Patent ID: 7882900

Claim:
A power tool capable of variable output speed comprising: a housing; a motor contained in the housing for outputting rotary power; an output shaft; a gear transmission mechanism disposed between the motor and the output shaft for transmission of the rotary power of the motor to the output shaft at each of a plurality of gear reduction ratios; a signal generator; and a control system electrically coupled to the signal generator and operatively engaged with the gear transmission mechanism such that when the signal generator is manually activated, an electric signal is generated and transmitted to the control system to cause the gear transmission mechanism to vary the gear reduction ratio; wherein the signal generator comprises a switch mounted on the housing; wherein the control system comprises: a control unit electrically connected to the switch to receive the electrical signal when the switch is activated; and a driving mechanism actuated by the control unit in response to the electrical signal to cause the gear transmission mechanism to vary the gear reduction ratio; wherein the gear transmission mechanism comprises: at least one gear train; and a movable member movable between a first position corresponding to a low gear reduction ratio and a second position corresponding to a high gear reduction ratio whereby to vary the gear reduction ratio; wherein the gear train is a planetary gear train which includes a plurality of planet gears, an adjacent planet carrier, and a rotationally fixed structure immovably associated with the housing; wherein the movable member comprises a ring gear which when located at the first position engages the planet gears and the planet carrier and when located at the second position engages the planet gears and the rotationally fixed structure.