Abstract: A control rod drive mechanism includes an outer tube, a guide tube disposed inside the outer tube, a ball tube, a hollow piston, and an electric motor. A gap between a ball traveling side of the screw shaft and the traveling balls and a gap between the traveling balls and a ball traveling side of the ball nut, having the gap coefficient (A) is given by: A=((2×R?L)?D)÷(2×R?L). The diameter of the traveling balls is determined to satisfy the expression given by A, in which A has the value of more than 0.03 and less than 0.17.

Abstract: A control rod drive mechanism includes an outer tube, a guide tube disposed inside the outer tube, a ball tube, a hollow piston, and an electric motor. A gap between a ball traveling side of the screw shaft and the traveling balls and a gap between the traveling balls and a ball traveling side of the ball nut, having the gap coefficient (A) is given by: A=((2×R?L)?D)÷(2×R?L). The diameter of the traveling balls is determined to satisfy the expression given by A, in which A has the value of more than 0.03 and less than 0.17.

Abstract: It is an object of the present invention to provide a gap expanding method of a ball screw for preventing an increase in torque relative to an electric motor due to a protruding bulge caused by generation of a scar on the traveling side of the screw shaft of the ball screw, and to provide a control rod drive mechanism incorporating the gap expanded ball screw. In a ball screw disposed in a control rod drive mechanism, the ball screw has a screw shaft and a ball nut engaged with the screw shaft via travelling balls, and the balls are each configured to have a diameter smaller than that of the existing ball, thereby expanding a gap between a ball traveling side of the screw shaft and the ball and a gap between the ball and a ball traveling side of the ball nut.

Abstract: It is an object of the present invention to provide a gap expanding method of a ball screw for preventing an increase in torque relative to an electric motor due to a protruding bulge caused by generation of a scar on the traveling side of the screw shaft of the ball screw, and to provide a control rod drive mechanism incorporating the gap expanded ball screw. In a ball screw disposed in a control rod drive mechanism, the ball screw has a screw shaft and a ball nut engaged with the screw shaft via travelling balls, and the balls are each configured to have a diameter smaller than that of the existing ball, thereby expanding a gap between a ball traveling side of the screw shaft and the ball and a gap between the ball and a ball traveling side of the ball nut.