Imagine lifting extremely heavy objects using your own physical force. Not only is it tremendously difficult but it is also very inefficient. This duty of carrying and moving around gigantic objects has been made easy by the principle of hydraulics. Using a hardly compressible fluid, the force exerted on one object can be easily (and almost effortlessly) transferred to another. With a cylinder (containing, say, oil as the incompressible liquid) connected to pistons with exactly fitting valves, you already have a simple hydraulics machine. Factors such as sizes of the pistons and compressibility of the fluid affect the amount of force transferred so designing a feasible hydraulics system can be very complicated.
One part of the modern hydraulics systems is called the hydraulic directional valve. This valve is an important part because it regulates the direction of the fluid being pumped, the pressure to be exerted and the rate of the fluid. Because hydraulic valves are exposed to high pressure, they are made of very durable materials such as steel. The seal around the valve is also made of a tough material so it can withstand the pressure and continuous up-and-down motion; if not, the piston will leak and the pressure within the system will not be controlled.
A hydraulic directional valve can have different designs. The most common valves would be the spool or the type that uses a sliding piston. Another design is called the Poppet valve where a ball, a cone or a flat plate, is used. This design is definitely applicable for hydraulics, however, because of the odd shapes, the system might not be as sturdy as the conventional designs. Another is called the rotary valves where a cross-shaped material rotates allowing and prohibiting the flow of liquid. Cartridge valves are a compact type and a block can contain more than just the directional valve.
There are at least two ways by which hydraulic directional valves can be operated. One method is referred to as pilot operated. For example, in a three-port valve, you'll have two ports across one port. As the valve slides, it opens one of the two ports allowing free flow of the liquid towards the opposite port. Opening and closing are alternating between the adjacent ports. Another method to operate a directional valve is through the use of solenoid, an object made of coil and an iron plunger. This material becomes magnetized and this condition moves the valve.
Inspecting a hydraulic system as well as its parts, including the hydraulic directional valve, is a crucial job. Keeping the whole system tight is essential to keep it efficiently working. Consumables should be replaced as soon as they show signs of breakage and the connections should be kept as tight as possible. The fluid reservoir should also be checked as the compressibility of the fluid might increase when the fluid is not clean or has been used for a long time. Bubbles should be avoided at all costs since it will hamper the sliding motion of the valve.