The escapement is the brain of a watch and it's precision greatly affects the watch's time keeping ability. While its motion would appear to be just another step in a linear progression, it's actually the hub of a cyclic action where each piece affects the entire mechanism's rate of motion. The escape wheel is named for it's function - it allows a patterned escape of energy from the mainspring.




The escapement's job boils down to these 2 functions:
- It releases the energy passed to it via the mainspring and wheel train at a regular rate. This energy drives the hands and all of the watch's functions.
- It uses this energy to power the balance wheel's oscillations by giving the roller jewel a nudge each time it swings by the end of the pallet fork.


Click HERE to see an animation of the escape wheel interacting with the pallet jewels.

The timekeeping will remain consistent (called "isochronism") as long as the power is sufficient to keep the balance & hairspring assembly oscillating at it's proper amplitude (degree of rotation). The rotational period of the balance wheel determines the interval that the escape lever rocks at, which in turn controls the rate at which the escape wheel intermittently revolves at, which then dictates the speed at which the entire wheel train rotates at and therefore the rate at which the mainspring unwinds. The mainspring is the start of the process, but it would unravel at a speed much too fast if it wasn't controlled by the very parts it supplies energy to. Each piece is vital to the overall performance of the watch.

There are 2 safety mechanisms in the escapement. First, the two banking pins (14 & 15) are designed to stop the lever from swinging too far (called "overbanking"). Some movements have adjustable banking pins, while others have solid banking walls that are part of the metal plate that the axels of the moving parts are mounted to. Second, the guard pin (13) and the safety roller (17) keep the lever from rocking before it is pushed by the roller jewel. When the balance & hairspring assembly rotates away from center, the guard pin slides against the the safety roller. As the balance & hairspring assembly spins, the lever can only rock when the guard pin has the clearance to move provided by the crescent-shaped passing hollow (19).

One other note: The finish on the escapement pieces, or any part of the movement for that matter, is not just for decoration. A high polished surface reduces friction, especially on the pivot of the escape lever. Smoother surfaces, accurately cut angles, properly set pallet jewels, and correct lubrication allow smoother motion with tighter tolerances, which will result in smaller variances in the daily rate (timekeeping) of a watch. Pallet jewels are set in place with shellac. The depth and angle at which they are set is critical to proper timekeeping. Countless hours are spent on just this in watchmaking school. When you hear the term "fit & finish" of a movement, this is what it is supposed to refer to.


***** If you wish to read about the motion in much more (and potentially dizzying) detail:
The energy from the unwinding mainspring flows through the wheel train which pushes the escape wheel. One of the escape wheel's teeth pushes the entry pallet jewel which causes the escape lever to rock in a clockwise direction on it's fulcrum. This causes the pallet fork end of the escape lever to move so the inside edge of the entry horn (prong) of the pallet fork pushes the roller jewel. The roller jewel is attached to the roller table, which is attached to the balance staff which is the axel that the balance and hairspring assembly rotates on, so this push ("impulse")causes the balance wheel to revolve CCW. This winds the hairspring. At the same time, the exit pallet jewel catches a different tooth of the escape wheel stopping the escape wheel and lever. When the hairspring reaches it's maximum tension, it stops the balance wheel's rotation and reverses it. This spins the entire balance & hairspring assembly in the CW direction and the roller jewel catches the inside edge of the entry horn on the pallet fork, pushing it back the other way. This causes the escape lever to rock in the opposite direction (now CCW), which causes the exit pallet jewel to slide off the tooth it was contacting and releases the escape wheel to turn under the force being supplied by the mainspring until the entry pallet jewel swings in and catches a different tooth on the escape wheel and stops it (called "locking"). All the while this is happening, the hairspring is still winding in the CW direction. When it reaches maximum tension in this direction, it again stops and reverses, causing the roller jewel to catch the inside edge of the exit horn on the pallet fork and push it the other way, which in turn causes the escape lever to rock in the other direction until it releases the entry pallet jewel from the tooth on the escape wheel it was engaged with. This releases the escape wheel to rotate another small increment (because of the force of the mainspring) until the rock of the lever swings the exit pallet jewel into position for it to catch another escape wheel tooth and halt the motion. The escape wheel starts and stops intermittently according to the rocking of the escape lever as each pallet jewel engages and disengages the escape wheel's teeth. This process will repeat itself as long as the mainspring has enough torque to drive the escape wheel.


Thanks to Steve / N2FHL for some clarity!