In November 1997, I though about building a clock with a pendulum
activated by the fall of bearing balls. The first thing to design was
the engine which was to sustain the pendulum's oscillation: how
to get enough energy from a ball to make this possible? The balls had
to be heavy enough to run a stable course but their size obviously
determines the clock's size. As a trade-off between weight and
size I arbitrarily picked a 19.8 millimetre diameter for the balls. I
could picture the running clock in my mind, and I believed I could
complete it in three months...
The engine's first prototypes were just unbelievable, at that time I thought that I needed as much gain as possible to achieve the largest possible amplitude: after a month of effort I had achieved a 60 centimetre swing with a 1 meter pendulum. That's when I stumbled across an old physics book from 1894 which explained that the isochronism of oscillations is only true for small amplitudes, not large ones. A month of work for nothing. I was now left with the reverse problem to solve: achieving the smallest possible amplitude with 20-millimetre balls. It took me another month to design the final version of the engine.
It runs on a simple principle: as it falls every ball pushes on the
pendulum in the direction of oscillation. To make sure that the kinetic
energy of the balls in the feeding ramp has no influence on the
engine's accuracy five right-angle speed-limiters slow them down before
being delivered to the engine.
The time reference is given by the pendulum's length: about one meter for a ball that falls every second. The pendulum's rods are made of invar a temperature-stable alloy. The bob is tooled in a 17 kilo brass ingot to provide enough inertia. Coarse tuning is done by screwing the bob up or down, fine tuning is done by adjusting the small auxiliary bob at the bottom.
To start the clock the pendulum must be swung with an amplitude large enough to admit one ball through the hole. This is done through a lever to achieve a first beat exactly in the oscillation plane.
As soon as the pendulum is released the movement is sustained by the seconds balls. The amplitude increases slowly up to a stable two centimetres after 10 minutes
Building the chassis, the bob, the display was a matter of five months, I started in January 1998. The chassis size is mainly determined by the balls' diameter. Sixty 19.8 millimetre balls come a little short of 1.2 meter (4 ft). Add 30 centimetres for the escape mechanism, 20 for the balls buffer, 10 for the moon phases and 50 for all the background mechanism. It all adds up to 2.2 metres high (7' 2'')....
My initial idea was to use electricity, electronics and optical sensors for all functions other than the pendulum's engine. But as months went by the idea of mixing mechanics and electronics was less and less appealing and at the end of may I finally decided to build an entirely mechanical clock.
That's when many problems popped up. Consider this one for example: how is it possible to have the sixtieth second ball increase the minutes count and at the same time reset the seconds count to zero? This is the trick I devised. (Fasten your seat belts).
1600 parts. 300 kilos (660 pounds). 95 ball bearings. Completed in 18 months.
Materials: brass, steel, stainless steel, silver, invar. Pyrex glass with sand engraving for the hours, minutes and seconds display. The cabinet is made of solid oak.