How Hydraulic Machines Work

A short guide to how hydraulic machines work. Information about hydraulic piston systems and examples of their function in everyday life.

One of the best places to get up close and personal lesson on large hydraulic machines is at a construction site. The thing that is most amazing about these machines is their sheer size. From backyard log splitters to the huge machines you see on construction sites, hydraulic equipment is amazing in its strength and agility. On any construction site you see hydraulically-operated machinery in the form of bulldozers, backhoes, shovels, loaders, fork lifts and cranes. In most other hydraulic systems, hydraulic cylinders and pistons are connected through valves to a pump supplying high-pressure oil

You see hydraulics at car service centers lifting the cars so that mechanics can work underneath them, and many elevators are hydraulically-operated using the same technique. Even the brakes in your car use hydraulics. . Hydraulics operates the control surfaces on any large airplane. Tracks are interesting. If you look at the tracks on any piece of large machinery, you will find that there is a hydraulic motor at one end, a free-spinning toothed wheel at the other, and then a set of rollers for the track to move over

The basic idea behind any hydraulic system is very simple: Force that is applied at one point is transmitted to another point using an incompressible fluid. The fluid is almost always an oil of some sort. The force is multiplied in the process. Two pistons fit into two glass cylinders filled with oil and connect to one another with an oil-filled pipe. If you apply a downward force to one piston then the force is transmitted to the second piston through the oil in the pipe. Since oil is incompressible, the efficiency is very good -- almost all of the applied force appears at the second piston. The great thing about hydraulic systems is that the pipe connecting the two cylinders can be any length and shape, allowing it to snake through all sorts of things separating the two pistons. The pipe can also fork, so that one master cylinder can drive more than one slave cylinder if desired.



The neat thing about hydraulic systems is that it is very easy to add force multiplication (or division) to the system. In a hydraulic system, all you do is change the size of one piston and cylinder relative to the other. Let's say he piston on the right is 9 times larger than the piston on the left. What that means is that any force applied to the left-hand piston will appear 9 times greater on the right-hand piston. So if you apply a 100-pound downward force to the left piston, a 900-pound upward force will appear on the right. The only catch is that you will have to depress the left piston 9 inches to raise the right piston 1 inch.

The brakes in your car are a good example of a basic piston-driven hydraulic system. When you depress the brake pedal in your car, it is pushing on the piston in the brake's master cylinder. Four slave pistons, one at each wheel, actuate to press the brake pads against the brake rotor to stop the car. (Actually, in almost all cars on the road today two master cylinders are driving two slave cylinders each. That way if one of the master cylinders has a problem or springs a leak, you can still stop the car.)

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