Here’s an overview of some common choices a designer faces when selecting valves:
2-position versus 3-position valves — In 2-position, 4-way directional valves, the two output ports are always in an opposing mode. When one is receiving inlet air, the other is connected to the exhaust port. When actuated, 3-position 4-way directional valves function the same way. However, a center or neutral position blocks all ports (pressure held), or connects both output ports to the exhausts (pressure released) when the valve is not being actuated.
Divide Cv by the factor listed at the appropriate pressure to calculate scfm.
Pressure-held, 3-position valves are ideal for “inching” operations where the cylinder rod needs to move to a desired position and then hold.
5-port, 4-way valves versus 4-port, 4-way valves — 5-port valves have separate exhaust ports for each cylinder port. If exhaust silencers with built-in speed controls are used, the speed of the cylinder motion may be individually controlled in each direction.
In 2-position, 4-way directional valves, when one outlett port is receiving inlet air, the other outlett port is connected to the exhaust port.
Five-port valves can also function as dual pressure valves where air flows from the exhaust ports to the cylinder and both cylinder ports use the inlet as a common exhaust. Vacuum may also be used in 5-ported valves.
3-way versus 4-way valves — 3-way valves have one power output and 4-way valves have two power outputs. Generally, 3-way valves operate single-acting cylinders and 4-way valves operate double-acting cylinders.
3-position, 4-way valves have a center position where all ports are blocked (pressure held) or both output ports connect to exhaust ports (pressure released). Pressure-held versions are suited for moving to, then holding, a position.
3-way normally closed versus 3-way normally open valves — Normally open valves allow air to pass when not actuated. Normally closed valves allow air to pass only when they are actuated.
Pressure-piloted versus bleed-piloted valves — Pressure piloting and bleed piloting refer to two different modes in which valves may be actuated. Pressure piloting positively actuates a directional valve by an external air signal that comes from a remote three-way valve. Air-pressure piloting provides an economical alternative to the use of electric switches and solenoids.
3-way valves have one power output and generally operate single-acting cylinders.
Bleed piloting uses internal air from the directional valve to feed the pilot valve. Air flows from the directional valve to the bleed valve. When the bleed valve is actuated, pressure drops in the directional valve pilot section. This causes a differential pressure and valve shift.
The main advantage of bleed piloting is that only one line enters the bleed valve. However, if the line is severed, a shift occurs. Pressure piloting is considered more positive and reliable.
4-way valves have two power outputs and generally operate double-acting cylinders.
Air actuation versus solenoid actuation — Solenoid actuation requires electric switches, wires, and all of the shielding necessary to reduce spark hazard and personal risk.
Air actuation requires only 3-way, air-pilot valves and tubing. There is no explosion, spark, or shock risk and the components are less expensive.
5-ported valves have separate exhaust ports for each cylinder port.
Feeding solenoids with an external air supply — Use this option when the air pressure passing through the directional valve’s power section is insufficient to shift the spool, when the medium passing through the power section would be detrimental to the solenoid operator, or where the operating medium could not be exhausted to the atmosphere.
Manual overrides — Manual overrides let users actuate directional valves without relying on the switches or pilot valves that normally are needed. In this way, a circuit may be tested without actually moving the machine elements.
Normally open valves permit airflow when not actuated.
Low-force actuation — Low-stress valves need only 6 to 8 oz. of force to initiate a signal. These valves reduce stress on worker’s hands. For example, Mead LTV 4-way valves operate on a pressure differential basis that lets them actuate with applied little force.
Converting air signals to electrical signals — Air-to-electric switches will turn an air signal into an electrical signal, which can be wired either normally open or closed.
Delaying a signal in the air circuit — Air timers can be used to delay an air signal. For instance, Mead offers models that provide up to two minute normally open or normally closed delay.
Normally closed valves permit airflow only when actuated.
Adjustable control of air flow — While flow-control valves perform this function, some directional valves have built-in flow controls. Exhaust silencers typically have built-in needle valves that also provide speed regulation.
Reducing piping and simplifying troubleshooting — When a group of valves is used in an application, order the valves stacked to take advantage of a common air inlet, common exhausts, and centralized controls.
Detented valves — A detented valve holds its position by some mechanical means such as a spring, ball, or cam. Most valves hold their position by means of the natural friction of the rubber seals. Where natural friction is low, such as in packless valves, or where it is insufficient for safety purposes, detented models are recommended. Also, detents are used to locate the middle position in three-position valves.
Shuttle valves — To direct air coming from either of two sources to a single destination, use a shuttle valve.
Safety — Use two-hand, anti-tiedown devices to keep workers’ hands out of hazardous locations.
For more information on Mead’s valves, switches, and controls, visit,www.mead-usa.com where you can also download a free copy of their Pneumatic Handbook.