A mechanical watch movement of the type wherein a balance-cock supports a balance arranged to pivot in bearings about a fixed axis, and a balance-spring is fixed at its inner end to the balance and at its outer end to a bent arm of a thin, blanked plate constituting a support member detachable from the remainder of the movement.
The invention concerns a timepiece with mainspring of the so-called fly tourbillon type, the tourbillon having a balance bridge (2) forming a cage with a collet (1) and a single protruding ball bearing (5) supporting said so-called fly tourbillon assembly. Said so-called fly tourbillon assembly is visible on the dial side (15) and is designed as an independent tourbillon module. Said tourbillon module is separable from the clockwork (13) by being detached on the rear side of the clockwork (13) and by being removed on that same side.
The collet is formed by a metal band (10) whose inner contour (11) delimits the recesses (11a, 11b, 11c) for driving the collet onto a balance staff (2) and whose outer contour (12) includes a function point (4) between the collet and the balance spring (9) located at the end of an arm (14) at a distance R from the centre 0 of the staff (2) greater than that of any other point (6, 8, 13, 15, 17) of the outer contour (12). It is characterised in that the inner contour (11) includes a discrete number of points of contact (1, 3, 5, 7) with the staff (2) distributed along identical or different angular apertures .alpha. and in that width "I" of the band (10) varies such that the compression forces of the contact points (1, 3, 5, 7) on the staff (2) do not substantially alter the distance R after driving in, leading to a friction torque allowing angular orientation of the collet on the staff (2) and not introducing any unbalance.
