Clutch member 3 is rotated at uniform velocity by gear 1 and p inion 2. Rotation is transmitted from member 3 to clutch member 4, pinion 5 and gear 6 when sliding key 7 engages slot a in member 3. Key 7 is kept from sliding downward and engaging the clutch by dog 8 wnich is keyed with lever 9 on shaft A. Shaft A turns in a fixed bearing. When gear 1 rotates, pin d periodically trips lever 9, turning it through a certain angle and thereby withdrawing dog 8 so that key 7 drops, due to the action of spring 10, into slot a of clutch member 3. This engages member 3 to clutch member 4 so that pinion 5 drives gear 6. When pin d has passed lever 9, a spring (not shown) returns lever 9 and dog 8 to the initial position in which the bevel face of key 7 comes into contact with the bevel face on dog 8 so that key 7 is forced upward out of slot a and the clutch members are disengaged.
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