The hydraulic valve is like a traffic control signal at a four-way intersection. It might allow east and westbound traffic lanes to continuously flow until the light changes, which would permit the north or southbound lanes to flow. The east and westbound traffic flow represents circulating hydraulic fluid. The hydraulic valve is representative of the traffic signal. Push the hydraulic valve and circulating fluid is diverted to extend a hydraulic ram. Pull the hydraulic valve and fluid is diverted to contract the same ram. The hydraulic valve is only a small part of the total system.
Hydraulic systems use very high pressure hydraulic fluid to perform various functions. Huge amounts of energy can be transferred over short distances much more efficiently with hydraulic systems than with electrical systems, especially when the desired motion is linear instead of rotational.
Hydraulic valves move and operate systems through the pressure of liquid. In many applications, using liquid to move machinery is preferable to drier methods, since the liquid will not cause the same wear on systems, does not require as many moving parts and often has a smoother, more precise motion that the more mechanic means. Of course, hydraulic systems also come caveats: They need to be both water and air tight to function correctly, and even small cracks or leaks can be disastrous for the system. This makes the valve the most important part of the hydraulic system, since it needs to open and shut with great precision and hold the liquid pressure with perfect accuracy.
While the root of hydraulic may be “hydro,” hydraulic systems do not use water, which is too corrosive and abrasive and does not have the proper pressure qualities to be used in hydraulic systems. Instead, various types of oil are used. Not only do these oils help the system to run smoothly without causing wear on the insides of the pipes, but they also have excellent compression qualities.
In hydraulic systems, the liquid used must be mostly incompressible, or very dense. Dense liquids have many molecules that can fit close together naturally and do not need a lot of space to branch out. When put under pressure, or when a force is applied, the liquid does not change or shrink—the molecules are already as close together as they can be, and more pressure will change neither the space between them nor their chemical composition. Compressible liquids, on the other hand, will collapse under pressure and draw its molecules closer together until the pressure is released. Incompressible oils simply transfer the energy of the pressure onward through the body of liquid.
This means that the pressure can be transferred along the liquid and used to move pistons, turn cranks and perform other types of work. Because the force will be extended throughout the channeled body of liquid, the pipes holding it need to be very strong, but they can also be a variety of shapes and sizes, making hydraulics useful for awkward spaces.
Hydraulic valves allow the liquid to enter or leave specific spaces in the hydraulic system. These valves are used in conjunction with hydraulic pumps and cylinders to control the flow of the fluid. Some valves are automatic, and when the fluid reaches a certain level or pressure inside the cylinder, these valves close, ensuring that the system has the required pressure in that particular part. Too much pressure can damage the system, but the valves also need to have a very reliable seal so no fluid can leak back along toward the pump and lower the pressure again. Other valves are controlled by computers and can be sealed or released at varying degrees depending on the task.