A mechanical watch can be divided into six systems:
- Power: a coiled flat spring (mainspring) wound to create potential energy.
- Train: a series of gears with specific ratios that transfer power.
- Escapement: a mechanism that regulates the release of power to both the indicator and timing systems.
- Timing: a method to maintain a precise regulated beat.
- Indicator: the pointers (hands) and dials.
- Support: the parts and structure that hold the mechanism together.
Mainspring
The flat, coiled spring that powers a watch is called a mainspring. Most watch mainsprings can, when fully wound, deliver 36-40 hours of power. A few special mainsprings can deliver 60 hours. Mainsprings are contained inside a barrel with toothed gears on the outside edge (mainspring barrel, main wheel, or 1st wheel). As the spring uncoils the barrel rotates and propels the train gears.
Power Train
A “train” is a sequential series of gears or gear assemblies. The power trail is composed of toothed wheels coupled with leaf pinions that are mathematically designed with specific ratios of rotation. The gear train transfers power from the mainspring to the escapement. The 2nd wheel (center wheel) rotates once every hour and the minute hand is attached to one end of the 2nd wheel arbor. The 4th wheel rotates once every minute and the sub-seconds hand is attached to its top pivot.
Lever Escapement
There are several types of escapement mechanisms, but the majority of watches made during the last 150 years are the straight-line lever-escapement type.
The escape wheel has feet instead of teeth. The 4th wheel engages and rotates the escape wheel, but the pallet fork (lever) stops and releases its rotation.
The pallet fork engages the escape wheel on one side and the roller jewel of the balance on the other. Shaped like the letter Y, the arbor of the pallet fork is positioned just below the junction of the V. As the escape wheel rotates it strikes the top of one arm and thereby moves the lever (base) of the Y-shaped fork to one side. This provides a burst or “impulse” of power that kicks the roller jewel or impulse pin to one side, thus causing the balance wheel to partially rotate and the hairspring to tighten.
At the same time the other arm of the Y locks the escape wheel and temporarily prevents the mainspring from unwinding. When the balance wheel rotates back, the roller jewel kicks the lever in the opposite direction, which releases the escape wheel and allows it to briefly begin its rotation before the sequence is repeated. The sound of the pallet fork releasing the escape wheel is “tick” and that of it engaging (stopping) is “tock.”
Timing
The heart of a watch is the balance assembly. It’s composed of an arbor known as a balance staff, onto which is centered:
- A balance wheel affixed with tiny screw weights so it is perfectly balanced (poised) flywheel,
- A hairspring, a flat concentric spring with one end fixed to the balance staff by a friction collet and the other to a rigid stud on the balance cock or plate.
- A roller table that holds an impulse pin or roller jewel that is engaged by the end of the pallet fork. antiquescollectibles.suite101.com/article.cfm/an_introduction_to_watch_jewels
The balance assembly is the most delicate and the most precise part of a watch. The lever pushes the roller jewel with enough impulse to cause the balance wheel, roller table, and staff to rotate 3/4 of the way (270°) and compressing the hairspring coil until tension stops the balance. The hairspring, now under tension, springs back and the balance travels 3/4 of a rotation in the opposite direction until the spring coil is expanded and tension once causes the balance wheel to stop and reverse direction.
As the assembly springs back the roller jewel strikes the end of the pallet, causing it to release the escape wheel. The assembly rotates back and forth (oscillates) 18,000 times an hour and regulates the time indicated by the hands.
Indicators
A watch has a main dial and often smaller subsidiary dials. The most common is the sub-seconds dial. Dials are divided into measured units: hours, minutes, seconds, and sometimes days. Metal pointers or hands are rotated by the mechanism to indicate time. collecting-jewelry-metalware.suite101.com/article.cfm/american_pocket_watch_dials
Support
The support system starts with the front or pillar plate to which the dial is attached and the rear plate or several partial plates sometimes called bridges. Screwed or pinned together, the plates keep the other systems in place, precisely aligned, and protected.
