high speed low torque condition in the illustrated embodiment the clamp 24 is retained in an initial position by the compression spring 25 where the pins 24 c are located at an upper section of the slots 12 due to the engagement between the pins 24 c and the annular groove 33 a of the shifting gear 33 the shifting gear 33 is located at a topmost position with respect to the frame 1 under this circumstance the trapezoid blocks 22 a of the pushing wheel 22 and the trapezoid portions 23 a of the sliding ring 23 completely engage each other angular position of the pushing wheel 22 with respect to the frame 1 is retained by the torsion spring 21 while axial position of the sliding ring 23 is retained by the shifting gear 33 which is retained by the compression spring 25 referring to fig3 and 4 when the a large torque is required the input gear 31 of the transmission mechanism 3 increases the torque transmitted to the front speed reduction gear 32 which in turn causes the planet gears 32 a of the front speed reduction gear 32 to impart a reaction torque in reverse direction to the inner teeth 22 b of the pushing wheel 22 so as to rotate the pushing wheel 22 nevertheless the rotation of the pushing wheel 22 is restrained by the force of the torsion spring 21 and the compressing spring 25 inclined side faces of the trapezoid blocks 22 a of the pushing wheel 22.