Mechanical principles › Complications › Display

RETROGRADE DISPLAY

Description

Unlike a traditional analog display, where the hand moves in a continuous circular motion, the scale of a value indicated by a retrograde display only occupies a segment of a circle.

When the hand of a retrograde display reaches the maximum value of its scale (e.g.: the retrograde date hand on July 31), it returns in the opposite direction, in an almost instantaneous motion, to the minimum value of its scale (to the 1st of the 1^st August for the date hand in our example). Retrograde display can apply to many indications (seconds, minutes, hours, date, days, etc.).

Several retrograde displays are thus frequently found in a single watch. This type of display allows information to be harmoniously distributed on a dial while often saving space on its surface.

Construction & Operation

Whatever the function to be displayed, the kinematics of the mechanism begins with a wheel, a star (wheel), rotating at the reference speed of the function to be displayed. For example, for a retrograde indication of the day of the week, this would be the 7-tooth star (wheel), making one revolution per week. For a date indicator, this would be the 31-tooth star (wheel), making one revolution per month. And for a hand for the retrograde seconds, it will be the seconds wheel (1), making one revolution in 60 seconds.

Whatever it may be, this first kinematic element carries on its axis the cam of the retrograde (2), whose shape evokes that of a snail shell. Let us imagine that we want a retrograde seconds hand, the retrograde cam will therefore make one revolution in 60 seconds. The rack’s feeler (3) is a lever that is kept in contact with the cam by a leaf spring (4) or a hairspring (balance spring).

The shape of the cam during its rotation drives the lever in a rotary motion of a limited angular value. Then, every minute, the rack falls instantly from the highest point to the lowest point of the cam. It thus travels the same angular value as that of its progression on the cam, but in the opposite direction.

Opposite the pivot point of the rack is the toothed sector that gives it its name. Its teeth mesh with the retrograde seconds pinion (6). An intermediate wheel (5) can be inserted between the rack and the retrograde seconds pinion in order to adapt the direction of the display.

The angular displacement of the rack and the gear ratio chosen between the teeth of the rack and those of the retrograde pinion make it possible to determine the angular value of the display (7).