and not conducive to automated manufacturing. With labour being in limited supply on the Isle of Man, Strix was an early adopter of automated assembly machinery, taking advantage of local engineers’ knowledge of cams and Bowden cables that resulted from the island’s motorcycling heritage.
Mechanical amplifier
Dr Taylor explains how he overcame the problem associated with tight tolerances: “What I came up with was a compact bimetallic disc with an integral built-in amplifier that gives ten times more movement. This extra movement comes at the expense of the force the disc can exert: whereas a plain disc exerts about one kilogram of force, the amplifier disc gives 400 grammes – but this is still more than enough to operate the switch” (Fig. 2). So successful was this invention that more than 5 billion bimetallic amplifying discs have been manufactured. Strix controls are incorporated in electric kettles from many of the world’s leading manufacturers. Design is not always a simple process, according to
Dr Taylor: “Ideas are easy, but making the practical embodiment is not always as straightforward.” This point is illustrated well by an example Dr Taylor
gives from the 1980s: “Plastic kettles became popular but they brought with them a problem that did not exist with copper or stainless steel kettles: plastic kettles melt if they overheat. At this time there was an increasing interest in safety standards and BSI (the British Standards Institution) decided that kettles must fail safely even in the event of the switch and thermal cutout failing to operate correctly. Initially manufacturers tried making the element out of thinner wire so that this would burn out in a safe manner, but these ‘rupturable elements’ were failing in the field; some types suffered 30 per cent failures within the guarantee period.
Fig. 2. Two bimetallic amplifying discs as used in millions of electric kettle controls.
“I therefore set out to design a three-level kettle
control: a switch to turn off the kettle when it boiled, a switch to turn off the kettle if it was switched on empty, and a switch to provide protection in the event of neither of the other two operating. Naturally, having always worked with bimetallic thermostats, this was the route I took. I designed a suitable three-level control, produced a prototype and it worked perfectly. Strix received orders for the control but, when it was installed in kettle, we were horrified to find that it did not work. It turned out that the time required for the heat soak to affect the third switch was also long enough for the surrounding plastic components to soften, relieving the spring pressure and preventing the ultimate protection from operating. A complete redesign of this third switch was required. “In this case the problem had been that the plastic
was softening, so I decided to turn this to my advantage. Instead of using a bimetal component, the new design featured a small plastic component that was in contact with the hottest part of the element. In fact this single- use thermal fuse was much simpler and cheaper to manufacture than the bimetallic thermostat, which was also a benefit. After all, you do not want to have a relatively expensive component if in 999 999 kettles out of a million it will never be needed. On the other hand, for the one in a million kettles in which the thermal fuse does operate, you must be absolutely certain that it will – even if all the other components in the kettle are starting to age and wear.” Of everything that Dr Taylor has invented, he is most proud of this thermal fuse. To keep his kettle-manufacturing customers loyal,
Dr Taylor has invented numerous features, including the lime scale filter, the 360-degree cordless connector and a control that switches off the kettle automatically either when it boils or at a lower temperature – typically for making coffee. In total, Dr Taylor has been granted over 150 patents. Dr Taylor is also a respected horologist. His interest
in early clocks stems from the 1970s when he was helping a Japanese manufacturer to improve the quality of its electric motors for automotive applications. He
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