Parts of a Block and Tackle

A block and tackle stands out as a common fixture in maritime and industrial settings.Gancho 2 image by Juanri from Fotolia.com

Pulleys provide a powerful mechanical advantage that allows you to lift heavy weights, but you need to arrange the pulleys correctly in order to get their full benefit. The block and tackle is a common arrangement of pulleys that provides significant lifting force in a compact and reliable arrangement. Learn the elements that form a block and tackle and you will be able to make good use of this useful machine.

The Blocks

A block and tackle actually incorporates at least two blocks. Blocks are wheels with grooves, or sheaves, through which someone can thread a rope. A block may have one sheave or multiple sheaves depending on how many times the line may pass through it. The most common arrangement of a block and tackle has one fixed block, which attaches another object and points downward, and a moving block which hangs freely below the fixed block. A more complex block and tackle may also include additional fixed and moving blocks. One of the fixed blocks may include a point where the line ends and attaches. One or more moving blocks will also include a hook or ring to which the weight can attach.

The line is a rope or cable which runs through the sheaves and attaches to a fixed block. The more times that the line threads through the sheaves, the greater the mechanical advantage that the block and tackle provides. This also, however, increases the amount of resistance on the line.

The free end of the line can either be rove to advantage, meaning that it runs into a moving block, or rove to disadvantage, meaning that it runs into a fixed block. A line that is rove to advantage applies force in the same direction that someone applies force to it. A line that is rove to disadvantage reverses the force that someone applies to it.

The standing part is the static object to which the block and tackle attaches. Any rigid object capable of supporting the weight of the block and tackle can be a standing part, but typically the block and tackle attaches to a mobile standing part such as a crane arm. A mobile standing part allows a user to move the axle of the block and tackle.