In a power transmission apparatus, an auxiliary clutch plate different in diameter from driving and driven clutch plates located in a clutch housing. When the driving and driven clutch plates are pressed against each other, the auxiliary clutch plate is pressed to enable transmission of a driving force of an engine to a driving wheel. When a pressing force exerted on the driving and driven clutch plates is released, a pressing force exerted on the auxiliary clutch plate is released to cut off transmission of the driving force of the engine to the driving wheel.

A power transmission device has a back torque transmission cam that brings driving clutch plates 6 and driven clutch plates 7 into press contact with each other. A second clutch member 4b is moved when a rotational force is input to a first clutch member 4a via the output shaft 3. The pressure member 5 is located at a non-actuation position. A torque transmission portion transmits a rotational force transmitted to the second clutch member 4b to the first clutch member 4a not via the back torque transmission cam.

A power transmission device has a back torque transmission cam that brings driving clutch plates 6 and driven clutch plates 7 into press contact with each other. This occurs by moving a second clutch member 4b when a rotational force is input to a first clutch member 4a, via the output shaft 3. A pressure member 5 is located at a non-actuation position. A back torque transmission cam can hold abutment between an interlocking member 9 and weight member 8 by moving the second clutch member 4b in a direction of being brought into proximity to the interlocking member 9.

A power transmission device has a back torque transmission cam that brings driving clutch plates 6 and driven clutch plates 7 into press contact with each other. This occurs by moving a second clutch member 4b when a rotational force is input to a first clutch member 4a, via the output shaft 3. A pressure member 5 is located at a non-actuation position. A back torque transmission cam can hold abutment between an interlocking member 9 and weight member 8 by moving the second clutch member 4b in a direction of being brought into proximity to the interlocking member 9.

A power transmission device has a first pressure member (5a), a second pressure member (5b) and a back-torque transmitting cam. The first pressure member (5a) presses the drive-side clutch plates (6) and the driven-side clutch plates (7) against each other. The second pressure member (5b) releases a press-contact force between the drive-side clutch plates (6) and the driven-side clutch plates (7). The back-torque transmitting cam moves the second pressure member (5b) relative to the first pressure member (5a) to keep pressing the drive-side clutch plates (6) and the driven-side clutch plates (7) against each other when a rotational force is input to the clutch member (4), via the output shaft (3), in a process where the weight member (8) moves from the radially-outer position to the radially-inner position and the first pressure member (5a) moves to follow the interlocking member (9).

A power transmission device has a first pressure member (5a), a second pressure member (5b) and a back-torque transmitting cam. The first pressure member (5a) presses the drive-side clutch plates (6) and the driven-side clutch plates (7) against each other. The second pressure member (5b) releases a press-contact force between the drive-side clutch plates (6) and the driven-side clutch plates (7). The back-torque transmitting cam moves the second pressure member (5b) relative to the first pressure member (5a) to keep pressing the drive-side clutch plates (6) and the driven-side clutch plates (7) against each other when a rotational force is input to the clutch member (4), via the output shaft (3), in a process where the weight member (8) moves from the radially-outer position to the radially-inner position and the first pressure member (5a) moves to follow the interlocking member (9).

A power transmission device has a first clutch member 4a coupled to an output member 3. A second clutch member 4b has a plurality of driven-side clutch plates 7. A back-torque transmitting cam presses the drive-side clutch plates 6 and the driven-side clutch plates 7 against each other by moving the second clutch member 4b when a rotational force is input to the first clutch member 4a. A cushioning member 12 is interposed between the first clutch member 4a and the second clutch member 4b. The cushioning member 12, by being compressed, applies an urging force while allowing movements of an interlocking member 9 and a pressure member 5 in a process where the interlocking member 9 moves and the pressure member 5 moves from the inactive position toward the active position.

A power transmission device has a first clutch member 4a coupled to an output member 3. A second clutch member 4b has a plurality of driven-side clutch plates 7. A back-torque transmitting cam presses the drive-side clutch plates 6 and the driven-side clutch plates 7 against each other by moving the second clutch member 4b when a rotational force is input to the first clutch member 4a. A cushioning member 12 is interposed between the first clutch member 4a and the second clutch member 4b. The cushioning member 12, by being compressed, applies an urging force while allowing movements of an interlocking member 9 and a pressure member 5 in a process where the interlocking member 9 moves and the pressure member 5 moves from the inactive position toward the active position.

In a power transmission apparatus, a centrifugal clutch includes a holder, a press, a first spherical body that is partially protruded from a first opening and in contact with a rolling contact surface of the press so as to be rollable thereon, and a second spherical body that is partially protruded from a second opening and in contact with a rolling contact surface of the holder so as to be rollable thereon. A through hole has a tapered shape from the first opening to the second opening. Disconnection of the first spherical body is prevented by an outer peripheral edge of one of the first opening and the second opening that has a smaller diameter.

In a power transmission apparatus, a centrifugal clutch includes a holder, a press, a first spherical body that is partially protruded from a first opening and in contact with a rolling contact surface of the press so as to be rollable thereon, and a second spherical body that is partially protruded from a second opening and in contact with a rolling contact surface of the holder so as to be rollable thereon. A through hole has a tapered shape from the first opening to the second opening. Disconnection of the first spherical body is prevented by an outer peripheral edge of one of the first opening and the second opening that has a smaller diameter.