Double snail sheave 1 has a double conical spiral groove and rotates about fixed axis D. Sheave 1 has two ropes, 2 and 3. From the position shown, rope 2, secured at point A, is being unwound from sheave 1 and wound on drum 4 which rotates about fixed axis E. At the same time, rope 3, secured at point B is being wound on sheave 1 and unwound from drum 5 which rotates about axis C. When rope 2 is on drum 4 and rope 3 on sheave 1, the sheave and drums are reversed.
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