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THE COUNTING AND TRANSMISSION MECHANISM

(The finishing Train)

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Video: Kinematics of the Counting and transmission mechanism (finishing Train)

1. Role

The Counting and transmission mechanism of a mechanical watch is the finishing Train.

Its role is to convey the energy received directly from the Motor organ (power source) (Barrel) to the Regulating organ (balance-hairspring) via the Escapement (distribution). Thanks to the chosen frequency of the regulating organ and the calculation of the gear ratios, each wheel (train) turns at a different, defined and precise speed. This quality makes it possible to distribute and display the various pieces of information on the Dial (hours, minutes, seconds, etc.) and to drive each of them at its own speed (counting).

2. Description and components

The counting and transmission mechanism is made up of several wheels.

Each wheel consists of a pinion in Steel, whose teeth are called “leaves”, and a wheel (generally in Brass) also toothed on its circumference. The pinion represents the axis of the wheel and the wheel’s plate is riveted to it in a fixed manner.

When the pinion makes one complete revolution, the wheel of the same wheel set also makes one complete revolution.

Depending on the construction of the Movement, the number of wheel sets in the going train may vary (generally between 4 and 6).

The Center wheel (sometimes called the great third wheel), driven directly by the Barrel and generally carrying the Hand of the minutes (1 rev/h).

The Third wheel (sometimes called the small third wheel) is an intermediate wheel set enabling the correct gear ratios to be obtained across the entire kinematic chain.

The Fourth wheel (seconds wheel) which is generally placed at the other end of the finishing train, carries (sometimes) the seconds hand and therefore rotates 60 times faster than the center wheel in order to complete one full rotation in exactly one minute.

The Escapement pinion. Although together with the Escapement wheel it forms the Escapement wheel assembly, only the pinion still belongs to the Counting and transmission mechanism. The Escapement wheel, for its part, belongs to the Distribution mechanism (the Escapement).

3. Operation

Multiplicative Train

The finishing train is a multiplicative 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 driving the pinion of the following wheel.

Gear ratios

Click on the animation to enlarge it

To understand, let’s take the example of a wheel with 100 teeth meshing with a pinion having 10 leaves.

When 10 teeth of the wheel have meshed with the 10 leaves of the pinion, the wheel will have made one-tenth of a turn, while the pinion will have made one full turn. From this point, we speak of a gear ratio of 10 (see the calculations tab of the Counting and transmission mechanism)

Rotation speed of the wheels

Taking into account the Frequency of the Regulating organ, one can easily determine the rotation speed of each wheel of the finishing Train (wheel train) by calculating the appropriate gear ratios (see the calculations tab of the finishing Train (wheel train)). Thus, the Center wheel, generally placed at the center of the Main plate, usually directly drives theHand of minutes. The train is therefore calculated so that the Centre wheel makes one revolution in 60 minutes. Similarly, the Fourth wheel (seconds wheel) frequently carries the seconds hand. In this case, the train is thus calculated so that the seconds wheel makes one revolution in 60 seconds, and the gear ratio between the Center wheel and the Fourth wheel (seconds wheel) is therefore 60 (the seconds wheel turns 60 times faster than the Center wheel).

Pointing (positioning) of the wheels

As long as they mesh correctly with each other, the positioning of the wheels on the Main plate is not subject to any particular rule. Although it is known that positioning the Axis of the various wheels along a straight line guarantees an ideal distribution of forces and optimal efficiency, this construction is rare due to the space constraints it imposes.

The Center wheel is generally placed at the center of the Main plate in order to carry the minute Hand. The Fourth wheel (seconds wheel), which may carry a seconds Hand, can be positioned so as to display, as desired, the seconds at 6 o’clock or at 9 o’clock, for example. Managing the positioning of the Train (wheel train) of the going train thus makes it possible to manage, in particular, the distribution of the Display on the surface of the Dial.

Efficiency

The going train is subject to friction from the gears and pivots in their bearings (jewels). It is generally considered that the going train (Counting and transmission mechanism) dissipates 30% of the Barrel’s nominal energy. The Escapement (Distribution mechanism) also absorbs 30% of the energy, so that only 30% of the Barrel’s nominal energy reaches the Regulating organ.

It is therefore essential to minimize as much as possible the friction of the finishing train. The choice and combination of Materials (wheel in Brass, pinion in Steel, jewels in corundum), the finishing of the components (polishing of the teeth, rolling of the pivots), the profile of the teeth, favoring rolling friction rather than sliding friction, and, of course, perfect lubrication make it possible to reduce the friction of the finishing train to a minimum.

4. Calculations of the Counting and transmission mechanism