A spring assisted drug delivery device (80, 90, 100) comprising - a housing, - a lead screw that is rotatably fixed with respect to the housing during dose setting and during dose delivery and that is axially moveable in a delivery direction relative to the housing for dose delivery, - a drive mechanism adapted and arranged to drive the lead screw in the delivery direction for dose delivery, - a spring, wherein the spring is arranged between two components (20, 23, 24, 25, 29) of the drive mechanism which are moveable with respect to one another and wherein, the spring is adapted and arranged to assist a user of the device (80, 90, 100) during a dose delivery operation such that a force the user must exert for performing the delivery operation is reduced, and characterized in that the device (80, 90, 100) further comprises a cartridge comprising a piston, wherein, for dose delivery, the piston is moveable with respect to the cartridge in the delivery direction by the lead screw, and wherein the components of the drive mechanism comprise - an inner sleeve which is axially moveable and rotatably fixed relative to the housing, - a mid-body which is axially fixed inside of the housing, - a drive nut, wherein the lead screw comprises a threaded shaft, wherein the drive nut is threadedly engaged with the threaded shaft, and wherein, for dose delivery, the drive nut is adapted to advance the lead screw with respect to the housing to dispense a fluid from the cartridge, - a dial link which is connected with the drive nut, wherein the dial link is axially moveable with respect to the drive nut and rotatably fixed relative to the drive nut, - a number sleeve which is threadedly engaged with the housing, wherein the spring is operatively coupled between the dial link and the drive nut, and wherein, for dose setting, the dial link is adapted to be screwed with respect to the housing in a direction opposite to the delivery direction for the first axial distance and the drive nut is adapted to be screwed with respect to the housing in the direction opposite to the delivery direction for the second axial distance, the second axial distance being smaller than the first axial distance, such that tension is created in the spring. - a housing, - a lead screw that is rotatably fixed with respect to the housing during dose setting and during dose delivery and that is axially moveable in a delivery direction relative to the housing for dose delivery, - a drive mechanism adapted and arranged to drive the lead screw in the delivery direction for dose delivery, - a spring, wherein the spring is arranged between two components (20, 23, 24, 25, 29) of the drive mechanism which are moveable with respect to one another and wherein, the spring is adapted and arranged to assist a user of the device (80, 90, 100) during a dose delivery operation such that a force the user must exert for performing the delivery operation is reduced, - an inner sleeve which is axially moveable and rotatably fixed relative to the housing, - a mid-body which is axially fixed inside of the housing, - a drive nut, wherein the lead screw comprises a threaded shaft, wherein the drive nut is threadedly engaged with the threaded shaft, and wherein, for dose delivery, the drive nut is adapted to advance the lead screw with respect to the housing to dispense a fluid from the cartridge, - a dial link which is connected with the drive nut, wherein the dial link is axially moveable with respect to the drive nut and rotatably fixed relative to the drive nut, - a number sleeve which is threadedly engaged with the housing,