same amount in the opposite rotational directions in other words the moment produced at the point p 1 around the center pivot 13 by the lever 11 due to the tension spring 15 is a moment produced by the tangential component force of the pressure normal to the cam face point p 1 around the central axis 3 similarly the moment produced at the point p 2 around the center pivot 19 by the lever 17 due to the tension spring 20 is a moment produced by the tangential component force of the pressure normal to the cam face point p 2 around the central axis 3 since the two moments are equal and directed in the opposite rotational directions it is possible to determine the tensional force of the springs 15 20 the distances between the pivot positions 13 19 of the levers 11 17 and the engaging points p 1 p 2 of the double cam 7 and the configuration of the double cam 7 so that such a condition of the two moments may be applied to all the rotational phases of the double cam 7 since there is actually a frictional force at the cam points p 1 p 2 in the tangential directions of the cam.