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Worm wheel 2 rotates about fixed axis A and meshes with worm 1 which rotates about fixed axis B. Turning about axis D on a pin carried by worm shaft 4 are vanes 3. Braking of shaft 4 begins when worm wheel 2 rotates at a certain definite speed and increases with the speed of the shaft because centrifugal force turns vanes 3 about axis D, overcoming the resistance of spring 5. For the transmission of rotation from wheel 2 to worm 1, the lead angle of the latter must be sufficiently large.