Have you ever marveled at modern technology? Although lots of modern technology and machinery are, in fact, very complicated, some are actually very sensible, once you weed out the bells and whistles.
The construction crane, for instance, is such a machine. The crane generally employs only three simple machines. The lever, the pulley, and the hydraulic cylinder.
In this article, we will briefly examine a very importance mechanism in the construction crane: the lever. Three subsequent articles, however, will investigate the role of the pulley, the hydraulic cylinder, and the concept of mechanical advantage, respectively, in construction cranes.
So, how do cranes work? To a greater or lesser extent, most cranes utilize the lever to lift exceptionally large loads. Almost all mounted cranes and many balanced cranes maximize lifting capacity with the lever.
These cranes use levers, or mechanical arms, that increase its strength. Although a complex system of ropes, chains, and pulleys usually accompany the mechanical arm, the lever itself is merely a simple machine.
The ancients have long used the lever in practice to build large temples, monuments, and fortifications. In fact, scholars contend that the Egyptians most likely used levers to construct the Great Pyramids.
However, most historians attribute the development of the geometric theory behind the lever to Archimedes. Archimedes, a Mathematician and Philosopher, lived in Ancient Greece around the third century B.C.E. Purportedly, he once quipped, “Give me a place to stand, and I shall move the Earth with a lever.”
The lever itself is a stable bar that rests on a pivot point, or fulcrum. You can press down on one end with some “effort” force to produce some resulting “work” force on the other end. The work force usually carries or holds the object being lifted.
Scientist classify all levers into three different groups. In class one levers, the fulcrum sits between the effort and work forces, as in a seesaw or crowbar. Class two levers are levers in which the the work force sits between the fulcrum and the effort force, like a wheelbarrow. And in class three levers, the effort force is applied between the fulcrum and the work force, as in tweezers.
But, again, how do cranes work? As we will see with the pulley and hydraulic cylinder, the lever manipulates a concept known as torque. Torque measures the distance over which a force is applied, or torque equals force times distance.
As Archimedes realized, manipulating torque provides greater lifting capacities. For example, consider a simple seesaw on a playground. The seesaw in ten feet long, and it pivots on a bar directly in the center of the seesaw board. One on side sits a 200 pound kid, and on the opposite side sits a scrawnier 100 pound kid.
The fatter kid will certainly push his side of the seesaw down to the ground, while the scrawny kid raises up. For the smaller kid, the must apply an extra 100 pounds of force to merely balance out the seesaw!
But what if he had magical abilities that allowed him to extend his side of the seesaw by 5 more feet. His ten-foot side of the seesaw, matched with his 100 pound weight, would allow him to balance the seesaw. And, theoretically, if he extended his side to a length greater than 10 feet, his side would slowly creep the ground, lifting the fatter kid off the ground.
Yet, again, how do cranes work? The lever, in part, manipulates torque allow cranes to lift very heavy loads. The more you spread the effort force over greater distances, the less “effort” force will be required to make the lift. Levers don’t only help scrawny children but also hundreds of engineers, architects, and construction workers who lift gigantic loads everyday!
Stay tuned for the next segment in our series “How Do Cranes Work?”, when we will explore the role of the pulley. Then we will move on to the hydraulic cylinder and the concept of mechanical advantage.