BREAK DOWN ROLL DEVICE IN TUBE MILL

PROBLEM TO BE SOLVED: To improve the circularity of a forming material and the thrust to the forming material by making break down upper and lower rolls into the combined rolls of assembling a center roll fitted in a single rotary driving shaft and a pair of split rolls inserting the above between them in the time of producing an electric resistance welded tube. SOLUTION: The composition of an upper roll 1 in a roll device is made in combined rolls assembled with a center roll 2 fitted and attached firmly to a single upper roll rotary driving shaft 7, and a pair of split rolls 3 arranged at the right and left symmetrical positions inserting the above between them. The cross-section profile of the forming working face of this roll is made of a circular-arc with a definite curvature of radius, a right and left symmetrical curve line, or an involute based on a polygon, and a part having a curve face specified to the curve at the position having the uniform gap with the lower roll. Further, the split roll 3 is made freely sliding on the rotary driving shaft 3, and it is driven eccentrically. More, a lower roll 4 is composed of a pair of split rolls 6 arranged on the right and left symmetrically with inserting the intermediate roll 5 between them.