A sheet feed mechanism includes a chassis for supporting a stack of sheets; a top sheet engaging member for engaging a top most sheet of the stack; a stack engaging structure for engaging the stack and biasing the top most sheet against the top sheet engaging member, the stack engaging structure hingedly connected to the chassis at a hinge axis; a friction surface extending from the stack engaging structure, the friction surface extending in a curvature parallel to a locus of the stack engaging structure about the hinge axis; a lock mechanism having a lock arm hingedly connected to the chassis at the hinge axis, the lock mechanism further having a biased contact foot for engaging the friction surface to retard a movement of the stack engaging structure about the hinge axis; and an actuator for engaging and disengaging the contact foot from the friction surface. The friction surface is arranged to extend beneath the biased contact foot, and the actuator is adapted to pivotally actuate the biased contact foot upwards and downwards to respectively disengage and engage the biased contact foot with the friction surface.