The Riefler Clock

Fig. 1  The Riefler clock

Fig. 2  The Riefler escapement showing also the pendulum suspension and knife-edge bearings.

The scientific community has always had the need for accurate clocks.  In the eighteenth and nineteenth centuries, the majority of observatory regulators were constructed using the Graham dead-beat escapement and mercury compensated seconds pendulum and of course were weight driven.  The need for ever greater accuracy of timekeeping led to the development of greatly improved escapements and at the end of the pendulum clock era when quartz and later atomic clocks were about to take over, there were to all intents and purposes only three contenders left.  These were the Synchronome company which produced the Shortt Free Pendulum Clock, Le Roy et Cie manufacturers of a precision clock which had a spring pallet escapement and Riefler of Munich.  All succeeded in bringing the humble pendulum to an unprecedented state of refinement and performance with a mean daily variation of about 0.1 seconds per day.  So accurate were some of these clocks that they were able to measure changes in local gravity caused by the tides, measure the earths precession, and were comparable with precision quartz clocks. 

In the Riefler Clock, the energy required to keep the pendulum swinging is supplied by bending the suspension spring.   The pendulum is not suspended from a fixed support, but instead the upper chops of the suspension spring rest on a bearer, which has two co-linear knife-edges on its underside.   The bending point of the suspension spring is in alignment with the line of contact of the knife-edges and the planes or supports on which they rest.   The spring will therefore be bent more than could be caused by the swing of the pendulum alone, and thus will provide the impulse for the next swing.  The suspension spring is used to discharge the twin functions of suspension and giving impulse.  It has a very good performance because the impulse and arc are nearly constant.  

In the Riefler escapement, the pendulum is free of entanglement with the escape wheel for roughly two-thirds of each swing, and the only work it has to do is to unlock the escape wheel once per second.  Even this operation is performed near the ideal place, which is when the pendulum is near the mid-swing.  

The Riefler escape wheel and pallets are of a special design.  There are actually two escape wheels mounted on the same shaft and two surfaces on each of the two pallet pins.   The front wheel has teeth rather like a dead-beat escapement, and operate on the flat surface of the pallet to lock the wheel.  The rear escape wheel has teeth cut in ratchet form with the sloping surface facing the direction of rotation.  The round part of each pallet is acted upon by this surface to give impulse.

This clock has been built in Meccano and has a fully functioning Riefler escapement gravity driven with Hope Jones Synchronome reset and maintaining power.  The pendulum bearer is mounted on knife edges with precision escapement.  The ModelPlan for this clock is clock is available from MW Models, Henley-on-Thames