Technique → Movement

THE CENTRE WHEEL

The centre wheel belongs to the counting and transmission organ, of which it is the first element in the gear train.

The centre wheel is one of the fundamental organs of the gear train in a mechanical watch. It takes its name from its usual position at the centre of the mainplate (and thus of the movement). Its arbor passes through the movement to receive, on the dial side, the minute hand via the cannon pinion. Its function is to transmit energy from the barrel to the subsequent wheels while providing the main time indication: the minutes and, indirectly, the hours.

Components

A centre wheel traditionally consists of a steel pinion and a brass wheel blank:

  • Centre wheel: Large in diameter, it occupies the central position of the movement. Its tooth count is calculated to establish the correct reduction ratio with the barrel and to ensure one full revolution per hour. The centre wheel (driving) engages the second pinion.

  • Centre pinion: Made of steel, it forms the arbor of the wheel. It is driven (driven element) by the toothing of the barrel. Its leaves are precisely cut to guarantee smooth running and minimise friction.

Materials

Traditionally, the centre wheel is made of gilded brass. The pinion is made of hardened, mirror-polished steel to improve its mechanical resistance. The pivots are burnished (surface work-hardened). They run in synthetic jewels (rubies) and are lubricated to reduce friction and wear.

The combination of a brass wheel with a steel pinion is optimal for minimising friction in the gear trains of a movement. This principle applies to most multiplicative and reducing trains in a watch (gear train, setting train, etc.).

Technical features

The wheel blank is firmly riveted to the pinion, creating a perfectly solid unit.
The toothing ratio of the centre wheel, and of the entire gear train, is calculated to ensure that the centre wheel completes one full revolution in sixty minutes — an essential condition for directly driving the cannon pinion and the minute hand and, by further reduction, the hour hand.

Role in the movement

The centre wheel is both a transmission and a display (counting) organ:

  • Transmission: It directly relays the energy from the barrel to the rest of the gear train (second wheel, third wheel, and escape wheel).

  • Display (counting): Its arbor, passing through the movement, carries the cannon pinion and the minute hand, forming the central axis around which the entire time display is organised.