JPS6337271B

PURPOSE:To enable to prevent the breakage of an iron tube by a method wherein the closing speed of a servomotor is switched to the same except the maximum closing speed of the servomotor upon breakage of a return wire. CONSTITUTION:When the servomotor is closed and the opening degree of a valve arrives at the degree of buckling, a switching valve 16 is switched through a switching valve push rod pulley 20. A proper allowance is provided at the side of the maximum servomotor opening during a normal operation to obtain a structure wherein the switching valve 16 is switched through the switching valve push rod pulley when the servomotor is opened excessively. When the return wire 7 is broken, a return shaft 12 rotates into the direction of opening, therefore, the servomotor is closed slowly by switching the switching valve 16 at the over-opening side of a servomotor opening degree setting cam 15 and thereby preventing the breakage of the iron tube.