The Difference between Pulleys and Sheaves
Pulleys have been around for a long time; they are one of the six types of simple machines, and their origins have been lost to memory. The basic concept allows loads to be lifted using much less force than would normally be applied, through a system of ropes and wheels. The term pulley is often used interchangeably with the term sheave (pronounced "shiv"); however, this usage is not technically correct. The sheave is the rotating part of the pulley system; it is the grooved wheel into which a rope fits.
A fixed pulley with a single sheave is used to change the direction in which a force is applied, but does not change the amount of force required. It is only with multiple sheaves that a pulley system offers a mechanical advantage. Each additional sheave halves the force required to move an object, but doubles the distance the rope must be pulled to achieve the same movement. Although the force exerted by the person is lessened, in the final calculation the same amount of work is done. What makes this system easier to use than raw manpower is the fact that the force needed is mitigated by the distance covered.
So, if more sheaves make the work easier, why don’t we see pulley systems with umpteen sets of sheaves and miles of rope making it possible for anyone to move anything? In the real world, friction intervenes. As more sheaves and more rope come together in a system, each additional set offers a smaller and smaller advantage, until there is no gain at all. The system becomes less efficient the larger it gets.
Despite the reduction in efficiency, several sheaves are often used in tandem in one pulley system. These are known as compound pulleys in their simplest form, and block and tackle in more complex varieties. In order to increase the efficiency of the systems with multiple sheaves, the sheaves are arranged above or below each other, often with a fixed axle between them.
An effective sheave must have the minimum surface area possible for the rope or cable with which it is being used, and must also be resistant to abrasions or warping which may cause the rope to fray when the system is put into effect. When designing or discussing a block and tackle, the most important thing to remember is… not to make the common mistake of confusing pulleys and sheaves!
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