The Grimthorpe Clock

Designed and built by Michael Adler

This clock is named after Lord Grimthorpe, who designed the escapement of Big Ben in London which was installed in 1859 at the Palace of Westminster.  The clock is unique, in that its designer had to face the problem of making the clock keep better time than any previous public clock, and had to drive the hands of four dials of unprecedented size, exposed to the full force of the weather on a high tower.  This meant that large surplus of energy must be provided in the train, not only enough to drive the pendulum and the dial work in normal times, but also with enough reserve to overcome the action of gusts of wind  on the hands and the stiffness of congealed oil in frosty weather.  The Astonomer Royal had stipulated that the clock should always strike the first blow of the hour within a second of the real time.

Grimthorpe's solution was the double three legged gravity escapement.  The pendulum is given a push on each of its vibrations, but at the extremity of its swing where it is most sensitive to disturbance   This might seem like anathema to a horologist who demands a free pendulum, or at any rate one that is free except at or near the middle of its swing.  But allowance must be given to the remarkable constancy of the push that each arm gives to the pendulum, because that push is governed only by gravity.  The clock has proved extremely successful, and is still the standard escapement for Turret Clocks.

This escapement was first built in Meccano by John Howe and he brought it to a Meccano meeting in 1983 and is a masterful piece of Meccano construction which faithfully reproduces the prototype design.  Alan Partidge was asked to photograph it, and he said that he would do more than that - he would turn it into a complete clock first and this was published in the Sheffield Meccano Guild magazine in March 1987.  

This clock has a number of interesting features and it differs in a number of ways from the original Partridge design which had a seconds pendulum of 29-30 inches.

1. The escapement is at the front of everything else, as is also the pendulum.  Thus the action is easily seen.  The original escapement is at it was originally designed by John Howe.

2.  The pendulum  is now a half seconds pendulum, which allows it to be turned into a Mantel Clock.

3.  The clock is powered by a Tensator spring, first brought to the knowledge of Meccanomen by Pat Briggs in the Meccano Newsmag No. 72 July 1995.  The idea was taken up by the late Keith Cameron, who built a Tensator spring-powered motor in 1996, and this was puiblished in CQ in June 1996. In the following issue, the Tensator spring was used to power a clock also designed by Keith Cameron.  More recently, a Tensator spring has been used to power an Arnfield clock, modified and built by Jack Partridge.

A Tensator spring is a unique piece of metal which has the remarkable quality of of wanting to restore itself to its original shape when wound against its natural curve onto a storage drum.  In so doing, there is no friction between the coils, and there is no change in torque throught the rewind process. These are exactly the qualities required from a clock spring, and obviates the necessity of a fusee to deliver even power from a normal spring.

The new Grimthorpe Clock is enclosed in a Meccano frame, and has glass enclosed panels.  It keeps very good time against an electronic timing device.

Michael Adler - December 2001 

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