The invention relates to a drive mechanism for use in a drug delivery device for dispensing a medicinal product comprising: a housing (154) having an external thread (150)； a piston rod (20") having a generally circular section； a dose dial sleeve (70"'), which is threadedly engaged with the external thread (150) of the housing (154) and being rotatable with respect to the housing (154)； a drive sleeve (30'), located between the housing (154) and the piston rod (20"), a clutch means (60") located between the drive sleeve (30') and the dose dial sleeve (70"'), in which e) when the dose dial sleeve (70"') and the drive sleeve (30') are de-coupled by the said clutch means (60"), rotation of said dose dial sleeve (70"') with respect to the said drive sleeve (30') is allowed； and f) when the dose dial sleeve (70"') and drive sleeve (30') are coupled by the said clutch means (60"), rotation of the dose dial sleeve (70"') with respect to the said drive sleeve (30') is prevented, A nut (40') is located between the drive sleeve (30') and the housing (154), the nut (40') having a thread that is engaged with a thread of the drive sleeve (30') and the nut (40') and the housing (154) being keyed together by means of longitudinally directed splines to prevent rotation between the nut (40') and the housing (154) while allowing relative longitudinal movement therebetween.
