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. In modern design it is common for the pallet mountings and the fork to be made as a single component.
As the escape wheel rotates, a tooth will slide across the sloping impulse plane of the entrance pallet. This will turn the pallets about their axis, which places the exit pallet into the path of the rotating escape wheel. Thus, once the tooth leaves the impulse plane of the entrance pallet, the wheel is only able to turn a small amount (called the drop) until a 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 half of the angle between two teeth.
The impulse received by the pallet as the tooth moves over the impulse face is transferred by the lever to the balance wheel via the ruby pin on the roller of the balance. The lever moves until it rests against the banking (either solid, or a pin); it is held in this position by the draw of the pallet jewels; this means that in order to unlock the wheel it must be turned backwards by a small amount.
After the drop, the balance wheel will rotate free of interference from the escapement until the impulse pin enters the fork again while moving in the opposite direction. This will unlock the escapement, which releases the escape wheel so that a tooth can slide over the impulse plane of the exit pallet, which transfers an impulse via the lever to the impulse pin. The escape wheel drops against until a tooth locks on the entrance pallet. The cycle then starts again.
Early lever escapements lacked draw (indeed some makers considered it injurious as a cause of extra friction in unlocking) as a result a jolt could result in the escapement unlocking.
Patent No. 7,553,068 Issued on June 30, Assigned to ETA SA Manufacture Horlogere Suisse for Lever Escapement (Swiss Inventors)
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