Sprag Clutch :

Sprag clutches are used to provide the disconnect mechanism between the starter motor and the engine. The clutch will transmit drive from the starter motor, but will disconnect the drive when the engine speed exceeds the starter. The clutch consists of two smooth concentric drive faces and between them a cage containing many elongated figure of eight shaped cams called "sprags". All the surfaces are hardened to reduce wear, and are lubricated by oil. The sprag are spring loaded in contact with the starter drive so that when the shaft starts to rotate the sprags stand up and contact the engine drive due to the cam action of their shape. See Figure 13.6. As engine RPM accelerates its drive will be faster than the starter motor and the clutch will automatically dis-engage as sprags get pushed back to their minimum height position.

Sprag clutches are used on most types of starter motor or in drives where one way drive is required such as helicopter gearboxes.