Introduction to Pneumatics for Home Haunting
Lesson 2
Air Valves
Air Cylinders may be most common image in pneumatics, but they would be useless without their counterparts: air valves.
Air valves change the direction of airflow to and from a cylinder. Without air valves, air pressure would be at a constant, and an air cylinder would either remain extended or retracted. By using an air valve, air is controlled, and thus, control of the action of the air cylinder is maintained.
To change the direction of airflow to and from the cylinder, we use a directional
control valve. The moving part in a directional control valve will connect and
disconnect internal flow passages within the valve body. This action results in a
control of airflow direction.
The typical directional control valve consists of a valve body with four internal flow passages within the valve body and a sliding spool.
Shifting the spool alternately connects a cylinder port to supply pressure or the
exhaust port. With the spool in the position where the supply pressure is connected
to port A and port B is connected to the exhaust port, the cylinder will extend. Then,
with the spool in the other extreme position, supply pressure is connected to port B
and port A is connected to the exhaust port, now the cylinder retracts. With a
directional control valve in a circuit, the cylinder's piston rod can be extended or
retracted and work performed.
Types of Valves for Home Haunting
Two Way Valve
A two-way directional valve consists of two ports connected to each other with
passages, which are connected and disconnected. In one extreme spool position, port
A is open to port B; the flow path through the valve is open. In the other extreme,
the large diameter of the spool closes the path between A and B; the flow path is
blocked. A two-way directional valve gives an on-off function.
Three Way Valve
A three-way directional valve consists of three ports connected through passages
within a valve body that are shown here as port A, port P and port Ex. If port A is
connected to an actuator, port P to a source of pressure and port Ex is open to
exhaust, the valve will control the flow of air to (and exhaust from) Port A.
The function of this valve is to pressurize and exhaust one actuator port. When the
spool of a three-way valve is in one extreme position, the pressure passage is
connected with the actuator passage. When in the other extreme position, the spool
connects the actuator passage with the exhaust passage.
Four Way Valve
Perhaps the most common directional valve in simple pneumatic systems consists of
pressure port, two actuator ports and one or more exhaust ports. These valves are
known as four-way valves since they have four distinct flow paths or "ways" within
the valve body.
A common application of four-ported four-way directional valve is to cause
reversible motion of a cylinder or motor. To perform this function, spool connects
the pressure port with one actuator port. At the same time, the spool connects the
other actuator port with the exhaust port. This is a four-ported four-way valve.
Five Port / Four Way Valve
Four-way valves are also available with five external ports, one pressure port, two
actuator ports, and two exhaust ports. Such valves provide the same basic control of
flow paths as the four-ported version, but have individual exhaust ports. In the fluid
power field this is referred to as a "five-ported, four-way valve." This type of valve
brings all flow paths to individual external ports. The pressure port is connected to
system pressure after a regulator. Actuator ports are connected to inlet and outlet
ports of a cylinder or motor. Each exhaust port serves an actuator port.
Regardless the type of valve, consideration will need to be made on the voltage...DC or AC.