Drum 2 rotates about fixed axis A. Driving link 1, designed as a treadle, oscillates about fixed axis B and is connected by turning pairs C to links 5 and 6 which, in turn, are connected by turning pairs D and E to links 7 and 8. Links 7 and 8 turn about axis A and are connected by turning pairs F and K to pawls 3 and 4. The dimensions of the links comply with the conditions: C͞D=C͞E and A͞D=A͞E. Pawls 3 and 4 are held in contact with the inside surface of drum 2 by springs 9 and 10. When treadle 1 is oscillated, pawls 3 and 4 alternately engage drum 2 and rotate it counterclockwise.