[AT] hydraulics question

Dudley Rupert drupert at premier1.net
Sun Jan 9 23:37:07 PST 2005


Cecil,

I am not a hydraulics expert but I have taken three maybe four two-way
hydraulic valves apart in the past two or three years, cleaned them up,
honed the cylinder bores, polished the pistons and installed new O-rings.
I've installed two of these valves on two different tractors to use for
controlling aftermarket 3-pt hitches and when these valves are in the
center/neutral position they will hold a load for days with no noticeable
compression of the hydraulic cylinders/lowering of the hitch.

If your' valves operate like mine (and I think probably all two-way self
centering hydraulic valves do operate pretty much the same) I would agree
with your' manual when it states that if, when the valves are in the
center/neutral position, oil is returning to the reservoir then something is
wrong in the valve.

However, I am not sure the opposite is necessarily true, at least not for
all valves.  That is, if oil is NOT flowing back from the valve to the
reservoir under the conditions described above I don't think one can
necessarily always conclude that the valve is Okay.  For example, as you
know the weight of a raised loader puts a down pressure on the lift
cylinders and, if there is a seal failure in the valve, this pressure could
force oil back down the cylinder up line, through the failed valve and back
up the cylinder down line.  That is, oil would just circulate "backwards"
between the valve and cylinders through the lines until the loader came to
rest on the ground.  However, judging from the valves that I have taken
apart I would say the most likely valve failure mode would be to let the oil
forced down the cylinder up line return to the reservoir and not circulate
back up to the cylinder.

The bottom line, as your' manual indicates, is one valve failure could be
the cause of both cylinders sagging an equal amount.  But, you've replaced
the valve so let's rule it out for the moment.  If there are no external
leaks between the valve and cylinders then it seems the seals in both
cylinders have failed.  If the seals in only one cylinder had failed then I
would think that the other cylinder could hold the loader up by itself
particularly if the bucket were empty.  I assume you've tried this and that
the loader will not stay up even when empty so we're left with two cylinders
to rebuild.

This was a long ramble and I am sure it hasn't told you anything you didn't
already know.  As Dean said in one of his earlier posts to Ralph it's fun to
play remote quarterback and I would add particularly so when you can do it
from a well lighted 70 degree computer room!

Good luck -

Dudley
Snohomish, Washington

-----Original Message-----
From: at-bounces at lists.antique-tractor.com
[mailto:at-bounces at lists.antique-tractor.com]On Behalf Of Cecil E Monson
Sent: Sunday, January 09, 2005 7:23 PM
To: Antique tractor email discussion group
Subject: [AT] hydraulics question

        I had a problem with the hydraulics on my JD 420 crawler with
a loader. For some reason, I had quite a bit of water in the hydraulic
fluid and apparently ran it that way, unsuspectingly, until I started
to have a problem with the bucket not staying up like it is supposed
to. I used this a lot when cutting firewood by hooking a chain around
a log, raising it up about 3 feet, and then cutting 18" chunks off it
until it was all cut up. Saved a lot of rolling the log to make second
cuts and so on. All of a sudden the bucket wouldn't stay up and that
was when I found the water.

        According to the manual, to test to see if the control valve
is not holding or if the cylinders are leaking past the seals, raise
the bucket and check to see if oil is being returned to the reservoir
if the bucket comes down by itself. If oil is being returned to the
reservoir, it is the control valve. If no oil goes back, the cylinders
are leaking internally. As oil was coming back during the test, I took
the control valve to a shop in New Jersey and had it tested. They said
it was no good and suggested a new one for $650. Instead, I ordered one
from Northern Hydraulics and installed it. Only problem is the bucket
still comes down - slowly but it doesn't stay up. I haven't tested it
again but suspect the control valve again. I don't see any load checks
on it and thought they should be externally mounted. The literature
says only one port has load checks but it does not hold when the lever
is in the neutral position.

        Does anyone know why a load check would not hold in the neutral
position and what good would it be anyhow if it only worked when being
used to lift or lower?

        Being as there are separate forward and return lines, could I
just install external load checks?

        I noticed when looking at the parts list for the loader, that
the cylinders have internal leather cups in them. I suppose this dates
the loader so far back the leather cups are now made of unobtainium.
Or are they still available somewhere?

        Shouldn't a new control valve with center neutral on the levers
hold a load on the cylinders without sagging?  Maybe all my cylinders
have blown gaskets and are leaking. Do you think this could happen to
all of them at once from the water? There are four fairly large cylinders
on this loader - two for the lift and two for the bucket.

        Any comments from anyone who works on his own hydraulics would
be appreciated.

Cecil
--
The nicest thing about telling the truth is you never have to wonder
what you said.

Cecil E Monson
Lucille Hand-Monson
Mountainville, New York   Just a little east of the North Pole

Allis Chalmers tractors and equipment

Free advice

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