Example Block and Tackle Configurations
Notice that there are two parts of the rope through each pulley in a block, and an additional part for the start of the rope that is attached to one of the blocks. If there are p pulleys in each of the blocks then there are 2p parts for one block and 2p+1 for the other block. This means if the block with the rope attachment is selected for the moving block, then the mechanical advantage is increased by one. This configuration requires the hauling rope to move in the same direction as the load.
The Gun tackle, Double tackle and Threefold purchase have the same number of pulleys in both blocks, one, two and three, respectively.
For example, consider a block and tackle with two sheaves on both the moving block and the fixed block. One block has four lines running through its sheaves, and the other block also has four lines including the part of the line being pulled or hauled, with a fifth line attached to a secure point on the block. If the hauling part is coming out of the fixed block, the block and tackle will have a mechanical advantage of four. If the tackle is reversed, so that the hauling part is coming from the moving block, the mechanical advantage is now five.
In the diagram on the right the mechanical advantage of the tackles shown is as follows:
- Gun Tackle: 2
- Luff Tackle: 3
- Double Tackle: 4
- Gyn Tackle: 5
- Threefold purchase: 6
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