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MMD > Archives > August 1999 > 1999.08.04 > 10Prev  Next


Testing Simplex Unit Pneumatics
By Craig Brougher

Bill Maxim writes:

> I am at the stage of testing the valves at the lower seat, using a
> test block received from PPCo many years ago.  On about 18 of the 88
> pneumatics, the pouch rises and lifts the valve off the seat when I
> suck on the supply tube (I'm not using the foot pump that came with
> the block).  This means that outside air is getting under the pouch
> somehow, but how?  I'm hoping not to have to tear each one completely
> apart again.  The harder I suck, the higher the valve rises.

There is only one path for air to get under your pouch, and that's
across the gasket.  Does that give you an idea?  If you have sealed the
pouch wells, then there is no other way for air to inflate the pouch,
except through the gasket interface, or caused by the tester after the
interface.

When a Simplex valve pneumatic has a void under or behind the gasket
and leaks out the side of the unit, then, it can raise the pouch.  For
example, if you have not used hot hide glue.  Also, when the test bleed
is partially blocked in its channel or marginal enough that when you
have sealed the pouches too tight, some will act proportional to the
vacuum you are supplying.

The reason the clogged test bleed or clogged test bleed channel may not
raise all the pouches you have is because possibly most of the pouches
have just enough leakage of their own to increase the overall pouch
bleed.  You may also have a little wood seepage to air from the test
stand, and it's a combination of leakages and blockages that are giving
you the problem.  Hook a bubble jar to your tester and "tune it" to
control the maximum bubble action with your tester bleed.  The connect
the jar to your own fixed bleed in a tube, and notice the differences.
Just relative testing like this is all you need to see if you have a
clog, or leaky wood.

However, you never want to seal pouches with phenoseal!  You probably
meant I suspect) pouch wells, and not pouches.  I don't see how a pouch
would even flex with phenoseal in it.

When you test any valve pneumatic, it should always be at the lowest
vacuum you expect to play on.  In this case, about 6." So make sure
they all work on 5", and with  a minimum of 4 feet of trackerbar
tubing.

Also make sure that the gaskets you are using are supple and com-
pressive, and not hard.  I've seen old gaskets do exactly what you
describe.  If you are using that PPCo. gasket material with black
rubber particles in it, get rid of it, and you'll probably solve the
problem.  It's junk.

Craig Brougher


(Message sent Wed 4 Aug 1999, 12:48:57 GMT, from time zone GMT-0500.)

Key Words in Subject:  Pneumatics, Simplex, Testing, Unit

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