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Very few pneumatic applications are ready to perform as desired
without accessory items to fine tune, augment and protect
components. TUBING
I am a fan of Nylon or Polyurethane plastic tubing and “Push IN”
or “One Touch” Fittings to connect the Valves and actuators, etc.
The tubing is available in about a dozen colors so that lines may
be color coded. Each line may be cut to fit with a tube cutter,
knife, scissors, etc. With a little effort to keep the cut ends
fairly square, just push the tube firmly into the fitting and you
are connected for modest vacuum to approximately 150 PSIG in most
cases. The fittings seal on the OD of the tubing with an O-ring that
may be replaced. They attach over the tubing and do not restrict the
full flow as do bibs, barbs and fittings that insert into the
tubing. The tubing is flexible with a modest bend radius and may be
bundled, tied, held in tube strengtheners etc. for neatness and
function. The push in fitting has no loose parts, is re-usable and
is easily disconnected. This is a blessing when the cylinder lines
should have been reversed.
Some applications require copper or stainless steel tubing. For
these I suggest you get out the tube bender and do the best you can.
Other applications may require specific materials for chemical
compatibility. All I can suggest is a positive attitude and caution
when male metal fittings are screwed into a female plastic port. The
taper of NPT threads may create hoop stress in the softer plastic.
ISO G threads with an O-ring face seal would be better if the thread
form is compatible try fittings in an acceptable plastic material
with a male thread.
HOSE (If you drag the air line on the floor try hose.)
For air tools and devices that are moved frequently rubber or
synthetic hose is tough, flexible and handles abrasion quite well.
Nylon tubing is specified by the OD size, hose size is designated by
the Inside Diameter (ID). The fittings will have an insert inside
the hose that reduces the flow that should be considered when
selecting a hose size. Hoses come in natural rubber, synthetics,
wire reinforced, you name it.
COUPLERS
The ubiquitous hose is often attached with a plug that mates a
coupler located in the in the air supply manifold or air
distribution line for that area. When disconnected the plug allows
compressed air within the hose to vent to atmosphere. Couplers
typically shut off when disconnected. As you know the smallest hole
or passage in an air line controls the total flow. SWAG-The entire
industry of North America would be 10% more productive if all
coupler and plug assemblies including hoses were replaced with one
size larger. If compressed air “don’t get no respect” the size of
the coupler and plug that control the amount of compressed air
available are seldom even considered.
Warning the following paragraph contains adult content. No lie;
a couple years ago the marketing consultant hired by our Coupler
supply company asked about ideas to increase sales. This particular
brand arguably lasts longer and leaks less. I suggested that to
glamorize his product he might make either prophylactics or balloons
with their company name, logo etc. available. A salesperson could
slip one over each coupler they see in any given plant or ask
maintenance to unplug hoses and slip the prophylactic over each
coupler Friday night. Monday morning the couplers that leaked would
either have an enlarged balloon or it may be broken or blown off.
This, I e-mailed to the consultant, would identify couplers that
should be replaced to stop the air loss, have a little fun and raise
the lowly quick coupler from a utilitarian device to the status of
sex object. I gave myself mental pats on the back for unusual
inspiration. The Guru who had asked for ideas out side the box
stopped all communication and I never heard from again. I offer this
idea to you. Go have a little fun.
AIR LINE FUSE
A device that will shut off flow if an air line breaks makes a
lot of sense with hoses and flexible lines. Safety glasses
wouldn’t stay on long with several pounds of hose whipping everything within
reach. The Air Fuse is designed to slow or close off the source of
fluid energy when the force of excess flow drives the sensing
mechanism forward to close.
CHECK VALVE
One way, can’t come back, stop valves and many other colorful
names are used to designate this humble devise that allows air to
flow one way but not return. The Check Valve is the fluid flow
equivalent of an electrical diode that allows free flow of current
in one direction only.
This handy device is used in dozens of different ways. For
example a check valve used with a reservoir will allow free flow
into the tank. During times when the pressure is low the check valve
keeps the higher pressure within the tank from bleeding back into
the system.
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CHECK VALVE (cont.)
The most common pneumatic check valves have a spring with light
force to close the valve. This establishes a “cracking pressure”
to open. Usually the manufacture of the check valves strive to keep
the cracking pressure low, one to two PSIG. Like a relief valve the
cracking pressure will cause an equivalent pressure drop. With 100
PSIG into the check valve with a 2 PSIG cracking pressure the
resultant pressure down stream or past the check valve will be 98
PSIG or slightly less. This is interesting and useful and some
manufacturers offer various levels of cracking pressure. If we put a
check valve with a 10 PSIG cracking pressure in one outlet of a Tee
or Wye and supply the third port with 100 PSIG our two outlet lines
will show approximately 100 PSIG and 90 PSIG respectively. A
constant Delta P of 10 PSID. FYI you may also do this with an inline
relief valve and readily adjust the differential pressure. For lower
cracking pressure , especially in larger lines consider a “Swing
Check” that may be mounted to permit gravity to close it.
SHUTLE VALVE
For various purposes more than one input or supply may be
required or desired. The simple Shuttle Valve is a selector/isolator
and provides a logical OR function. Pressure applied to one port OR
the other OR both will produce output pressure or flow. The inlet
flow paths are approximately equal. If two inputs of different
pressure are applied simultaneously the higher pressure will drive
the shuttle to close off the lower pressure and flow through. With a
shuttle valve I may latch a valve on. One signal may pilot the
valve. When operated the output from the valve itself can be
connected to the pilot shuttle valve to hold it on. To reset the
pressure must be removed from both the pilot signal and the valve
that is operated. A shuttle valve typically shifts the shuttle away
from the inlet pressure to block the other inlet and flow through.
If the pressure is exhausted it will follow the inlet path back to
exhaust at the source.
QUICK EXHAUST VALVE
In forward flow from say 100 PSIG through the valve to extend a
cylinder or fill a volume the air is forced from a constant source.
When the exhaust path is opened to atmosphere the 100 PSIG initial
pressure in the volume decays quickly and 63% of the initial
pressure in the volume is lost in the first time constant. The
volume will not be entirely vented until after 5 time constants.
Obviously the final third or the pressure has less velocity an flow
as the pressure driving it decays. What do you mean what do I mean?
I mean the exhaust flow takes longer than the fill flow. Rather than
wait the extra moments for the lower pressure exhausting air to
return through long lines inlet lines a quick exhaust valve
connected directly to the cylinder or volume is often used to dump
pressure immediately without the natural line loss.
The quick exhaust is not symmetrical like the shuttle valve.
Incoming air pushes the shuttle to block exhaust and flow through to
the cylinder or volume. When the fill pressure is removed and a
slight exhaust decreases pressure in the inlet path the pressure in
the volume forces the shuttle over and dumps the cylinder to exhaust
right now.
FLOW CONTROL VALVE
A typical flow control valve has dual flow paths. One is
controlled with a needle valve or flow restricting mechanism. The
other path has a check valve. Most often the flow control is
oriented with free flow into the cylinder. The exhaust flow from the
cylinder is most often metered as it comes out of the cylinder. Many
say “It’s a sin to meter in, when in doubt meter out”. Flow control
valves are a great to control how fast a pneumatic cylinder rod
travels however they can cut system performance.
1. If located between the valve and the cylinder the cracking
pressure of the check valve will trim off a couple PSIG.
2. If located between the valve and the cylinder the Cv of the
free flow path must meet or exceed the Cv of the power valve.
With a Flow Control Valve between the valve and cylinder the flow
is somewhat controlled or affected in both directions.
CYLINDER SPEED CONTROL
The absolute best place to control cylinder movement is at the
exhaust ports of a five port spool valve. Nothing is installed
between the Valve and Cylinder to cause pressure drop. This allows
blinding speed in one direction and sophisticated control in tne
other.
Speed control mufflers most often have a crude taper so finesse
in adjustment may be difficult. A quality needle valve at the
exhaust port should give excellent control.
MUFFLERS
The Metal shell mufflers with high flow and good noise
suppression are worth a few extra dollars. These almost never build
up crud and increased back pressure as sintered mufflers may. OHSA
specifies a noise limit for the decibels of exhausting air. It
behooves you to find a muffler that will quiet the noise and keep
dirt out of your valves exhaust and minimize back pressure.
The definition of pneumatic accessories might be “All those cost
extra items required to complete the pneumatic system.” There are
many more. If your favorite did not make this short list or you have
any questions let us know @ the Fluid Power Journal “All About Air”
department. |
