hydraulic valve - opens on set pressure, closes no pressure

This is like an "unloader valve" (?) - which does exist - but with additional characteristics(?)
The need...
I've got a hypothetical on-paper hydraulic device.
For fatigue-testing - while "the hydraulic cylinder is always bigger than the sample you are trying to test" * has always meant a machine with a frame and parts distributed along a central axis, dwarfing the size of the sample it's testing * it also means the sample will always fit *inside* the hydraulic cylinder which is testing it
I cycled up a high hill to get that inspiration, by the way, if you were wondering...
For fatigue testing samples - it has to tension and release millions of times.
If I had this valve I mention, you connect the cylinder directly to a pump - the higher its capacity the faster - more strokes per second - it will go - "strokes per second" - with "the valve" at the outlet, dumping the oil in the cylinder and flow of the pump for the time being back to the tank.
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?
There are computer-controlled systems with a pressure transducer and the "dump" valve opening on command. These are the "servo-hydraulic" systems which are familiar to many. That might be the option it would be necessary to use, in reality.
However - still curious if there is a stand-alone valve device which does what's wanted.
For accurate pressure control, the only thing I could think of was to use a balanced open-close valve (sliding "bobbin" ?) - but with one end pressurised by a "reference pressure system" with its own small pump, large accumulator and pressure relief valve returning to the tank. With the cylinder pressure routed to the other side of the "balanced valve". So when the cylinder pressure exceeds the reference pressure by only a small amount, the valve moves over to rapidly fully open a big dump line to tank. Then there has to be another mechanism / valve which only trips for the valve to return to closed when the cylinder pressure is about the same as atmospheric. If proven to work well, the almost constant pressure in the reference system could be taken as the peak pressure the cylinder reaches. That reference pressure is freely adjustable by turning the adjuster squeezing the spring on the relief valve of the "reference" system.
Thanks for considering.
Rich Smith
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"Richard Smith" wrote in message
This is like an "unloader valve" (?) - which does exist - but with additional characteristics(?)
The need...
I've got a hypothetical on-paper hydraulic device.
For fatigue-testing - while "the hydraulic cylinder is always bigger than the sample you are trying to test" * has always meant a machine with a frame and parts distributed along a central axis, dwarfing the size of the sample it's testing * it also means the sample will always fit *inside* the hydraulic cylinder which is testing it
I cycled up a high hill to get that inspiration, by the way, if you were wondering...
For fatigue testing samples - it has to tension and release millions of times.
If I had this valve I mention, you connect the cylinder directly to a pump - the higher its capacity the faster - more strokes per second - it will go - "strokes per second" - with "the valve" at the outlet, dumping the oil in the cylinder and flow of the pump for the time being back to the tank.
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?
There are computer-controlled systems with a pressure transducer and the "dump" valve opening on command. These are the "servo-hydraulic" systems which are familiar to many. That might be the option it would be necessary to use, in reality.
However - still curious if there is a stand-alone valve device which does what's wanted.
For accurate pressure control, the only thing I could think of was to use a balanced open-close valve (sliding "bobbin" ?) - but with one end pressurised by a "reference pressure system" with its own small pump, large accumulator and pressure relief valve returning to the tank. With the cylinder pressure routed to the other side of the "balanced valve". So when the cylinder pressure exceeds the reference pressure by only a small amount, the valve moves over to rapidly fully open a big dump line to tank. Then there has to be another mechanism / valve which only trips for the valve to return to closed when the cylinder pressure is about the same as atmospheric. If proven to work well, the almost constant pressure in the reference system could be taken as the peak pressure the cylinder reaches. That reference pressure is freely adjustable by turning the adjuster squeezing the spring on the relief valve of the "reference" system.
Thanks for considering.
Rich Smith
----------------------
As I understand it, you want a bistable valve with variable hysteresis between its opening and closing pressures.
I recently tricked up a relay for my solar panels that acts that way. Relays do anyway but aren't adjustable, I have it switching resistance in or out of series with the coil to set the pull-in and drop-out voltages. The reason is to protect digital meters from the voltage range just below their minimum supply requirement at dawn and dusk, where they operate strangely.
For your problem a second pilot cylinder could change the tension of the relief valve spring that opposes the pressure. You might need a small accumulator and restrictor orifice to delay the pressure change at the pilot to ensure the valve completes each operation instead of chattering between states.
The generic name for a bistable device with memory is "flip-flop". https://www.hydraulicspneumatics.com/technologies/pneumatic-valves/article/21122363/basics-of-pneumatic-logic
The solution is easy with electrical control by relays and solenoid valves. You can either sense pressure or use time delays.
https://en.wikipedia.org/wiki/Relay_logic In the example STOP/START circuit CR1 (ControlRelay1) is bistable, it remains in whichever state the last button press left it. The symbol that looks like a capacitor is a relay contact and the circles are relay coils, solenoids, motors, etc.
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wrote:

Look at "sequence valves."
For example: http://valveproducts.net/pressure-sequence-valve/operation-principle-of-pressure-sequence-valve

--
Ned Simmons

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"Ned Simmons" wrote in message wrote:

Look at "sequence valves."
For example: http://valveproducts.net/pressure-sequence-valve/operation-principle-of-pressure-sequence-valve

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

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

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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 ??
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
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wrote:

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.

--
Ned Simmons

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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
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"Richard Smith" wrote in message writes:
... 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?
-----------------
I don't know about that valve but I do have lots of R&D experience on difficult projects.
Generally you design and build what you predict (hope) will work, then measure and correct until you run out of time, money or patience, and declare it "good enough". The limit is how accurately you can measure.
If the valve is inconsistent you may simply need a better filter, or different oil viscosity. A recording of cycle intervals and peak pressures would show you if a problem develops and perhaps hint at why, i.e. was the change sudden or gradual.
I save measurement data into text files that can be loaded into a spreadsheet as *.csv for analysis.
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On Fri, 21 May 2021 14:15:03 -0400, "Jim Wilkins"

It's easy to run out of all three quickly fiddling with a new hydraulic design. They're messy, difficult to change, and often don't allow easy access to the parameters you'd like to measure when debugging.
--
Ned Simmons

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"Ned Simmons" wrote in message wrote: >Generally you design and build what you predict (hope) will work, then

It's easy to run out of all three quickly fiddling with a new hydraulic design. They're messy, difficult to change, and often don't allow easy access to the parameters you'd like to measure when debugging.
--
Ned Simmons

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wrote:

I've designed hydraulic systems that included sequence valves, but I wouldn't want to make a prediction on how repeatable the cracking pressure might be, especially at high cycle rates relative to the size of the device. It's also likely to drift with oil temp, though that's a slow-changing parameter that could be compensated for manually. Probably best to speak to manufacturers, Eaton/Vickers, for example.
--
Ned Simmons

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Ned - that's what I "feared" - it will likely settle down to a stable pattern, but what that is will drift around in time, with many variables, including ambient temperature, etc.
Running 24hrs a day for many days, you couldn't keep fine-tuning to stay on one condition.
I'm seeing why servo-hydraulic with digital logic control on the basis of pressure transducer(s) is the way things are done.
By the way Ned - "drift" affects welding - not sure how much that is your thing. "drift" is central to a funny (?) story.
This Company owner knew I wanted to get ahead, so ordered the Foreman - "Don't let Richard weld - he doesn't know how to". Well, this job comes in with far thicker plate than anyone has ever met - even the "old-timers" with 40+ years experience. They couldn't get their welds to "stick" in those slot-welds. None knew spray-transfer. Co. had one really good machine, and I flipped it up into spray and was putting in these slot welds easier than shelling peas. These big structurals were up on plinths, so I was up there in the middle of the shop, on top of these things, putting in the welds no-one else could.
No-one came and asked me how I did it.
Reason - they knew I'd say "Don't know. I can't weld. I just pick up the torch and it seems to work".
The reason they couldn't just copy my setting, or order me go buy striped paint while they take the torch and continue on that setting is - conditions drift... You have to tune and get back to the right condition every few minutes. Variables? Line voltage? Ambient temperature? How long since the machine was turned on? etc.
If you drift downwards, the wire will stub and the weld will transition to dip transfer. Get too long and it can burn-back to the torch which will instantly destroy the contactor tip and probably the shroud. You'll spend a long time rebuilding your torch (N.Am. "gun") if you don't know how spray works.
So yes, drift...
Again Ned - thanks, appreciated.
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"Richard Smith" wrote in message ... This Company owner knew I wanted to get ahead, so ordered the Foreman - "Don't let Richard weld - he doesn't know how to". Well, this job comes in with far thicker plate than anyone has ever met - even the "old-timers" with 40+ years experience. They couldn't get their welds to "stick" in those slot-welds. None knew spray-transfer. Co. had one really good machine, and I flipped it up into spray and was putting in these slot welds easier than shelling peas. These big structurals were up on plinths, so I was up there in the middle of the shop, on top of these things, putting in the welds no-one else could. ...
--------------------------
You could get a high-paying job welding submarine hulls, and be invited aboard for the test dive.
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There is a submarine base nearby. Very exacting recruitment.
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"Ned Simmons" wrote in message

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
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On Thu, 20 May 2021 07:22:42 -0400, "Jim Wilkins"

I don't entirely follow that paragraph. See my reply to Richard.
Re a device with memory, a directional valve with two detented solenoid actuators would do the trick. One solenoid controlled by a pressure switch that closes at the high pressure setting, the other by a pressure switch that closes at the low pressure setting. Or the hydraulic equivalent.
https://www.festo-didactic.com/int-en/services/symbols/fluid-power-hydraulic/valves/directional-control-valves-4-2-way-valve-with-two-solenoid-coils,directly-actuated,with-detent-pulse-valve.htm?fbid=aW50LmVuLjU1Ny4xNy4zMi4xMjMxLjY3Nzk
--
Ned Simmons

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Ned, Jim, everyone - massive thanks. Rich S
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wrote:

This sounds very much like the mechanism of a hydraulic shake-table driver, used for vibration testing of all kinds of equipment.
One manufacturer is Unholtz-Dickie. Look into their history, and patents assigned to them and their predecessors.
Like "Fluid-operated vibration test exciter" to John Dickie, patent US2773482A. This is basically a siren driving a shuttle piston back and forth. If the shuttle piston is prevented from moving, it will generate a cyclic stress. The addition of a dead weight to this allows the cyclic stress to ride atop a static stress.
Joe Gwinn
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"Joe Gwinn" wrote in message wrote:

This sounds very much like the mechanism of a hydraulic shake-table driver, used for vibration testing of all kinds of equipment.
One manufacturer is Unholtz-Dickie. Look into their history, and patents assigned to them and their predecessors.
Like "Fluid-operated vibration test exciter" to John Dickie, patent US2773482A. This is basically a siren driving a shuttle piston back and forth. If the shuttle piston is prevented from moving, it will generate a cyclic stress. The addition of a dead weight to this allows the cyclic stress to ride atop a static stress.
Joe Gwinn
----------------------------
I considered an oscillator-based solution but didn't suggest it because they may require specialized instruments, dataloggers, digital storage oscilloscopes and spectrum analyzers, to test and debug. It's much easier to test a system that can be stopped or run slowly.
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