Beat Conversion - Tales From The Bench

Tri-lobed jewel mounted to escape wheel shaft

FERDINAND GEITNER - A very interesting movement came across my desk the other day. A nice 8 day desk clock made by Omega, Caliber Number 120. As expected it was a fully jeweled quality movement with one very special feature. With a standard Swiss lever escapement, the balance oscillates at 18000 beats per hour or five beats per second. The escape wheel has fifteen teeth so it turns one full revolution in three seconds or five teeth per second. Watch factories often opt for faster beats of the balance as it gives the second hand a sweeping rather than ticking motion and tends to be more accurate.

Close-up of fork engaged with the tri-lobed jewel

In this case a jewel which is polished into a triangular shape was added onto the shaft of the escape wheel. The jewel engages with the fork of a larger "pallet" with specially shaped pallet stones. These are designed to push on a gear with sixty specially shaped teeth advancing the wheel one tooth at a time every time the triangular jewel on the escape wheel makes a third of a revolution. This gear acts like a center seconds wheel going through the center wheel and has its own bridge which it shares with the special “pallet” lever.

The special pallets, fork and wheel

Each time the escape wheel turns one third of a revolution its triangular jewel moves the pushing pallet in one and then the other direction. Advancing the center seconds wheel one step every five escape wheel teeth and showing seconds impulses on the dial turning a standard fast beating lever escapement into a motion like a chronometer with seconds impulses.

The Omega movement