Fluid Power. Open triangles at valve Ports P and E show entry and exit points for compressed air to and from the valve.

Transcription

1 Control of Single-Acting Cylinder. Figure The 3-way directional valve used here has a solenoid actuator and spring return on its spool. The single-acting cylinder extends by air pressure and retracts by spring force. When the solenoid is energized (top figure), the valve spool moves downward. Air is connected to the cylinder and the exhaust connection on the valve is blocked. The cylinder extends, compressing the return spring. When the valve solenoid is de-energized, the valve spool moves upward. Inlet air is blocked, while the cylinder is vented to atmosphere through the valve ports. The internal spring causes the cylinder piston to retract. Open triangles at valve Ports P and E show entry and exit points for compressed air to and from the valve. Air Motor Control. Figure A 3-way directional valve may be used instead of the shutoff valve of Figure 3-11 to start and stop a nonreversing air motor, with the advantage that when stopped, both motor ports are vented to atmosphere. A reversing air motor would require a 4-way valve. A solenoid 3-way valve is shown here. Other types of actuators could be used if appropriate. When the solenoid is de-energized (top view), inlet air pressure is cut off, and the motor is in a free floating condition with both ports open. With solenoid energized, air pressure is connected to the motor. If this circuit is used to control hydraulic motors it should be limited to low power applications because the incoming oil supply is stopped. On high power hydraulic motor applications some means should be provided to unload the oil supply. One good method is to use a 4-way rather than a 3-way valve to control the motor. 1

2 SHUTTLE VALVE (DOUBLE CHECK) Shuttle Valve. Figure This valve is officially termed a "Double Check Valve With Cross Bleed". It has two inlets and one outlet. Since no official port markings have been assigned, the letters "P1" and "P2" are used for the two inlets, and "A" for the outlet. This is the same marking used for a 3-way, 2-pressure valve, which also has two inlets. The purpose of a shuttle valve is to permit flow into Outlet A from either P1 or P2 (but not both at the same time) while keeping the two inlets isolated from one another. The valve will pass flow in the reverse direction to either P1 or P2, depending on the position of the shuttle at that moment. The floating poppet which is called the "shuttle" is free to move back and forth, closing off the inlet which has the lower pressure and opening the inlet which has the higher pressure. A pressure difference of 1 PSI or less between the two inlets is usually sufficient to shift the shuttle. Shuttle Valve Application. Figure If Valve 1 is actuated, air coming through this valve enters Port P1 of the shuttle valve, pushes the shuttle over to block Port P2, and moves into the cylinder to extend it. Shuttle action prevents loss of air to atmosphere through Valve 2 exhaust. When Valve 1 is released, air flows back through the shuttle valve and vents to atmosphere through the exhaust of Valve 1. The cylinder can be operated in like manner with Valve 2. Directional Valves 1 and 2 are 3-way directional type as described on Page

3 FLOW CONTROL VALVES Flow Control Valves. Figures 3-63, 3-64, Usually built from brass or steel bar stock and designed for installation in cylinder or fluid motor connecting lines to control the rate of fluid flow, hence the travel speed of the cylinder or motor. Flow is controlled in one direction only, flowing toward the right. Fluid flowing toward the left can pass in free flow through the internal check valve. Two flow control valves are used to control speed individually in each direction of a double-acting cylinder or reversible fluid motor. Their use in controlling single-acting rams is shown in Figure Graphic Symbol. Figure 3-63 is the ANSI (American National Standards Institute) graphic symbol for use on circuit drawings. Also shown are the NFPA (National Fluid Power Association) port markings, "P" for controlled flow inlet, "F" for free flow inlet. Flow control valves are usually marked with an arrow which shows CONTROLLED flow direction. They must be installed correctly to work properly, and if necessary their free-flow direction can be determined by applying an air hose to each port while the needle valve is screwed all the way in. Mouth pressure should not be used because most check valves crack at 3 PSI or higher, and a person usually cannot develop enough mouth pressure to open them. Pressure Compensated Flow Control Valves. A more precise flow control for hydraulic cylinders, usually those operating at 1000 PSI and over, which has a variable orifice which automatically adjusts to keep the flow rate constant through the valve under changing load conditions. They are not suitable for air or air-over-oil applications. 3

4 DUAL EXHAUST AIR VALVES Four-way valves built for compressed air service usually have two exhaust connections, one from each end of the spool (see diagram below). These valves are called "dual exhaust valves". Exhaust Port EA exhausts cylinder Port A, and exhaust Port EB exhausts cylinder Port B. Examples of nomenclature are shown below and in Figure 4-6 on the opposite page. The purpose in bringing out both internal exhaust passages is two-fold: valves can be manufactured more economically, and more important, these exhaust connections can be used separately to restrict cylinder speed in each direction of travel. This is illustrated in Figure 4-6 on the opposite page. and in Figure 4-44 on Page 149. FIGURE 4-5. Dual exhaust air valve connected to a double-acting air cylinder. The graphic symbol is constructed in the same way as related on Pages 128 and 129 for a single exhaust valve. The actuators shown above or those shown on Pages 138 to 140 may be used on either end of the valve symbol. 4

5 Figure 4-6. A needle valve may be screwed into each exhaust outlet connection, EA and EB, of a dual exhaust air valve to obtain individual speed control in both directions of cylinder movement. A needle valve in EA restricts air coming from cylinder Port A and reduces speed of retraction without affecting extension speed. A needle valve in EB restricts air coming from cylinder Port B and reduces speed of extension. Refer to pictorial cutaway on opposite page for path of air flow through 4-way valve. POPULAR 4-WAY VALVES FOR COMPRESSED AIR Graphic diagrams under this heading are primarily intended to show end actuators for compressed air valves. Flow arrows inside the valve blocks are those for dual exhaust 4-way valves which are the most common type for compressed air. Actuator symbols are equally valid for single exhaust valves. This list shows actuator types most popular but does not cover all variations offered by some manufacturers. Four-way valves of some brands may be reconnected or may have unused ports plugged for use as 2-way, 3-way, or 5-way valves. However, certain other brands, if used this way, would have their packings unseated or washed out. Always consult the valve manufacturers literature, if it is available, before using a 4-way valve for any other type service. These actuator symbols were taken from Specification ANSI Y A copy of this specification may be purchased from the National Fluid Power Association, P.O. Box 49, Thiensville, Wisconsin GENERAL PURPOSE symbol may be used on one or both ends of valve symbol. It actuates the spool in both directions. It may be used if exact type of actuator is not known or not yet selected. It can purposely be used to avoid limiting a valve to a certain type of actuator, or when the actuator is optional. 5

6 SOLENOID VALVES. To keep the spool in shifted position, current must be maintained on the electric solenoid of all valves which have spring centered or spring returned spools. Valves which do not have spool springs will shift on a momentary impulse of electric current, and will stay shifted until returned by an actuator on the opposite end (solenoid, pilot signal, manual, etc.). Single solenoid, 2-position, spring return. Valve spool springs back to original position when solenoid is deenergized. Dual exhaust model shown. Single solenoid, 2-position, no springs. Spool is returned by a pilot pressure signal from another valve after solenoid has been de-energized. Double solenoid, two position action. Valve shifts and remains shifted when one solenoid or the other is momentarily energized. Double solenoid, 3-position spool, spring centered. Porting in center neutral depends on spool selected. MANUALLY OPERATED VALVES include all valves in which the spool is shifted manually by an operator. Manual lever actuators are used on valve sizes to 1 inch. For higher flow, a 1/4" manual lever valve may be used to control a larger pilot-operated main valve. Push button or palm button actuators are not practical on valves larger than 1/4 inch. They have no lever advantage, and the high shifting force causes operator fatigue. Palm button valves may use either the actuator symbol shown here or the one shown on Page 107. Symbols shown are for dual exhaust valves. See Page 130 for symbol construction. Two-position spool stays in shifted position, held by packing friction, when manual lever or button is released by the operator. Two-position spool springs back to original position when operator releases lever. Three-position spool spring centered. Porting in center neutral depends on spool type selected. Dual exhaust shown. Three-position spool held by center position or 3-position click detent. Spool stays in position to which shifted. Porting in center neutral depends on spool type selected. PILOT-OPERATED VALVES 6

7 Includes all valves where the main spool is shifted by application of pressure to the end of the spool, either directly on the spool area or through a piston, diaphragm, or cylinder. A pilot-operated main valve is controlled from a remote point by an operator using a miniature 3-way valve to pressurize and vent the pilot lines. Two-position, double pilot, no spring. Spool stays either position when pilot pressure is vented. Two-position, spring return. Spool returns to original position when pilot pressure is vented. Three position, spring centered. Spool goes to center when both pilot connections are vented. MECHANICALLY ACTUATED VALVES These symbols may be used for any valve in which the spool is directly actuated by a traveling member of the machine, and includes cam roller, remote clevis, and stem actuated types. Mechanically actuated valves of small size can be used to pilot operate larger valves. Care should be exercised in mounting these valves in a position where they will not be damaged if the machine member should accidentally over travel. Cam or mechanically operated, two-position spring returned spool. Dual exhaust type shown. Cam or mechanically operated, two-position with spool return by remote pilot pressure signal from another source. Dual exhaust model shown. FOOT OPERATED VALVES Includes pedal valves which are operated with toe only, and treadle valves which can be operated with toe in one direction and heel in the other. Foot operated valves are furnished only in small sizes, but can be used to pilot operate larger valves. 7

8 Pedal actuated (treadle optional), 2-position spool is returned to original position by spring force when pedal is released. Pedal actuated (treadle optional). 2-position spool returns to original position when a remote pressure signal is received from another source. Treadle actuated 2-position spool held in each valving position by packing friction when treadle is at rest. Not suitable for pedal actuation. Treadle actuated 3-position spring centered spool. Not suitable for pedal actuation. Closed center spool shown. Treadle actuated 3-position spool with click detents. Not suitable for pedal actuation. Closed center spool shown. 8