Do you realize that you can make a clock with 3 gear wheels, and a wheel on the top like a circular saw? Now this wheel, and the two beneath it, are mounted on steel shafts with pinions cut in them; little gear wheels, really, so that they can mesh with each other. The big wheel at the bottom, or great wheel as it's called, carries the winding mechanism. At the very top, there's a two legged piece that drops in and out of the saw toothed wheel. This, together with the wheel, is known as the Escapement. And there's your clock.



Yes, I know, there's more to it than that; there's a method of gearing the hour wheel down to run one twelfth the speed of the minute hand, but very basically the clock has 3 wheels, three pinions and old saw tooth!

The gear ratio on the average antique grandfather clock, from the Great Wheel to saw tooth is roughly 1:750.

Remember your applied maths? The clock has tremendous velocity ratio, but very little mechanical advantage.

Now of course we've been talking only about the time gear train. Clocks that strike have another train, and those that chime the quarter hours have yet another train. The velocity ratio/mechanical advantage rule is very much the same with the strike and chime trains, as with the time train.

This was shown to devastating effect the day Big Ben exploded! The clock tower of Big Ben, so named for the massive strike bell, was designed by a gentleman named Augustus Pugin. This was his last design, following which the poor man went mad a short time before his death.

On the 5th. of August, 1976, the poor old clock, which was built in 1859, decided that enough was enough! The arbor of the speed regulator on the chime train, variously called a governor or fly, suddenly broke due to metal fatigue. The mighty four ton weights, suddenly free of restraint, thundered downward, spinning the unregulated gear train with them as they went at tremendous speed. If anyone had been standing in the tower at the time, they would have been cut and sliced into little pieces. There was so much shrapnel that it was thought at first to be an I.R.A. bomb!

Her Majesty the Queen insisted that the clock be repaired before the opening of Parliament, and on the 9th. May, 1977, Big Ben was once again calling out the time to London and the world.

The pendulum of the great clock measures 12.8 feet in length and weighs about 660 lbs. There's sometimes a misconception about the old pennies that are placed on the pendulum tray. It's thought that it's the weight of the pennies that regulate the timing of the clock. In fact, they very slightly change the effective length of the pendulum. One penny has the effect of changing the time by 0.4 seconds per day.

Just as a matter of interest, I made a chiming clock myself when I was a practising clockmaker. It chimed two tunes; the Westminster and the Winchester, the latter more properly named the St. Michael's. The interesting part was that I bought a set of tuned bells, and the tenor strike bell, from a firm in London who'd been in the business of making bells since 1571. The first Elizabeth was on the throne then. Tempus fugit!!

Copyright (c) 2009 Mike Bond