Behind the back of a mechanical chronograph you’ll find of the most beautiful sights in watchmaking. *A complexity of wheels, cams and levers working together to actuate and drive a chronograph second hand. *Sometimes, they’ll be driving even more finely sliced fraction-of-a-second timekeeping. *And all this, just so you can time your breakfast egg or how long your tedious boss is yapping in a meeting.
You already know the history of the chronograph (what do you mean, you’ve not read Ilya’s excellent article at Chronography 1: A History?); now it’s time to unscrew that back and take a look at the engine and what makes it go.

Let’s assume that you’ve delved into a mechanical (rather than quartz) chrono. *Inside, you’ll usually find one of two types of movement; a column wheel movement or one of the two subtypes of cam-actuated varieties. It’s all about how they start, stop and transmit the power from the mainspring to the chrono mechanism.
The received WISdom is that column wheels are finely honed and balanced Bugatti Royale engines whereas cam and levers are Chevy V8s. *The truth is, as ever in Watchworld, more complicated.
Here’s how to spot them

Column wheel (or “roue à colonnes”); look for a small, castellated wheel in the movement, shaped a little like the top of a chess rook. *Sometimes it’s even blued – as in ETA’s A08.231 that you’ll find in some top-end Longines. *Unless you’ve got the back off a Patek Philippe CH 29-535 PS. *In which case you’re very lucky, but you won’t see the wheel; Patek have thoughtfully covered it with a small fractionally eccentric adjuster cap.

Cam and lever (or “coulisse”); like the ETA/Valjoux 7750 or Shuttle cam (or “navette”), like the Lemania 5100. *In both cases, look for an eccentric cam and a series of tiny metal pawls, levers and arms.
A cam and lever has – near enough – the same number of parts as a column wheel set-up. *But it costs rather less to produce as the tolerance of the parts doesn’t need to be quite so precise. A shuttle cam is simpler – and thus cheaper – still.

How a column wheel chrono works

When your boss starts droning, you push the ‘start’ button. *This, in turn, moves the column wheel operating lever which moves the wheel one click or turret. *That then moves the clutch rocker away from its detent. *The rocker then engages with the oscillating pinion with the chronograph wheel and the hand starts moving. Click to stop and the column wheel turns the other way, stopping the mechanism.

Ah, the very zenith of chronographical excellence. *Lots of parts. *Lots of serious hand-finishing to ensure perfect tolerances. *Expensive, you say? *Well, it can be. *You could nearly buy a house and a 911 to park in the drive for the price of that Patek CH 29-535. But not all column wheel chronos are stratospheric; how about the Seagull ST19? *Yours for around $389. Or you could plump for the original Seiko cal. 6139 from the late 1960s. But it’s true; most column wheels will leave your wallet looking a little wilted. *Think Omega cal.321 from the very first Speedies, Rolex’s cal. 4130 and the completely gorgeous, drool-inducing Zenith El Primero (who said anything about being unbiased?).
How a cam and lever chrono works

Widely thought of as a the column wheel’s poor relation. *A sort of horological embarrassing hick-from-the-sticks cousin. *But are cam and lever movements really second-raters?
Take a look at the exemplar cam and lever, the Omega cal. 861. Because of the way it operates, its sharp, triangular teeth let two wheels mesh together efficiently, imparting power to the chronograph. *But whilst one wheel is always in motion, the one with which it meshes isn’t. *This results in wear. *Not good. *At least, that’s what you’d think.
CAL. 861 Image Credit: Omega Forum User KRLYUZH

In fact, even if you’re timing that dreary boss most mornings for a few years, the wear won’t really worry you. *It didn’t worry NASA, after all. *But watchmakers dislike wear, so they developed a new way to actuate the chrono mechanism. *Exhibit A – the almost ubiquitous Valjoux/ETA 7750 and its clever oscillating pinion. *In this movement there’s no vulgar clashing of wheel teeth, just neat, orderly meshing.
Back to the Valjoux 7750. *Push the ‘start’ button. *At the end of a tiny, pivoted metal lever there’s an oscillating pinion. *This moves in response to mesh with the chronograph wheel and round goes the hand. *Not so bad, is it?

So why do cams get such a bad name? *There seem to be three reasons: their apparent tendency to have fractionally jumpy second hands when the ‘start’ button gets pushed, slightly harder-to-push pushers and their (relative) cheapness. In other words, there’s a bit of watch snobbery at play.
The bit about the pushers is true though. You’ll get a smoother push with a column wheel movement.
The whole jumping second hand thing is, in fact, nothing to do with the cam actuation mechanism. *It’s simply because most cam-actuated chronos use a horizontal clutch; and horizontal clutches can make a chrono second hand jump a little because of the way their gearwheel teeth mesh.
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