Hydraulic machines work by using the power and pressure of moving fluids, such as water (the name, "hydraulics," refers to "hydro," or water. It is similar to a similar type of machine which works by the movement or pressure of a gas, known as pneumatics. Hydraulic systems are extremely common today. They are used in construction equipment (the moving arms of machines like excavators and bulldozers), aircraft control surfaces, and automobile brakes.
Basically, all hydraulic machines work by applying pressure against one part of a pipe, cylinder, or other object filled with a fluid, so that the same pressure is exerted at the other end. Like solids, it is difficult or impossible to compress certain fluids. Hydraulics do not have to use water for this purpose: many, for instance, use oil, which is non-compressible. Instead, given the option, it will flow in the direction of least resistance. If a piston pushes against the oil at one end of a cylinder, the same pressure can then be translated through the flowing medium and exerted against another piston at the other end. These are known, respectively, as the master cylinder (which moves the fluid) and the slave cylinder (which is moved by the fluid).
Because of fluid dynamics, hydraulic machines can also take advantage of multiplication of force. If a narrow master cylinder (say, one inch wide) exerts pressure against a very wide slave cylinder (say, nine inches wide), then it must move nine inches to force the slave cylinder to move one inch - but in doing so, it will result in nine times as much force being exerted over that one inch.
Car brakes, for instance, work on this system. In that case, most vehicles have multiple master and slave cylinders in their brake system, so that they will remain at least somewhat effective even if one component breaks down. When a driver presses down on the brake pedal, that force pushes down a piston in the braking system. This is translated through the braking system fluid into pressure against several pistons at the other end, which are shoved outwards. Each of these are connected to a braking pad, which, when pushed outward, applies pressure to the wheel.
Large hydraulic systems are more complex, but operate on the same basic principles. In an excavator, for instance, the engine operates some very powerful pumps that drive long hydraulic pistons to operate the machine's arm.
- Sources and More Information -
HowStuffWorks. "How Hydraulic Machines Work."
University of Alaska Fairbanks. "How Hydraulics Work."