In general, this type of electrical power tool includes a structure in which rotative power of an electric motor as a drive source is decelerated by a speed change device to output a necessary rotation torque. In many cases, a planetary gear train is used as the speed change device.
For example, in the screw tightening machine, a low torque is sufficient at the beginning of tightening, but a higher rotation torque is gradually needed as a tightening operation progresses. Therefore, a function that is required from the point of view of carrying out a quick and reliable screw tightening is to reduce a reduction ratio of the speed change device so as to output a high speed low torque at the beginning of the tightening operation, and to increase the reduction ratio of the speed change device so as to output a low speed high torque in the middle of the tightening operation. Moreover, in terms of usability, it is required that, in the middle of the tightening operation, the reduction ratio is automatically switched at a point in which a tightening resistance (an external torque) applied to an output shaft reaches a certain value.
The following Japanese Pat. No. 3289958 teaches a screw tightening machine in which a speed change device having two-stage planetary gear trains is interposed between an output shaft of an electric motor and a spindle provided with a screw tightening bit. According to the speed change device, at the beginning of a screw tightening operation, a carrier of a first stage planetary gear and a carrier of a second stage planetary gear are directly connected via an internal gear of the second stage planetary gear train. As a result, a high speed low torque is output, so that a quick screw tightening operation can be performed. When a user increases a pushing force applied to the screw tightening machine as the screw tightening operation is proceeded, the internal gear of the second stage planetary gear train is relatively displaced in an axial direction, so as to be separated from the carrier of the first stage planetary gear train, while rotation thereof is fixed, thereby causing a deceleration in the second stage planetary gear. As a result, a reduction ratio of the speed change device can be increased, so as to output a low speed high torque. Thus, a reliable screw tightening operation can be performed.
The following Japanese Patent No. 3084138 teaches a reset mechanism that functions to return a low speed high torque output condition changed by an automatic speed change to a high speed low torque output condition corresponding to an initial condition. According to the prior art reset mechanism, a speed change device can be returned to the initial condition (the high speed low torque output condition) utilizing return motion of a switch lever which motion is performed to stop operation of a main body portion. Therefore, the speed change device can be reset to the initial condition without a special manipulation by the user of the screw tightening machine.