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. It is found in two principal arrangements, the right angled or English form and the straight line or swiss form. In the former the escape wheel pivot, balance staff pivot and pallet staff pivots form a right angled triangle, in the latter they are colinear.

The invention of the lever escapement is attributed to Thomas Mudge and its modern form was developed by subsequent workers including Breguet. It is a detached escapement, which means that the time keeping element runs entirely free from the escapement during a portion of the operating cycle.

The escape wheel is controlled by the pallets fork, and also drives the fork. The escape wheel has specially shaped teeth of either ratchet or club form, which interact with the two jewelled pallets called the entrance and exit pallet. These pallets are attached solidly to the lever which has at its end a fork to receive the impulse pin of the balance roller.

As the escape wheel rotates, a tooth will slide across the sloping impulse plane on the entrance pallet. This will turn the pallets so as to place the exit pallet into the path of the rotating escape wheet, preventing the escape wheel from rotating more than 1/2 tooth. The wheel is said to be locked on the exit pallet. The impulse is transfered by the lever to the balance via the implulse pin on the roller of the balance. The lever moves until it rests against a banking pin where it is held by the draw of the pallet jewels.

The balance will rotate free of interence 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 turns until the next tooth locks on the entrance pallet.

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. Until the late 1970s, pin-lever watches were common. 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.

Mechanical movements have largely been replaced by electrically-operated quartz watches which are cheaper, just as reliable and more accurate.

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. Depending on the watch, this process happens at an exact rate between 2.5-5 times per second, causing the second hand to pulse forward 5-10 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 vs 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 watches do not have a traditional regulator, instead relying on moveable weights to fine-tune the balance for rate changes.