Lever escapement
The lever escapement is the typical movement found in most wristwatches, pocket watches and many small mechanical non-pendulum clocks. 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 1970’s, pin-lever watches were common. These are nearly identical in operation, except that most or all of the jewels are replaced by plain steel parts.
Mechanical movements have largely been replaced by battery-operated quartz watches which are cheaper, as reliable and more accurate.
How a typical lever escapement 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 forth wheel drives the escapement wheel.
The escapement wheel is controlled by the pallet fork, and also drives the fork. The escapement wheel has coarse, specially shaped teeth that mesh with one of the two jewels or pins of the fork. The fork pivots in a way that allows one jewel to move out of the way of a tooth of the escapement wheel but at the same time moves another in the way, allowing the escapement wheel to move by one-half tooth. When this happens, the escapement wheel gives an impulse to the fork, pushing it in the same direction with a bit more force. This causes the fork to rock to the other end of it’s travel, where it stops the escapement wheel until it is nudged again.
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. When the fork is impulsed by the escapement wheel, it sends this impulse to a pin on the balance wheel, causing the balance to rotate. The balance will swing until the hairspring causes it to slow and then reverse direction. As the balance rotates back around, the pin will nudge the fork, releasing it from the escapement wheel, which will cause the escapement to nudge the fork, which causes the fork to push the pin a bit harder in the same direction it is now traveling, accelerating the balance again until the hairspring again causes it to reverse directions again, etc… 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 will not 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.