| 摘要 |
A rotary shaft ( 3 ) and a rotation driven member ( 4 ) are so connected as to be capable of transmitting motive power when the rotary shaft ( 3 ) rotates relatively to the rotation driven member ( 4 ). A roller ( 6 ) gets displaced to a first position where a frictional force with an outer race ( 5 ) and the rotation driven member ( 4 ) decreases, thereby permitting transmission of a rotational force from the rotary shaft ( 3 ) to the rotation driven member ( 4 ). When the rotation driven member ( 4 ) rotates relatively to the rotary shaft ( 3 ), the roller ( 6 ) gets displaced to a second position where the frictional force with the outer race ( 5 ) and the rotation driven member ( 4 ) increases, thereby rotationally fixing the rotation driven member ( 4 ) with respect to the outer race ( 5 ). The power transmission from a brake apparatus (e.g., a parking brake apparatus) to an electric motor ( 2 ) can be hindered while permitting the power transmission to the brake apparatus from the electric motor ( 2 ) by using neither a worm exhibiting a relatively low transmission efficiency nor a worm wheel. In a state where the brake apparatus exhibits its braking force, the roller ( 6 ) receives the force from the brake apparatus and is thereby biased toward the second position, whereby a construction of a power transmission mechanism ( 50 ) can be simplified and costs can be reduced to such an extent.
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