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.