To provide a portable power tool having a motor speed control circuit capable of virtually increasing the effective torque output to the given-size motor and a train of gears.
The power tool 11 is equipped with a train of gears 20, which connects the output spindle of a motor 16 to a tool bit attachment chuck 22, and the train of gears has a certain degree of loosening arranged to absorb the torque from the motor before its conveyance to the tool bit. The control circuit 12 of the power tool alternately makes the motor on and off in the pulse long enough to relax the train of gears during the off period, so that the motor can get off to a "running start", when power is applied again. For this shift to the ratcheting action mode, various methods for detection of the fixed threshold-value electric current, detection of fixed increases in motor electric current, and detection of the fixed declining speed in motor electric current are disclosed.
Jun Tsuruta
Shimichi Akihisa
Masaya Nishiyama
Higuchi Souji