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.

The Going Train - A Melodrama in Three Parts

When we last looked in, the energy in our mechanical watch or clock had entered the device by means of a purposeful winding or by taking advantage of the potential energy in a raised weight as it descended to the earth. The energy was then stored in a tightly wound mainspring (in the case of the watch), wound around an arbor and connected to a barrel, which was both free to spin and capped with gear teeth. The barrel, which comprises all of those elements - the mainspring, arbor, and the gear-capped cylinder - interlocks with the all-important going train, which is the subject of this week's adventures in horology.

The Cast of Characters:

- The Mainspring Barrel  - a strong, steady type, as we saw in last week's episode. Strives to keep things moving smoothly, but can be a bit pushy.

The mainspring barrel, with and without the mainspring. Photo from Wikimedia Commons, but lacking a source. If this is your photo, please contact me and I will credit you!

- The Center Wheel - a newcomer to the story. In lockstep with the Mainspring Barrel, it keeps a very regular schedule. The Center Wheel has the honor of holding the Minute Hand, which gets paraded around once an hour like clockwork. Also charged with driving the Third Wheel.

- The Third Wheel - an awkward but necessary character. Is only included in the train to make the math work, and he knows it. Takes his duty of driving the Fourth Wheel very seriously, however.

-  The Fourth Wheel  - The Fourth Wheel has the dubious distinction of carrying the second hand - dubious because of the diminishing number of clocks that choose to flaunt a second hand. Worries that he will soon be obsolete. Also troubling: his second function of driving the escape wheel could easily be taken over by the Third Wheel, who has upward aspirations.

He's very shy.

- The Escape Wheel  - a spiky fellow, but absolutely necessary member of the Train. Has an impeccable sense of time and knows just when to give the Balance Wheel a push to keep it in the game. Very tight with the Escapement - they have a whole system worked out to make sure things happen on time.

Courtesy of mfrasca at en.wikipedia.

- The Pinions  - Small but mighty gears, without them the math simply would not work out. They feel strong attachments to their respective main gears and ensure that communication between all the biggest players comes off without a hitch.

In a watch, the pinion is on the outside. But you get the idea - it's a smaller gear rotating with the main gear to dial up or dial down the speed of rotation.

The Plot:

The reckless and headstrong Mainspring is hell-bent on pouring all of its power into the timepiece at once. There is much consternation and frenzy - too much energy in the watch at once will destroy the lovely Hairspring and rip apart her equally entrancing sister, the Balance - and without the Balance, there can be no Time! When it appears that no solution is forthcoming, at the last minute the valiant Gear Train leaps into action.

The Action:


The spring is wound and the stage is set. The mainspring pushes inexorably on the Mainspring Barrel, which grabs hold of the Center Wheel's pinion and forces it into lockstep. The dutiful pinion transmits the force of the Mainspring Barrel to the Center Wheel.

Due to the loyal pinion, the Center Wheel does not turn with nearly the force of the Mainspring Barrel, and lets the world know of its success by spinning the minute hand around the clock face at the rate of exactly one time per hour. The Center Wheel captures the attention of the Third Wheel's pinion, which spins the Third Wheel and distributes the Mainspring Barrel's energy even further. Already things are looking up for our heroines, the Hairspring and the Balance.

The Third Wheel spins the Fourth Wheel's pinion, raising the rate of rotation in its excitement. The Fourth wheel spins frenetically around at one turn per minute. In the midst of this frantic rotation, the Fourth Wheel finds the time to drive the escape wheel - the last gear to stand between the Mainspring Barrel and the Balance.

The Escape Wheel looks odd - its teeth are spiky, and it's made of different stuff than a proper watch gear. But the Escape Wheel is a steady fellow. He knows that without him, the rest of the watch gears would rotate with abandon, expending the Mainspring Barrel's energy with no regard for Time at all. The Escape Wheel is a regulator - he rotates precisely one tooth at a time in an arrangement he's worked out with his friend the Anchor. The Anchor is precisely that - it keeps even the Escape Wheel in check. Once the Escape Wheel has been brought down to a steady tick-tock rhythm, it sends just a bit of its energy down do the Balance - just enough to keep her spinning, rather than destroying her and the Hairspring in the process.


The Denouement: 
Thanks to these brave gears, the force of the Mainspring Barrel has been dialed down to a manageable force, and is even useful in keeping the Balance - she who actually keeps the time - running. Along the way, we passed over the contributions of the Motion Work and the underlying input from Gear Ratios.

All topics for the future.

Until next time!