Chapter 2 Pneumatic Valves LECTURE BY: DR. ALLAN MELVIN PREPARED BY: MOHD SHAHRIL SHARIFF
Pneumatics Component Classifications Power element Control Element Processing Element Input Element Supply Element Control Valves
Pneumatic Control Valve ü Valve are defined as devices to control/regulate the commencement, termination and direction and also the pressure/rate of flow of a fluid under pressure which is delivered by a compressor or is stored in a vessel(receiver tank) ü The form of control energy will be dictated by the valve’s mode of actuation and may be manual, mechanical, electrical, hydraulic or pneumatic. ü Valve available for pneumatic control can be classified according to their function: v. Directional control valves v. Non return valves v. Flow control valves v. Pressure control valves v. Shut-off valves v. Combination valve
Directional Control Valves (DCV) The function of directional control valve (DCV) is to control the direction of flow in the pneumatic circuit. DCVs are used to §start, §stop and §regulate the direction of air flow and § help in the distribution of air in the required line.
Directional Control Valves (DCV)(cont. ) There are commonly two positions of DCVs: Ø The normal position (non-operated) is position with existing reset, e. g. spring, refers to the switching position assumed by the moving parts of the valve, if the valve is not connected. Ø The initial position is the switching position assumed by the moving parts of a valve after valve has been installed in a system and the system pressure has been switched on and possibly also the electrical voltage, and with which the designated switching program starts
Directional Control Valves (DCV)(cont. )
Types of Direction Control Valves Criterion Parameter Based on the number of ports 2 -way valve, 3 -way valve, 4 -way valve, 5 -way valve Based on the number of position 2 -position, 3 -position Based on method of actuation of valve & return actuation. Manual, mechanical, electrical, pneumatic, combination Poppet/seat valve Based on construction Ball seat valve Disc seat valve Sliding valve Longitudinal slide valve(spool valve) Rotary valve/Plate slide valve
A. Based on the Number of Ports §The number of ports in a directional control valve is identified by the term way. Thus, for example, a valve with four ports is a four-way valve. §The ports are marked based on numbering or lettering system. Port ISO 5599 Lettering system Pressure port 1 P Exhaust port 3 R (3/2 DCV) Exhaust ports 5, 3 R, S (5/2 DCV) 2 A(3/2 DCV) 2, 4 B, A (4/2 or 5/2 DCV) Pilot line opens flow 1 to 2 12 Z (single pilot 3/2 DCV) Pilot line opens flow 1 to 4 14 Z ( 5/2 DCV) Signal output/working port Signal Outputs/working ports
B. Based on the Number of Positions §The positions in a directional control valve determines the number of alternative flow conditions the valve can provide. §The number of switching positions of the valve is shown by number of squares §Two-position valves provide two different flow conditions. Open or closed passages from the inlet ports to the outlet ports are changed in each position. §A three-position valve provides three different flow conditions between its port.
B. Based on the Number of Positions (cont. ) §There are several variations in the flow pattern of the center/mid position of three positions directional control valve. The center configuration affects the circuit behavior when the valve is placed in the center/neutral position. §Below show the graphic symbols for 4 -way and 5 -way valves. Pressure closed/ 4 -way valve 5 -way valve
Directional Control Valve: Ports & Switching Positions
Symbols & Functions of DCV Type of Valve Symbol Function 2/2 DCV Drives the air motor and pneumatic equipment 3/2 DCV N/C Drive a single acting cylinder or acts as a switch "on / off". 3/2 DCV N/O Drive a single acting cylinder 4/2 DCV Drive the Double-acting cylinder with one exhaust to release air. 4/3 DCV close centre Drive the Double-acting cylinder with the ability to stop the cylinder in any position to prevent air in the cylinder from the exit.
Symbols & Functions of DCV Type of Valve Symbol Function 5/2 DCV Drives the double acting cylinder with individual exhaust 5/3 DCV Exhaust centre Double-acting cylinder drive with the ability to stop the cylinder at any position by releasing the air in the cylinder. 5/3 DCV Close centre Double-acting cylinder drive with the ability to stop the cylinder in any position to prevent air in the cylinder from the exit. 5/3 DCV Pressure centre/mid centre Stopping in the middle of the cylinder rod to balance the air simultaneously in the bows and rear cylinders
Example of Directional Control Valve 2/2 Directional Control Valve, Normally closed
Example of Directional Control Valve 3/2 Directional Control Valve (normal position)
Example of Directional Control Valve 5/2 Directional Control Valve (normal position)
C. Based On The Method of Actuation of Valves & Return Actuation method of valve Manual Type of control Powered by the operator by pressing the buttons provided General operated Push button operated Lever operated Detent lever operated(hold position) Mechanical Valve is actuated by a mechanical mechanism such as a switch wheel and the cylinder rod. Foot pedal operated Spring return Spring centered Roller operated Idler roller operated Symbol Example Figure construction
C. Based On The Method of Actuation(cont. ) Actuation method of valve Pneumatic Valve is actuated by compressed air which acts move the wind channel. Electrical Actuated by a solenoid valve which is generated by electricity Combined Valve actuated by combination of double solenoid and pneumatic(pilot) Type of control Direct pneumatic (pneumatically operated) Indirect pneumatic Single solenoid Double solenoid, double pilot operated , manual override Symbol Example figure construction
C. Based On The Method of Actuation(cont. ) Pneumatic method(direct) Pneumatic method (indirect)
C. Based On The Method of Actuation(cont. ) Pneumatic method(direct) Combined method
D. Based On Construction Valve construction are categorized as follows: Ball seat valve Poppet valves Disc seat valve Longitudinal slide valve (spool Slide valves valve) Rotary valve/Plate slide valve
Poppet Direction Control Valves §In a poppet valve, simple discs, cones or balls are used in conjunction with simple valve seats to control flow. §The Poppet valve is a simple and effective design used mainly in 2/2 and 3/2 DCV functions. §The poppet seal will give long life (not subjected to sliding friction)
Poppet Direction Control Valves (cont. )
Slide Direction Control Valve §In slide valves, the individual connections are linked together or closed by means of spools, flat slide or plate slide valves to control flow. §Two types of Slide Direction Control Valve: v. Longitudinal slide valve (spool valve) v. Rotary valves/Plate slide valve
(a) Spool Direction Control Valve §A spool has a number of major and minor diameters called lands and valleys. §The lands seal with the valve bore and the valleys connect valve ports to control flow direction §Available in most functions 3/2, 3/3, 5/2, 5/3, etc §Wide range of styles, sizes, operators and mounting arrangements. §Fully force balanced §Suit a multiple range of applications.
(a) Spool Direction Control Valve (cont. ) disc sealer (red) §Dynamic seal type has the seals (disc sealer) on the spool §Glandless type have no sliding seals §Static seal type has the seals fixed in the valve bore Sealer (red)
Example of Spool Valve 5/3 way DCV, closed centre (normal position)
Example of Spool Valve (cont. ) Spool right, port 1 is connected to port 4, port 2 is connected to port 3
Example of Spool Valve (cont. ) Spool left, port 1 is connected to port 2, port 4 is connected to port 5
(b) Rotary valve/Plate slide valve § The rotary valve has a round core with one or more passages or recesses in it. § The core is mounted within a stationary sleeve. As the core is rotated within the stationary sleeve, the passages or recesses connect or block the ports in the sleeve. § The ports in the sleeve are connected to the appropriate lines of the fluid system.
(b) Rotary valves (cont. ) §Plate side valves are mostly operated manually as other types of actuation only be implemented with difficulty. Different position of core and sleeve for various middle/centre position of 4/3 way Direction control valve Closed center Open center Tandem center Floating/vented center
Example of Directional Control Valve 3/2 DCV normally closed, single pneumatically operated, spring return 5/2 DCV, double pneumatically operated, manual override 4/3 DCV closed centre, double solenoid operated, spring centered
Non-Return Valve (NRV) �Non return valves permit flow of air in one direction only, the other direction through the valve being at all times blocked to the air flow. �Among the various types of non-return valves available are as follows i. Check valve ii. Spring-loaded check valve iii. Shuttle valve iv. Quick exhaust valve v. Two pressure valve vi. Shut-off valve
i) Check Valve If the inlet pressure at 1 is higher than the outlet pressure at 2, then the check valve allows the flow to pass, otherwise it blocks the flow. free flow No flow
ii) Check valve-Spring loaded If the inlet pressure at 1 is higher than the outlet pressure at 2 and force of the internal spring, then the check valve allows the flow to pass, otherwise it blocks the flow. free flow No flow
iii) Shuttle Valve (OR) • 2 input and 1 output • Allow a signal to flow in one direction • Output is acquired if one of input supply pressure • For circuit with more than 1 input signal (safety purpose)
iii) Shuttle Valve (OR)-(cont. ) Circuit Diagram with Shuttle Valve Actuated
iv) Quick Exhaust Valves • 1 input, 1 output and 1 exhaust • To increase speed of piston movement (at port 3) silencer • 1 input, 1 output, 1 exhaust without silencer
iv) Quick Exhaust Valves (cont. ) Quick Exhaust Valve during Forward Motion Sealing disc Quick Exhaust Valve during Return Motion
v) Two Pressure Valve (AND) § 2 inputs (12 and 14) and 1 output (2). § Output is acquired if both inputs supply pressure. 12 2 14
v. Two Pressure Valve (AND)-(cont. )
v. Two Pressure Valve (AND)-(cont. )
Flow Control Valve/Choke Valve §Flow control valve influence the volumetric flow of the compressed air in both direction. §Function: ØTo control speed of actuators. ØTo control a constant speed of actuators. ØAs a cushioning tool of actuators ØTo control heavy load §Two types of flow control valves: a. Throttle valve b. One-way flow control valve
i. Throttle Valve §Throttle valves are normally adjustable and the setting can be locked in position. §Throttle valves are used for speed control of cylinder. §Care must be taken that the throttle valves does not close fully.
ii. One-way Flow Control Valve §Air flow is throttled in 1 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 the opened check valve
ii. One-way Flow Control Valve (cont. ) Supply Air Throttling • Supply air entering the cylinder through the working ports, undergoes throttling as the non return valve is closed in the direction of flow. • During exhaust , the compressed air leaving the cylinder is by passed through the non return valve and escapes freely as it does not under go throttling • Supply air throttling can be used for single acting cylinder and small volume cylinder
ii. One-way Flow Control Valve (cont. ) Exhaust Air Throttling • Supply air flows freely to the cylinder through the bypassage of the non return valve. The supply air does not under go any throttling • Exhaust air leaving the cylinder has to under go throttling as the non return valve is closed in the return direction • Used for double acting cylinder • Not suitable for small volume cylinders and cylinders with short strokes as effective pressure cannot build up sufficiently.
Pressure Control Valve 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. There are basically three (3) types of pressure control valves: §Pressure release valve/relief valve/safety valve/limiting valve §Pressure sequence valve §Pressure regulating valve/reducing valve (REFER Chapter 1)
(i) Pressure Release Valve/Relief Valve/Safety § 1 input and 1 exhaust §These valves perform a safety relief function by opening to the atmosphere if a predetermined pressure is exceeded in the system, thus releasing the excess pressure.
(ii) Pressure Sequence Valve • Sequence Valve generates a pneumatic signal if the sensing pressure [signal input-port 12] is more than the desired set pressure • This generated out put signal is used to control the movement of cylinder by using it as a set signal or reset signal to the final control valve to obtain forward or return motion respectively • This is a combination valve, having two sections. One of the section is a 3/2 directional control and the other a pressure control valve
(ii) Pressure Sequence Valve (cont. ) Pilot spool valve Adjustable Pressure Sequence Valve. Actuated
Example 2. 1
Solution Example 2. 1 All valves are unactuated in the initial position. Pressure is applied at the piston rod side of the cylinder and the piston rod remain in the retracted status. Actuation of the push button switches the valve 3/2 DCV to flow and a signal is applied at the control port 14 of the double pilot valve 5/2 DCV. The valve 5/2 DCV switches, pressure is applied at the piston side of the cylinder and piston rod advances. The switching status of the double pilot valve 5/2 DCV remain intact if the push button is released. When the piston rod reaches the workpiece, travel is stopped and pressure starts to build up on the piston side. The increasing pressure causes the force of the die to increase.
Solution Example 2. 1 (cont. ) The control port 12 of the pressure sequence valve is connected to the pressure line on the piston side of the cylinder. When the pressure in the cylinder reaches the valve set on the pressure sequence valve, the 3/2 DCV valve switches. A signal is now applied at the control port 12 the 5/2 DCV. The valve 5/2 DCV switches, pressure is applied at the piston rod side of the cylinder and the piston rod retracts. During retraction, the response pressure set on the pressure sequence valve is not met and the pressure sequence valve return to its initial position.
(iii) Pressure Regulating Valve §It is to maintain constant pressure even with fluctuating supply. §The input pressure must be greater than the required output pressure
Shut-off Valves §Shut-off valve are non-adjustable valves, which release or shut off flow in both directions. §Typical example are the stop cock and ball cock. Stop cock
Combination Valve(Time Delay Valve) • Pneumatic Timers are used to create time delay of signals (5 -30 seconds) in pilot operated circuits. • Available as Normally Closed Timers and Normally Open Timers. • Usually Pneumatic timers are On Delay Timers delay of signals is very commonly experienced in applications such as Bonding of two pieces. • Normally Open Pneumatic Timers are used as safety device in Two Hand Blocks
Time Delay Valve (cont. ) A Pneumatic Timer is a combination valve which consists of three parts 1. 3/2 way pneumatically operated directional control valve [NC or NO], 2. A one way flow control valve and 3. An accumulator/air reservoir
Time Delay Valve (cont. ) Working Principle of Time Delay Valve [N. C] §Signal input is supplied at port 1 and delayed signal out put is taken at 2. §The signal control air flow into at port 12 through one-way flow control valve and air reservoir §When the necessary control pressure has built up in the air reservoir, the pilot spool of 3/2 way valve is moved downwards. §This blocks the passage from 2 to 3 and thus air can flow from 1 to 2.
Time Delay Valve (cont. ) Time respond with delay-on Two methods to increase a delay when using the delay time valve: § Increase the size of accumulator/additional of external accumulator. § Decrease the valve opening of one way flow control valve
Example 2. 2
Solution Example 2. 2