Unit Ten Directional Control Valves Directional control valves fall into one of two categories: Shear Action Poppet Lapped Spool Packed Spool Packed Bore Bonded Spool Metal Face Bonded
DCV Flow Path Terminology Valve type Valve use 2-way 3-way 4-way 5-way On-Off functions for fluid supply Single acting cylinders, uni-directional motors Double acting cylinders, bi-directional motors Pneumatic systems with dual air pressure The above descriptions refer to the total number of directions in which a fluid can travel through a directional control valve.
DCV Flow Path Terminology 2-Position 3-Position 4-Position The positions refer to which direction the fluid happens to be going when a valve is “shifted.” The position corresponds to a envelope in the schematic symbol.
DCV Schematic Symbols The basic valve symbol is a blank envelope. DCVs have at least two envelops which corresponds with a 2-position valve. A 3-position valve will have three envelopes and so forth.
DCV Schematic Symbols Arrows are added to indicate a direction that fluid can travel through the valve at that point when the valve is in “that” position. Each arrow crossing represents two ports. The example above indicates four ports but not necessarily wher they are located on the actual valve.
DCV Schematic Symbols This is an example of a 3-way valve. Note the little “T” emblem which denotes a blocked port.
DCV Schematic Symbols This is an example of a 2-way valve. The single arrow indicates one inlet port and one outlet port.
Relating Symbols to Components The cutaway diagram shows how a “spool” can connect and disconnect passages.
Relating Symbols to Components If shifted to the envelope on the left, ports A is connected to the port which is being fed by the pump, port P. At the same time port B is connected to the tank port T. A DCV used in pneumatics would use EX which is short for exhaust port.
Relating Symbols to Components If shifted to the envelope on the left, the spool connects port P with port B, and port A with port T which gives a reversal of the previous flow paths.
4-Way Valves in a Circuit Whether in a hydraulic circuit or a pneumatic circuit, 4-way DCVs function the same. The two triangles at the bottom represent the supply source, hydraulic on the left and pneumatic on the right. The direction of the cylinder in both cases is determined by the envelope chosen. Envelopes are shifted by an “operator.”
3-Way Valves in a Circuit In the circuits above, each cylinder has something to push it back. The ram on the left has the weight of the ram and gravity to return it to the starting position. On the left, a spring returns the rod to its starting point. 3-way valves have other functions as well.
2-Way Valves in a Circuit The 2-way DCV is used here to isolate fluid flow from the 4-way DCV until needed. The current condition shows no flow going to the 4-way DCVs.
DCV Operators An operator is a device that physically moves a spool or poppet. These are examples of the kinds of operators that might be found on DCVs.
Valve Operators Manual operators such as the push button, hand lever, and foot pedal, use human power for actuation. Mechanical operators use a machine component to contact a roller or ball bearing. Pilot operators use a fluid to move the spool. A solenoid uses electro-mechanical force to move a spool or poppet.
Solenoid Operation Push Pin Air Gap Spool A solenoid is an electrical-mechanical device that takes electrical energy and produces a linear force by use of magnetism. It is basically a winding of wire around a metal core. Many times when a valve fails to shift, the problem can be traced to a faulty solenoid. A spring, not shown, holds the spool against the solenoid push pin when the solenoid is de-energized.
Solenoid Operation When the solenoid is energized, the air gap is closed quickly and a force is developed in the direction of the valve spool. The force developed is determined by the size of the coil. If the air gap is not closed, the coil will burn up due to “inrush” current.
Spring Operator In DCVs and other valves, springs are used to “return” or”off-set” the spool or poppet to ensure a home or rest position. When reading a schematic symbol, remember that the envelop to which the spring is attached is the home position. The terms normally and normally closed relate to whether or not the valve will permit flow in a given position. If the valve will not allow flow in the normal position we say it is normally closed or not passing. The opposite is said for the normally open position and we say normally passing.
Other Operators Detents Detents are used to hold a valve spool in place. They serve a safety function in that the spool is held positively in place to keep it from moving due to vibration or possibly from gravity where the valve is mounted in an up right position. Shown above is a ball and spring detent but there are other designs as well.
Sub Plate mounted 4-Way Valves Sub plate or sub base mounting is just a convenient way of attaching a DCV to a system and reducing the time required to replace it should the need arise. The DCV is held into place by four fasteners and a seal is made with O-rings. The time savings is in to having to remove pipe connections.
Solenoid Controlled, Pilot Operated DCV Although a valve could be shifted directly by the force of a solenoid, large flow DCVs are most often shifted using fluid at system pressure. Large flow valves demand larger shifting forces and it no longer practical to use a solenoid. Illustrated on the left is a typical mounting arrangement for such a valve. On the left is the schematic symbol. Can you tell what the normal position is?
Piggy-Back DCV Piggy-backing is a term used to describe a two valves connected as a unit where a small valve controls the operation of a larger valve. This arrangement makes used of the powerful force of fluids in order to actuate a large flow DCV.
Center Conditions Open Center This spool is sometimes called a “motor” spool because it is often used to control a hydraulic motor and minimize shock that may be encountered as the valve returns to the center position. It is usually not recommended for cylinders because of the possibility of drift while in the center position.
Center Conditions Closed Center While in the center position the closed center valve allows no flow to the system because all ports are blocked off from one another. This spool is used where multiple stop positions are required.
Center Conditions Float Center The chief advantage of a float center valve is the ability of the actuator move freely. It is used to control cylinders on mobile equipment where a blade cylinder must be permitted to move freely in order to scrape or level.
Center Conditions Tandem Center The tandem center spool is an energy conservative design used on low power equipment. When in the center position this valve allows the flow of the pump to return to tank at low pressure thus reducing system pressure.
Completed DCV Symbols 3-position, Blocked Center 4-way valve, Direct Solenoid Operation and Spring Centered 3-position, Open Center 4-way valve, Direct Solenoid Operation and Spring Centered 3-position, Tandem Center 4-way valve, Direct Solenoid Operation and Spring centered
Other DCV Symbols Flow Through Center DCV Four Position DCV This two valves are commonly found in mobile hydraulics and each has advantages in the application where it is found.
Deceleration Valve Although considered a DCV, the deceleration valve operates by shutting down flow as an actuator approaches the point of operation. This would prevent a tool crash.
Pneumatic Valves The operating principles of pneumatic valves are identical to the ones used in hydraulics but there does exist a difference in the schematic symbology. First, you may notice that the arrows are “canted” or at an angle. This serves only to Distinguish them from hydraulic DCVs and nothing more. Second, these valves are often referred to as 4-way valves although they have 5 ports. The reason is that there is an additional exhaust port, usually not found in hydraulic valves.