The present invention relates to a drive mechanism which is suitable for an injection device (1； 101), especially a pen type drug delivery device. The mechanism comprises a housing (4, 5； 104, 105), a dial member (6； 106), a torsion spring (12； 112) fixed between the housing (4, 5； 104, 105) and the dial member (6； 106) such that energy is accumulated in the torsion spring (12； 112) upon rotation of the dial member (6； 106) relative to the housing (4, 5； 104, 105), a drive member (8； 108) engaging a lead screw (9； 109), a first clutch (18； 118) rotationally coupling the drive member (8； 108) and the housing (4, 5； 104, 105) in a coupled state and allowing relative rotation between the drive member (8； 108) and the housing (4, 5； 104, 105) in a de-coupled state, and a second clutch (19； 119) rotationally coupling the drive member (8； 108) and the dial member (6； 106) in a coupled state and allowing relative clockwise and anti-clockwise rotation between the drive member (8； 108) and the dial member (6； 106) in a de-coupled state. The second clutch (19； 119) comprises a ratchet clicker with teeth (19a, 19b； 119a, 119b) having in the clockwise and the anti-clockwise direction different ramped tooth angles such that the teeth (19a, 19b； 119a, 119b) are allowed to override each other in the de-coupled state of the second clutch (19； 119) with a different resistance in the clockwise and the anti-clockwise direction.本發明有關一驅動機構，其係適合用於一注射裝置(1；101)、尤其一筆型藥品輸送裝置。該機構包括一外殼(4、5；104、105)；一刻度盤構件(6；106)；一扭轉彈簧(12；112)，被固定於該外殼(4、5；104、105)及該刻度盤構件(6；106)之間，使得當該刻度盤構件(6；106)相對該外殼(4、5；104、105)旋轉時，能量被蓄積於該扭轉彈簧(12；112)中；一驅動構件(8；108)，嚙合一導螺桿(9；109)；一第一接合器(18；118)，在一耦接狀態中旋轉式耦接該驅動構件(8；108)與該外殼(4、5；104、105)，且於一退耦狀態中允許該驅動構件(8；108)與該外殼(4、5；104、105)間之相對旋轉；及一第二接合器(19；119)，在一耦接狀態中旋轉式耦接該驅動構件(8；108)與該刻度盤構件(6；106)，且於一退耦狀態中允許該驅動構件(8；108)與該刻度盤構件(6；106)間之相對順時針及逆時針方向旋轉。該第二接合器(19；119)包括一具有齒部(19a、19b；119a、119b)的棘輪鎖閂器，該等齒部在該順時針及該逆時針方向中具有不同的傾斜齒部角度，使得該等齒部(19a、19b；119a、119b)被允許在該第二接合器(19；119)之退耦狀態中彼此重疊，而於該順時針及該逆時針
