A pulley is a mechanism with a wheel and a simple frame that can be connected to something, either a fixed object or a movable object. The purpose of the pulley is to decrease friction when redirecting the pull/force of a rope, chain, or some equivalent thing. The double pulley system works by transferring energy to make it easier to move heavy objects by the use of two pulleys. A rope or belt is placed around a wheel, and then when the rope is pulled, the object that needs to be moved moves with minimal effort. A simple pulley is hung from a fixed point that is strong enough to support the weight to be loaded. This is usually above where the load must be hauled to and where the person or machine will be pulling, so that the load will stop in a convenient location. The second pulley is hooked to the load. The rope is tied to the bottom ring of the first pulley, threaded through the second pulley, threaded through the first pulley, and the free, pulling end is taken away to where it will be pulled. The rope is pulled, the sheaves turn, and the load is hoisted up.
Reeving blocks to set up a double pulley system requires a little thought. Let us show you how to set up pulley works but before you have to decide if you will use a double pulley system with one sheave—or or a double pulley system with two sheaves in each block.
Rigging Two Single-Sheave Pulleys
1) Lay the blocks on the deck, 2 to 3 feet apart, with one block on its side and the other on its face. The hooks should point away from each other. The block lying on its side will be the upper block of the tackle; the bottom block will attach to the load.
2) Reeve the line from the left, through the sheave of the block lying on its side.
3) Reeve the rope through the sheave of the block lying on its face.
4) Bring the rope up to the becket of the block lying on its side and secure it to the becket with a becket bend or an eye splice.
5) Use the block that has the rope belayed to its becket as the uppermost block in your tackle to gain the best mechanical advantage.
Reeving Two Double Pulleys
1) Lay the blocks on the deck, 2 to 3 feet apart, with one block on its side and the other on its face.