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Richard Smith[_4_] Richard Smith[_4_] is offline
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Default hydraulic valve - opens on set pressure, closes no pressure

Ned Simmons writes:

On Thu, 20 May 2021 12:01:50 +0100, Richard Smith
wrote:

Ned Simmons writes:

On Wed, 19 May 2021 12:43:21 -0400, "Jim Wilkins"
wrote:

Ned Simmons" wrote in message
m...

On Wed, 19 May 2021 07:18:38 +0100, Richard Smith
wrote:



The set pressure of opening means you reach an aim maximum tension in
the sample.
That that valve stays fully open until the hydraulic pressure drops to
(very near) zero completely unloads the sample to no load.
The valve closes and the cycle repeats, etc.

Does such a valve exist?

Look at "sequence valves."

For example:
http://valveproducts.net/pressure-se...sequence-valve


Rich Smith

--
Ned Simmons
---------------------------

You need some bistable hydraulic, mechanical or electrical memory device
that remembers if the pressure should be increasing or decreasing after the
limit sensors stop signaling the limit condition, and operates the valves
accordingly, and that sequence valve could be the trigger that switches it
at the high pressure side.

The proper sequence valve may do it all. See Figure 2 on the page I
pointed to. Connect the IN port to a tee at the cylinder's port; the
OUT port resturns to tank. It'll act like a relief valve when the
pressure reaches the preset, but unlike a normal relief, won't reclose
until the pressure drops to a very low value.

--
Ned Simmons


Ned - I thought this is it.

Then I realised (?) - the full flow of the pump will always be the
minimum flowing through valve - which will defeat the closing action
we are counting on? This device, the "kickdown valve", is for filling
say a hydraulic cylinder, where the flow comes to a definitive stop at
full stroke? It avoid the energy loss of pumping oil past in-effect
an "intermediate-pressure" relief valve.

I suspect that constant flow from the pump would defeat it ??


Good question, but I don't think so, as long as the valve and the
return piping are sized such that the pressure at the OUT port (at
full flow) is low enough that the "light spring" in Fig 2 can force
the spool closed.

In other words: the pressure rises to the set point; the "control
relief poppet" opens, releasing the balancing pressure on the back
side of the main spool; the spool shifts open, and the pressure at the
IN port drops; the control poppet closes, but; the "kickdown jet" is
now open and bleeds the balancing pressure from the back of the spool,
until; delta P across the spool * spool area spring force and the
valve closes.

I hope that's right. Whether this is a practical way to control your
device in the real world is another matter.


I'll try to do sketches.

I have thought of a circuit I believe would act quickly at the set
pressure - giving the set pressure and no more. I ran with the idea
of having "separate reference pressure system" where a small pump,
large accumulator and pressure in it freely set via an adjustable
pressure relief valve dumping back to the reference-system tank. I'll
try to sketch that too.

Rich S


--
Ned Simmons


Hi Ned.

Your real-world experience enables you to answer this question?

You've already raised the point

"... Whether this is a practical way to control your device in the real
world is another matter."

I was thinking of constancy of pressure at which it triggers.
This thing is perfect when sequencing.
For a fatigue test, hydraulic pressure at trigger = peak force.
This is crucial to the test - for the peak force to be known and constant.
Would this device settle down to triggering at a very constant
pressure, do you reckon?
Within reason, so long as it stays constant, what that pressure is is
just fine - plot the "F=PA" force on the "S-N curve / plot" for
comparability of data.

(That's why I designed a parallel small system "reference pressure"
concept - so the peak force / pressure is accurately at, but neither
more or less, than a constant reference pressure)

Rich Smith