A power transmission device has an interlocking member 9 moving a pressure member 5 from an inactive position to an active position. A release spring (m) applies an urging force, while allowing movements of the interlocking member 9 and the pressure member 5, until drive-side clutch plates 6 and driven-side clutch plates 7 reach an engaged state before the drive-side clutch plates 6 and the driven-side clutch plates 7 are pressed against each other. A clutch spring 11 is compressed in a process where the interlocking member 9 moves after the drive-side clutch plates 6 and the driven-side clutch plates 7 have reached the engaged state. The clutch spring applying a press-contact force between the drive-side clutch plates 6 and the driven-side clutch plates 7 while allowing movements of the interlocking member 9 and the pressure member 5. The set load of the clutch spring 11 is set to be smaller than the maximum load of the release spring (m).

During movement of a mass of a centrifugal clutch a radially inner position to a radially outer position and a resulting increase in torque transmitted from an input gear to an output shaft, a power transmission apparatus includes a first torque region where the apparatus restricts operation of a pressing assist cam and a second torque region where the apparatus allows operation of the pressing assist cam.

During movement of a mass of a centrifugal clutch a radially inner position to a radially outer position and a resulting increase in torque transmitted from an input gear to an output shaft, a power transmission apparatus includes a first torque region where the apparatus restricts operation of a pressing assist cam and a second torque region where the apparatus allows operation of the pressing assist cam.

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, and springs to urge masses from a radially outer position to a radially inner position. The masses are each housed in an associated one of housing portions arranged in a circumferential direction of the holder such that the masses are radially movable. More than one spring is provided in the circumferential direction between an inner peripheral wall surface of each of the housing portions and an associated one of the masses to urge the associated mass from the radially outer position to the radially inner position.

A power transmission apparatus a centrifugal clutch including a holder and a press. The holder and the press are each circumferentially provided with protrusions fitted and attached to cut-outs of a clutch housing. The holder is an integrated assembly into which a mass that is being held is integrated. The integrated assembly and the press are non-connected separate components and are independently movable toward or away from each other.

During movement of a mass of a centrifugal clutch a radially inner position to a radially outer position and a resulting increase in torque transmitted from an input gear to an output shaft, a power transmission apparatus includes a first torque region where the apparatus restricts operation of a pressing assist cam and a second torque region where the apparatus allows operation of the pressing assist cam.

A power transmission device has an interlocking member 9 moving a pressure member 5 from an inactive position to an active position. A release spring (m) applies an urging force, while allowing movements of the interlocking member 9 and the pressure member 5, until drive-side clutch plates 6 and driven-side clutch plates 7 reach an engaged state before the drive-side clutch plates 6 and the driven-side clutch plates 7 are pressed against each other. A clutch spring 11 is compressed in a process where the interlocking member 9 moves after the drive-side clutch plates 6 and the driven-side clutch plates 7 have reached the engaged state. The clutch spring applying a press-contact force between the drive-side clutch plates 6 and the driven-side clutch plates 7 while allowing movements of the interlocking member 9 and the pressure member 5. The set load of the clutch spring 11 is set to be smaller than the maximum load of the release spring (m).

A clutch (100) having: a fixed hub (201), a movable hub (202), a plurality of discs (204). The fixed hub (201) and the movable hub (202) axially slide, getting farther or closer so as to transmit a variable axial load onto the discs (204). The clutch (100) has a pressure plate assembly (208) including a centrifugal assembly (210), having: a rotatable mass holder (211) and a plurality of mass elements (212) radially disposed. Each mass element (212) has a pivot (301) and is configured for a displacement (1002) by pivoting under centrifugal effects. The mass elements (212) are configured for bringing the movable hub (202) closer to the fixed hub (201), so as to increase the variable axial load and allowing the clutch (100) (automatic) engagement. Each mass element (212) has a main body (302) and a column element (303) which connects the pivot (301) with the main body (302).

A clutch (3) includes: a clutch housing (32) which is provided with a drive plate (33) to be movable in an axial direction; a clutch hub (34) which transmits a rotary motive power to an output shaft; a clutch sleeve (35) which is provided with a driven plate (36) to be movable in the axial direction; and a pressure disk (40) which biases the drive plate (33) and the driven plate (36) toward the clutch hub (34), further includes a back torque limiter (7) which moves the pressure disk (40) in a direction away from the clutch hub (34) when back torque is applied, and the clutch hub (34) is provided with a restricting apparatus which restricts movement of the pressure disk (40) in the direction away from the clutch hub (34) when a number of rotations of the output shaft is less than a predetermined value.