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Disks 1 and 2 rotate freely about fixed axis A. Pawl 4 turns about fixed axis B. When pawl 4 is turned out of engagement with disk 1, the disk turns one half revolution counterclockwise due to the action of spiral spring 3 which has one end fastened to disk 2 and the other to disk 1. When disk 2 is turned in the same direction one half revolution, spring 3 is wound up again as in the initial position. Disk 2 is locked by pawl 5 which turns about fixed axis C and is held in engagement with disk 2 by spring 6.