Abstract: A screw actuator may comprise: a barrel; a piston rod movable in the barrel in an extending direction and in a retracting direction; an elongated screw in the bore of the barrel and rotatable by a motor drive; a nut coupled to the piston rod and movable in the bore of the barrel in the extending direction and in the retracting direction responsive to rotation of the elongated screw; and a plurality of lock segments in the piston rod and movable radially outward and radially inward in response to movement of the nut. The plurality of lock segments engage the barrel when moved radially outward and do not engage the barrel when moved radially inward, whereby the piston rod is lockable to the barrel by the plurality of lock segments for bearing loads applied thereto.