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).

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.

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.

An improved adjuster for a clutch having an expanding friction disk. The improved adjuster allows desirable characteristics of a clutch pressure plate to be maintained while allowing an operator to bias the pressure plate into proper position to accommodate the function of the expanding friction disk.

An improved adjuster for a clutch having an expanding friction disk. The improved adjuster allows desirable characteristics of a clutch pressure plate to be maintained while allowing an operator to bias the pressure plate into proper position to accommodate the function of the expanding friction disk.

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.

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 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.

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.