A block and tackle system is a simple mechanical system of pulleys and ropes generally used for lifting or pulling heavy loads for small distances. The name comes from old nautical terminology, where pulleys were called "blocks" and rope was called "tackle." A block and tackle system is composed of two or more pulleys linked with a rope or chain. It allows the movement of heavy objects with less force than is usually necessary. Long, low-powered pulling movements are turned into short, high-powered pulling movements.
Things You'll Need
- 2 enclosed pulleys (with the wheel inside a frame)
- 2 large screw-ended hooks
- Large eye hook
Screw a hook into one end of both pulleys. Screw an eyehook into the opposite end of just one of the pulleys.
Attach the pulley with hooks in both ends to an overhead support. This is what the block and tackle will be braced against to provide its lift. You could use a tree branch, roof beam or construction support. An easy way to attach this pulley, called the fixed block, is to tie a rope around the support and hook the block's hook over the ropes. Be sure the ropes will bear the weight of whatever you're lifting.
Thread one end of the rope into the fixed block. Pull enough of it through so that the rope's weight doesn't pull it back out of the block when you let go of it.
Thread the other end of the rope through another pulley and back up to the fixed block. Tie the rope to the eye hook at the bottom of this block. This second pulley is called the traveling block. To use the block and tackle, attach a weight to the hook on the traveling block. Pull on the rope coming out of the fixed block. As you pull on the rope, the tension is the same everywhere along the rope. This means that the traveling block is being lifted on either side by the same tension that you are pulling with. This doubling of your lifting force is called a 2-to-1 mechanical advantage. The trade-off is that you have to pull twice as far as the weight itself moves, so that you use the same total energy.
Tips & Warnings
- This simple block and tackle arrangement is called a gun tackle. A luff tackle uses three pulleys for a 3-to-1 mechanical advantage, tripling your lifting force. Other block and tackle systems can be constructed with even more pulleys for correspondingly greater lifting power.
- Photo Credit ship's block and tackle image by David Smith from Fotolia.com
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