CONTROL ELEMENTS
DIRECTIONAL CONTROL VALVES (D C VALVES)
Directional control valves are devices which influence the path taken by an air stream. Normally this involves one or all of the following:
- Allowing the passage of air and directing it to particular air lines
- Canceling air signals as required by blocking their passage and / or
- Relieving the air to atmosphere.
Designation of DC Valves
DC Valves are designated by
Its number of controlled connections or Ways,
Number of switching positions,
Mode of actuation and
Mode of reset.
Classification of DC Valves
DC valves basically classified as follows:
Poppet valves
- Ball seat valve
- Disc seat valves
Slide valves
- Longitudinal slide valve
- Longitudinal flat slide valve
- Plate slide valve
Poppet valves
With poppet valves the connections are opened and closed by means of balls, discs, plates or cones. The valve seats are usually sealed simply using elastic seals. Seat valves have few parts which are subject to wear and hence they have a long service life. They are insensitive to dirt and are robust. The actuating force, however, is relatively high as it is necessary to overcome the force of the built – in reset spring and the air pressure.
Slide Valves
In Slide valves, the individual connections are linked together or closed by means of spool slides, spool flat slides or sliding disc valves.
2/2 way valve
The 2/2 way valve has two ports and two positions. It is rarely used except as on-off valve, since its only function is to enable signal flow through and cannot release the air to atmosphere once in closed position. If air is to be released on closing, then the 3/2 way valve must be used.
The 2/2 way is normally of the ball seat construction and it is generally manually operated or electrically operated via a solenoid.
3/2 way valve
The 3/2 way valve is a signal generating valve, with the characteristics that a signal on the output side of the valve can be generated and also cancelled .The 3/2 way valve has three ports and two positions . The addition of the exhaust port 3(R) enables the signal generated via the passage generated through the 3/2 way valve to be cancelled. The valve connects the output signal 2(A) to exhaust 3(R) and atmosphere in the initial position.
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3/2 way valve
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The valve shown below is constructed on the disc seat principle. The sealing is simple but effective. The response time is short and a small movement results in a large cross-sectional area being available for air flow.
Like the ball seat valves, they are insensitive to dirt and thus have a long service life. The 3/2 way valves are used for controls employing single acting cylinders or for generating signals supplied to processing elements and final control elements.
The valves can be actuated manually, mechanically, pneumatically and electrically. Difference actuation methods can be applied to suit the application (Refer appendix – A)
The pneumatically actuated 3/2 way valve is operated by an air signal at 12(z), using no internal auxiliary air assistance. This is referred to as singe pilot operation since there is only one control signal and the valve has spring return. In the initial position, the valve is normally closed since the 1(p) port is blocked by the disc seat and the 2(A) port is exhausted to atmosphere.
The pneumatically actuated valve can be used as a final control element, with indirect control. The signal for extension of the cylinder is initiated indirectly by a pushbutton valve which supplies the control signal to the final control element. The valve can be of relatively large size in comparison to the pushbutton signal element and the signal can be fitted at the remote distance from valve1.
4/2 way valve
The 4/2 way valve has four ports and two positions. An example of the 4/2 way valve is the plate slide valve with hand or foot actuation. It is difficult to fit other means of actuation to these valves. By turning two discs, channels are connected with one another.
5/2 way valve
The 5/2 way valve has five ports and two positions. The 5/2 way valve is used primarily as a final control element for the control of cylinders. An example of the 5/2 way valve, the longitudinal slide valve, uses a pilot spool as a control component. This connects or separates the corresponding lines by means of longitudinal movements. The required actuating force is lower because there are minimal opposing forces due to compressed air or spring.
Sealing presents a problem in this type of Slide valve. The type of fit known in hydraulics as metal to metal requires the spool to fit precisely in the bore of the housing. In pneumatic valves, the gap between spool and housing bore should not exceed 0.002- 0.004 mm, as otherwise the leakage losses will be too great. To save these expensive fitting costs, the spool is often sealed with O-rings or double – cup packing or the bore of the housing is sealed with O-rings. To avoid damaging the seals, the connecting ports can be distributed around the circumference of the spool housing.
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| Spool valve with sleeve |
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Spool valve with oval rings
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Matched metal spool and sleeve valve
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The 5/2 way double air pilot valve has the characteristics of memory control. The last switched position is retained until a new switching position is initiated by a unique pilot signal from the opposite side to the last signal. This new position is memorized until another unique signal occurs.
Plane slide valve
Flow through the ports is controlled by the position of a slide made of metal, Nylon or other plastic. The slide is moved by an elastomer sealed air operated spool.
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Plane slide valve
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Solenoid operation
The electrical operation of a pneumatic valve is effected by a solenoid and plunger, and therefore the units are generally known as solenoid valves.
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Solenoid operation
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AUXILIARY VALVES
Non - return valves or Check valve
These valves can stop the flow in one direction. In the opposite direction the flow is free with a minimal pressure drop due to the resistance of the valve. Blocking of the one direction can be effected by cones, balls or discs.
Quick exhaust valve
This valve used to increase the piston speed of cylinders. This enables lengthy return times to be avoided, particularly with single acting cylinders. The principal of operation is to allow the cylinder to retract or extend at its near maximum speed by reducing the resistance to flow of the exhausting air during motion of the cylinder via a large orifice opening. The valve has a supply connection (P) and an outlet (A). In this direction of flow the air is passed freely via the opening of the check valve component. Port R is blocked by the disc. If the air is supplied to the (A) port the disc seals the (P) port and the air is expelled to atmosphere through the silenced large orifice(R).
Flow control valves
Flow control valves also called as throttle valve influence the volumetric flow of the compressed air in both directions.
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Flow control valves
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Flow control valves are normally adjustable and the setting can be locked in position. These valves are used for speed control of cylinders within a range of values. Care must be taken that the flow control valve is not closed fully, cutting off air from the system.
One way flow control valve
The throttling device shown here is commonly known as a one way flow control valve. With this type of valve, the air flow is throttled in one direction only. A check valve blocks the flow of air in the bypass leg and the air can flow only through the regulated cross- section. In the opposite direction the air can flow freely through opened check valve. These valves are used for the speed regulation of actuators and if possible, should be mounted directly on the cylinder.
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One way flow control valve
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Shuttle Valve (OR function)
This non-return element has two inlets and one outlet. If compressed air is applied to the first inlet, the valve seat seals the opposing inlet. A signal is generated at the outlet. When the air flow is reversed i.e. a cylinder or valve is exhausted, the seat remains in its previously assumed position because of the pressure conditions.
This valve is also called an OR component. If a cylinder or control valve is to be actuated from to or more positions, a shuttle valve should be used.
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| Shuttle Valve |
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| Two Pressure Valve |
Two pressure valve (AND function)
The two pressure valve has two inlets and one outlet. Compressed air flows through the valve only if signals are applied to both inlets. One input signal blocks the flow. If signals are applied to both X and Y, the signal which is last applied passes to the outlet.
The two pressure valve is used mainly for interlocking controls, check functions and logic operations.
COMMENTS