210. watch-knowledge


A cycloid reducer to display the hours and minutes [Video]

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A lecturer at the Haute Ecole Arc Ingénierie and his students have designed a new mechanical system for a complication watch movement. The idea is to replace some of the wheels and pinions of a mechanical watch with a cycloid reducer.

The cycloid reducer, invented in 1926 by the German Lorenz Braren and then patented in 1930, was initially used in heavy mechanics, in particular for crushing machines and the rotation of large parabolas. It comprises four principal elements: the input arbour, the disc-cam, the crown and the output arbour.

A cycloid reducer to display the hours and minutes

The principle

An eccentric is keyed onto the input arbour, the ring of which is fitted with cylindrical rollers. This assembly is set on ball bearings in the input flange.

There is a boring in the centre of the disc-cam, which fits over the rollers of the eccentric and around the edge of the disc-cam is a succession of cycloidal scallops, the number of which determines the reduction ratio. In the middle, there is a series of borings which connect with the output arbour.

The internal diameter of the ring-shaped crown bears a series of axes, being 1 more in number than that of the cycloidal scallops of the disc-cam. The inside of the output arbour is fitted with the reducer plate provided with pins enclosed in sheaths. It is these pin-and-sheath assemblies which engage in the scallops reamed around the edge of the disc-cam.

The primary movement applied to the input arbour and its eccentric causes the disc-cam to roll on the axes of the crown. As the number of axes is 1 higher than the number of cycloidal scallops of the disc-cam, the disc-cam moves back by one scallop (one step) at each rotation of the eccentric. The uniform rotational movement of the disc-cam is transmitted to the output arbour via the pin-and-sheath assemblies of the arbour plate. The reduction ratio can be considerable, 85:1 for a single stage.

Exceptional yield

The yield of the cycloid reducer is exceptional (95% per stage) and the transmission of the movement involves only bearings. There is no friction on the sides of the teeth! This advantage and the low inertia of the cycloid reducer makes it possible to use low power motors.

Professor Christian Robert had the idea of applying the principle of the cycloid reducer to a complication watch movement as part of the micro-technical design course for watchmaking, taught to the students at the Haute Ecole Arc Ingénierie.

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