The present invention relates to a medical delivery device comprising a housing (10) a medicament container (16) arranged to be placed in said housing a stopper (18) movable within said container a medicament dispensing means attachable to said housing and to said container through which medicament can be expelled characterized in that the device further comprises a power unit (22) comprising a cover (24), a volute spring (26) arranged within said cover, and a pressure member (28) having a pressure pad (30) in contact with the stopper, a guide rod (34) extending into the volute spring and an annular ledge (32) in contact with the edges of the volute spring activation means (36) mechanically connected to the cover and to the housing and a spring locking means mechanically connected to said activation means, wherein said spring locking means (39) comprises a deformable ring-shaped member having (40) an inner diameter somewhat larger than the annular ledge, such that said ring-shaped member presents an oval shape when it is in contact with the annular ledge for locking the volute spring in a tensioned state.
