Drive mechanism for an injection device

摘要

A drive mechanism for an injection device is disclosed in which piston means are selectively driven to expel medicament from within a medicament cartridge. The drive mechanism comprises a drive means (270), first means (278) associated with the drive means, second gear means (274) being moveable between a first position in wich only movement along its longitudinal axis is permitted and a second position in which the second gear means (274) is free to rotate about the longitudinal axis, the piston means (280) being associated with the second gear means (274), a housing (276) within which the first gear means (278) and the second gear means (274) are constrained for movement, and biassing means (282) located between the housing (276) and a second gear means (274) to bias the second gear means (274) to the first position.