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 systems are everywhere. Your car uses hydraulics in the power steering, brake system and transmission. Every piece of heavy construction equipment you can think of uses hydraulics, including graders, front-end loaders, dump trucks, cranes and more. Factories use hydraulic equipment for lifting, positioning, pressing, and forming their products. Robotic equipment is heavily dependent on hydraulic systems.Hydraulic Valves , Hydraulic Pressure Control Valves

         A basic hydraulic system will have a pump, a pressure control valve, one or more control valves, one or more pistons or hydraulic motors that transfer the hydraulic power into motion and a return system that collects the low-pressure hydraulic fluid and feeds it back to the pump for re-use. See an illustration of this system in Figure 1. This article focuses on the hydraulic control valve illustrated in Figure 1.

Basic Hydraulic Valve

1. 
   Figure 2: Basic Hydraulic Valve
   Hydraulic valves are often called spool valves because the moving part of the valve is reminiscent of a spool of thread. The design of a hydraulic valve allows it to control both the supply and return lines on the equipment being operated. This way it is able to operate equipment in both directions by reversing the high pressure and low pressure connections. The spool is attached to levers or a solenoid that position it to operate the equipment.

   Operating the Hydraulic Valve

2. 
   Figure 3: Operating the hydraulic valve
   When the operator wants to raise his truck he moves the operating lever so that the illustrated valve spool moves to the right. This connects the high pressure to the bottom of the cylinder, lifting the dump on the truck. It also connects the top of the cylinder to the return line so that hydraulic fluid can be pushed out and return to the tank. This is illustrated in Figure 3.

   Reversing the Hydraulic Valve

3. 
   Figure 4: Reversing the Hydraulic Valve
   After the load has been dumped, the operator moves the spool in the opposite direction as shown in Figure 4. This connects the high pressure hydraulic fluid to the top of the cylinder, pushing it down, and allows the oil at the bottom of the cylinder to return to the tank.

   Ending Operation

4. When he is done, the operator releases the control lever and the valve returns to its center position as in Figure 2.