The great divide in chronographs is between cam chronograph movements, in which the pushers rotate a cam (or coulisse) which in turn moves a series of levers, and column wheel chronographs, in which the pushers rotate the column wheel which then operates the levers. The column wheel provides smoother pusher operation and is the higher-end option. It’s a convenient way of moving several levers simultaneously. Rotating the column wheel through a minimal angle causes the spring-loaded levers to fall into the gaps, or to be pushed out and rest on the columns. Why several levers? You need one to start and stop the chronograph mechanism, one to brake the chronograph seconds gear when you stop the chronograph, and (in a mono-pusher chronograph) one to reset the hands. The positions of the column wheel are determined by a set of teeth under the column wheel that interact with a detent spring. Each time you press the start-stop pusher, you rotate the column wheel one step, and the cllick that you feel in the pusher is compressing and releasing the detent spring. Below, detail of the A. Lange & Söhne Datograph.
Amongst column-wheel chronographs, there is another differentiation, between horizontal and vertical clutch control. In a column-wheel chronograph with a horizontal clutch, when the chronograph is running, the central chronograph seconds hand is powered by the fourth wheel, the gear driving the seconds hand of the watch. They are linked by an intermediate gear, mounted on a bridge that can swing so that that it meshes with the chronograph seconds gear to start the chronograph, and unmeshes when you stop it. The horizontal clutch is not really like a clutch at all. It’s more like a crash gearbox in a car. The chronograph seconds gear often has a very fine tooth profile designed to facilitate meshing, but even so, every so often when you start the chronograph, the teeth collide and cause a slight judder in the chronograph seconds hand as it starts, or a slight delay. Below, a column-wheel chronograph with horizontal clutch by Audemars Piguet.
In the detail below, you can see the fourth wheel at top right, which drives the continuous seconds, and which meshes with the intermediate gear below. When the start pusher of the chronograph is pressed, the dark semicircular bridge swings fractionally left so that the teeth of the intermediate gear mesh with the very fine teeth of the chronograph seconds gear on the left. The column wheel is at bottom right.
There is another downside to the horizontal clutch: the fine teeth of the chronograph seconds gear are not a perfect match to the teeth of the intermediate wheel, and so the system absorbs more energy than a normal gear train. As a result, when the chronograph is running the amplitude of the balance wheel’s swings decreases by as much as 30°, and this can cause a change in the precision of the watch. So it’s not the sort of chronograph you want to leave operating indefinitely, partly because the system is more subject to wear. The horizontal clutch has a visual advantage: the mechanism can be displayed through the caseback, or on the dial side as in the Louis Moinet Memoris. You see everything that is going on.
The vertical clutch on the other hand is more like the clutch in a car. The seconds gear and the chronograph seconds gear are one on top of the other. When the chronograph is started, they are pushed together, and rotate together, held by friction. They effectively form a single wheel. Below, the Rolex Daytona vertical clutch.
There are several advantages with respect to the horizontal clutch. There is virtually no extra energy absorption when the chronoograph is running, and in some watches of this type, the balance wheel amplitude actually increases slightly. So, no detrimental effect on power reserve. There are no problems with extra wear. So you can leave the chronograph running all the time. This is useful in chronographs like the Piaget Polo S which doesn’t have a continuous seconds hand. In chronographs with a combined hour and minute totalizer, such as the IWC Pilot’s Watch Chronograph Top Gun the chronograph can be started at an appropriate time and left running to provide a second time zone.
The disadvantages of the vertical clutch are that it is more demanding to make, so it’s more expensive; it’s harder to perform maintenance; and the mechanism is pretty much invisible when seen through the caseback. As it consists of a series of discs stacked in the vertical plane, it is possibly more difficult to incorporate into an ultra-thin movement. Below, part of the Patek Philippe CH 28-520 HU movement with vertical clutch, used in the World Time Chronograph Ref. 5930.