In order to use compressed air, you first have to compress
it. It’s that compressibility of air that can create problems when you are
trying to use it to stroke an air cylinder smoothly or with consistent stroke
timing.
To product consistent, smooth cylinder rod travel, or a
consistent speed-of-stroke from end to end, cycle after cycle, then a
traditional air cylinder just won’t do.
Changing conditions in your external cylinder load (friction
– sideload for example), seal wear inside the cylinder, variations in the
supply air pressure, pulsation from the compressor and many other factors will
contribute to varying cylinder rod speed and smoothness.
Each time one of these factors inhibits the rod or tooling
travel – even by a little bit - there will be a momentary hesitation in that
rod travel as the air pressure inside the cylinder builds to the level
necessary to overcome that inhibition. This changes cylinder rod speed and
timing inconsistently and continuously.
The use of pneumatic flow controls will do much to reduce
the impact of the speed and stroke time variations but they cannot ensure that
your cylinder stroke and timing will be consistent all the time, as the air
that they are throttling will compress as it’s being restricted, resulting in
changing stroke speed and time. These changes will be less than if no flow
controls were used, but if you need really consistent cylinder operation, then
you need to consider alternates.
Consider a hydraulic system.
A hydraulic cylinder uses oil which is theoretically
incompressible. With hydraulic flow controls lowering the cylinder speed and
time-of-stroke below the available hydraulic force from the power-pack, you can
be virtually certain of consistent stroking every time.
There are many circumstances, not the least of which is
cost, that make an all-hydraulic system unacceptable. If that is the case, then the next option to deliver stroke-to-stroke
consistent speed and timing is to use an air over oil system.
This system will use air pressure to drive oil into a
cylinder and, since oil is not compressible, flow controls controlling the oil
exiting the cylinder will ensure a smooth, consistent stroke every cycle.
An air –oil system
will consist of:
-an air - hydraulic cylinder of choice
-two air - oil tanks
-two hydraulic flow controls
-a two position four or five ported air valve or two 2
positions 3-way valves
-necessary lines and fittings to connect the oil tanks to
cylinder and valve to the oil tanks
-hydraulic oil
Check with the air cylinder vendor to ensure that their
cylinder can be used in an air-oil application. Most can. The pressures
generated are usually well within the safety factor for pneumatic cylinders
which are much less costly than hydraulic cylinders.
Each of the oil tanks must contain enough oil to fill the
cylinder during a complete stroke (an extension or retraction) without
completely emptying the tank.
Each of the cylinder ports is connected to a fitting at the
bottom of it’s own oil tank.
The air valve is connected to the air/oil tanks, one working
port to each tank. The lines are connected to the fitting on the top of the air
– oil tank.
The oil tanks must be installed higher than the cylinder
they are supplying.
When the air valve is shifted, air flows down one air line
to it’s air/oil tank. The compressed air fills the top of the air – oil
pressure tank, and exerts force on the oil in that tank. The oil then flows
through the line to the cylinder, either extending or retracting the rod,
depending on which line it flows to.
The flow controls, one installed on each of the cylinder
lines, will dampen the flow of the oil to provide smooth, consistent stroking of
the cylinder rod.
When the valve is shifted in the other direction, the air
flows down the other valve line to the other tank, and the cycle repeats.
Each time the valve shifts, the oil being driven into the
cylinder from one tank pushes the cylinder piston towards the other end of the
cylinder, and that piston drives the oil on the other side back up the line to
the other air – oil tank.
With a properly installed air over oil system as described
will provide the cylinder stroke speed and consistency that you desire for your
application.
Depending on the cylinder cycle speed, each time the valve
shifts, a minute amount of oil may exhaust with the air. A re-classifier should be plumbed to the
valve’s exhaust port(s). This device will strip the exhaust air of oil for
re-use or disposal.
Trying to drive an air – oil system at too high a speed
could cause the oil to boil in the tank generating a significant amount of air
bubbles in the oil, which will be
detrimental to the use of the system to control cylinder speed and time.
There are many qualified FPDA (Fluid Power Distributors of
America) suppliers of air – oil systems, and they will be able to match the
system to your requirements.
And as always, if you have questions, please don’t hesitate
to contact me through the contact page on my site.
Bill Wade's experience in compressed air and
other industries spans decades; from field sales positions through to the
corporate presidential office. His sales agency represents a select group of
industrial firms. Mr. Wade writes about all facets of compressed air at http://www.about-air-compressors.com.
