Dorrin K Mace, Horologost

Dorrin K Mace, Horologost
The Clock Man in a pensive moment

Tuesday, June 11, 2013

The Lever Escapement

     The Level Escapement was developed after the pendulum regulated clock.  The lever escapement allows the time piece to operate with great accuracy in any orientation.  Unlike the pendulum that must be perpendicular and cannot tolerate excessive vibration or movement.  Thus the lever escapement allowed the development of clocks that could be carried.  i.e. Carriage clocks, pocket watches, and later wrist watched
     The lever escapement is a type of escapement that is detached, so that the time-keeping element of the watch or clock runs entirely free of interference from the escapement during most of the operating cycle. It is used in the movement of most mechanical wristwatches, pocket watches and many small mechanical non-pendulum clocks. The invention of the lever escapement is attributed to Thomas Mudge, and its modern form was developed by subsequent workers including Breguet and Massey.
     The rotation of the escape wheel is controlled by the pallets. The escape wheel has specially shaped teeth of either ratchet or club form, which interact with the two jewels called the entrance and exit pallets. The escape wheel, except in unusual cases, has 15 teeth and is made of steel. These pallets are attached solidly to the lever, which has at its end a fork to receive the ruby impulse pin of the balance roller which is fixed to the balance wheel shaft. The balance wheel is returned towards to its static center position by an attached balance spring. In modern design it is common for the pallet mountings and the fork to be made as a single component. The lever is mounted on a shaft and is free to rotate between two fixed banking pins.
At rest one of the escape wheel teeth will be locked against a pallet. As shown in the diagram, the escape wheel rotates clockwise and the entrance tooth is locked in place against the entrance pallet, the lever held in place by the left banking pin. The impulse pin is located within the lever fork and the balance wheel is near its center position. To get started, the lever fork must receive a small impulse from the anti-clockwise rotation of the balance wheel via the impulse pin (say by being shaken) which rotates the lever slightly clockwise off the left banking pin. This unlocks the entrance pallet allowing the wheel to rotate clockwise.
     As the powered escape wheel rotates clockwise, the entrance tooth slides across the sloping impulse plane of the entrance pallet. This turns the pallets about their axis, which places the exit pallet into the path of the rotating escape wheel. Once the entrance tooth leaves the impulse plane of the entrance pallet, the wheel is able to turn a small amount (called the drop) until the exit tooth of the escape wheel lands on the locking face of the exit pallet. The wheel is said to be locked on the exit pallet. From the release from the entrance pallet to this point, the escape wheel will have turned through exactly one half of the 24 degree angle between two teeth.  The impulse received by the entrance pallet as the tooth moves over the impulse face is transferred by the lever to the balance wheel via the ruby impulse pin on the roller of the balance wheel. The lever moves until it rests against the right banking pin; it is held in this position by the force of the exit tooth against the exit pallet jewel (called the draw). This means that in order to unlock the wheel it must be turned backwards by a small amount, which is done by the return momentum of the balance wheel via the impulse pin.  After the exit tooth locks, the balance wheel rotates anti-clockwise, free of interference from the escapement until the balance spring pulls it back clockwise, and the impulse pin re-enters the fork. This will unlock the escapement, releasing the escape wheel so that the exit tooth can slide over the impulse plane of the exit pallet, which transfers a clockwise impulse to the balance wheel's impulse pin via the lever fork, while pushing the lever up against the left banking pin. The escape wheel drops again until the entrance tooth locks on the entrance pallet now being held in place by the left banking pin via the lever. The balance wheel continues clockwise, again free from interference until it is pulled back by the balance spring to the center position. The cycle then starts again.
     Each back and forth movement of the balance wheel from and back to its center position corresponds to a drop of one tooth (called a beat). A typical watch lever escapement beats at 18,000 or more beats per hour. Each beat gives the balance wheel an impulse, so there are two impulses per cycle. Despite being locked at rest most of the time, the escape wheel rotates typically at an average of 10 rpm or more.

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