A special clock was designed and built by members of the BHI to celebrate 150 years
A three pendulum clock with almost free escapement was designed by Jim Arnfield and built by a small team and is on display at the BHI at Upton Hall, Newark.
The escapement drives three pendulums. It rotates to drive each pendulum in turn and so is three dimensional.
The pendulums are on knife edges. Their supports are arranged in a triangle in a platform which has a central recess in which the escapement rotates. Each pendulum has an impulse lever at its upper end pointing inwards at right angles to its knife edges. Each impulse lever is visited in turn by the impulse arm of the escapement.
The escapement is mounted in a vertical central shaft which bears two arms which rotate with the shaft. The upper is the locking arm. It is caught at its tip by a stop (pointing downwards) underneath the pendulum support platform, positioned so that it (and the impulse lever below in line with it) come to a stop opposite a pendulum impulse lever.
Just below the locking arm is the impulse arm. There is a small roller at one end which rolls against the pendulum lever. As the impulse and locking arms rotate into impulse position, it is reset just in time. It thus comes to lie just above the tip of the pendulum impulse arm. Its other end has a small lever which catches on a reset arm mounted on the pendulum support platform between the escapement stops. This catch is the heart and soul of the escapement. As the impulse arm catches it as it is flying around between impulses, the reset arm is depressed, but at the same time the impulse arm at this end is also depressed which means that its impulse or business end is raised - to a higher level ready for impulse and locked there. I asked Jim Arnfield how on earth he had conceived of this arrangement and his answer was that he seems to have a gift for imagining mechanisms.
As the pendulum is just completing its swing before impulse, its impulse arm tip catches under the impulse roller and lifts the impulse arm slightly. As a result its other end is lowered - and just enough to clear the reset arm. The reset arm of course springs back (to its stop) to its previous position ready for the next operation.
The impulse arm is now ready to give impulse, and does so and its roller then falls off the end of the pendulum impulse arm. It continues to fall further and its other end then raises the end of the locking arm to cause unlocking. The cycle is then ready to repeat.
Meccano Upton BHI 150 Anniversary Clock showing three pendulums
Detail of the escapement
The clock is weight driven and the weight is a heavy circular ring outside the pendulums, suspended from three cords each attached to one circular drum supported on rollers. This drum is linked to the escape shaft via a differential through which the electrical rewind also operates.
The time is read on two rings above the escapement.
Michael Adler BHI