1. Field of the Invention
The present invention generally relates to electric power tools. More particularly, the present invention relates to an electric power tool, such as an electric screwdriver or driver-drill, employing an epicycle reduction gear unit to provide three-speed transmission for the spindle.
2. Description of the Related Art
A known type of electric screwdriver includes a housing, a motor, and an epicycle reduction gear unit with a plurality of axially arranged stages each including an internal gear, a plurality of planetary gears revolving on the internal gear, and a carrier supporting the planetary gears. Attached to the front end of the housing in this known tool is a spindle to which the rotation of the motor is transmittable via the reduction gear unit, which also reduces the speed of the rotation during the transmission.
U.S. Pat. No. 6,431,289, the content of which is incorporated herein by reference, discloses such an electric screwdriver that employs a speed change mechanism to allow the operator to select from three rotational speeds for the spindle. More particularly, two internal gears within the epicycle reduction gear unit are disposed so as to be axially slidable between two positions. Further, a selector is operated from the outside of the housing to switch the positions of the internal gears. This causes integral or independent rotation of the planetary gears and the carriers depending on the positions of the internal gears so as to provide three spindle speeds.
While the foregoing arrangement achieves its intended objective, it is not free from certain problems and inconveniences. For example, the speed change mechanism must move the two internal gears to perform its function. Additionally, to effect such movement, a wire clip mounted on each of the two internal gears is fitted in a cam groove in a selector cam. This selector cam is provided outside a sleeve that houses the reduction gear unit. The selector cam in turn is moved in axial directions with a switch member mounted outside the cam. Accordingly, this arrangement significantly increases the number of components required and thus complicates the structure and the assembly of the power tool.