Podcast How Watches Work E5 – Understanding Mechanical Chronographs

David takes Allen inside the fascinating world of mechanical chronographs, and Allen finally “gets it.”  Vertical vs. horizontal clutches, column wheel vs. cam actuators, reset functions, and much more are covered here, leaving the listener with a foundational understanding of mechanical stop watches.

The chronograph is a watch complication that allows the wear to record elapsed seconds, parts thereof, and possibly hours and minutes depending on the exact features of the watch. The term comes from the greek roots chrono meaning time and graph meaning to write. An abundantly useful complication, it can be used to time your steak, your boiled egg, your motor race or help you navigate on the sea, in the air and even in space.

The modern chronograph was invented in 1815 by French watchmaker, Louis Moinet, who set out to make a watch for astronomy. The pocket watch he created and called his ‘Compteur de Tierces’ or ‘Counter of Thirds’ used an ultra-high-frequency balance beating at 216,000 vibrations per hour. That’s 10 times faster than a modern Seiko… in 1816! Because of this high frequency, the watch could measure elapsed time to the nearest 1/60th of a second.

Pressing the crown started Moinet’s chronograph running and the large sweep hand rotateed once per second (remember it is measuring  down to 1/60th of a second). An additional subdial and smaller hand recorded the elapsed seconds. Releasing the crown stopped the chronograph and depressing a second button reset it back to 0.

As Moinet observed the stars and planets traveling across his telescope’s viewpoint, he would measure the time the heavenly body took to cross between two crosshairs on the viewer. Recording the day-to-day timing variations for the same object crossing his telescope viewport, he could draw conclusions on the relative positions of nearby celestial bodies and even the gravitation effect of other unseen objects.

The chronograph complication made its way into wristwatches care of Longines with the 13.33Z movement in 1913. The 13.33Z features a horizontal clutch and a column wheel, terms we will describe later, and therefore looks very much like many more recent chronograph movements such as can be seen in Speedmasters and Tag Heuer chronographs. Most chronographs, then and now, follow the convention of having the start/stop button at 2 o’clock and the reset button at 4 o’clock.

The problem all chronograph complications need to solve in order to act as a stopwatch is the same. You will remember from episode 2 of How Watches Work, that the going train of the watch is running continuously and provides the rotation needed for a second hand to turn. Therefore the issue is how to start and stop this hand which is normally solidly connected to the constantly running gear train. The going train cannot be stopped as it drives the balance and the escapement which is the time-keeping part of the watch. If the going train is stopped, the time keeping aspect of the watch will loose time. This is why stop watches are simpler than chronographs. A chronograph must be a stop watch AND continue to tell time precisely.

So, a way must be found to connect and disconnect the second hand from the train. There are two approaches, the first uses a horizontal clutch and the second, a vertical clutch.

With a horizontal clutch, an additional gear is placed on top of the fourth wheel. Remember, that the fourth wheel is the wheel in the gear train that rotates once per minute and typically drives the second hand on the dial side via an arbor (axle) that extends though the dial. By extending the arbor in the other direction and placing an additional wheel on top, the movement gains an additional wheel turning once per minute that can be used to divert drive to another part of the watch mechanism with a useful rotational velocity for counting seconds.

The next addition required is a central gear (known as the chronograph wheel) and arbor connected through the dial to the sweep hand. This is the hand that will start turning when the start button is pressed and stop when it is pressed again. The final part of the solution is an intermediate wheel to link the additional wheel on the fourth wheel abor to the chronograph wheel but only when the start/stop button is pressed. 

When a horizontal clutch chronograph is started, the intermediate wheel is moved so that it connects the wheel running on the fourth wheel arbor to the central chronograph wheel. Once connected, or meshed, the chronograph hand begins to turn driven by the going train and the sweep hand on the dial starts to count elapsed seconds.  When the button is pressed again, the intermediate wheel moves back and disengages from the chronograph wheel so that the sweep hand stops where it is. A brake is usually part of the mechanism to keep the hand in position once stopped. 

The horizontal clutch design has two minor disadvantages. The first is because the chronograph wheel is indirectly driven by gear train. An indirectly driven wheel is one that is not under constant torque from the mainspring and so when it moves, it tends to shake a little. The effect is usually small but often noticeable. It can be minimized using a spring but that increases friction and compromises the power reserve.   

The second disadvantage of the horizontal clutch is that there is a potential delay in the time between pressing the button and the intermediate wheel meshing with the chronograph wheel to start turning the hand. The teeth of the intermediate wheel are shaped into small triangles to minimize this delay by allowing the teeth to mesh as quickly as possible. The horizontal clutch always represents a slight compromising in the speed of time recording but that does not stop watch companies such as Patek Philippe using horizontal clutches in their high end chronographs.

The vertical clutch alleviates the first disadvantage of the horizontal clutch by ensuring that the chronograph wheel is always driven directly by the mainspring. Through the use of a vertical clutch within the chronograph wheel, the chronograph arbor is connected and disconnected from the gear train. The vertical clutch here is very similar to the clutch in an automotive manual transmission that disengages drive between the engine and the wheels so that gears can be changed. The design is similar although vastly smaller, with pressure plates and springs.

The fast acting springs in the vertical clutch also help with the second disadvantage of the horizontal clutch resulting in a sweep hand that starts and stops more precisely when the button is pressed and does not wobble as it turns. Of course, with advantages come disadvantages one is that the vertical clutch arrangement hides much of the chronograph mechanism from view below a large plate above the vertical clutch. Watches with vertical clutches are typically more expensive to service since the vertical clutch cannot usually be serviced and must be entirely replaced if the springs become loose or fail completely.