The present invention relates to a drive mechanism which is suitable for an injection device, especially a pen type drug delivery device and to such a drug delivery device. The mechanism comprises a spring driven rotatable drive member (120), a rotatable driven member (40), a clutch (41, 121) rotationally coupling the driven member (40) and the drive member (120) in a coupled state and allowing relative clockwise and anti-clockwise rotation between the driven member (40) and the drive member (120) in a decoupled state, and a spring (130) biasing the clutch (41, 121) into its coupled state and allowing relative axial movement between the drive member (120) and the driven member (40) into the decoupled state of the clutch () against the bias of the spring (130). The clutch comprises a first ring of crown teeth (121) on the drive member (120) and a second ring of corresponding crown teeth (41) on the driven member (40) with each crown tooth (41, 121) having in the clockwise and the anti-clockwise direction different ramped tooth angles such that the teeth (41, 121) are allowed to override each other in the de-coupled state of the clutch with a different resistance in the clockwise and the anti-clockwise direction. The material of the drive member (120) may be polybutylene terephthalate (PBT) or polyoxymethylene (POM) and the material of the rotatable driven member (40) may be PBT, POM or polycarbonate (PC). Further, the coefficient of friction (μ) between the teeth (41, 121) may be between 0.07 and 0.13 and the teeth (41, 121) may have a surface roughness (Ra) between 0.5 and 1.2.本發明涉及一種適用於注射裝置，尤其是筆型藥物輸送裝置的驅動機構，並涉及這種藥物輸送裝置。驅動機構包括彈簧驅動的可旋轉驅動構件(120)、可旋轉從動構件(40)、在聯結狀態下將從動構件(40)和驅動構件(120)在轉向上聯結而在分離狀態下允許從動元件(40)和驅動元件(120)之間順時針和逆時針相對旋轉的離合器(41，121)以及彈簧(130)，彈簧(130)將離合器(41，121)偏置到其聯結狀態，並允許驅動構件(120)和從動構件(40)之間相對軸向運動，以克服彈簧(130)的偏向力進入離合器(41，121)的分離狀態。離合器包括驅動構件(120)上的第一冠齒環(121)和從動構件(40)上對應的第二冠齒環(121)，沿順時針和逆時針方向每個冠齒(41，121)具有不同的斜坡齒角度，使得這些齒(41，121)被允許在離合器的分離狀態下相互越過，沿順時針方向和逆時針方向具有不同
