How to Make a Block & Tackle Lift
A block and tackle is a simple machine composed of a series of pulleys rigged to provide the user with a mechanical advantage (MA). Mechanical advantage is the difference between the force applied to a machine and the force that the machine applies to an object. You apply less force over a greater distance to the machine, and the machine applies more force over a shorter distance. Put simply, when using a block and tackle, you pull less weight each time you pull, but you will be pulling more rope. The complexity of the block and tackle determines the mechanical advantage. Each pulley added to the system will increase the MA of the system by one, increasing the amount of rope that must be pulled, and decreasing the force necessary to pull it. Does this Spark an idea?
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Instructions
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1
Determine the mechanical advantage necessary to lift your object by using FA=(1/MA)*FO, where FA is the force applied by the user, MA is the mechanical advantage of the system and FO is the force (or weight) of the object being lifted. For example, if you intend to lift a 300-lb. object and you are capable of exerting 100 lbs. of downward force, you need a block and tackle with a mechanical advantage of three, seeing as 100=(1/3)*300.
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2
Calculate the number of pulleys you need. You will need one pulley per MA. Each pulley should have a safe working load of SWL = (2/MA)*W, where W is the weight of the object being lifted. The pulleys will have to be wide enough to allow your rope to pass through them.
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3
Calculate how much rope you need: L=(MA+1)*H, where L is the total length of rope you need and H is the height of the lifting point. Your rope's safe working load should be greater than the weight of the object you are lifting. You also need mounting hardware suited to the object you are lifting and the point you are lifting it from. In most cases, you can use a threaded hook to secure your pulleys in place. You need one set of threaded hooks, washers and nuts for every other pulley.
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4
Secure the first pulley to the lifting point using your mounting materials. In most cases this will consist of drilling a hole through the mounting point, inserting a threaded hook, screwing a nut and washer onto the other side of the hook to hold it in place and hanging the pulley on the hook. Pass the lifting end of the rope through the pulley and to the ground. You now have a simple lift with a MA of one.
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Secure the second pulley to the object being lifted. Drill into the object as in Step 4 or pass a rope around the object and secure the pulley to that. Pass the lifting end of the rope through the pulley. When properly secured, this machine has a MA of two.
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Repeat Steps 4 and 5 as necessary to add MA. Each time you repeat one of these steps, you increase the MA of your system by one. Add each additional pulley right next to the pulley placed before it. The rope should flow neatly between the pulleys and should not cross itself.
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Secure the lifting end of the rope to the object being lifted if the last pulley you installed was at the lifting point. Secure the lifting end of the rope to the lifting point if the last pulley you installed was on the object.
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Pull down on the working end of the rope to lift your object.
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Tips & Warnings
Make sure that the point you are using to lift (such as a tree limb or a building's rafters) is capable of supporting the full weight of the object you are lifting.
Always use safety precautions when working at height or with heavy objects. Be sure to operate within the safe working loads of all of your materials.
References
- Photo Credit pulley of a boat image by timur1970 from Fotolia.com
