Chapter 4: Control components in Hydraulic system

1 fluid power Control Systems. ( For private ciruculation only) Jagadeesha T, Assistant Professor, Mechanical Engineering Department, NIT Calicut Chapter 4: 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.

2 There are 3 basic types of valves. 1. Directional Control valves 2. Pressure Control valves 3. 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.

3 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. fluid power Control Systems. ( For private ciruculation only) Jagadeesha T, Assistant Professor, Mechanical Engineering Department, NIT Calicut Directional Control valves can be classified in a number of ways: 1.

4 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 The designation of the directional Control valve refers to the number of working ports and the number of switching positions. Thus a valve with 2 service ports and 2 switching positions is designated as 2 / 2 way valve. A P Fig 2 /2 valve symbol A valve with 3 service ports and 2 position is designated as 2 / 3 way valve.

5 A P T Fig 2 / 3 valve symbol fluid power Control Systems. ( For private ciruculation only) Jagadeesha T, Assistant Professor, Mechanical Engineering Department, NIT Calicut A valve with 4 service ports and 2 position is designated as 2 / 4 valve. A B P T Fig 2 / 4 valve symbol A valve with 4 Service ports and 3 Switching position is designated as 3 / 4 way valve.

6 Fig 4 shows an example of open centered position. A B P T Fig 3/ 4 valve symbol : Spool positions as well as their corresponding actuating elements are labelled with numbers 1, 2 as shown in figure 5. A valve with 2 spool position is shown and also a valve with 3 spool positions. In directional Control valves with 3 spool position, the central position is the neutral position ( or mid position or zero or null position ). The neutral position is the position in which the moving parts are assumed to be inactive, but affected by a force ( spring ) The ports are designated as follows: P = Pressure Port ( Pump Port ) T = Tank Port A , B = User Ports 1 2 1 0 2 2- Position valve 3- Position valve Fig Basic symbol for directional Control valves fluid power Control Systems.

7 ( For private ciruculation only) Jagadeesha T, Assistant Professor, Mechanical Engineering Department, NIT Calicut Observe that the graphical symbol shows only one tank port T even though the physical design may have two since it is only concerned with the function of a component and not its internal design. The tank port is the port of the valve that is piped back to the Hydraulic oil tank. Therefore, each tank port provides the same function. The spool valve working ports are inlet from the pump, outlets to the cylinder, and exhaust to tank. These ports are generally identified as follows : P = pressure ; A or B = actuator and T = tank. 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.

8 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 / 2 DCV (Poppet design) :- A A P P a.

9 Valve Closed b. Valve Opened Fig 2 / 2 DCV Poppet Design Figure shows a ball poppet type 2 / 2 DCV. It is essentially a check valve as it allows free flow of fluid only in one direction (P to A) as the valve is opened hydraulically and hence the pump Port P is connected to port A as shown in fig b. In the other direction the valve is closed by the ball poppet ( note the fluid pressure from A pushes the ball to its seat) and hence the flow from the port A is blocked ( fig a.). The symbol for this type of design is same as that of check valve.(fig ) No flow Free flow Fig Symbol of 2/2 poppet valve ( Check valve ) fluid power Control Systems.

10 ( For private ciruculation only) Jagadeesha T, Assistant Professor, Mechanical Engineering Department, NIT Calicut 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.