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