How hydraulic machines work

In any plant, you see hydraulically operated machinery such as actuators, punch presses or forklifts. In this edition of "How Stuff Works," Marshall Brain explains how.


Plant Engineering - January 2001


How Hydraulic Machines Work


Adapted from

In any plant, you see hydraulically operated machinery in the form of actuators, punch presses, forklifts, and injection molding machines. Hydraulics operate the control surfaces on any large airplane. You see hydraulics at car service centers lifting 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!

The basic idea
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 oil of some sort. The force is almost always multiplied in the process. The picture below shows the simplest possible hydraulic system.

In this drawing, two pistons fit into two cylinders filled with oil and are connected to one another with an oil-filled pipe. If you apply a downward force to one piston (the left one in this drawing), 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 be split, 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. Trading force for distance is very common in mechanical systems. The same is true for hydraulic systems. In a hydraulic system, all that you do is change the size of one piston and cylinder relative to the other.

To determine the multiplication factor, start by looking at the size of the pistons. Assume that the piston on the left is 2 in. in diameter, while the piston on the right is 6 in. in diameter. The area of the two pistons is pr2. The area of the left piston is therefore 3.14 sq in., while the area of the piston on the right is 28.26 sq in. The piston on the right is nine times larger than the piston on the left. What that means is that any force applied to the left piston will appear nine times greater on the right piston. So if you apply a 100-lb downward force to the left piston, a 900-lb upward force will appear on the right. The only catch is that you will have to move the left piston 9 in. to raise the right hand piston 1 in.

The brakes in your car are a good example of a basic piston-driven hydraulic system. When you press 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.

Many hydraulic systems, hydraulic cylinders, and pistons are connected through valves to a pump supplying high-pressure oil. For example, a hydraulic metal-cutting punch press has valves that direct the oil to either side of the main hydraulic ram. When an operator presses the button to lower the ram, oil is directed through the valve to the top of the ram, forcing it downward, pressing the cutter through the metal and into the die.

When the ram reaches a predetermined limit of travel, it actuates a switch, which effectively changes the position of the valve. This directs the oil to the bottom side of the ram, forcing it upward to the home position.


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