Watches in Depth - The Chronograph ¶
(Cet article n'est pas disponible en Français)
One of the most popular complications in watchmaking is the chronograph, the watch that doubles as a stopwatch as well as a timekeeper. It’s been one of the most prolific complication in the 20th century, second only to the basic calendar complication, and are accessible from entry-level Swiss brands all the way up to the top of the watchmaker’s craft. The chronograph has a long history in fine watchmaking, and has long been a symbol of mechanical prowess combined with everyday functionality. After having become so popular and commonplace, many people overlook just how complicated a fine chronograph mechanism really is.
The term chronograph literally means “time writer”. It is a device to measure elapsed time from a set point. A counter is started with the push of a button, after which the time is “written” on a series of subdials (also called registers). The timer can then be stopped to show the elapsed time, and restarted (or reset) to continue timing (or begin again). The simplest chronographs measure only the elapsed seconds with a central sweep hand (meaning a seconds hand with the hour and minute hands in the middle of the dial, rather than on a smaller subdial like a pocketwatch. The second hand only runs when the chronograph is running, otherwise it is stationary at 12 o’clock). The next step up in function adds the ability to measure minutes and hours as well. Some can even measure days and weeks. Every chrono shares the same ability to be started, stopped and reset by the user. How they go about this is where things get complicated.
Early chronograph designs emerged in the 18th century, with production beginning in the 19th century. Early designs had a problem where pressing the reset button while the chronograph was running would cause the gear train to jump and damage the mechanism. To prevent accidental damage, a safety in the form of a column (or castle) wheel actuator was developed. A column wheel is a tall gear with triangular teeth that is turned by a series of levers that are connected to the chronograph buttons. When a button is pushed, the wheel turns and activates the functions in a specific order – start, stop and reset. Pushing the wrong button at the wrong time doesn’t engage the wheel, preventing damage. The column wheel would become one of the most popular forms of chronograph systems in the 20th century with the Valjoux manual-wind chronograph calibres, and has found a resurgence in modern designs in haute-gamme chronograph movements.
Column wheel movements are expensive to produce and require very fine adjustment to function properly, so for mass-produced inexpensive movements a simpler solution was required. Several options were introduced; the most popular are cam and lever (or coulisse), and shuttle cam (or navette). Essentially they replace the column wheel with a series of levers, pawls and arms that mesh together with an eccentric cam. Adjustment and repair is much simpler and production is cheaper too, as the tolerances can be a little looser than in a column wheel. While not really any “worse” than a column wheel (the simpler repair is certainly a plus), modern tastes and marketing have made column wheel designs the top tier of chronographs and workhorse lever-actuated movements have become common in less expensive mass-produced watches. The ubiquitous Valjoux 7750 uses cam and lever, while Lemania-based calibres often have navette systems.
All of this is well and good, but how does a chronograph actually function? Two ways are common. Basically a series of gears connected to the chronograph hands are held in mesh with a driving wheel on a movable arm kept adjacent to the central pinion of the movement. When the chronograph is engaged, the driving wheel (which runs off the seconds wheel of the base movement) is pushed into the central gear. It meshes and begins turning the chronograph counters through a central wheel. If this sounds a bit crude, with a stationary gear getting jammed into a turning one, you are right. The centre wheel of the chrono has small teeth, much finer than the drive gear, to ensure it meshes reasonably well without skipping (or breaking) teeth. Unfortunately it isn’t perfect, and that is why you sometimes see a slight jump of the second hand when starting the chronograph. Alternately an oscillating pinion is used, where a tall pinion gear is constantly meshed with the fourth wheel of the movement and engaging the chrono pushes the driving wheel onto the upper part of the pinion to start the functions.
When you stop the chronograph the driving wheel is pulled away from the central wheel (or the oscillating pinion is disengaged) and a brake is applied to the gears. Pushing the reset engages a series of spring-loaded hammers that snap the gears (and thus the hands) back to their zero positions.
Back to the problem of jumping hands – because of the engagement system, with a non-turning gear hitting a moving one, chronographs can sometimes have a degree of imprecision. Starting the chrono may cause the hands to jump slightly, sometimes as much as half a second (it doesn’t sound like much, but a half a second error is a lot in precision timing). The gears can also wear heavily over time if the chronograph is used constantly. The solution is to have the gears meshed all the time, with a clutch that engages or disengages the mechanism. This is called a vertical clutch system, which along with the column wheel occupies the top tier of the chronograph market.
The vertical clutch is a tiny device that has a pinion running through the centre. An arm holds the clutch together while the chronograph is stopped, when the chrono is started the arm moves away to let the clutch open and allow the pinion to turn. It really is like a tiny automotive clutch. No gears are moved and the wheel teeth are always meshed together, so excess wear and jumping hands are eliminated. Additionally accuracy is much improved; a lesser chronograph can lose amplitude and thus accuracy when the chronograph is engaged. Unfortunately the vertical clutch is an expensive and delicate mechanism, so it is relegated to the upper end of watchmaking. It is also difficult to repair; if the clutch breaks it must be replaced with a new unit.
Technical details aside, chronographs can assume many different forms in terms of their function and their operation. An early form of activation that has found a modern resurgence is the monopusher chronograph; as the name suggests, there is only one button to handle starting, stopping and resetting. All well and good unless you need to measure split times by continuing the chronograph after taking a reading, which a monopusher can’t do. For that the more common dual-pusher setup was developed, with one pusher to start and stop and another for resetting. Today this is the most common form of chronograph.
There are a variety of chronograph sub-categories that offer different functions and complications. The most common is the standard chronograph that measures elapsed time on different subdials. Some more complicated pieces, like the IWC DaVinci chronograph, can measure hours and minutes together on a single subdial that looks like a second time zone while the chrono is running. A regatta timer is a reverse chronograph – rather than measuring elapsed time, it counts down from a set point (usually ten minutes maximum) to determine starting times in a yacht regatta. A rattrapante is similar to a standard chrono but adds a split-second hand that is stopped by pressing a third button. The split hand stops while the main seconds hand continues running, pressing the button again makes the split hand catch up to the main hand (hence “rattrapante”). A flyback is a chronograph that can be instantly reset without stopping the chronograph, at which point it will immediately begin timing again. A foudrayante show real-time split seconds with a rapidly spinning hand that makes a revolution every second. Any of these complications may be combined or supplemented by calendar mechanisms or GMT functions. The most complicated watches in the world are often chronographs that have perpetual calendars as well as tourbillons or minute repeaters. As you add more functions the movement becomes exponentially more complicated to produce and adjust, and the prices of highly complicated chronographs reflects this.
Another distinction to be made is the layout of the chronograph. The classic chronograph layout is bicompax (two register) and tricompax (three register) – bicompax laid out at 3 and 9 o’clock, tricompax at 3, 6 and 9 o’clock. For many decades these were the standard layouts, and almost all handwound chronograph movements featured this layout, but the rise of the Valjoux 7750 as the most popular chronograph calibre has seen the introduction of a rotated layout where the subdials are at 12, 9 and 6 o’clock. There are a few oddball movements (notably from Omega and Eberhardt) that feature more than three subdials, but normally three (seconds, minutes, hours) is the maximum you will encounter.
Something seen on many chronographs is the tachymetre scale (see the Rolex Daytona above for an example). A tachymetre is a fixed scale showing units of speed per hour; it will either be printed on the chapter ring of the dial, or on an external bezel surrounding the crystal. To measure speed, you start the chrono at the beginning of a flying mile or kilometre, then stop it at the end of the mile. The second hand of the chrono will point to a number on the tachymetre – that is the average speed in miles per hour (for a flying mile) or kilometres per hour (for, you guessed it, a flying kilometre). Other scales have existed over time. One that was popular up to the 1950s was the telemetre, which functions like a tachymetre but is used to measure distance by sound (example, start the chrono when you see a lightning flash and stop when you hear the thunderclap. The measurement will be you distance from the lightning strike). The oddest and rarest scale is for pulsations, designed for doctors to measure a patient’s pulse. These are scaled to multiply the seconds by the beat – you start the chrono, count 15 beats, and then stop. The number indicated is beats per minute.
The chronograph is one of the most popular complications in the market today. I use my personal chronograph as a pasta timer, a speed indicator, and a parking-meter reminder. The astronauts of Apollo 11 used a mechanical chronograph to time their descent onto the surface of the moon; using a chronograph to time their loop around the dark side of the moon saved the crew of Apollo 13. From cooking to space missions, the chronograph is one of the most useful complications out there. Many people take the chronograph for granted, choosing it for its appearance of complexity – under the skin the chronograph is a remarkable complication with a long history that deserves more appreciation.
Stay tuned for more Watches in Depth articles here on the Baily Blog.