How Cranes Work, Part 4: Mechanical Advantage

We recently examined the importance of simple machines and their roles in construction cranes through three articles on the lever, the pulley, and the hydraulic cylinder. The three previous articles illustrated how each of these simple machines manipulate torque to increase lifting capacities while minimizing the amount of effort required to lift them. In this fourth and final segment on the science behind construction cranes, we will discuss the concept of mechanical advantage and why it's important.

Almost all construction sites require heavy lifting. If they must move an extremely heavy load, they will likely employ a crane. The crane's greatest ability is to lift enormous objects; this much is obvious. However, how cranes do this is fairly complicated, as cranes employ a number of simple machines to lift large loads. In any event, the goal of the crane, and simple machines in general, is to minimize the force needed to lift monstrous loads.

Ultimately, cranes minimize the force applied, or the input force, to create the greatest lifting force, or the output force. This goal is simply known as mechanical advantage: exerting the lowest force possible to maximize lifting potential.

We may define mechanical advantage in two ways. Mechanical advantage ("MA") equals the output force divided by the input force. You may also measure it by dividing the distance over which the effort or input force is applied by the distance over which the resulting force acts, or the distance over which the heavy object moves.

Consider this example. You may have a lever 30 feet in length, as the fulcrum sits 10 feet from one end. You may press down on the longer, 20-foot arm to raise an object with the shorter, 10-foot arm. In this case, the MA equals 2. This is also known as the ideal mechanical advantage ("IMA") because there is no friction. Likewise, you may apply 6,000 pounds of input force into some machine which results in 24,000 pounds of output force. If there is no friction, the IMA equals 4,000.

Unfortunately, there are almost never any instances in which friction is absent. When considering friction, you measure an actual mechanical advantage ("AMA"), which equals the resistance force of the machine divided by the effort or applied force. The resistance force not only includes the weight of the object being moved but also the amount of friction. In a real life example, you may use a machine to lift a heavy object that weighs 80 Newtons. There exists, however, a friction force of 40 Newtons. An effort force of 20 Newtons may lift the object, but the AMA equals 2, as friction force is a negative force.

Likewise, you can measure the mechanical efficiency of a machine when you divide the AMA by the IMA. In the last example, the IMA would equal 2.5. Therefore, the mechanical efficiency of the system 0.8, or 80%. Mechanical efficiency is a good tool for measuring how effective a construction crane, or any simple machine for that matter, will lift a heavy load.

In conclusion, mechanical advantage measures the abilities of the simple machines covered in the first three articles. Mechanical advantage also measures the ability of particular cranes that employ a number of simple machines. The lever, the pulley, and the hydraulic cylinder all maximize the use of torque in heavy lifts, but mechanical advantage is a method by which we compare these machines. This science is crucial to the way cranes work, and likewise, this science makes us able to complete magnificent works in the construction industry.

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

The Science Behind Cranes: Part 3, The Hydraulic Cylinder

In our first two editions, we briefly saw how cranes employ the simple machines known as levers and pulleys to maximize lifting capacity. Today's installment will cover the role of the hydraulic cylinder, and how it compares to the lever and pulley. Our next and final edition will review perhaps the most importance concept behind the physics of cranes, otherwise known as mechanical advantage.

Now, what exactly is a hydraulic cylinder? Well, simply put, a hydraulic cylinder is a cylinder, or a circular prism, that is completely filled with a fluid, most often an oil, that has two pistons. The pistons can be connected to the cylinder in a number of configurations.

Assuming each piston is the same size and weight and there is no friction, when something presses down on one of the pistons the other piston will move up at an equal force, speed, and distance. For example, if you press one piston down three inches, the second piston will shoot up three inches.

The greatest advantage to a hydraulic cylinder is that you may easily redirect forces from one plane to another. For example, one piston may be connected horizontally while the second may be positioned vertically. Levers and pulleys, as we saw before, do not translate direction this easily, and any force applied will result in a force on the same plane in the opposite direction. For example, moving a lever arm downward will move the opposite arm upward, and vice versa. However, the hydraulic cylinder will allow a force to be translated into a number of direction, such as up, down, forward, backward, left, or right.

On the other hand, the hydraulic cylinder can multiply forces by maximizing torque, as we saw with the lever and pulley. If one piston has an area of 6 square units, and another piston has a 2 square units, then the force pushing down on the smaller piston will appear 3 times greater on the larger piston. For example, if one pushes the 2-square-unit piston down with a force of 500 pounds, then the 6-square-unit piston receive a push with the force of 1500 pounds. However, the distance the larger piston moves will be 3 times less than the distance the smaller piston moved to create 1500 pounds of force.

Also similar to the lever and pulley, the hydraulic cylinder is used in almost all cranes. The cylinder may be applied directly to lift a heavy load, or it may aid another mechanism that directly lifts the load. A cylinder could be used in lever arms on cranes, or it may be used to move a jib or beam that acts as the lifting mechanism for the crane.

In conclusion, the hydraulic cylinder is much like the pulley and lever for its frequent use in cranes and its manipulation of torque. However, the hydraulic cylinder sets itself apart because of its ability to redirect forces to different planes. However, all three, the lever, pulley, and hydraulic cylinder, collectively maximize the mechanical advantage in lifting large objects. In the next installment, we will examine exactly what mechanical advantage is and how it's applied to cranes.

Serving in the crane industry for decades, Barnhart Crane and Rigging Companying provides the best in Crane Service andMachinery Moving.