Rigging pulleys to form a block-and-tackle is a ready response to a load too heavy to lift or pull. After reeving a block to rig the block-and-tackle, moving the load takes less effort. "Reeving" is the act of threading a line through a block, so that it travels over a pulley. The mechanical advantage imparted by the pulleys of a block-and-tackle allows two pulleys in each block, totalling four, plus the line you pull on, to give a five to one advantage. With a five to one mechanical advantage, lifting 45.4 Kilogram requires no more effort than lifting 9.07 Kilogram.
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Things you need
- 2-sheave blocks
Lay the two blocks on the ground with their bottoms--the end without a hook--toward each other. Reeve a line through the sheave closest to the ground in either block. "Sheave" is the technical name for a pulley in a block. This block becomes the "head block," or the block that will remain stationary.
Take the end of the line, properly called the "bitter end," to the other block and reeve it through the sheave closest to the ground. This is the block that will become the "heel block," attached to the load that moves when the pulling the rope.
Pull the bitter end back to the head block and reeve it through the remaining sheave. Because the sheaves closest to the ground were rove first, those lines are not in the way of the rest of the rigging process. After passing the bitter end over the final sheave in the head block, carry it to the heel block once more.
Reeve the line through the final sheave in the heel block and carry the bitter end to the ring on the base of the head block. Secure the line to that ring by a knot or an eye splice and the block-and-tackle is rigged. Hook the head block above the load to be lifted and hook the load to the heel block.
Tips and warnings
- You needn't worry if the lines appear twisted; when you hoist the head block, the heel block will untwist them naturally.
- Always choose a pulley that's sized for the rope you use. For fibre ropes, multiply the circumference of the rope--the distance around, not across--by three to determine the size of the pulley you need. For wire rope, multiply the rope's diameter, the distance across, by 20.
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