Thursday, January 24, 2008


A block with a line led over the sheave makes applying power by changing the direction of the pull easier. Used with line and another block, it becomes a tackle and increases the power applied on the hauling part. Tackles are designated according to their uses and the number of sheaves in the blocks that are used to make the tackle. The various types of tackle are rove with different size blocks and all have a limited lifting capacity depending on the number of sheaves, the size blocks and the size line used. The tackles are named for their use or from their makeup.
A single whip tackle consists of a single fixed block with a line passed over its sheave. This tackle has no mechanical advantage.
The gun tackle, named for its use on old sailing ships to haul the cannons back to their gun port after firing, consists of one single-sheave fixed block and one single-sheave movable block.
The luff tackle is made up of one double-sheaved block and one single-sheaved block.
The twofold purchase is made up of two double-sheaved blocks.
The double luff tackle is made up of one double-sheaved block and one triple-sheaved block.
The threefold purchase is made up of two three-sheaved blocks.