Patent ID: 8303007

Claim:
A drive mechanism comprising: a rotary drive source; a feed screw that transmits a rotary drive force generated from the rotary drive source to a displacement member that is displaceable in an axial direction; and a pressing member connected to the displacement member, that presses a workpiece by being displaced by the displacement member; the feed screw forming a self-locking mechanism, which prevents the displacement member from being displaced by a reactive force received by the pressing member from the workpiece when the rotary drive source stops generating the rotary drive force and the workpiece is pressed by the pressing member, wherein the self-locking mechanism is releasable by transmittal of the rotary drive force from the rotary drive source to the feed screw, and in the self-locking mechanism, where μ refers to a coefficient of friction of meshed portions between male threads disposed on the feed screw and female threads disposed on the displacement member that convert the rotary drive force into an advancing or retracting force of the displacement member, and θ refers to an angle between a direction of an axial force received by the feed screw when the workpiece is pressed by the pressing member and a direction of a normal force at the meshed portions between the male threads and the female threads, μ and θ are set such that the relational expression μ≧sin θ/cos θ is satisfied.