PURPOSE: To secure torque between rated speed and maximum torque speed so easily in a way of changing a torque curve freely by forming a torque spring and a torque adjusting screw into two sets each, and changing each spring constant of two types of torque springs.

CONSTITUTION: A tone spring unit 9 is provided with a high speed torque spring 10 and a low speed torque spring 11. In this constitution, spring force of the high speed torque spring 10 is made adjustable with a high speed torque adjusting screw 12, while spring force of the low speed torque spring 11 is made adjustable with a low speed torque screw 13.