Lever escapement

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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 Lever watch movement

Until the late 1970s, a cheaper version pin-lever watches were common, also called Rosskopf watches, by the name of its inventor Georges Frederic Roskopf. These are nearly identical in operation, except that the fork jewels are replaced by plain steel parts. Most pin-lever watches have no jewels, or a single, mostly useless jewel.

Those cheap mechanical movements have largely been replaced by electrically operated quartz watches, which are cheaper, just as reliable and more accurate.

On the other hand, lever mechanical movements of high quality have become a standard for luxury watches.

How a typical lever escapement movement works:

The crown and stem turn the keyless works, which when in the wind position turns the inside loops of the mainspring coil. The mainspring is inside the barrel, with the outside of the mainspring attached to the barrel. The barrel turns the center wheel once per hour — this wheel has a shaft that goes through the dial. On the dial side the cannon pinion is attached with a friction fit (allowing it to slide when setting the hands) and the minute hand is attached to the cannon pinion. There is a small wheel driven by the minute wheel that in turn drives the hour wheel and hand once for every 12 revolutions of the minute hand.

The center wheel drives the third wheel, which in turn drives the fourth wheel. On most watches, the fourth wheel is geared to rotate once per minute, and on most watches with "sub seconds" (seconds on a small subdial between the center and edge of the watch) rather than the more common center seconds, the second hand is attached directly to this wheel. The fourth wheel drives the escapement wheel or escape wheel.

The combination of hairspring stiffness and length works with the diameter and mass of the balance wheel to precisely control the rate of the watch according to Hooke's law. Depending on the watch, this process happens at an exact rate between 2.5 and 5 times per second, causing the second hand to pulse forward 5 to 10 times per second. The speed of this process is almost independent of the rest of the watch. More or less force (as in the difference between a fully wound mainspring and a nearly unwound) will change how far the balance wheel swings, but under normal operation will not significantly change how long it takes to complete a cycle.

If either the effective length or strength of the hairspring are changed, or the mass or diameter of the balance are changed, the rate of the watch will change. Most watches are regulated with a moveable regulator that grips the spring at a point near the outside end, changing the effective length. Some watches have screws around the balance — by adding or removing washers under these screws, coarse adjustments to rate can be made. Some high-end "Free-Sprung" watches do not have a traditional regulator, instead relying on moveable weights to fine-tune the balance

See also

• pallet fork