Mechanical Watches, Mechanism

All mechanical watches have these five parts:

• A mainspring that stores mechanical energy to power the watch.
• A gear train, called the going train, which has the dual function of transmitting the force of the mainspring to the balance wheel, and adding up the swings of the balance wheel to get units of seconds, minutes and hours, etc. A separate part of the gear train called the keyless work allows the user to wind the mainspring and enables the hands to be moved to set the time.
• A balance wheel which oscillates back and forth. Each swing of the balance wheel takes precisely the same amount of time. This is the timekeeping element in the watch.
• An escapement mechanism which has the dual function of keeping the balance wheel vibrating by giving it an impulse each swing, and allowing the clock's gears to advance or 'escape' by a set amount with each swing. It is this part that produces the characteristic 'ticking' sound of the mechanical watch.
• An indicating dial, usually a traditional clock face with rotating hands, to display the time in human-readable form.

Additional functions on a watch besides the basic timekeeping ones are traditionally called complications. Mechanical watches may have these complications:

• Automatic winding or self-winding - in order to relieve the need to wind the watch, this device winds the watch's mainspring automatically using the natural motions of the wrist, with a rotating weight mechanism.
• Calendar - displays the date, and often weekday, month, and year. Simple calendar watches don't account for the different lengths of the months, requiring the user to reset the date 5 times a year, but perpetual calendar watches account for this, and even leap years.An annual calendar doesn't make the leap year adjustment, so the date must be reset on March 1 every fourth year.
• Alarm - a bell or buzzer that can be set to go off at a given time.
• Chronograph - a watch with additional stopwatch functions. Buttons on the case start and stop the second hand and reset it to zero, and usually several subdials display the elapsed time in larger units.
• Hacking feature - found on military watches, a mechanism that stops the second hand while the watch is being set. This enables watches to be synchronized to the precise second. This is now a very common feature on many watches.
• Moon phase dial - shows the phase of the moon with a moon face on a rotating disk.
• Wind indicator or power reserve indicator - mostly found on automatic watches, a subdial that shows how much power is left in the mainspring, usually in terms of hours left to run.
• Repeater - a watch that chimes the hours audibly at the press of a button. This rare complication was originally used before artificial lighting to check what time it was in the dark. These complex mechanisms are now only found as novelties in extremely expensive luxury watches.
• Tourbillon - this expensive feature was originally designed to make the watch more accurate, but is now simply a demonstration of watchmaking virtuosity. In an ordinary watch the balance wheel oscillates at different rates when the watch is in different positions, causing inaccuracy. In a tourbillon, the balance wheel is mounted in a rotating cage so it will experience all positions equally. The mechanism is usually exposed on the face to show it off.

How it works

The mechanical watch is a mature technology, and virtually all ordinary watch movements have the same parts and work the same way.

The spiral mainspring that powers the watch is inside a cylindrical barrel, with the outer end of the mainspring attached to the barrel. The barrel has gear teeth around the outside that turn 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. The cannon pinion drives a small 12-to-1 reduction gearing called the motion work that turns 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. In watches with the second hand in a subdial on the face, the fourth wheel is geared to rotate once per minute, and the second hand is attached directly to this wheel. In watches with a center second hand, the fourth wheel turns a gear on a shaft projecting through the center of the minute hand shaft, which turns the second hand.

The fourth wheel also drives the escape wheel of the lever escapement. The escape wheel teeth alternately catch on two fingers called pallets on the arms of the pallet lever, which rocks back and forth. The other end of the lever has a fork which engages with an upright impulse pin on the balance wheel shaft. Each time the balance wheel swings, it moves the lever, which releases one tooth of the escape wheel, allowing the watch's wheels to advance by a fixed amount, moving the hands forward. As the escape wheel turns, its tooth pushes against the lever, which gives the balance wheel a brief push, keeping it swinging back and forth.

The balance wheel keeps time for the watch. It consists of a weighted wheel which rotates back and forth, which is returned toward its center position by a fine spiral spring, the balance spring. The mass of the balance wheel combines with the stiffness of the spring to precisely control the period of each swing or 'beat' of the wheel. Most watch balance wheels oscillate at 5, 6, 8, or 10 beats per second. In most watches there is a regulator lever on the balance spring which is used to adjust the rate of the watch. It has two curb pins which embrace the last turn of the spring, and can be slid up or down the spring to control its effective length.

A separate set of gears called the keyless work winds the mainspring when the crown is rotated, and when the crown is pulled out a short distance allow the hands to be turned to set the watch. The stem attached to the crown has a gear called the clutch or castle wheel, with two rings of teeth that project axially from the ends. When the stem is pushed in, the outer teeth turn the ratchet wheel on top of the mainspring barrel, which turns the shaft that the inner end of the mainspring is attached to, winding the mainspring tighter around the shaft. A springloaded pawl or click presses against the ratchet teeth, preventing the mainspring from unwinding. When the stem is pulled out, the inner teeth of the castle wheel engage with a gear which turns the minute wheel. When the crown is turned, the friction coupling of the cannon pinion allows the hands to be rotated.

3345, Watches Australia.