Control components in Hydraulic system
One of the most important functions in any fluid power system is control. If control components are not properly selected, the entire system will fail to deliver the required output. Elements for the control of energy and other control in fluid power system are generally called “Valves”. It is important to know the primary function and operation of the various types of control components. This type of knowledge is not only required for a good functioning system, but it also leads to the discovery of innovative ways to improve a fluid power system for a given application
The selection of these control components not only involves the type, but also the size, the actuating method and remote control capability. There are 3 basic types of valves.
1. Directional control valves
1. Pressure control valves
2. Flow control valves.
Directional control valves are essentially used for distribution of energy in a fluid power system. They establish the path through which a fluid traverses a given circuit. For example they control the direction of motion of a hydraulic cylinder or motor. These valves are used to control the start, stop and change in direction of flow of pressurized fluid.
Pressure may gradually buildup due to decrease in fluid demand or due to sudden surge as valves opens or closes. Pressure control valves protect the system against such overpressure. Pressure relief valve, pressure reducing, sequence, unloading and counterbalance valve are different types of pressure control valves.
In addition, fluid flow rate must be controlled in various lines of a hydraulic circuit. For example, the control of actuator speeds depends on flow rates. This type of control is accomplished through the use of flow control valves.

Directional control valves
As the name implies directional control valves are used to control the direction of flow in a hydraulic circuit. They are used to extend, retract, position or reciprocate hydraulic cylinder and other components for linear motion. Valves contains ports that are external openings for fluid to enter and leave via connecting pipelines, The number of ports on a directional control valve (DCV ) is usually identified by the term “ way”. For example, a valve with four ports is named as four-way valve.

Directional control valves can be classified in a number of ways:
1. According to type of construction :
• Poppet valves
• Spool valves
2. According to number of working ports :
• Two- way valves
• Three – way valves
• Four- way valves.
3. According to number of Switching position:
• Two – position
• Three - position
4. According to Actuating mechanism:
• Manual actuation
• Mechanical actuation
• Solenoid ( Electrical ) actuation
• Hydraulic ( Pilot ) actuation
• Pneumatic actuation
• Indirect actuation

1. Poppet Valves: Directional poppet valves consists of a housing bore in which one or more suitably formed seating elements ( moveable ) in the form of balls, cones are situated. When the operating pressure increases the valve becomes more tightly seated in this design. The main advantage of poppet valves are;
- No Leakage as it provides absolute sealing.
- Long useful life, as there are no leakage of oil flows.
- May be used with even the highest pressures, as no hydraulic sticking (pressure dependent deformation ) and leakages occurs in the valve.
The disadvantages of these valves are;
- Large pressure losses due to short strokes
- Pressure collapse during switching phase due to negative overlap ( connection of pump, actuator and tank at the same time ).

2. Spool valves: The spool valve consists of a spool which is a cylindrical member that has large- diameter lands machined to slide in a very close- fitting bore of the valve body. The spool valves are sealed along the clearance between the moving spool and the housing. The degree of sealing depends on the size of the gap, the viscosity of the fluid and especially on the level of pressure. Especially at high pressures ( up to 350 bar) leakage occurs to such a extent that it must be taken into account when determining the system efficiency. The amount of leakage is primarily dependent on the gap between spool and housing. Hence as the operating pressure increases the gap must be reduced or the length of overlap increased. The radial clearance is usually less than 20 Microns. The grooves between the lands provide the flow passage between ports.