About Block and Tackle Hoists

A block and tackle hoist is a classic piece of muscle power engineering, and a solid example of elementary mechanics. Given that all it requires is a pair of sturdy hands, a rope, and two pulleys, it is a handy principle to understand for anyone in a job that might require lifting a heavy object on the fly. It is also a basic mechanism for anyone studying engineering, sailing, mechanics or physics.

Identification

The block and tackle is any mechanical system involving at least two pulleys (or "blocks") with a rope or cable running between them. The typical configuration is to put one pulley on a fixed point, and one on the object to be hoisted (the "moving" block of pulley).

Function

The block and tackle system is a means of lifting heavy objects using muscle power.

Effects

A block and tackle works through mechanical advantage, or the multiplication that a mechanism gives to the force applied to it. In the case of the block and tackle, that mechanism is the pulley, and the mechanical advantage is determined by the number of ropes/cables being pulled through the moving pulley. The popular conception of the pulley has two lines, granting a mechanical advantage of two, meaning 100 lbs. could be lifted using 50 lbs. of tension.

Considerations

The increased power offered by a block and tackle is partly negated by the force of friction, namely the resistance of the ropes/cables running through the pulleys.

Features

There are two basic terms to describe how a block and tackle is rigged. One is for it to be rigged to advantage. This means that the rope or cable is running through the moving pulley, and is being pulled in the same direction that the object is supposed to be hauled. Rigged to disadvantage means that the rope is being pulled through the fixed pulley, and pulled in a direction other than that in which the object is to be moved. As a practical matter, pulleys often need to be rigged to disadvantage, such as when raising a heavy object: it is easier for the haulers to pull the rope down, and depending on the location of the object being hauled, possibly easier to use the fixed pulley over the moving pulley.

History

The block and tackle is thought to have been invented by Archimedes, the Greek philosopher from Syracuse. They were widely used in the days when all work was done with muscle power, and continue to be in use today where motorized tools are not practical or available. The block and tackle is most commonly associated with sailing ships.