The Hydraulic Crane – how it works

Anyone who has seen a crane effortlessly lifting an enormous piece of equipment has certainly witnessed the power of hydraulics. Perhaps a few of those observers have even wondered “how does it do that?”. Although the precise movement of multi-ton objects seems like magic, it is accomplished through the thoughtful application of a few relatively simple concepts from physical science. Foremost is the idea that “liquids don’t squish”.

Hydraulic systems are all designed around the idea that fluids are at their maximum density. Plainly said, if you increase the pressure on a fluid, its molecules don’t get any closer together. This is distinctly different than a gas, which can be squeezed to increase its density (if you squeeze a gas hard enough, it changes states to become a liquid).

What does all this talk of density and pressure really mean for the crane operator? Well, if you have a tube filled with fluid and you push a plunger in on one end, the fluid pushes out the other end with exactly the same amount of force. That, basically is how a hydraulic ram works. When it is time to do the heavy lifting, the crane operator engages a pump that increases hydraulic pressure against a plunger, the other end of which is supporting the long arm of the crane. As the pressure builds, to over 3,000 pounds per square inch, the plunger in is able to extend against the weight of even the heaviest of loads.

Now, beyond supporting the boom under a tremendous load, most crane operators have some desire to manipulate the load, to move it from here to there, and to keep the crane from tipping over while they work. These tasks are often also accomplished with hydraulic systems. Usually another hydraulic pump moves fluid to rotate the crane’s boom around thus allowing the lateral movement of the load. Hydraulic pressure, often from this same steering pump, is used to extend and retract outriggers that support the base of the crane so that the load can be moved farther away from the base.

Sitting in his cab, the crane operator is directing pumps and valves, using hydraulic fluid to push and pull plungers, typically called rams, that translate fluid force into motion. Although it looks like magic and can be breathtaking to watch, hydraulic crane work is the careful application of physical principles to fluid systems.