The technology behind your wristwatch...

An introduction to mechanical watches by Matt V.

What actually is it that "drives" your mechanical wristwatch? After all, there's no batteries to exchange, right? So something's got to be providing the energy that drives your little timepiece and you're about to find out what that is.

Well, the answer is rather simple and applies to both manual wind watches as well as automatics: the pressure of an unwinding mainspring is turning the wheels and pinions of the geartrain, which are responsible for the motion of the hands that indicate time. The speed in which this happens is controlled by the escapement and balance wheel on the other end of the geartrain. But let's start our tour with the powerplant inside your watch.

1) The Mainspring

Let's find the little bugger first, shall we?

On the picture below you can see the crown and stem (also known as winding shaft) leading to the inside of a mechanical movement.
Now what exactly happens if you start turning the crown to wind the watch?

On this picture you can see a smaller wheel called the transmission wheel. When you start turning that crown, the "keyless works" (which cannot be seen in this shot) and crown wheel start to rotate this wheel. It than interconnects with the larger wheel you can see above it in the photo.

Here it is: the so called "ratchet wheel".

This photo below shows it even better. So if you turn the crown and stem, the transmission wheel starts turning the ratchet wheel and the post in the middle with the screw that holds the ratchet wheel attached to it (arbor) will start to wind up the mainspring hidden underneath. So what keeps it from violently unwinding in the opposite direction the exact moment you loosen your grip on the crown?

Look gain closely: the small metal piece with the two prongs on the side, the so called "click spring"! It will allow the ratchet wheel to turn in one direction by moving the prongs. But when that direction changes, it will actually lock the ratchet wheel and keep it from rotating back into the other direction. It actually is responsible for the "clicking" sound when you wind the watch. So this little piece is key to "lock" the power into the mainspring when you wind your watch.

But if this wheel only turns in one direction, how can it drive the watch? And where does the mainspring hide? Let's remove the ratchet wheel to find out!

The ratchet wheel is gone in this picture and you can clearly see the square top of the barrel arbor and the hole for the screw that attached the ratchet wheel to it. It is poking though a hole in the plate called the top barrel hole.

What's hidden underneath can be seen here. The so called "barrel". In modern watches, the mainspring is typically sealed inside the barrel which would than have a "barrel cover" on top. Inside, the mainspring is wound up. One end is connected to the arbor in the center, the other end to the wall of the barrel.

So the barrel itself moves and rotates when the mainspring unwinds. And if you look at the "teeth" on the bottom of the barrel: they actually form what's called the "main wheel".

And the part that stores and releases all of the energy is finally shown here: the mainspring! Can you identify the "eye" on one end (in the middle) that interlocks with the barrel arbor hook? And the "folded" end that attaches to the barrel itself (out of focus in the front)?

This piece of metal, about 200-300mm long in the past was a common source of problems. Metal fatigue caused it to break or to loose its force when unwinding, literally "tiring". During a service, the watchmaker will inspect the mainspring, clean/lubricate it again. And often when they discover an older style mainspring, they will try to replace it with one of the modern "unbreakable" mainsprings.

The length and space the mainspring has to unwind determine the power reserve of a watch. I guess all of you have seen the "8jours" or 8 day watches that will keep running for up to 8 days when wound. These watches will use larger barrels/mainsprings and some of them actually have 2 barrels working together.

Oh, and before I forget: automatic watches basically use the same principle, just the mechanics to wind the mainspring look a little different and more complicated. In order to avoid "overwinding" the mainspring, they use a clutch that releases the mainspring in case its fully wound and the rotor continues to attempt windng it. So keep in mind when you wind your manual wind watches: when you feel resistance, stop winding or you can "overwind" the watch, rip the mainspring from the arbor or damage it in a different fashion. Another tip: try to wind it every day around the same time...

Hope you liked this little trip into watchmaker territory!

Cheers

Matt V.