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.
Monday, November 29, 2010
The Science Of How Cranes Work: The Lever, Part I
Have you ever wondered how certain technological items? Well, this article, plus the next three parts, serve to explain the science behind construction cranes. First, we will explain how a lever increases the crane's ability to lift really heavy loads. The next articles will investigate the role of the lever, the hydraulic cylinder, and the concept of mechanical advantage in the science behind construction cranes.
To a greater or lesser extent, all cranes employ the lever to lift really heavy loads. Balance cranes and all mounted cranes optimize lifting capacity through the lever. These cranes have a mechanical arm that acts as such a lever. Although the arm is usually accompanied by a complex system of pulleys, ropes, and chains, the lever is classified as a simple machine.
Scholars insist that the ancients practically applied the lever in the building of large temples, monuments, and fortifications. In fact, many conjecture that the Egyptians utilized the lever in the building of the Great Pyramids. However, most attribute the geometrical and mathematical theory behind the lever to the ancient Greeks, most particular Archimedes in the third century B.C.E. He famously quipped, "Give me a place to stand, and I shall move the Earth with a lever."
Since then, architects and engineers throughout history have optimized the lever for particular lifting purposes. A lever is defined as a rigid "bar" that rests on a pivot point, or fulcrum, where you apply an "effort" force to create a resulting "work" force that lifts some object.
Physicists categorize levers into three classes. First class describes levers where the fulcrum rests between the effort and lifting forces, as one sees in a seesaw or crowbar. Second class defines levers in which the load forces sits between the fulcrum and the applied force, such as a wheelbarrow. And finally, third class indicates levers in which one applies the effort force between the fulcrum and the load. For example, a set of tweezers is an example of third-class levers.
These classes define all possible levers, but why does this matter? Well, different classes of levers can lift loads of varying weights for numerous purposes. Most importantly, these levers manipulate the mathematical concept of Torque. In Physics, torque equals the effort force times the distance over which the force is applied. For example, applying 40 pounds of effort over five feet is much harder than applying a mere two pounds of effort over 100 feet. Both applications require the same amount of Torque to lift some object, but the second requires much less "effort" force for humans to apply. It literally requires less effort. This is why, for example, pulling a nail out of a board by hand is much harder than using a crowbar.
Be sure to catch our next article on pulleys in construction cranes.
Serving in the crane industry for decades, Barnhart Crane and Rigging Companying provides the best in Crane Service andMachinery Moving.
To a greater or lesser extent, all cranes employ the lever to lift really heavy loads. Balance cranes and all mounted cranes optimize lifting capacity through the lever. These cranes have a mechanical arm that acts as such a lever. Although the arm is usually accompanied by a complex system of pulleys, ropes, and chains, the lever is classified as a simple machine.
Scholars insist that the ancients practically applied the lever in the building of large temples, monuments, and fortifications. In fact, many conjecture that the Egyptians utilized the lever in the building of the Great Pyramids. However, most attribute the geometrical and mathematical theory behind the lever to the ancient Greeks, most particular Archimedes in the third century B.C.E. He famously quipped, "Give me a place to stand, and I shall move the Earth with a lever."
Since then, architects and engineers throughout history have optimized the lever for particular lifting purposes. A lever is defined as a rigid "bar" that rests on a pivot point, or fulcrum, where you apply an "effort" force to create a resulting "work" force that lifts some object.
Physicists categorize levers into three classes. First class describes levers where the fulcrum rests between the effort and lifting forces, as one sees in a seesaw or crowbar. Second class defines levers in which the load forces sits between the fulcrum and the applied force, such as a wheelbarrow. And finally, third class indicates levers in which one applies the effort force between the fulcrum and the load. For example, a set of tweezers is an example of third-class levers.
These classes define all possible levers, but why does this matter? Well, different classes of levers can lift loads of varying weights for numerous purposes. Most importantly, these levers manipulate the mathematical concept of Torque. In Physics, torque equals the effort force times the distance over which the force is applied. For example, applying 40 pounds of effort over five feet is much harder than applying a mere two pounds of effort over 100 feet. Both applications require the same amount of Torque to lift some object, but the second requires much less "effort" force for humans to apply. It literally requires less effort. This is why, for example, pulling a nail out of a board by hand is much harder than using a crowbar.
Be sure to catch our next article on pulleys in construction cranes.
Serving in the crane industry for decades, Barnhart Crane and Rigging Companying provides the best in Crane Service andMachinery Moving.
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