chin strap 3 cam 10 is rotatably mounted on shell 1 by axis 16 drive part 19 is formed joined to the side of chin strap 3 which in this case is roughly circular even though this shape is not restrictive and shows a displacement track 14 for a drive body 13 joined to cam 10 this part 19 can be added to chin strap 3 or form a single part with the latter which it then forms part of track 14 has a radial part 14 a which when the drive device 13 is positioned in this part allows part 19 to apply a drive force on cam 10 track 14 also has a tangential part 14 b in which by tangential orientation of the track relative to the direction of rotational movement of part 19 no effort is transmitted to drive device 13 thus cam 10 is driven on a first phase of raising chin strap 3 when drive body 13 is present in radial part 14 a of displacement track 14 of part 19 during a subsequent raising phase after the continued rotation of part 19 body 13 reaches tangential part 14 b of displacement track 14 of part 19 which causes no further rotation of cam 10 cavity 15 is arranged in cam 10 so as to form a wall for the application and guidance of upper stop 17 and lower stop 18 executed in relief at the two ends of oblong hole 9 of lateral support 8 the coacting of stops 17 18 and cavity 15 ensures the position of the side support 8 changes relative to rotational axis 5 in fig1 rotational axis 5 is at the level of the upper stop 17.