Mechanical Watch Basics
written by ulackfocus
A watch is an intricate machine. For any machine to function it must be supplied with energy. A mechanical watch is "fueled" by it's wearer, either by manual winding or with the motion of your wrist depending on the type of movement. The force of these actions is retained in the mainspring. The mainspring is attached to the barrel (case) it sits in on one end and hooked to the arbor (a shaft through the center of the barrel) at the other end.
basic barrel configuration with arbor in the center and cutaway to show the mainspring:
The mainspring coils tightly around the arbor when wound thereby storing energy. It is always trying to unwind itself to relieve tension. This causes the barrel to rotate around the arbor. The outside edge of the barrel has serrated teeth which mesh with the center pinion. This motion drives the pinion which is attached to the first wheel of the gear train.
wheel train assembly:
The wheel train consists of several different gears and pinions that are designed to transmit the energy from the barrel to the escape wheel. Their ratios are calculated to match the frequency (measured in bph - beats per hour) of the balance assembly's oscillation and they vary in size & number of teeth as needed.
pallet and escape wheel:
The energy is then transferred from the escape wheel to the pallet which moves it in one direction. It rocks on a pivot so the energy is transferred down the lever to the pallet fork. The pallet fork moves in the opposite direction of the pallet-jeweled end of the lever that was moved by the escape wheel. The fork makes contact with the balance shaft. It's movement pushes the shaft, which in turn rotates the balance wheel and hairspring assembly.
balance wheel & hairspring assembly:
The hairspring is wound by this spinning motion, increasing tension until it stops the balance wheel from rotating in one direction and reverses it. This spins the balance shaft in the opposite direction which results in the pallet fork being pushed in the opposite direction until the next tooth on the escape wheel catches a pallet jewel and halts it. When the hairspring reaches it's maximum tension in this direction, it again reverses. This creates a rocking motion of the pallet causing the balance & hairspring assembly to oscillate back & forth (side note: this is the motion you see in an "open heart" watch through the dial or through a display back when the rotor isn't obscuring the balance wheel)
Here's a link to see the motion recreated: Swiss Lever Watch Escapement in Motion
To keep the pallet fork from swinging completely out of the mechanism there are stoppers located on either side of the escape lever's shaft near the end where the pallet fork contacts the balance shaft. (see the 3rd drawing above) These are called banking pins. Some movements have adjustable pins while others simply have small walls rising from the main plate instead. (side note #2: the rhythmic impacts created by the pallet & escape wheel is the ticking sound you hear) The balance & hairspring assembly regulate consistency and pass the energy through gears to the hands which are mounted on the cannon pinion.
Here's a link to an old film by Hamilton that will illustrate some of the basics: YouTube - Cómo Funciona un Reloj Mecánico