How Cranes Work, Part 2: The Pulley

We recently touched on the role of the lever in construction cranes. In this edition, however, we will briefly look at how pulleys increase lifting capacity. Our final two article, then, will focus on the hydraulic cylinder and the concept of mechanical advantage.

As with the lever, Archimedes is credited with the earliest formal theoretical development of the pulley. According to Plutarch, Archimedes claimed that he could move the world if he had enough pulleys, a very similar statement to his proposal to move the Earth with a lever. The story continues when King Hieron asks Archimedes to move a large ship in Hieron's navy. On the appointed day, Archimedes set up his system of pulleys, the King loaded the ship full of passengers and cargo, and then Archimedes sat from a distance and pulled the rope. The result? Plutarch explains the shipped moved along "as smoothly and evenly as if she had been in the sea."

To the ancients, this was mere novelty, but today, this is basic science. To explain it crudely, pulleys distribute weight through different segments of rope to make lifting heavy objects easier. Let's say you have a large object you wish to lift. You reach down and attempt to lift it with your own strength, but you can't. So, to make this easier, we attached a pulley to the large load. Then we attach a rope to the ceiling and pull that rope through the pulley. We lift up on the rope, and we finally lift the object. We can do this because the rope on the ceiling supplies half of the force needed to lift the object while we apply the other half.

How does this work? Well, the pulley allows you to distribute the weight over two rope segments, the rope connected from the ceiling to the pulley and the other part of the rope from the pulley to you. As a result, the ceiling provides half of the applied force needed to lift the object while you supply the other half. Although the distribution of weight changes with how many pulleys you add and where you add more pulleys, but generally, the more pulleys you add, the easier heavy objects are to lift.

But the number of pulleys isn't the only factor in improving lift capacities. In fact, the configuration of the pulley is very important too. There are three types of pulleys: fixed, movable, and combined. Fixed pulleys have a fixed axle around which the rope, wire, chain, etc. is looped. Movable pulleys have a free-moving axle, which maximizes the lifting capacity. Combined pulleys are obviously a combination of fixed and movable pulleys. Although movable pulleys provide the most power, certain situations and loads can only allow certain types of configurations. Different lifting conditions require different pulley systems.

But why does this matter to construction cranes? Almost all cranes employ pulleys to some degree. The best example, however, is the jib crane which connects a pulley and the load you wish to lift. The more you wrap the cable through the pulley and the load, the higher amount of lifting capacity you can achieve.

Next, we shall see how hydraulic cranes are used in construction cranes and the science behind it. The final article shall discuss the concept of mechanical advantage.

This article is brought to you by Barnhart Crane & Rigging Company, providing quality Crane Service and Machinery Movingfor the heavy construction industry.