Glossary · Terminology
Conical Geometry
Definition
In watchmaking, the term conical describes at least three distinct families of technical problems that share almost nothing in common except their shape. Confusing them is a frequent mistake, even among those who have written about watches for years.
The first family concerns motion transmission across non-parallel planes. A conical gear has teeth arranged on a cone rather than a cylinder: when two conical gears engage on intersecting axes, the motion changes direction. In watchmaking this becomes essential whenever the dial is inclined relative to the movement plane. The most discussed case is the MB&F Legacy Machine FlyingT, where the hours-minutes dial is oriented at 50° so it can only be read by the wearer: conical gears are the only way to carry the torque from the vertical movement to the angled display. The same logic appears in the LMX, the SP One and the Hautlence Sphere Series, where four conical gears on axes intersecting at 21° manage the multi-axis rotation of a jumping sphere. Without conical geometry, none of these displays would exist.
The second family is the conical tourbillon. Eric Coudray, working for HYT, revived an idea by Walter Prendel from 1928 and built a cage in which the balance spiral is inclined at 30° to the horizontal plane, the escape wheel at 15° and the lever at 23°. Complete rotation occurs in 30 seconds rather than 60. The stated objective is to compensate for gravity effects across multiple planes; whether it does this better than a traditional tourbillon is an open question, and honestly I don't have sufficient data to answer with certainty.
The third family is purely geometric: bezels, crowns and cases with profiles that taper toward the outer edge. From a functional standpoint, a conical bezel offers better finger grip than a flat bezel because the protruding edge serves as a handle. Visually, it accentuates the perceived thickness of the case. It should not be confused with knurling or fluting, which are surface textures independent of shape.
The conical jewel of the Greubel Forsey Hand Made 2 belongs to its own category: a synthetic ruby cut into a cone, hand-finished over approximately four months, which rotates and rises axially during winding while serving as a power reserve indicator. It is an example of how conical geometry can solve a three-dimensional visualization problem in an extremely confined space.
Frequently asked questions
Why does a movement use conical gears instead of traditional cylindrical gears?
Conical gears transmit motion between axes that intersect at an angle. They're necessary whenever a display is inclined relative to the movement's plane—like in the 50° dials of the FlyingT or in Hautlence's multi-axis jumping spheres. With cylindrical gears, you cannot change direction so compactly.
Does a conical tourbillon compensate for gravity better than a flat tourbillon?
Theory suggests yes, because it acts on multiple planes. In practice, chronometric comparison data between conical tourbillons and conventional tourbillons aren't published systematically. It's a legitimate question that, honestly, cannot be answered with certainty based on available information.
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