PISTON ROD DRIVE MECHANISM

The present invention provides a piston rod displacement mechanism for a pen injection device, comprising: a tubular housing (2) extending along a reference axis and comprising an interior housing surface (3) with a first engagement structure (4), a nut member (5) fixed within the tubular housing (2), a piston rod (15) comprising a first non-self-locking thread (18) having a first thread direction and a first thread pitch and being engaged with the nut member (5), and a second non-self-locking thread (17) superposed on the first non-self-locking thread (18), the second non-self-locking thread (17) having a second thread direction and a second thread pitch, and a dose dial sleeve (20) operable to advance the piston rod (15) in the nut member (5), the dose dial sleeve (20) comprising an exterior sleeve surface (21) with a second engagement structure (24) being in sliding engagement with the first engagement structure (4), and an interior sleeve surface (23) with a third engagement structure (29) being engaged with the second non-self-locking thread (17). One of the first engagement structure (4) and the second engagement structure (24) comprises a closed-circuit track configuration and the other of the first engagement structure and the second engagement structure comprises a track follower, and the closed-circuit track configuration comprises a helical track portion (24h) having a helical track direction equalling the second thread direction and a helical track pitch equalling the second thread pitch, and an axial track portion (24a) extending between a distal track end (24d) and a proximal track end (24p) and being connected to the helical track portion (24h) at the distal track end (24d). La présente invention concerne un mécanisme de déplacement de tige de piston pour un dispositif d'injection pour stylo, comprenant : un boîtier tubulaire (2) s'étendant le long d'un axe de référence et comprenant une surface de boîtier intérieure (3) avec une première structure de mise en prise (4), un élément d'écrou (5) fixé à l'intérieur du boîtier tubulaire (2), une tige de piston (15) comprenant un premier fil non autobloquant (18) ayant une première direction de filetage et un premier pas de filetage et étant en prise avec l'élément d'écrou (5), et un second fil non autobloquant (17) superposé sur le premier fil non autobloquant (18), le deuxième fil non autobloquant (17) ayant une seconde direction de filetage et un second pas de filetage, et un manchon de sélecti