Mechanical principles → Basic Principles

COUNTING AND TRANSMISSION ORGAN

Its role is to transmit the energy received directly from the power source (the barrel) to the regulating organ (the balance and hairspring) via the escapement (transmission). Using the selected oscillation frequency of the regulating organ and the calculated gear ratios, each wheel in the gear train rotates at a specific, defined, and precise speed. This precision enables the watch to measure and display the various indications on the dial (hours, minutes, seconds, etc.), ensuring that each is driven at the correct rate (counting).

The counting and transmission mechanism consists of various wheels (mobiles).

Each mobile comprises a steel pinion, whose teeth are known as leaves, and a wheel (typically made of brass) with teeth on its circumference. The pinion forms the axis of the mobile, and the wheel plate is riveted to it, creating a solid assembly.

When the pinion completes one full revolution, the wheel attached to the same mobile also completes one full revolution.

Depending on the construction of the movement, the number of mobiles in the gear train can vary, typically between four and six.

  • The centre wheel mobile is driven directly by the barrel and usually carries the minute hand (1 revolution per hour).

  • The second wheel mobile serves as an intermediate mobile, enabling the correct gear ratios across the kinematic chain.

  • The third wheel mobile usually sits at the opposite end of the gear train. It sometimes carries the seconds hand, rotating 60 times faster than the centre wheel to make one full revolution every minute.

  • The escape pinion. Although it forms, together with the escape wheel the escapement mobile, only the pinion is still considered part of the counting and transmission mechanism. The escape wheel itself belongs to the distribution organ (the escapement).

Multiplicative Gear Train

The finishing train is a multiplicative gear train, which means that, in the kinematic chain, each wheel rotates faster than the one preceding it, according to the driving-wheel/driven-pinion principle. Each wheel drives the pinion of the following wheel.

Gear Ratios

To illustrate, let us take the example of a wheel with 100 teeth meshing with a pinion having 10 leaves.

When 10 teeth of the wheel have engaged with the 10 leaves of the pinion, the wheel will have completed one-tenth of a revolution, while the pinion will have completed a full revolution. This corresponds to a gear ratio of 10 (cf. “Counting and Transmission Train” calculations tab).

Rotational Speed of the Wheels

Taking into account the frequency of the regulating organ, it is easy to determine the rotational speed of each wheel in the gear train by calculating the appropriate gear ratios (cf. “Gear Train Calculations” tab). The centre wheel, usually located at the centre of the mainplate, typically drives the minute hand directly. The train is therefore calculated so that the centre wheel makes one revolution in 60 minutes. Similarly, the third wheel often carries the seconds hand. In this case, the train is calculated so that the third wheel makes one revolution in 60 seconds, and the gear ratio between the centre wheel and the third wheel is thus 60 (the third wheel rotates 60 times faster than the centre wheel).

Positioning of the Wheels

Provided that they mesh correctly, the positioning of the wheels on the mainplate is subject to no strict rule. While it is known that aligning the arbors of the various wheels along a straight line ensures optimal distribution of forces and maximum efficiency, this construction is rare due to the spatial constraints it imposes.

The centre wheel is generally placed at the centre of the mainplate to carry the minute hand. The third wheel, which may carry a seconds hand, can be positioned so as to display the seconds, for example, at 6 o’clock or at 9 o’clock. Thus, the arrangement of the wheels in the gear train allows the layout of the dial display to be managed.

Efficiency

The gear train is subject to friction in the gear teeth and in the pivots within their bearings (jewels). It is generally considered that the gear train (counting and transmission organ) dissipates around 30% of the mainspring barrel’s nominal energy. The escapement (distribution organ) also absorbs about 30% of the energy, so that only about 30% of the mainspring barrel’s nominal energy reaches the regulating organ.

It is therefore essential to minimise the friction of the gear train as far as possible. The choice and combination of materials (brass wheels, steel pinions, corundum jewels), the finishing of the components (polishing of the teeth, burnishing of the pivots), tooth profiles designed to favour rolling friction rather than sliding friction, and of course, perfect lubrication, all help reduce the gear train’s friction to a minimum.