DISTANCE OF CENTRES

The distance of centres is the distance separating the centres of rotation (the axes) of two neighbouring components, in particular of two gears meshing with one another. In horology, it is a fundamental geometric parameter of the train (wheel train): the distance of centres between a wheel and the pinion it drives must correspond precisely to the sum of the pitch radii of the two toothings. A distance of centres that is too large separates the teeth, reduces the meshing depth and encourages disengagement; too small, it causes the toothings to “bottom out”, creates hard spots, and increases friction and wear. Respecting the distance of centres, to within a few microns, therefore determines the efficiency, smoothness and reliability of the transmission, and by extension the Amplitude of the Balance. The distances of centres are set by the position of the pivot holes in the Main plate and bridges, hence the extreme requirement for the drilling and positioning of these holes (and jewels). During a Testing or a strip-down inspection, checking the gearing involves checking the distances of centres and the play. It is a cornerstone of the construction of the Movement.