Last Stop on the Going Train

The last time I wrote about the inner workings of a mechanical watch, I ended with the fourth wheel, which interacts with the final piece of the puzzle, the escapement/balance combination. The combination of gears that makes up the going train is all very well and good, but it's the escapement and the balance that make all those tiny mechanical parts into an actual timekeeping device.

An actual timekeeping device. Source:  "Nature" journal, September 22, 1887, pp. 485. For all the tiny parts and clever engineering, mechanical watches are awfully old. 

The escapement is what keeps all of the energy in the mainspring from dissipating all at once. Imagine a heavy weight raised up to the ceiling with a pulley. You're holding on to the other end of the rope. If you let go of the rope, all the energy you put into the weight to get it into the air is released all at once, and the weight puts a hole in your floor. Instead, you can choose to let it down slowly by applying some downward force to the rope. The energy released is the same, but it's over a longer period of time, and you don't do any structural damage to the building. Which method you choose depends on how much you like your floorboards or your neighbors.

The same thing happens with a mechanical watch or clock. The wound mainspring is like the weight/pulley system when the weight is all the way up at the ceiling. You, if you decide to pull back on the rope and slow the pulley's descent, are like the escapement: you control the release of potential energy in the system. If there were no escapement, all of the energy in the spring would be released in a very short amount of time. (Quick physics note: energy per unit time is defined as power, or P = W/t. The maximum energy in this system is constant [due to the physical limitations of the mainspring], so as time gets very small, power gets very big. Thus, a hole in your floor.)

Oops.

By regulating the power output at a very precise rate, the escapement turns all those gears into a useful way to keep track of time. The escape wheel is shaped differently than the rest of the gears. The teeth look a little bit like those things that keep you from driving back into parking garages once you've left.

Courtesy of Guardian Traffic Control Systems.

And with good reason - their purpose is really the same: to allow motion in one direction, but not the opposite. In a watch, the fourth wheel meshes with the escape wheel's pinion (gear with fewer teeth than the main gear that spins on the same axle and with the same angular frequency as the main gear). The pinion turns the axle, which turns the escape wheel. However, the bow-shaped bit on top of the escape wheel prevents it from spinning freely. The anchor (bow shaped bit) rocks back and forth to allow only one tooth at a time to escape.

             

Every time the anchor swings in one direction, it also gives a little push to what's called the balance wheel.

Credit to Chris Burks for the photo.

The brass-colored wheel in the bottom center is the balance wheel. It's attached to the thin spring above it called the hairspring. The balance wheel spins until the force of the hairspring pushes it back in the other direction. This is the equivalent of having a pendulum in a standard wall clock. The time to complete one rotation of the wheel or one swing of the pendulum is constant*, so the rate at which the escapement lets the mainspring's energy escape is steady. Et voila, a timekeeping device.

So I thought I was done with tracing the energy through a watch, but take a look at this picture from horologist.com:

What's a crown wheel? or a click spring? or a click, for that matter?

There is more work to be done here.

Time and a Half

 I've never been one for small aspirations. As a kid, if they told me to build a model building out of paper, I was
 going to use the longest, thinnest paper available and hold the thing together by sheer force of will. After college, I
 wanted to work in science museum, so I picked the best one I knew of, got an (unpaid) internship, and flew
 across the country to start working two months later. When I decided to learn to snowboard, I took the ski lift to
 the top of the bunny slope, strapped in, eavesdropped on a kids' lesson to learn how to stand up, and went off to
 go snowboarding.

 I mention this because I know my own tendencies, and I suspect that my next project is beyond the scope of
 anything else I've tackled so far. I want to make a wooden clock.

 I chose a gear for my first SketchUp tutorial for a reason. I have a major soft spot for anything clockwork
 (from *before* it was popular, thankyouverymuch) and a serious penchant for little
 fiddly projects. I think precision makes things beautiful, and if you've ever seen the inside of a mechanical
 watch, you'll know that it's been precision-ed to death. So, a clock it is. Wooden because metalwork is a little
 beyond me right now and besides, look at this thing. It's gorgeous. This particular one is done by an immensely talented woodworker named Wayne Sutter.

This is gorgeous, and it's not even finished in this shot. Go buy a clock from him - we need more craftspeople like this.

 Clocks, especially mechanical ones, are amazing and complex works of careful engineering. Sounds like a
 great amateur project, doesn't it?

Image from Wikimedia Commons. Yay public domain!

 Clocks from the inside out
 Initial impression: Holy crap, there's a lot of vocab here. Right off, we have: mainspring, arbor, going
 barrel, ratchet, pawl, pinion, wheel train, escapement, remontoire, and balance wheel, just to name a few.

From a mechanical perspective, I think that the easiest way to tackle a watch movement (the whole thing in the back of a clock or watch that makes the thing go) is to trace the energy. Energy goes into the watch when you wind it, and out of it as the stored energy is expended in making the hands turn and the escapement oscillate (and the alarm go, and the cuckoo bird cuckoo, etc.). When you wind a watch, you spin a post, or turn a key that turns a post, called an arbor. The arbor connects to a spiraling strip of metal called the mainspring.

The mainspring.
Image from Wikimedia Commons.

 The mainspring is attached to the arbor from the inside of the spiral, making the whole thing look a little like a spinning top, or maybe a barred galaxy if you're creative. 

The arbor/spindle/massive beams of radiation would be coming out of the center of that.

The other end end of the mainspring attaches to a short, fat cylinder called the going barrel.

Also Wikimedia.

The barrel is free to spin, which is a good thing, because the barrel cylinder is capped with what is essentially a solid gear. This gear is used to interact with the gear train, which is the bit that makes the hands and other useful parts move. Its most important function, though is to power the balance, the little wheel that does for a wristwatch what a pendulum does for a grandfather clock.  

A real-life example: the going barrel in this watch is capped by the gear you can see in this picture. It also happens to be upside down.

Suppose you got bored at this point and thought, what the hell, we've got a power source and something that spins - let's just put some hands on it and let 'er rip! You'd have a timekeeping device of some sort, but right now there's nothing to counteract the force of that spring - you'd wind it, let it go, and it would release all its energy all at once. You'd measure time in spring-unwindings instead of seconds. 

That's where rest of the gear train comes in. That'll have to wait for part two, though. In the meantime, here's some watch porn for all you horologists out there. 

...that even sounds dirty.