As discussed previously, the coupling clutch is the main interface between the chronograph train and the regular train of wheels. When the user engages the chronograph, the pillar wheel or cam will cause the coupling clutch to move it’s position so that it’s intermediate wheel engages with the Chronograph Runner wheel. Until the chronograph is engaged, the Coupling Clutch will be held in it’s home position under tension with a spring.
So let’s examine the movement of the Coupling Clutch in detail. First of all, please observe that the pivot point of the coupling clutch is here. Please note that this is not a screw and should not be touched when disassembling the chronograph. It’s an eccentric and its used to adjust the tooth pitch between the Driving Wheel and the coupling clutch wheel and we shall discuss chronograph adjustments in another lesson.
The coupling clutch is secured with one screw here which is shouldered to allow free movement during operation.
The coupling clutch is held under tension by a spring. The force from the spring causes the coupling clutch to bear in the direction of the chronograph runner wheel at all times.
At the opposite end of the pivot point, the coupling clutch has an extended leg which interacts with the pillar wheel.
When the chronograph is disengaged, the tip of this extended leg will be resting against one of the pillars of the pillar wheel which prevents the coupling clutch wheel from further travel towards the chronograph runner wheel.
When the pillar wheel rotates by one division, the tip of the leg will drop between two pillars which allows the coupling clutch to move into an engaged position with the chronograph runner wheel.
Another eccentric here arrests the coupling clutch and allows for fine adjustment of the tooth pitch between the coupling clutch wheel and the chronograph runner.