While a liquid and a gas may appear to be very different from each other, each is a fluid (matter that flows). The two behave similarly in many ways. Both, for example, can produce great pushing power in one place when pushed on, or put under pressure in another place far away. Hydraulic and pneumatic systems make use of the behavior of liquids and gases under pressure. Such systems have advantages that have brought them into ever wider use—just about anywhere machines are at work. Every pneumatic system has pneumatic symbols of it own.
Pneumatic systems work very much like hydraulic systems, but they use a gas—usually air—under pressure instead of a liquid. (The term "pneumatic" comes from the Greek word for air, pneuma.) Many pneumatic systems use cylinders, a pump (called a compressor), and valves almost identical to those in hydraulic systems. However, because pneumatic systems operate at much lower pressures than hydraulic systems, they are best suited to lighter tasks.
There are other important differences. Unlike a liquid, air can be compressed. Thus the air in a pneumatic cylinder expands and compresses as the load on the cylinder changes. As a result, a pneumatic cylinder is much springier and less precise than a hydraulic cylinder.
This can be a disadvantage if precise and consistent control is needed. But it can be an advantage if the system is used to grasp or hold an object, as some robots are required to do. A hydraulic system has no "give"—one most choose between too little pressure, which might let the object drop, and too much, which might crush it. In contrast, the springiness of pneumatic systems makes them ideal for robotic grippers.
Pneumatic systems operate the air brakes on trains, buses, and trucks. Air brakes were invented by George Westinghouse, an American manufacturer, in 1868 and have saved countless lives over the years. These systems do not use air pressure to apply the brakes. Rather, the brakes are spring-loaded so that they remain applied until air pressure in the brake cylinders releases them. If a railroad car comes unhooked from a train, for example, air pressure is lost and the brakes automatically stop the car. The operation of all sorts of air brakes is based on pneumatic symbols.
Pneumatic tools are power tools driven by pneumatic symbols. The air is fed to them through flexible hoses from a compressor, which continually draws more air from its surroundings—reservoirs and return valves are not needed. A valve between the air line and the tool lets the operator regulate the tool's power.
Pneumatic tools have a number of advantages over electrical tools. They are simple and rugged. They do not overheat because they are cooled by the air flowing through them. Like hydraulic tools, they do not spark or produce electric shocks. And because the power source can be located some distance away instead of in the tool itself, a pneumatic tool can be more powerful than an electric tool of the same size and weight.
Pneumatics can also be used for transport. In industry, pneumatic conveyors are used to move all kinds of materials—foodstuffs, coal, dry cement. In these conveyors, the materials are blown through tubes by streams of compressed air. Some banks and large stores use pneumatic conveyors to quickly send papers, enclosed in metal capsules, from one office to another. In some of the earliest subway trains, the same principle was even used to carry people—subway cars were shot through snugly fitting tubes by blasts of compressed air.
Hydraulics also come under the category of pneumatics. The word "hydraulic" comes from the Greek word for water pipe, hydraulis. Today machines that use liquids to do work—including waterwheels, turbines, and pumps—are called hydraulic machines. But in recent years, the term "hydraulics" has come to refer mostly to hydraulic energy transfer systems, which are also called fluid power systems. In these systems, a fluid is used to transfer power from its source to the point where it is used.
A hydraulic system could be made to work with any liquid, but most systems use oil or special hydraulic fluids. These fluids do not freeze in cold weather and keep parts of the system well lubricated. Hydraulics also have pneumatic symbols, but here they are known with a different name.
The basic principle on which hydraulic systems depend was discovered by the French scientist Blaise Pascal (1623–62) and is now called Pascal's law. It states that pressure exerted on a confined liquid is transmitted equally throughout the liquid. Suppose, for example, that water is confined in a long tube. If you put pressure on the tube at one end, the water will push against the sides of the tube with equal pressure all the way down the tube's length. Pascal's contributions were so important that the international unit of pressure, the pascal, was named for him. (In the United States, however, pressure is generally measured in pounds per square inch, or psi.)
To see how Pascal's law works in hydraulics, imagine a very simple hydraulic system. It consists of two identical, fluid-filled cylinders, connected by a tube. In each cylinder is a snug-fitting, movable plug called a piston. If you push down on one piston, it will force fluid through the tube to the other cylinder and push its piston up. This piston will be pushed up with exactly the same force as that with which the first piston was pushed down. If you push down on piston A with a force of 10 pounds, for instance, you can lift a 10-pound weight on piston B.
The tube between the cylinders can be of any length. It can be bent to go around corners, or it can be flexible, so that the cylinders can be moved about. Thus a hydraulic system can be used for remote control—that is, to operate machinery that is far away or hard to reach. Because the fluid cannot be compressed, the piston in the slave cylinder (at the remote location) will always move exactly as much as you move the piston in the master cylinder (at the power source). This makes very precise control possible even at a distance.