INTERNAL CLAMP DEVICE

PROBLEM TO BE SOLVED: To improve the dimensional tolerance of a steel tube, to expand and straighten its flat shape and to enable an excellent circumferential welding by forming the bottom part of the plunger fixed a strap holder into an inclined surface and engaging a wedge rod cylinder with the inclined surface. SOLUTION: An internal clamp device 1 is butted to the rear part of the predetermined place of the circumferential welding in the steel tube 20 and advanced to the center of a backing 25. The cylinder of a clamp part 15 is extended and internal clamp device 1 is fixed inside the steel tube 20. A hydraulic pump 16 is driven to advance the wedge rod cylinder 2, whereby a plunger 4, abutted to a slide metal 3 screwed the inclined surface of the wedge rod cylinder 2, is extended. At this time, the backing 25 is pressed to the inner peripheral surface of the steel tube 20 in accordance with the link angle formed with the wedge rod cylinder 2 and plunger 4. In this way, a load is applied to the wedge rod cylinder 2 until a prescribed inner circumference is obtained, whereby the tube is expanded. The inner circumferential welding for the steel tube 20 is executed after a prescribed inner circumference of the steel tube 20 is obtained.