Double three-legged gravity escapement - animation(The following text is reproduced with permission from Chris McKay's excellent book "Big Ben", to be published in 2010)
"The escapement is the key part of a clock that connects the train of gears to the pendulum. Working together, the pendulum and escapement define the accuracy of a clock’s timekeeping and it is Denison’s double three-legged gravity escapement which was finally fitted to the Great Clock that produced an exceptionally accurate clock. In simple terms, Denison’s escapement along with a compensation pendulum, produced a clock that kept time to within a few seconds a week. In horological terms Denison’s escapement was one of the most significant inventions of the 19th century. In a subtle way it supported the growing need for accurate public timekeeping as society relied more on railways for communication. From the 1860s onwards many thousands of turret clocks were made and installed in factories, churches, town halls, hospitals, prisons, workhouses etc. Many of these clocks had a gravity escapement; the economic boom fuelled the turret clockmaking industry and the clocks installed brought order and timekeeping and thus fuelled social and economic progress".
"What Denison’s gravity escapement did was to always provide a constant impulse to the pendulum irrespective of weather conditions, so the pendulum arc remained the same hence timekeeping was constant. This was achieved by raising a small weighted arm that was then ‘lowered’ onto the pendulum to give it a push or impulse. Since the arm was a constant weight and always descended the same distance under the influence of gravity, the impulse given to the pendulum was constant. In fact two arms were used, one on each side of the pendulum".
Key to the animation: The two "gravity arms" are coloured blue and red. The two three-legged escape wheels are also coloured blue and red. They work in two parallel planes so that the blue wheel only impacts the locking block on the blue arm and the red wheel only impacts the red arm. In a real escapement these impacts give rise to loud audible "ticks" and these are indicated by the appearance of a * beside the locking blocks.
The three black lifting pins are key to the operation of the escapement. They cause the weighted gravity arms to be raised by an amount indicated by the pair of parallel lines on each side of the escapement. This gain in potential energy is the energy given to the pendulum on each cycle. For the Trinity Clock a mass of around 50 grams is lifted through 3mm each 1.5 seconds - which works out to 1mW of power. The driving power from the falling weight is about 12mW, so there is a substantial excess of power used to drive the escapement. Much of this energy is dissipated in the acceleration and deceleration of the frictional "fly" attached to the escape wheels. As W.G. Schoof said of the double three-legged gravity escapement: "its working may be likened to the doings of a captain taking thirty tons of coal on board, for a trip that only requires one ton, and engaging a man called “fly” to throw the other twenty-nine tons overboard".