Lever escapement

Lever escapement

The lever escapement is a key component of the typical movement found in 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. It is a detached escapement, which means that the time-keeping element runs entirely free of interference from the escapement during a portion of the operating cycle.

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.

Draw

The reliability of the modern lever escapement depends upon draw; the pallets are angled so that the escape wheel must recoil a small amount during the unlocking; this holds the lever against the banking during the detached portion of the operating cycle. Draw angle is typically about 15 degrees to the radial.

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.

Lever watch movement

Most modern mechanical watches are jeweled lever watches, using manmade ruby or sapphire jewels for the high-wear areas of the watch.

Pin pallet escapement

A cheaper, less accurate version of the lever escapement is used in alarm clocks, kitchen timers, mantel clocks and, until the late 1970s, cheap watches, called the Roskopf, pin-lever, or pin-pallet escapement after Georges Frederic Roskopf, who invented it in 1867. It functions similarly to the lever, except that the lever pallet jewels are replaced by vertical metal pins. In a lever escapement, the pallets have two angled faces, the locking face and the impulse face, which must be carefully adjusted to the correct angles. In the pin pallet escapement, these two faces are designed into the shape of the escape wheel teeth instead, eliminating complicated adjustments. The pins are located symmetrically on the lever, making beat adjustment simpler. Watches that used these escapements were called pin lever watches, and have been superseded by cheap quartz watches.

Future directions

One recent trend in escapement design is the use of new materials, many borrowed from the semiconductor fabrication industry. A problem with the lever escapement is friction. The escape wheel tooth slides along the face of the pallet, causing friction, so the pallets and teeth must be lubricated. The oil eventually thickens, causing inaccuracy, and requiring cleaning and reoiling of the movement about every 4 years. A solution is to make the escape wheel and other parts out of harder materials than steel, eliminating the need for lubrication. Materials being tried include silicon (called silicium in the watch industry), nickel phosphorus, diamond, and diamond-on-silicon. Ulysse Nardin in 2001 and Patek Philippe in 2005 introduced watches with silicon escape wheels.

References

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