A power tool includes a motor, a speed reducer and a speed-reducing-ratio change mechanism. The speed reducer includes planetary gear mechanisms arranged in multiple stages. The speed-reducing-ratio change mechanism is configured to change a speed reducing ratio of the speed reducer in response to a change of a rotating direction of a motor shaft. At least two stages of the planetary gear mechanisms are configured such that an internal gear in each stage selectively functions as a fixed element. The speed-reducing-ratio change mechanism includes a one-way clutch and a lock mechanism configured to non-rotatably lock the internal gears of the at least two stages when the one-way clutch does not transmit rotation, and to rotate the internal gears of the at least two stages when the one-way clutch transmits rotation. The at least two stages include a speed-increasing planetary gear mechanism and a speed-reducing planetary gear mechanism.

A transmission includes a transmission case provided with an internal gear an inner wall thereof and a multistage planetary gear mechanism housed in the transmission case. Each stage of the multistage planetary gear mechanism includes a retainer, a sun gear and a plurality of planetary gears mounted on the retainer. The plurality of the planetary gears in the last planetary gear stage of the multistage planetary gear mechanism includes four planetary gears annularly arranged between the corresponding sun gear and the inner gear in an uneven manner.

A transmission includes a transmission case provided with an internal gear an inner wall thereof and a multistage planetary gear mechanism housed in the transmission case. Each stage of the multistage planetary gear mechanism includes a retainer, a sun gear and a plurality of planetary gears mounted on the retainer. The plurality of the planetary gears in the last planetary gear stage of the multistage planetary gear mechanism includes four planetary gears annularly arranged between the corresponding sun gear and the inner gear in an uneven manner.

A planetary gear actuator including a sun gear, a rotary internal gear being concentric with the sun gear, a fixed internal gear being concentric with the sun gear, compound planetary gears, and a carrier connected to each of the compound planetary gears so as to be rotatable relative to each of compound planetary gears may be provided, Each of the compound planetary gears may include a first planet gear engaged with the sun gear, a second planet gear at one side of the first planet gear, engaged with the rotary internal gear, and having a smaller number of teeth than the first planet gear, and a third planet gear at another side of the first planet gear, engaged with the fixed internal gear, and having a smaller number of teeth than the first planet gear. One compound planetary gear may overlap another compound planetary gear in a thickness direction.

A planetary gear actuator including a sun gear, a rotary internal gear being concentric with the sun gear, a fixed internal gear being concentric with the sun gear, compound planetary gears, and a carrier connected to each of the compound planetary gears so as to be rotatable relative to each of compound planetary gears may be provided, Each of the compound planetary gears may include a first planet gear engaged with the sun gear, a second planet gear at one side of the first planet gear, engaged with the rotary internal gear, and having a smaller number of teeth than the first planet gear, and a third planet gear at another side of the first planet gear, engaged with the fixed internal gear, and having a smaller number of teeth than the first planet gear. One compound planetary gear may overlap another compound planetary gear in a thickness direction.

A universal driving device includes a sun gear rotatably provided; a ring gear disposed on a rotation plane coplanar to a rotation plane of the sun gear and provided so that a rotation shaft of the ring gear is movable relative to a rotation shaft of the sun gear; and a gear train engaging the sun gear to the ring gear and configured to allow a relative motion between the rotation shaft of the sun gear and the rotation shaft of the ring gear and define a continuous power transmission state between the sun gear and the ring gear.

A power transmission apparatus may include a first motor-generator including a first stator fixed to a first housing, and a first rotor; a second motor-generator including a second stator fixed to a second housing and a second rotor; a first planetary gear set including first and third rotation elements connected to the first rotor and a first wheel, respectively, and a third rotation element; a second planetary gear set including fourth, fifth and sixth rotation elements connected to the second rotor, a second wheel, and the third rotation element, respectively; a first clutch selectively locking up the first planetary gear set by selectively connecting two out of the first to third rotation elements; a second clutch selectively locking up the second planetary gear set by selectively connecting two of the fourth to sixth rotation elements; and a brake selectively fixing the third and sixth rotation elements to a third housing.

A power transmission apparatus may include a first motor-generator including a first stator fixed to a first housing, and a first rotor; a second motor-generator including a second stator fixed to a second housing and a second rotor; a first planetary gear set including first and third rotation elements connected to the first rotor and a first wheel, respectively, and a third rotation element; a second planetary gear set including fourth, fifth and sixth rotation elements connected to the second rotor, a second wheel, and the third rotation element, respectively; a first clutch selectively locking up the first planetary gear set by selectively connecting two out of the first to third rotation elements; a second clutch selectively locking up the second planetary gear set by selectively connecting two of the fourth to sixth rotation elements; and a brake selectively fixing the third and sixth rotation elements to a third housing.

Mechanisms or apparatus convert a number of inputs via a number of input members into a number of output movements of an output structure, providing control in three degrees-of-freedom (DOF), for example control over roll, pitch and yaw of the output structure. Inputs may be rotations about a common axis of rotation, for example via a first ring, a second ring, and one or more plates, concentrically array. Rotation of the first ring may control a first DOF, rotation of the first ring may control a second DOF, and rotation of the plate may control all three DOF. Three concentrically arrayed tubular shafts may be employed, providing a through-passage or cable fluid conduit run to accommodate wires, optical fibers, fluid carrying conduits. Such may be particularly advantageous when employed as part of a robot, or other device with a tool or sensor or transducer located at or proximate a distal end thereof.

Mechanisms or apparatus convert a number of inputs via a number of input members into a number of output movements of an output structure, providing control in three degrees-of-freedom (DOF), for example control over roll, pitch and yaw of the output structure. Inputs may be rotations about a common axis of rotation, for example via a first ring, a second ring, and one or more plates, concentrically array. Rotation of the first ring may control a first DOF, rotation of the first ring may control a second DOF, and rotation of the plate may control all three DOF. Three concentrically arrayed tubular shafts may be employed, providing a through-passage or cable fluid conduit run to accommodate wires, optical fibers, fluid carrying conduits. Such may be particularly advantageous when employed as part of a robot, or other device with a tool or sensor or transducer located at or proximate a distal end thereof.