The cam chronograph is the most widespread control system in twentieth-century mechanical watchmaking, and still equips high-volume movements like the Valjoux 7750 and Omega Speedmaster's caliber 1861. The principle is straightforward: one or more cams — eccentric profiles fixed on a shaft — rotate when you press the pushbutton and translate that rotary motion into a precise sequence of actions: engaging the chronograph wheel, braking, and reset.
The substantial difference compared to the column wheel system concerns the geometry of control. The column wheel selects positions through alternating teeth and columns, guaranteeing a crisper feel at the pushbutton and millimetric synchronization between levers. The cam system achieves the same result with fewer components, less precise geometries to machine, and more generous manufacturing tolerances. Result: lower production cost, superior robustness under stress, more accessible maintenance. For this reason cam control dominated chronographs intended for professional and military use from the 1960s onward.
At the bench, you feel the difference. A well-regulated column wheel chronograph has an almost silent start and homogeneous pushbutton travel. A cam chronograph may have slight variable resistance along the travel, especially when the cams begin to show wear on their contact profiles. It's not a structural defect: it's the nature of the mechanism. With proper lubrication and intact cam profiles, the system functions for decades without service.
One aspect I often see undervalued: the reset. In cam systems the chronograph seconds hand returns to zero following the profile of the reset cam. If that surface is worn or the contact heel is deformed, the hand won't land exactly on zero. It's not an irreparable problem, but it requires component replacement, not simple adjustment. In column wheel systems the reset is mechanically more controlled, though not immune to similar issues.
The Valjoux 7750 and its direct derivatives — Sellita SW510, ETA 7750 in all variants — represent the modern reference for the cam system. Omega's caliber 1861, descended from the 321 through the 861, uses it in manual form. Hamilton's H-51-Si caliber is another recent example. None of these movements is inferior in reliability: they're different, and that difference has precise technical meaning.