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  Reader's letter | December 2007 | Clocks Magazine

Fly function

I would like to add a comment to Ian Beilby’s article on ‘Beginner’s Guide to Clock Repair’ Part 11 in the August issue, in which he states that ‘the fly … is simply an air brake, fitted to govern the speed of the striking’.

Clock restorers should be aware that in addition to speed control, the fly has one other very important function to perform and that is to disperse the kinetic energy of rotation of the entire strike train.

It will be noticed that all striking clocks, from 30-hour longcase to miniature carriage, have a spring clutch arrangement for holding the fly to its arbor. The reason for this is that when the strike train is brought to an abrupt halt at the end of the strike sequence, all that rotational kinetic energy of the whole train (supplied initially, of course, by the falling weight or the unwinding spring) is taken on board by the fly, the blades of which should slip a little around their arbor and thus convert it into heat by friction. This heat (miniscule though it is) is then passed on to the local atmosphere.

Restorers should always check that at least some slippage takes place at the end of striking, or the energy will be otherwise absorbed by the adjacent components as transient strain energy which will then be given back out in the form of recoil. However, it would take an awful lot of striking clocks to replace the central heating in winter!

Barrie T Fitton, UK

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