How a Block and Tackle Works

Photo by Graham Lindsay
Photo by Graham Lindsay

A block and tackle is a simple mechanical device widely used for lifting heavy weights. The typical block and tackle consists of two blocks in which pulleys are mounted, and a rope or chain (the tackle) strung through the pulleys. A small force applied to the tackle over a relatively long distance is used to lift a heavy weight a short distance.

In the simple block and tackle shown to the left in the illustration on the following page, the upper block has two pulleys and is attached to a support. The lower block has one pulley and is attached to the load to be lifted. Since there are three ropes supporting the load, each supports % of the load, and so the force applied to the tackle to move the load must also be equal to at least % of the load.

If the tackle is pulled a distance of 3 feet, the load will be raised a distance of 1 foot. Since the force applied to the tackle will move a load 3 times as large, the mechanical advantage of the system is 3. A convenient method of establishing the mechanical advantage of this type of block and tackle is to count the number of ropes supporting the load.

A modification of the block and tackle, called a differential block and tackle (illustrated at the right of the above figure), has two pulleys of slightly different diameter that are fastened together and mounted in the upper block. When force is applied to the free-hanging loop, the chain supporting the load on the right moves up by a greater distance than the chain supporting the load on the left moves down. The load moves up by % the net difference of the distances that the chain on the left and the chain on the right move.

The system is self-locking and thus prevents the load from slipping when force is not being applied to the free-hanging loop. The smaller the difference in the radii of the fixed pulleys, the greater the lifting force produced and the slower the lift. The mechanical advantage is normally high; a value of 60 or higher is not uncommon.

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