A simple pulley is designed to magnify the force of pulling weight so that the strength of one man can do the work of ten. This ingenious device uses wheels and belts to change the direction of the pulling force and, when multiplied, can create a pulley system capable of moving nearly anything. Even the most complex pulley system can be broken down into the basic building blocks of pulley design.
The axle is the backbone of a pulley system. It can be a fixed axle (mounted to something solid) or a free axle (able to freely swing), but must be made of solid material in order to take the force of pulling heavy loads. It can be as simple as a single steel rod running through a single wheel or as complex as a series of metal bars encased in a pulley housing. The axle provides the pivot point for the direction of force and must be smooth and round to allow wheels or gears to rotate smoothly in the direction of the pulling force.
The wheel of a pulley can be a number of things. A simple wheel with grooves along its outer circumference works for simple rope pulleys while a many-toothed gear may rotate on the axle of a complex pulley system. Whatever the case, the wheel must be able to spin freely and smoothly in order for the force magnification to work effectively. The wheels of a pulley may be exposed or housed in a metal, plastic or wooden casing to protect the wheel itself and the axle from the elements. Some wheels are solid material with ball bearings mounted on the inner circle to facilitate spinning on the axle.
The "belt" of the pulley can be almost anything long, strong and rope-like. Rope, chain, rubber belt, hose and even woven fabric is used in pulley systems. The practicality of a material may come into question when force is applied, but the basic mechanics of the pulley belt is the transfer of the pulling force from a short pull at the direct object to a longer pull at a different angle, making the work much easier to do. Strong chains and ropes are common in hand-driven pulley systems while specialized belt and high-tension chains are used in machine-driven pulleys.
The final piece of the basic pulley system is the hook and connecting mechanism. For a pulley to work, it must be affixed to something removed from both the worker and the load that needs to be moved. Hooks allow portable pulleys to be moved or loads to be attached to pulleys that operate against the load (crane and machine pulleys). Connectors are used to connect differing pulleys together so that a single belt loop can be formed, compounding the pulleys into what is called a block and tackle (more than one pulley connected together). With strong hook and pulley connections, the load can be moved safely and with no worry of pulleys snapping.