A sun gear of a speed reducer is provided coaxially with and integrally rotatably with a rotor of a prime mover. Multiple planetary gears are disposed in a circumferential direction of the sun gear, and are capable of revolving in the circumferential direction of the sun gear while rotating in a state of meshing with the sun gear. A carrier rotatably supports the planetary gears, and is rotatable relative to the sun gear. A first ring gear is fixed to a housing, and has a first ring gear tooth portion capable of meshing with the planetary gears. A second ring gear is rotatable integrally with a rotation portion, and has a second ring gear tooth portion capable of meshing with the planetary gears. The second ring gear tooth portion is different in number of teeth from the first ring gear tooth portion.

A prime mover includes a stator fixed to a housing and a rotor relatively rotatable relative to the stator. The prime mover outputs torque from the rotor by being supplied with electric power. A speed reducer reduces the torque of the prime mover and outputs the reduced torque. One bearing portion rotatably supports the rotor. The speed reducer includes an input portion that is coaxial and integrally rotatable with the rotor and receives the torque from the rotor.

A state changing unit is provided radially outward of a translation portion and movable in an axial direction relative to the translation portion. The state changing unit is in contact with or separated from the clutch. The state changing unit receives a force in the axial direction from the translation portion such that the state changing unit is pressed against the clutch. The state changing unit is capable of changing a state of the clutch to an engaged state or a disengaged state according to a position of the translation portion in the axial direction relative to the housing. A movement restriction portion is provided on the translation portion and located between the state changing unit and the clutch. The movement restriction portion contacts the state changing unit and restricts movement of the state changing unit relative to the translation portion toward the clutch.

A robot includes a first link having a first longitudinal axis operatively coupled to a second link having a second longitudinal axis such that rotation of the first link relative to the second link alternatively performs the following effects: elongation of the first link; pivoting of the first longitudinal axis relative to the second longitudinal axis; and rotation of the first longitudinal axis relative to the second longitudinal axis.

A rotational translation unit includes a rotation portion that rotates relative to a housing upon receiving an input of torque output from a speed reducer, and a translation portion that moves relative to the housing in an axial direction according to rotation of the rotation portion relative to the housing. A clutch is provided between a first transmission portion and a second transmission portion that are rotatable relative to the housing, and the clutch allows transmission of the torque between the first transmission portion and the second transmission portion in an engaged state of the clutch, and interrupts the transmission of the torque between the first transmission portion and the second transmission portion in a disengaged state of the clutch. A return spring is capable of urging the rotational translation unit in a direction away from the clutch relative to the housing.

A prime mover includes a stator fixed to the housing and a rotor rotatable relative to the stator. The prime mover outputs torque by being supplied with electric power. A speed reducer reduces the torque of the prime mover and outputs the reduced torque. A rotational translation unit includes a rotation portion that rotates relative to the housing upon receiving the torque from the speed reducer, and a translation portion that moves relative to the housing in an axial direction. A clutch allows torque transmission in an engaged state, and interrupts the torque transmission in a disengaged state. A state changing unit changes the state of the clutch by receiving a force along the axial direction from the translation portion. Cogging torque generated between the rotor and the stator is set to such a magnitude that the rotor is stoppable at any rotation position relative to the stator.

Embodiments of the disclosed gear transmission comprise an input shaft and a coaxial output shaft, a planetary gear assembly configured with a sun gear fixedly articulated to the input shaft, a ring gear of the planetary gear assembly fixed at a hub of a transmission plate configured with a peripheral gear ring, a gear ratio changing mechanism comprising a pinion gear engaged with the peripheral gear ring of the transmission plate and rotatable about a radial axis, and a transmission pinion gear coupled to an axle rotatably articulated with the pinion gear. The transmission pinion gear being displaceable along the axle and configured for engaging with a rotatable gear plate. The gear plate is coaxially articulated with the output shaft and the gear plate is configured at an inside face thereof with a plurality of coaxially disposed gear rings. The transmission pinion gear is engageable with a gear ring of the gear plate, and the gear ratio changing mechanism comprises a manipulator configured for selective radial displacing the transmission pinion gear between gear rings of the gear plate. The axle is configured to rotate about its longitudinal axis and simultaneously rotate about a central axis of the input shaft in a direction opposed to the direction of rotation of the transmission plate.

Embodiments of the disclosed gear transmission comprise an input shaft and a coaxial output shaft, a planetary gear assembly configured with a sun gear fixedly articulated to the input shaft, a ring gear of the planetary gear assembly fixed at a hub of a transmission plate configured with a peripheral gear ring, a gear ratio changing mechanism comprising a pinion gear engaged with the peripheral gear ring of the transmission plate and rotatable about a radial axis, and a transmission pinion gear coupled to an axle rotatably articulated with the pinion gear. The transmission pinion gear being displaceable along the axle and configured for engaging with a rotatable gear plate. The gear plate is coaxially articulated with the output shaft and the gear plate is configured at an inside face thereof with a plurality of coaxially disposed gear rings. The transmission pinion gear is engageable with a gear ring of the gear plate, and the gear ratio changing mechanism comprises a manipulator configured for selective radial displacing the transmission pinion gear between gear rings of the gear plate. The axle is configured to rotate about its longitudinal axis and simultaneously rotate about a central axis of the input shaft in a direction opposed to the direction of rotation of the transmission plate.

A prime mover includes a stator fixed to a housing and a rotor relatively rotatable relative to the stator. The prime mover outputs torque from the rotor by being supplied with electric power. A speed reducer reduces the torque of the prime mover and outputs the reduced torque. One bearing portion rotatably supports the rotor. The speed reducer includes an input portion that is coaxial and integrally rotatable with the rotor and receives the torque from the rotor.

A selective motion transfer mechanism acts to transmit rotary motion between a common element and a selected one of multiple channel elements. The common element can be positioned in alignment with any one of the channel elements, and when so aligned, rotation of one of the aligned elements causes rotation of the other.