Block and tackle rope through the block. The snatch block is also the load lifting pulley in certain arrangements. Block and tackle A block and tackle is a system of two or more pulleys with a rope or cable threaded between them, usually used to lift heavy loads. The pulleys are assembled together to form blocks and then blocks are paired so that one is fixed and one moves with the load. The rope is threaded through the pulleys to provide mechanical advantage that amplifies the force applied to the rope. Hero of Alexandria described cranes formed from assemblies of pulleys in the
Block and tackle more precise estimate of efficiency is possible by use of the sheave friction factor, formula_12 (which may be obtainable from the manufacturer or published tables). The relevant equation is: Typical formula_12 values are 1.04 for roller bearing sheaves and 1.09 for plain bearing sheaves (with wire rope). The increased force produced by a tackle is offset by both the increased length of rope needed and the friction in the system. In order to raise a block and tackle with a mechanical advantage of 6 a distance of 1 metre, it is necessary to pull 6 metres of rope through the