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What do you do when you've undertaken the diagnosis steps
written about in my article titled " Compressed air leaking? Is it the
valve or is it the cylinder?" in this E-zine, and you have determined that
it is the air cylinder that is the problem?
The seals inside the air cylinder that allow the cylinder to
function and those that keep the air inside the cylinder where it is supposed
to be wear out over time through normal cylinder use.
When they do, your air cylinder could leak air to atmosphere
past the rod seals. It might be losing compressed air faster through rod seal
leaks than it can flow into the cylinder port from the air line. The cylinder
won't produce the force you need for your application if that is the case, or,
may not do the work required due to air blowing by the piston seals internally.
If the air cylinder is a non-repairable type, recognizable
by the cylinder barrel being "rolled" into a groove on the end caps,
then you cannot take this type apart, and it is time to discard it and buy a
new one.
If your application is high cycle, and the cylinder is 1
1/2" bore and larger, it might be a cost effective idea to replace the
"throw-away" cylinder with a repairable type. A repairable cylinder
will be higher priced up front, yet the savings could be realized through a
much longer service life with, as necessary, simple seal changes to keep the
cylinder operating at peak performance.
Let us assume that your cylinder is repairable, perhaps an
NFPA type, and it isn't working.
Rather than first taking the cylinder to a shop to determine
the problem, take it apart in your shop. Usually all this will entail is to
remove the nuts or bolts on the end of the tie rods on the rod-end of the
cylinder, and pull that cap off.
The cylinder rod will be extending through the rod-end cap,
so as you remove that cap, you will get the rod, piston and end cap as a unit.
Examine the cylinder rod. Is it scored or scratched? If so,
then these imperfections will likely be cutting the seals where the rod passes
through the rod-end cap. If it is those seals that are leaking now, replacing
them will only solve the problem for a short time before they start to leak
again.
Why is the rod scored? Has external debris being carried
back to the rod wiper and lodged there? That may be what is scratching the rod.
If there is no debris, check that the seals inside the rod bearing are not worn
to the point where there's metal to metal contact abrading the surface of the
rod?
If the rod is scored, replace it. Most piston rods are threaded and screw into the piston, with
some sort of sealing compound to prevent the rod from vibrating loose. Examine
where the rod meets the piston to be sure that it has not been tack welded.
Weigh the cost of a new rod against the cost of replacing the whole cylinder to
see which course of action is the right one for you.
After removing the blind-end cap, you should be left with
the empty cylinder barrel. Hold this up to a light and look through the barrel.
You are looking for scratches or marks on the inside. In particular, look for
signs of wear on one side of the inside of the cylinder barrel. This is an
indication that your cylinder rod is being side-loaded as it is working.
Sideloading the rod as it extends and retracts will increase
wear on one side of the rod-bearing seals, almost guaranteeing that you will
create a leak sooner rather than later. Over time, this side-load may damage
the rod bearing and even the rod beyond repair.
Remember that an air cylinder is a linear actuator, and the
load that it is moving must be isolated from causing other than linear forces
on the rod. Side-loading the rod will
destroy the cylinder. If side-loading is creating these cylinder problems, you have a choice between the expense of
externally guiding the load so that tangential forces are not affecting the rod
travel, or accepting that side-load is shortening the life of the cylinder, and
paying to replace it more often.
Examine the piston itself and see if there are visible cuts
or nicks on the piston seals, if there is evidence of wear or if debris is
lodged in the seals which might scratch the inner cylinder barrel wall or
prevent a tight seal.
If the cylinder barrel is scored or badly worn, replace it.
If the air cylinder components are not damaged, scratched or
worn, then it is likely that the cylinder just needs a seal kit replacement
which will be a relatively low cost fix for your cylinder. A seal kit replacement
will provide millions of additional cycles of trouble free cylinder use.
Most NFPA cylinder manufacturers offer seal kits for their
cylinders, and their commercial distributors should be able to get the kit for
you. If changing the seal kit is beyond your time or skill capabilities, most
distributors offer cylinder repair services.
Knowing the condition of the rod, barrel and seals of the
cylinder before you take the it to the shop for repair will help you help them
diagnose the cylinder problem and save you time and money.
As always, if you have any questions, please send me a
message from the contact screen at my web site. I'm happy to help if I can. 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.
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