Shuttle Valve?

                A shuttle valve allows two alternate flow sources to be connected in a one-branch circuit. The valve has two inlets P1 and P2 and one outlet A. Outlet A receives flow from an inlet that is at a higher pressure. Figure 1.5 shows the operation of a shuttle valve. If the pressure at P1 is greater than that at P2, the ball slides to the right and allows P1 to send flow to outlet A. If the pressure at P2 is greater than that at P1, the ball slides to the left and P2 supplies flow to outlet A .

               One application for a shuttle valve is to have a primary pump inlet P1 and a secondary pump inlet P2 connected to the system outlet A The secondary pump acts as a backup, supplying flow to the system if the primary pump loses pressure. A shuttle valve is called an “OR” valve because receiving a pressure input signal from either P1 or P2 causes a pressure output signal to be sent to A

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Pilot-Operated check Valve?

               A pilot-operated valve along with its symbol is shown in Fig. This type of check valve always permits free flow in one direction but permits flow in the normally blocked opposite direction only if the pilot pressure is applied at the pilot pressure point of the valve. The check valve poppet has the pilot piston attached to the threaded poppet stem by a nut. The light spring holds the poppet seated in a no-flow condition by pushing against the pilot piston. The purpose of the separate drain port is to prevent oil from creating a pressure build-up at the bottom of the piston. The dashed line in the graphical symbol represents the pilot pressure line connected to the pilot pressure port of the valve. Pilot check valves are used for locking hydraulic cylinders in position

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Advantages and Disadvantages of a Poppet Valve?

Advantages of a poppet valve 

 Virtually zero leakage in closed position.

 Poppet elements do not stick even when left under pressure for long periods
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 Fast, consistent response time: typically 15 ms.

Disadvantages of a Poppet Valve A poppet valve has the following disadvantages:

 Axial pressure balance is impossible and considerable force may be needed to open the poppet against the flow at a high pressure. This limits valves that have direct mechanical actuation to low flow duties.

 Generally individual poppets are required for each flow path that significantly increases the complexity of multi-port valves
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 Lapping and super finishing of valves add cost.

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Check Valve?

               The simplest DCV is a check valve. A check valve allows flow in one direction, but blocks the flow in the opposite direction. It is a two-way valve because it contains two ports

               provides two schematic drawings showing the operation of a poppet check valve. A poppet is a specially shaped plug element held on a valve seat by a light spring. Fluid flows through the valve in the space between the seat and poppet. In the free flow direction, the fluid pressure overcomes the spring force. If the flow is attempted in the opposite direction, the fluid pressure pushes the poppet in the closed position. Therefore, no flow is permitted.

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Graphical symbols of Actuating Devices?

What is meant by Actuating Devices?

                      Direction control valves may be actuated by a variety of methods. Actuation is the method of moving the valve element from one position to another. There are four basic methods of actuation: Manual, mechanical, solenoid-operated and pilot-operated. Several combinations of actuation are possible using these four basic methods. 

 Manually operated: In manually operated DCVs, the spool is shifted manually by moving a handle pushing a button or stepping on a foot pedal. When the handle is not operated, the spool returns to its original position by means of a spring.

 Mechanically operated: The spool is shifted by mechanical linkages such as cam and rollers.

 Solenoid operated: When an electric coil or a solenoid is energized, it creates a magnetic force that pulls the armature into the coil. This causes the armature to push the spool of the valve.

 Pilot operated: A DCV can also be shifted by applying a pilot signal (either hydraulic or pneumatic) against a piston at either end of the valve spool. When pilot pressure is introduced, it pushes the piston to shift the spool.

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4/3 Valves ?

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