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A reduction gear mechanism, incorporating a speed changing mechanism, for use with a motor-driven drill. The mechanism includes at least one planetary speed reducer having a sun gear to which rotation of an electric motor is transmitted, a plurality of planet gears arranged around the sun gear and e

A reduction gear mechanism, incorporating a speed changing mechanism, for use with a motor-driven drill. The mechanism includes at least one planetary speed reducer having a sun gear to which rotation of an electric motor is transmitted, a plurality of planet gears arranged around the sun gear and engaged with the sun gear, an internal gear internally engaged with the planet gears, and a planet gear holding plate which pivotally supports the planet gears on one side of it and has an output shaft on the other side of it. The internal gear of at least one planetary speed reducer includes a cylindrical internal gear. The reduction gear includes mechanisms for locking the cylindrical internal gear to prevent rotation of the cylindrical internal gear and for engaging the cylindrical internal gear with the planet gear holding plate so that the cylindrical internal gear and the planet gear holding plate rotate as a single unit.

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In a reduction gear mechanism with a speed changing mechanism for a motor-driven drill including a gear case, at least one planetary speed reducer comprising a sun gear to which rotation of an electric motor is transmitted, a plurality of planet gears arranged around said sun gear and engaged with s

In a reduction gear mechanism with a speed changing mechanism for a motor-driven drill including a gear case, at least one planetary speed reducer comprising a sun gear to which rotation of an electric motor is transmitted, a plurality of planet gears arranged around said sun gear and engaged with said sun gears, an internal gear internally engaged with said planet gear, and a planet gear holding plate which rotatably supports said planet gears on one side thereof and has an output shaft on the other side thereof, the improvement wherein: said internal gear of at least one planetary speed reducer comprises a slidably mounted cylindrical internal gear, said cylindrical gears comprising a set of internal gear teeth engaged with said planet gear and a set of outer gear teeth, an annular groove being formed in an outer surface of said internal gear; said planet gear holding plate has a set of gear teeth engaged with said internal gear teeth of said internal gear in a first position of said internal gear to allow said internal gear to rotate with said planet gear holding plate; a separate set of fixed teeth is fixed to a structure within which said sun gear rotates, and said set of outer gear teeth of said internal gear is engaged in a second position of said internal gear with said fixed teeth to rotatably lock said internal gear, and wherein there is provided a lever for sliding said internal gear between said first and second position, said lever extending in a semicircle around said internal gear, said gear case extending circumferentially around said internal gear, said lever being pivotally mounted to said gear case, and said lever having pins on opposite sides thereof passing through windows in said gear case and slidably received in said annular groove at opposed points around the circumference of said internal gear.