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.La présente invention concerne un mécanisme dentraînement qui convient à un dispositif dinjection, notamment un dispositif dadministration de médicaments de type stylo, et ce dispositif dadministration de médicaments. Le mécanisme comprend un élément dentraînement rotatif entraîné par un ressort (120), un élément entraîné rotatif (40), un embrayage (41, 121) couplant en rotation lélément entra
