hydraulic valve - opens on set pressure, closes no pressure

[Richard Smith]

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

[Jim Wilkins]

"Richard Smith" wrote in message news:***@richards-air-2.home...

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.

[Ned Simmons]

Post by Richard Smith
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-sequence-valve/operation-principle-of-pressure-sequence-valve

Post by Richard Smith
Rich Smith

[Jim Wilkins]

Post by Richard Smith
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-sequence-valve/operation-principle-of-pressure-sequence-valve

Post by Richard Smith
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.

Is this commercial or Heath Robinson?

[Ned Simmons]

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

Post by Ned Simmons

Post by Richard Smith
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."
http://valveproducts.net/pressure-sequence-valve/operation-principle-of-pressure-sequence-valve

Post by Richard Smith
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.

[Richard Smith]

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

Post by Ned Simmons

Post by Richard Smith
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."
http://valveproducts.net/pressure-sequence-valve/operation-principle-of-pressure-sequence-valve

Post by Richard Smith
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 ??

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]

Post by Richard Smith

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

Post by Ned Simmons

Post by Richard Smith
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."
http://valveproducts.net/pressure-sequence-valve/operation-principle-of-pressure-sequence-valve

Post by Richard Smith
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.

Post by Richard Smith
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

[Richard Smith]

Post by Ned Simmons

Post by Richard Smith

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

Post by Ned Simmons

Post by Richard Smith
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."
http://valveproducts.net/pressure-sequence-valve/operation-principle-of-pressure-sequence-valve

Post by Richard Smith
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.

Post by Richard Smith
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

[Jim Wilkins]

"Richard Smith" wrote in message news:***@void.com...
Ned Simmons <***@nedsim.com> 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.

[Ned Simmons]

On Fri, 21 May 2021 14:15:03 -0400, "Jim Wilkins"

Post by Richard Smith
...
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.

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.

[Jim Wilkins]

"Ned Simmons" wrote in message news:***@4ax.com...

On Fri, 21 May 2021 14:15:03 -0400, "Jim Wilkins"

Post by Jim Wilkins
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.

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

-------------------

I bought the largest used baking pan Goodwill had, for working on small
engines and hydraulics.

A low-profile drain for a pan or bucket can be made by turning down one end
of a brass push-on tubing connector into a tubular rivet.

[Ned Simmons]

Post by Richard Smith

Post by Ned Simmons

Post by Richard Smith

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

Post by Ned Simmons

Post by Richard Smith
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."
http://valveproducts.net/pressure-sequence-valve/operation-principle-of-pressure-sequence-valve

Post by Richard Smith
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.

Post by Richard Smith
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

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.

[Richard Smith]

Post by Ned Simmons

... ....

...

...

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

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.

[Jim Wilkins]

"Richard Smith" wrote in message news:***@void.com...
...
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.

[Richard Smith]

Post by Jim Wilkins
You could get a high-paying job welding submarine hulls, and be
invited aboard for the test dive.

There is a submarine base nearby.
Very exacting recruitment.

[Jim Wilkins]

"Ned Simmons" wrote in message news:***@4ax.com...

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

Post by Ned Simmons

Post by Richard Smith
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."
http://valveproducts.net/pressure-sequence-valve/operation-principle-of-pressure-sequence-valve

Post by Richard Smith
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
-----------------------------------

I'll take your expert word for it. My brain is still in a primitive BC
state. (Before Coffee)
https://www.hydraulicspneumatics.com/technologies/hydraulic-valves/article/21885033/are-you-taking-advantage-of-sequence-valves

"When flow stops, spring force closes the main poppet because pressure has
equalized."

[Ned Simmons]

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

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

Post by Ned Simmons

Post by Richard Smith
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."
http://valveproducts.net/pressure-sequence-valve/operation-principle-of-pressure-sequence-valve

Post by Richard Smith
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
-----------------------------------
I'll take your expert word for it. My brain is still in a primitive BC
state. (Before Coffee)
https://www.hydraulicspneumatics.com/technologies/hydraulic-valves/article/21885033/are-you-taking-advantage-of-sequence-valves
"When flow stops, spring force closes the main poppet because pressure has
equalized."

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

[Richard Smith]

Ned, Jim, everyone - massive thanks. Rich S

[Joe Gwinn]

Post by Richard Smith
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?

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

[Jim Wilkins]

Post by Richard Smith
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?

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.

[Richard Smith]

Post by Joe Gwinn

Post by Richard Smith
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?

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.

That's a "vibrophore", isn't it, if you apply that conept ot fatigue
testing machines?

Electro-mechanical device.

You would always use one of these if you could, for the project I'm
planning (?) Test rates to 150Hz and higher. Energy consumption so low
many will plug into a "domestic" wall socket.

Never met one in real life. Would desperately like to.
Idea of running a sample to 20 Million cycles no problem is like a
dream come true.

But the problem is when you go beyond "research test samples" to
testing representations of full-sized welds.

The biggest machines are 100Tonnes-force (1000kN; 1MN).

I've indicated the discussions would get very favourable if a 250kN
(25Tonne-force) "vibrophore" were available.

The rig I've sketched is for if say you needed to test a weld to
hundreds of tonnes cyclic stress range.

Rich S

[Joe Gwinn]

On Wed, 19 May 2021 17:42:41 -0400, "Jim Wilkins"
[snip]

Post by Joe Gwinn
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.

In the patent reference above, the oscillation cycle is controlled by
an external "variable speed motor" of unspecified kind, designated is
item 25 in the figures and accompanying text.

If the motor runs slow, so does the oscillation cycle, in direct
proportion.

Joe Gwinn

[Jim Wilkins]

"Joe Gwinn" wrote in message news:***@4ax.com...
...
If the motor runs slow, so does the oscillation cycle, in direct
proportion.

Joe Gwinn
----------------------
If the expected fatigue life is 2 million cycles, the test time at 1 cycle
per second is three weeks. I think a good solution would be a closed loop
based on a pressure sensor that shows when the high and low limit pressures
have been reached, so the controller can switch between the fill and dump
solenoid valves as rapidly as fluid flow permits.

If I had to build a prototype of the tester the controller would be an old
laptop (or desktop) with a printer port, the data bits driving a
successive-approximation A/D converter to measure the pressure sensor and
two control bits operating the fill and dump solenoid valves.

QBasic running in DOS gives full unhindered access to all of the printer
port bits for input and output, unlike Windows. An Arduino could also work
but the laptop has the advantages of a huge hard drive to store data, the
keyboard for control, and the LCD on which QBasic can display the cycle
count and a graph of the pressure.
http://www.nicolasbize.com/blog/30-years-later-qbasic-is-still-the-best/

This simple resistor network outputs a voltage proportional to the binary
code from the port bits:
https://www.tek.com/blog/tutorial-digital-analog-conversion-r-2r-dac

The other electronics are an analog comparator (LM311) driving a printer
port status bit that tells if the sensor output voltage is more or less than
the DAC output, and the two high current solenoid valve drivers.

[Richard Smith]

Post by Joe Gwinn
...
If the motor runs slow, so does the oscillation cycle, in direct
proportion.
Joe Gwinn
----------------------
If the expected fatigue life is 2 million cycles, the test time at 1
cycle per second is three weeks. I think a good solution would be a
closed loop based on a pressure sensor that shows when the high and
low limit pressures have been reached, so the controller can switch
between the fill and dump solenoid valves as rapidly as fluid flow
permits.
If I had to build a prototype of the tester the controller would be an
old laptop (or desktop) with a printer port, the data bits driving a
successive-approximation A/D converter to measure the pressure sensor
and two control bits operating the fill and dump solenoid valves.
QBasic running in DOS gives full unhindered access to all of the
printer port bits for input and output, unlike Windows. An Arduino
could also work but the laptop has the advantages of a huge hard drive
to store data, the keyboard for control, and the LCD on which QBasic
can display the cycle count and a graph of the pressure.
http://www.nicolasbize.com/blog/30-years-later-qbasic-is-still-the-best/
This simple resistor network outputs a voltage proportional to the
https://www.tek.com/blog/tutorial-digital-analog-conversion-r-2r-dac
The other electronics are an analog comparator (LM311) driving a
printer port status bit that tells if the sensor output voltage is
more or less than the DAC output, and the two high current solenoid
valve drivers.

Stunning detail.
I see that with
* test method and hardware
* control and data acquisition
2million cycles would be a start. With well-performing welds, you'd
want to get out to 20million cycles or more
(200Million is something around what a ship or bridge gets in its
entire period of service, but can be a bit of an ask in test).
I've programmed in Basic. Access to devices / ports - hadn't thought
about it being that convenient and tailor-made.

[Jim Wilkins]

"Richard Smith" wrote in message news:***@void.com...
...
I've programmed in Basic. Access to devices / ports - hadn't thought
about it being that convenient and tailor-made.

----------------------

I can't claim credit for the idea, it was the engineer's suggestion when we
(Unitrode / Texas Instruments) wanted to design and operate evaluation
boards for new user-configurable ICs from customers' unmodified laboratory
computers, which at the time were typically former office desktops and
laptops running Win98. We started with Visual Basic but quickly found that
it lacks the hardware control instructions of QBasic, and Windows polls the
printer port to detect newly attached devices. DOS + QB give full read/write
access to the I/O address space, interrupted only to update the clock.

The only change to the computer was setting the BIOS to boot from a DOS
floppy if present, else to Windows. The same can be done with a USB flash
drive using HPUSBFW. FAT32 USB flash drives are big enough to store programs
and large data files without access to the NTFS internal drive though older
FAT32 hard drives could handle either DOS or Windows, up through XP.
https://www.handheldgroup.com/knowledge-base/create-a-bootable-usb-drive/

The computer boots normally with the flash drive removed and you can read
any data log files the QB program created, such as cycle intervals which
might increase when the sample began to stretch, and indicate the point of
failure if you can't otherwise sense it.

I had previously assembled one-time computer to hardware interfaces with
added plug-in boards, a purchased digital I/O card and a 16 bit A/D
converter for the Macintosh that I designed. The printer port and DOS/QB
method turned out to be easier for relatively simple tasks. I'm also very
familiar with relay ladder logic controls if you choose to go that way.

Do you have the equipment or machinist friends to consider custom machining
as part of solutions? I couldn't do nearly as much without my lathe and
milling machine.

[Richard Smith]

Post by Richard Smith
...
I've programmed in Basic. Access to devices / ports - hadn't thought
about it being that convenient and tailor-made.
----------------------
I can't claim credit for the idea, it was the engineer's suggestion
when we (Unitrode / Texas Instruments) wanted to design and operate
evaluation boards for new user-configurable ICs from customers'
unmodified laboratory computers, which at the time were typically
former office desktops and laptops running Win98. We started with
Visual Basic but quickly found that it lacks the hardware control
instructions of QBasic, and Windows polls the printer port to detect
newly attached devices. DOS + QB give full read/write access to the
I/O address space, interrupted only to update the clock.
The only change to the computer was setting the BIOS to boot from a
DOS floppy if present, else to Windows. The same can be done with a
USB flash drive using HPUSBFW. FAT32 USB flash drives are big enough
to store programs and large data files without access to the NTFS
internal drive though older FAT32 hard drives could handle either DOS
or Windows, up through XP.
https://www.handheldgroup.com/knowledge-base/create-a-bootable-usb-drive/
The computer boots normally with the flash drive removed and you can
read any data log files the QB program created, such as cycle
intervals which might increase when the sample began to stretch, and
indicate the point of failure if you can't otherwise sense it.
I had previously assembled one-time computer to hardware interfaces
with added plug-in boards, a purchased digital I/O card and a 16 bit
A/D converter for the Macintosh that I designed. The printer port and
DOS/QB method turned out to be easier for relatively simple tasks. I'm
also very familiar with relay ladder logic controls if you choose to
go that way.
Do you have the equipment or machinist friends to consider custom
machining as part of solutions? I couldn't do nearly as much without
my lathe and milling machine.

Machining, making parts and equipment... My cider-maker / cider
supplier could feature in that... Can see a need to trade favours...


Digression into computing.
I often put "emacs" on a CD-ROM, take it by IT-support and invite them
to check it (fully up-to-date virus check software, etc, etc.)
If it's a write-once CD-ROM, it can't be altered ever again.
So if they trust that is all I will ever put in the drive of "my"
networked computer, that's sound in terms of IT security.

The reason for this is;
I have a range of suites of functions for various engineering
tasks which run in the test-processor I am using now - the famous /
well-known "emacs".
They will inject the answer straight into the document you are
writing. You can quote the function and the values you fed to it, and
the answer it gave. Complete record.
"sketching" your way to often high-value decisions.
Plus I do most other text-based thing in emacs.
For examples I didn't just type
"
Thermo-Mechanically Controlled-Processed High-Strength Low-Alloy
"
I typed tmcpqc hslaqc
When you are typing documents with huge strings of standard quotes of
Standards, Company Specifications, etc, that can save a huge amount of
time and effort, and leave your mind clear to think of the big
picture.
Boot off a CD-ROM? Could do?

[Jim Wilkins]

"Richard Smith" wrote in message news:***@void.com...

"Jim Wilkins" <***@gmail.com> writes:
...

Post by Jim Wilkins
Do you have the equipment or machinist friends to consider custom
machining as part of solutions? ...

Machining, making parts and equipment... My cider-maker / cider
supplier could feature in that... Can see a need to trade favours...

-----------
So not easily, perhaps shortening a bolt but not to the full custom extent I
would otherwise suggest, like the small piston in the cylinder end cap.
Experimenting is difficult when you are limited to only what you can afford
to buy. My shop is what an inventor would have dreamed of in 1960 though
perhaps not today, after 50~60 years of wear.

How about electronic test equipment? Capturing the brief peak value from an
analog pressure sensor during rapid cycling will be difficult without a
digital storage oscilloscope.

https://www.aliexpress.com/price/200-bar-pressure-sensor_price.html

[Richard Smith]

Post by Richard Smith
...

Post by Jim Wilkins
Do you have the equipment or machinist friends to consider custom
machining as part of solutions? ...

Machining, making parts and equipment... My cider-maker / cider
supplier could feature in that... Can see a need to trade favours...
-----------
So not easily, perhaps shortening a bolt but not to the full custom
extent I would otherwise suggest, like the small piston in the
cylinder end cap. Experimenting is difficult when you are limited to
only what you can afford to buy. My shop is what an inventor would
have dreamed of in 1960 though perhaps not today, after 50~60 years of
wear.
How about electronic test equipment? Capturing the brief peak value
from an analog pressure sensor during rapid cycling will be difficult
without a digital storage oscilloscope.
https://www.aliexpress.com/price/200-bar-pressure-sensor_price.html

Could be more than that...
Device with an eccentric crank?
Simple and presents a huge bearing area to take high forces.

[Jim Wilkins]

"Richard Smith" wrote in message news:***@void.com...
..
Boot off a CD-ROM? Could do?

_____________
Can do easily.

Hit the key during startup that enters the BIOS setup and put the CD-ROM
ahead of the hard drive in the Boot Sequence. On my Dells it's F2 before the
self-test completes. F12 lets me select any bootable source for this session
only.

My Lenovo laptop is a little different. It came in a Fastboot mode that
bypassed the chance to enter the BIOS.
https://support.lenovo.com/us/en/solutions/ht501793-how-to-turn-on-or-off-fast-startup-in-windows-1081

[Richard Smith]

Post by Richard Smith
..
Boot off a CD-ROM? Could do?
_____________
Can do easily.
...

There's a mini-computer called a "Raspberry Pi". Designed by group
endeavour of hobbyists. Meant for education. Uses a "merchant"
embedded-chip for eg. washing machines as its processor.
Might look if that offers a way.
Lots of devices been matched to it.

However, note you "PC architecture / QBasic" way.

[Jim Wilkins]

Post by Richard Smith
..
Boot off a CD-ROM? Could do?
_____________
Can do easily.
...

There's a mini-computer called a "Raspberry Pi". Designed by group
endeavour of hobbyists. Meant for education. Uses a "merchant"
embedded-chip for eg. washing machines as its processor.
Might look if that offers a way.
Lots of devices been matched to it.

However, note you "PC architecture / QBasic" way.

----------------------------
Sure. The Pi has the advantage that you can buy and plug in an A/D converter
to read the pressure sensor, and a keyboard and display to monitor the test,
no technician bench skills required. I haven't played with one and don't
know the learning curve for it. Are you generally familiar with computer
hardware and software architecture?

As for the eccentric, I considered it because you could vary the piston
stroke, but you'd need a lathe to make it, the pump drive is more than a
friend could whip out in a spare hour. A pump built into the cylinder end
might give the fastest cycle rate because there's no flow restriction. You
could fine tune the peak pressure during operation with a screw that
displaces oil. A cheap used tie-rod cylinder with a scratched bore could be
cut down to be your pressure chamber. Cylinder rebuilders can provide the
tubing in any length.

On my tractor's homebrew bucket loader attachment hydraulics I turned the
head of a bolt round and grooved it for an O ring, so it can screw into or
out of the oil space without leaking. It operates the variable pressure
relief valve I made to replace the fixed relief the control valve came with.
The tractor's front tires turned out to be the weakest link that limited how
high I could set the pressure.

The decision comes down to what you can build or buy. I've spent significant
time and money becoming able to build what I or the customer wanted,
electrical, optical and mechanical. Right now I'm upgrading my sawmill and
its overhead gantry hoist to handle a larger log than it was designed for.

The sawmill is a large horizontal bandsaw made from salvaged motorcycle
wheels. Thursday was lost to the miserable task of prying off the drive
wheel's tubeless tire, scrubbing the corroded bead seating surfaces and
getting it to hold air. The slow air leak reduced the blade's initial 500 lb
tension and let it wander in the >20 inch long cut. I learned manual tire
repair at the Hardway School (an Army motor pool) on split rim truck tires.
As the tire store manager told me, a 2" right angle air sander is the
perfect tool for cleaning up a leaking rim. My tractor's rusted wheels will
be its next victims.

Would fatigue cracking in oil be similar enough to cracking in air, which
oxidizes freshly exposed steel?

[Leon Fisk]

On Sat, 22 May 2021 09:01:35 -0400
"Jim Wilkins" <***@gmail.com> wrote:

<snip>

Post by Jim Wilkins
Would fatigue cracking in oil be similar enough to cracking in air, which
oxidizes freshly exposed steel?

And what about heat? Flexing metal creates heat. If you accelerate
flexing to speed up failure detection you will likely create heat that
would not be present in its actual use...

[Jim Wilkins]

"Leon Fisk" wrote in message news:s8b55e$59o$***@dont-email.me...

On Sat, 22 May 2021 09:01:35 -0400
"Jim Wilkins" <***@gmail.com> wrote:

<snip>

Post by Jim Wilkins
Would fatigue cracking in oil be similar enough to cracking in air, which
oxidizes freshly exposed steel?

And what about heat? Flexing metal creates heat. If you accelerate
flexing to speed up failure detection you will likely create heat that
would not be present in its actual use...
--
Leon Fisk
Grand Rapids MI

----------------------

That's a good point, however since the sample is in an oil bath its
temperature is easy to control with equipment that can be added later, if
needed.
https://www.wattco.com/product_category/band-heaters/

[Richard Smith]

Post by Leon Fisk
And what about heat? Flexing metal creates heat. If you accelerate
flexing to speed up failure detection you will likely create heat that
would not be present in its actual use...
--
Leon Fisk
Grand Rapids MI

Heating is pretty negligible.
On a resonant machine you can test at up to around 300Hz and there is
no drift in values obtained compared to at much slower rates.
You have full elastic energy recovery.

Servo-hydraulic - not recovering elastic energy with the drive heats
the oil away from the sample where you dump the pressure - and you
dump that heat through "raditiators" (forced convectors transfering
oil-to-air).

But the sample is seeing full energy recovery / negligible energy
loss.

[Richard Smith]

Post by Jim Wilkins
...
Would fatigue cracking in oil be similar enough to cracking in air,
which oxidizes freshly exposed steel?

I think so - very little difference.
Well my instinct says for environment in general that
* negligible difference at high stresses
* possible significant effects at low stresses / huge numbers of
cycles to crack / break
I suspect the difference between in-air and in-oil would be
negligible.

Another thing I must test though.

Rich S

[Jim Wilkins]

Post by Jim Wilkins
...
Would fatigue cracking in oil be similar enough to cracking in air,
which oxidizes freshly exposed steel?

I think so - very little difference.
Well my instinct says for environment in general that
* negligible difference at high stresses
* possible significant effects at low stresses / huge numbers of
cycles to crack / break
I suspect the difference between in-air and in-oil would be
negligible.

Another thing I must test though.

Rich S

--------------

Racing engine builders might know.

[Richard Smith]

Post by Jim Wilkins
...
As for the eccentric, I considered it because you could vary the
piston stroke, but you'd need a lathe to make it, the pump drive is
more than a friend could whip out in a spare hour. A pump built into
the cylinder end might give the fastest cycle rate because there's no
flow restriction. You could fine tune the peak pressure during
operation with a screw that displaces oil. A cheap used tie-rod
cylinder with a scratched bore could be cut down to be your pressure
chamber. Cylinder rebuilders can provide the tubing in any length.
On my tractor's homebrew bucket loader attachment hydraulics I turned
the head of a bolt round and grooved it for an O ring, so it can screw
into or out of the oil space without leaking. It operates the variable
pressure relief valve I made to replace the fixed relief the control
valve came with. The tractor's front tires turned out to be the
weakest link that limited how high I could set the pressure.

Variable - you have many eccentrics, each with a slightly different
throw?

I thought - one eccentric, but many different "pistons" with their
"collar" they go through into the fluid volume.

Yes one thought is that the mechanism could be built onto the lower cylinder end.

With the "bar" driven by the eccentric simply pushing in and out of
the cylinder volume.
For the "170Tonne-force" test it would need to displace about 270cc -
would be 70mm diameter and stroke (or some other combination of
diameter and stroke which gives that displacement).

One advantage of this arrangement is, seeing as it's so stiff, plus
bits can't fly around with being inside the cylinder, the test rate
could be high. Fastest induction motor speed? 3000rpm on 50Hz supply
= 50Hz test rate :-)
That would be 60Hz on N.Am. supply.

Post by Jim Wilkins
The decision comes down to what you can build or buy. I've spent
significant time and money becoming able to build what I or the
customer wanted, electrical, optical and mechanical.

Knowing the metallurgical and fatigue stuff has cost me a lot - money
in various ways and time ...

[Jim Wilkins]

...eccentric...

Variable - you have many eccentrics, each with a slightly different
throw?

I thought - one eccentric, but many different "pistons" with their
"collar" they go through into the fluid volume.

-----------------------

One circular eccentric disk, mounted on an offset pivot pin so it can be
centered or swung out as needed. The clamp for the swinging side might have
to include a custom stepped bushing to withstand the torque, rather than
just a bolt that holds by friction, but its surface finishes aren't critical
like the eccentric's. Lathes don't necessarily leave surfaces good enough to
be running bearings, that's extra hand work.

It's easy to turn two cylindrical surfaces with different centers when
holding the work in a 4-jaw lathe chuck.
http://s3.cnccookbook.com/CCLatheEccentricTurning.htm

The eccentric disk could be a slice of hydraulic cylinder rod with a
case-hardened, chromed and polished surface. The strap could be lined with
replaceable slices of Oilite bushing. You don't need the historical accuracy
many British model engineers strive for.
http://www.stockbridgelocomotiveworks.com/2019/10/14/eccentrics-and-valve-gear/

You have ideas that need machine tools to create. Good new ones and hired
custom shop work are quite expensive, so I saved by finding older industrial
machines which had become obsolete and too worn to be economical in a
production shop, like a lathe made in 1965. You have an excellent
equivalent:
http://www.myford-lathes.com/used.htm
I have no experience with the current imports.

I can't easily hit the tolerances on a customer's drawing but I can still
make two pieces fit each other although they may not be quite to spec, so my
antique machines are fine for making single devices for my own use (and for
fixing each other). From the reference above:
"I turned the final diameter on a good portion of the bar and then machined
each eccentric one at a time, individually match-fitting each eccentric
strap."

The trick is that two parts of a complementary operation may not be equally
difficult, for example the piston is easier to turn and finish than the
cylinder, so make the difficult one first and fit the easier one to it. The
boring head he used on the eccentric has a micrometer adjusting screw to
change size, I have an identical one. Many shortcuts are possible when you
control the design.

I made the prototype of an inch-long diode laser and lens mount in a few
evenings that later cost $4000 apiece from a job shop that normally made
parts for BAE. Mine wasn't quite as well finished but it worked and proved
my ideas. First I needed approval to charge it as overtime, but the project
engineer knew how expensive the company's main machine shop was. I suspect
part of the high cost was due to the electrical engineers' inexperience with
mechanical design and machining.

[Jim Wilkins]

"Jim Wilkins" wrote in message news:s8di01$fb0$***@dont-email.me...

...
http://s3.cnccookbook.com/CCLatheEccentricTurning.htm

-----------------

I made a split bushing to hold short 3/8" Grade 8 bolts in a 5C collet like
in the "I love this tooling." example shown. Grooves near the ends hold O
rings that keep it together. Neither the hex head, shank nor threads run
true to each other so I have to clamp the bolt by the section I'm modifying.
OK, they are technically hex head cap screws when they go in a tapped hole
but 'bolt' is their common name.

The bushing's center hole is 0.370" diameter to match the shanks of the
bolts at the store, and before splitting I tapped it 3/8-16 to grab the
thread crests without damaging them, as rolled threads can be larger than
the shank. It lets me modify bolts to within as little as 1/2" from the
head, for instance to trim back the slightly-long shank so a nut bears on
the parts being clamped, or to cut a root-diameter pilot and pointed end on
the threads for simultaneous alignment of multiple 40 Lb parts with 0.370"
bolts in 0.375" holes. Those piloted bolts are longer and go at the outer
ends to stop the trolley after using them to align the center splice. Most
of the bolts joining my gantry track sections had to be modified to put
their shanks in the shear planes and clear the moving trolley.

My previous 5C bolt-holding fixtures are bushings with tapped holes, but
this version is more versatile and clamps tight on either shank or threads.
I split it with a hacksaw after scribing the end with a tool point aligned
with the 5C splits.

[Richard Smith]

Post by Jim Wilkins
...
One circular eccentric disk, mounted on an offset pivot pin so it can
be centered or swung out as needed. The clamp for the swinging side
might have to include a custom stepped bushing to withstand the
torque, rather than just a bolt that holds by friction, but its
surface finishes aren't critical like the eccentric's. ...
...

I haven't got this for certain.
Not grasped the idea for sure, yet.

Forces would be in Tonnes to tens of Tonnes.
The size of the eccentric - both what it will withstand as a bend or
shear, and preesenting enough bearing surface for the bearing to take
the load - can make these things quite big. Maybe 200mm diameter with
190mm shaft for 10mm "throw", for some enormous tests in high yield
steels.
As I calculate / estimate as best I can.

If it came to making one of these things, there would have been a lot
of proof-of-principle with "testing lab. scale" samples, and the
stakes would be quite remarkable by that stage, if we got there.

The main point is to have plans in place.
In a political world, you have to have everything covered, so every
interjection, objection, etc. is smoothly put in its place.
As I've experienced.
I've certainly had the skill of predicting what the ploys might be
tested. Having big efforts to derail the plan slapped down in
seconds.

So it's about being able to see a way ahead, far along a perceived
path. What would actually be going on, where you would actually be by
then, what method you would use given experience had by then but ahead
of you now - that might be a different story.
But for now - you are "covered"...

With the beam tests, you can tune the testing force by moving the beam
end supports in and out - present different spans. So one fixed throw
/ movement drive would cover all purposes.

For the "hydraulic inner fatigue test", I think different diameter
"spuds" sliding in and out of the fluid volume, on a fixed eccentric
drive, might be easiest?

[Jim Wilkins]

Post by Jim Wilkins
...
One circular eccentric disk, mounted on an offset pivot pin so it can
be centered or swung out as needed. The clamp for the swinging side
might have to include a custom stepped bushing to withstand the
torque, rather than just a bolt that holds by friction, but its
surface finishes aren't critical like the eccentric's. ...
jsw

I haven't got this for certain.
Not grasped the idea for sure, yet.

Forces would be in Tonnes to tens of Tonnes.
The size of the eccentric - both what it will withstand as a bend or
shear, and preesenting enough bearing surface for the bearing to take
the load - can make these things quite big. Maybe 200mm diameter with
190mm shaft for 10mm "throw", for some enormous tests in high yield
steels.
As I calculate / estimate as best I can.
RS

[[[[[[[[[[[[[[[[[[[
Lets say the 200mm eccentric is pinned to its mounting plate with two
diametrically opposed 25mm round bushings. If the bushings are straight
cylinders the eccentric disk runs in a true circle. If both bushings have a
5mm offset step in the middle to shift the eccentric disk sideways the throw
is 10mm. The lathe setup to turn the bushings is the 5C collet block in a
4-jaw chuck, as in the posted reference.

The bushings provide high shear and bearing strength. They would be slotted
and keyed to prevent rotation. Making them is a reasonably simple lathe
operation. To change the throw you make bushings with different offsets
rather than changing the eccentric disk.

Two identical stepped bushings is simpler to describe, one stepped and one
parallel is more rigid, triangle vs parallelogram.
jsw
]]]]]]]]]]]]]]]]]]]

If it came to making one of these things, there would have been a lot
of proof-of-principle with "testing lab. scale" samples, and the
stakes would be quite remarkable by that stage, if we got there.

The main point is to have plans in place.
In a political world, you have to have everything covered, so every
interjection, objection, etc. is smoothly put in its place.
As I've experienced.
I've certainly had the skill of predicting what the ploys might be
tested. Having big efforts to derail the plan slapped down in
seconds.

So it's about being able to see a way ahead, far along a perceived
path. What would actually be going on, where you would actually be by
then, what method you would use given experience had by then but ahead
of you now - that might be a different story.
But for now - you are "covered"...
RS

[[[[[[[[[[[[[[[[[[[
Been there, done that, I was the lab rat who built the proof-of-principles
from sketches and gave the engineers simple, practical and economical ways
to convert their concepts into products. I provided you several different
approaches you could pursue, and mentioned possible sticking points like
difficulty of fabrication or programming, and the skills and equipment to
moved past them.

A very handy trick I picked up to counter objections is to memorize the
squares and reciprocals of numbers up to 32 so I could do engineering math
rapidly in my head and present an off-the-cuff mathematical basis for my
arguments. Above 31 [sqrt(1000)] they pair with lower numbers, 1/25 =
0.040, 1/40 = 0.025. Engineers not from the slide rule era don't learn the
simplifying tricks we had to.

For example I was running an error rate test on a satellite link when the
chief engineer demanded to know how much longer I would be tying up the
channel. The test was 10 million bits at 2400 per second. Knowing that 1/24
is 0.041666... I mentally figured the test duration as 4166.7 seconds, then
converted that to 1 hour (3600), 9 minutes (540) and 26.7 seconds and gave
him the answer. He pulled out his calculator, hesitated, then admitted he
didn't know where to start and left me alone to complete the test.

I understand the physics but it's still weird to see something stored and
later retrieved intact from empty space.
jsw
]]]]]]]]]]]]]]]]]]]

With the beam tests, you can tune the testing force by moving the beam
end supports in and out - present different spans. So one fixed throw
/ movement drive would cover all purposes.

For the "hydraulic inner fatigue test", I think different diameter
"spuds" sliding in and out of the fluid volume, on a fixed eccentric
drive, might be easiest?
RS

[[[[[[[[[[[[[[[[[[[
The most demanding part to make is the piston/cylinder sealing surface,
especially if you don't have machine tools. Any spud screwed into the end of
the piston displaces a fixed volume of oil. There may be another way but I
think the easy answer is to change the stroke length (lever?) unless you
collect an appropriate assortment of polished rod stock and seals (car shock
absorbers, gas cylinders etc). Modifying the rods into pistons and making
packing glands to retain the seals are lathe operations.
jsw

[Richard Smith]

... Modifying the rods into pistons and making packing glands to
retain the seals are lathe operations. ...

I took it a lathe would be available.
Cylindrical "spud", cylindrical bush.

Maybe the "hydraulic fluid" could be grease - thick - tranmitting to
oil or water through a rubber membrane. So the piston/bush sees
viscous grease not thin oil.

[Jim Wilkins]

... Modifying the rods into pistons and making packing glands to
retain the seals are lathe operations. ...

I took it a lathe would be available.
Cylindrical "spud", cylindrical bush.

[[[[[[[[[[[[[[[[[
It's difficult to create innovative mechanical solutions with only stock
parts meant to solve standard problems. At Segway which was an engineer's
playground by official policy the CNC lathe and milling machine were almost
always tied up making someone's wild idea. I used their manual lathe or my
home shop machines but my main task was custom electronics.

https://news.yale.edu/2008/12/05/students-learn-how-kiss-frogs-and-take-risks-morrell-s-class
Electric model airplanes were practically a second product there, and they
taught me the care and feeding of Lithium batteries.
Unfortunately like bridge building the team dissolved and sought new
challenges once the project was complete.
]]]]]]]]]]]]]]]]]

Maybe the "hydraulic fluid" could be grease - thick - tranmitting to
oil or water through a rubber membrane. So the piston/bush sees
viscous grease not thin oil.

[[[[[[[[[[[[
It's known that high pressure oil seals work if done right, so I'd bin that
with the minor issues that can be solved later by throwing enough money at
them, and concentrate on identifying and resolving the show-stoppers.

I'm glad I read the Amazon reviews before applying the thick black tire bead
sealer goop. A reviewer advised to let it set somewhat before inflating the
tire lest it spray out. I did, and only soapy water rubber lube sprayed me
before the bead seated when I inflated it.

Here is a racing engineer and technology historian who might know about
metal fatigue in oil:
https://www.calum-douglas.com/

[Richard Smith]

Post by Jim Wilkins
...
It's known that high pressure oil seals work if done right, so I'd bin
that with the minor issues that can be solved later by throwing enough
money at them, ...

I'd wish - but lots of slightly different sizes - and you are going to
be able to find a seal for each diameter?

Maybe one could get a list of stock seal diameters, and find that yes,
the pressure and therefore test force you'd end up with would be just
fine - sits just where it needs to be to plot the S-N curve for
fatigue performance. Throwing aside minor vanities like getting the
stress which should mean the mean-average sample break is at exactly
250k cycles, etc.

Rich S

[Jim Wilkins]

Post by Jim Wilkins
...
It's known that high pressure oil seals work if done right, so I'd bin
that with the minor issues that can be solved later by throwing enough
money at them, ...

I'd wish - but lots of slightly different sizes - and you are going to
be able to find a seal for each diameter?

Maybe one could get a list of stock seal diameters, and find that yes,
the pressure and therefore test force you'd end up with would be just
fine - sits just where it needs to be to plot the S-N curve for
fatigue performance. Throwing aside minor vanities like getting the
stress which should mean the mean-average sample break is at exactly
250k cycles, etc.

Rich S

-----------------------

This is a US company selling inch-sized products, but it shows what to look
for.
https://www.baileyhydraulics.com/Product-Type/Hydraulic-Cylinder-Components-

When I needed new seals for leaky used Porta-Power-type cylinders I went to
a pump rebuilder who found seals that were close enough in the catalog of a
supplier I don't remember. My cheaply made Chinese cylinders were somewhat
non-standard so I had to machine the pistons a bit. I've been told they are
typical of import hydraulics that can't be repaired (without remachining).

The cylinders were $10 and $15 each and the right very compact size for my
bucket loader design so they were worth the trouble.

I suspect that seals are available for all cataloged sizes of chromed and
polished hydraulic rod stock, a part I can't make. Then you only need to
make (or buy) the packing gland that adapts the seal to the cylinder end
cap, and a crosshead to keep the piston running straight, both fairly easy
on a thread-cutting lathe. They could be combined into one part. Instead of
machining a hex you can drill two holes for a pin spanner wrench to tighten
it.

Last night the air cleaner cover latch on the Chinese engine of my sawmill
broke off so I made a stainless sheet metal replacement. I've stopped
hunting for spare parts that are difficult or impossible to find and no
better than the originals.

A -good- lathe is an investment that may appreciate. Collectors restore the
better examples of mine, like classic cars.
https://www.tinshackrestoration.com/2015/01/19/south-bend-heavy-10-lathe-2/
"Too funny! You bought a great lathe, and with all of those vintage Rovers
you better learn to make parts quick! Thanks for the note!"

[Richard Smith]

Post by Jim Wilkins
...
[[[[[[[[[[[[[[[[[
It's difficult to create innovative mechanical solutions with only
stock parts meant to solve standard problems. At Segway which was an
engineer's playground by official policy the CNC lathe and milling
machine were almost always tied up making someone's wild idea. I used
their manual lathe or my home shop machines but my main task was
custom electronics.
https://news.yale.edu/2008/12/05/students-learn-how-kiss-frogs-and-take-risks-morrell-s-class
Electric model airplanes were practically a second product there, and
they taught me the care and feeding of Lithium batteries.
Unfortunately like bridge building the team dissolved and sought new
challenges once the project was complete.
]]]]]]]]]]]]]]]]]
Maybe the "hydraulic fluid" could be grease - thick - tranmitting to
oil or water through a rubber membrane. So the piston/bush sees
viscous grease not thin oil.
[[[[[[[[[[[[
It's known that high pressure oil seals work if done right, so I'd bin
that with the minor issues that can be solved later by throwing enough
money at them, and concentrate on identifying and resolving the
show-stoppers.
I'm glad I read the Amazon reviews before applying the thick black
tire bead sealer goop. A reviewer advised to let it set somewhat
before inflating the tire lest it spray out. I did, and only soapy
water rubber lube sprayed me before the bead seated when I inflated
it.
Here is a racing engineer and technology historian who might know
https://www.calum-douglas.com/

Off-topic but...

Two
great leads in there!

Engine Development and how done.
Empires were at stake then and technical folk got resources - amazing
what they did.

The Calum Douglas lead - never met or seen a pic. of the brittle lacquer
method for stress distribution in-action. ****! it's effective!
The "perspex / polarised-light" method - I wanted to use that when I
knew a Finite Element Analysis modelling engineer must be talking
<...nonsense!...>, before I could FEA. I actually blagged some
"perspex" from a nearby company - but the Finite Element Analysis
engineer then made such a bad political slip-up that we lost the job
it was for anyway.

"Yale" / Segway lead...
The one about engineers "kissing a lot of frogs to find a
prince". Exactly! Yes, that's the reality of a scientist too!
You have to be able to think and process things that way, to be a
human having an effective channel of perception into the Natural
World.
I almost envy people living in what they think is a determinate world.

[Jim Wilkins]

"Richard Smith" wrote in message news:***@richards-air-2.home...

"Yale" / Segway lead...
The one about engineers "kissing a lot of frogs to find a
prince". Exactly! Yes, that's the reality of a scientist too!
You have to be able to think and process things that way, to be a
human having an effective channel of perception into the Natural
World.
I almost envy people living in what they think is a determinate world.

---------------------

I have a BS in Chemistry so I understand how a scientist must think and
operate. It's a very hands-on discipline that applies to almost any industry
and we received a broader training than I've seen in other types of engineer
and scientist, such as Materials Science (the properties of steel) and two
years of Physics.

The profs told us a BS degree doesn't prepares us to do useful work
immediately, only to understand the explanations wherever we go. Advanced
degrees were essential, they said self-servingly. (Mitre was like that too.)
I was chosen for summer research grants and did learn more about real life
applications, and what it's like to hole up in the lab all night and not see
other humans for weeks at a time. I was disturbingly comfortable with that.

At graduation time I still needed 4 more credits in any subject and, being
somewhat burned out by then, signed up for 6 credit, certain-to-pass summer
theatre as a carpenter, and was packed in with as many touchy and demanding
humans as I could stand for 12-16 hours a day, good training I suppose. I
was dancing on stage when Armstrong landed on the moon.

When I graduated the grad school draft deferment had ended, taking with it
my Chemistry career, but the Army was glad to find someone they could train
to maintain complex electronics along the lines of your Colossus machine.
The few who survived that school all had technical degrees. The integrated
circuit was enabling electronics' Great Leap Forward and I got in it at the
start.

[Jim Wilkins]

"Richard Smith" wrote in message news:***@richards-air-2.home...
...
For fatigue testing samples - it has to tension and release millions
of times.
...
----------------------

If you want simple, a 3-port rotary valve would alternately let oil in and
out of the cylinder. You could run it slowly until you determine how fast
you can spin it and still attain full and minimum pressure. The valve could
operate a revolution or cycle counter. If you can tolerate a little leakage
it's simple enough for a model engineer to make. ( I have too many machining
jobs for the sawmill and its gantry hoist in the queue to offer. )

[Richard Smith]

Hi to everyone.
You folk are amazing!
I've been here on my own, and you come along knowing lots of solutions.
I'm in information-overload with all the great leads you've given.
Plus I've just done a week and a half of calculations on testing
methods and really really need to give my head time-out.

You asked about this device.
I'll share with you
http://www.weldsmith.co.uk/tech/fatgres/210511_fatgt_hyd_inner/210511_fatgt_hyd_inner.html
It's an idea for a metal fatigue test, particularly of welds.

So in this rig, the sample is inaccessible, inside the hydraulic
cylinder immersed in the hydraulic fluid.

For the advantages you get, the disadvantages are "nothing".
This test could be running for days to 10's of days, by the way.
It's "high stakes".

Even where you can stop the test and approach the sample, you don't,
in reality. Gets you nothing additional.

The test stops when the sample breaks...

Welding the sample to the "pistons" has more advantages than
disadvantages (one would reckon).
Small and stiff.

A "contraption"? Yes. However - so long as you do a "scattergun"
approach, you have a significant likelihood that some try will work.
You have to be very open-minded, because you can have "dead-certs"
which don't work, and "shouldn't work" which delivers and then some.
You can find the science was wrong, or incomplete, and by giving it a
try anyway you find that out, as well as getting results.

I'll get my head around the info - the links and hints.

Like be back in a little while...

Best wishes all,
Rich S

[Jim Wilkins]

"Richard Smith" wrote in message news:***@richards-air-2.home...
...
You asked about this device.
I'll share with you
http://www.weldsmith.co.uk/tech/fatgres/210511_fatgt_hyd_inner/210511_fatgt_hyd_inner.html
It's an idea for a metal fatigue test, particularly of welds.

So in this rig, the sample is inaccessible, inside the hydraulic
cylinder immersed in the hydraulic fluid.

For the advantages you get, the disadvantages are "nothing".
This test could be running for days to 10's of days, by the way.
It's "high stakes".
---------------------------

You might be able to significantly reduce the 80KW power demand if the
pressure source is a small flywheel-driven piston built into the fixed end
of the cylinder to minimize flow friction loss, that absorbs the pressure
energy as it retracts. You'd add oil/bleed air until a pressure sensor
signal peaks at the desired pressure.

This could require some custom machining and knowledge of using an
oscilloscope. Are those acceptable for you?

When I was in the automated testing business we had to figure times for high
count operations such as testing each cell on a memory chip wafer, often in
conversation without a calculator. There are 86,400 seconds in a day. 1
million seconds (cycles?) is 11.57 days. A micro-year is 31.5 seconds. 1 US
billion (10^9) seconds is 31.7 years.

[Richard Smith]

Post by Richard Smith
...
You asked about this device.
I'll share with you
http://www.weldsmith.co.uk/tech/fatgres/210511_fatgt_hyd_inner/210511_fatgt_hyd_inner.html
It's an idea for a metal fatigue test, particularly of welds.
So in this rig, the sample is inaccessible, inside the hydraulic
cylinder immersed in the hydraulic fluid.
For the advantages you get, the disadvantages are "nothing".
This test could be running for days to 10's of days, by the way.
It's "high stakes".
---------------------------
You might be able to significantly reduce the 80KW power demand if the
pressure source is a small flywheel-driven piston built into the fixed
end of the cylinder to minimize flow friction loss, that absorbs the
pressure energy as it retracts. You'd add oil/bleed air until a
pressure sensor signal peaks at the desired pressure.
This could require some custom machining and knowledge of using an
oscilloscope. Are those acceptable for you?
When I was in the automated testing business we had to figure times
for high count operations such as testing each cell on a memory chip
wafer, often in conversation without a calculator. There are 86,400
seconds in a day. 1 million seconds (cycles?) is 11.57 days. A
micro-year is 31.5 seconds. 1 US billion (10^9) seconds is 31.7 years.
http://youtu.be/hj7LRuusFqo

Exactly so.

I have already done these calculations for my "beam configuration"
fatigue test.
Static version of it - tensile test
http://www.weldsmith.co.uk/tech/struct/210122_fwtest_rig/210122_fwtest_testrig.html

The benefit you have there are the highly predictable Euler-Bernoulli
beam calculations for long sections.
I found one fixed stroke actuator would cover every need.


The problem with the "hydraulic inner fatigue test" is it is difficult
to know with much accuracy how much hydraulic oil it is going to take
per stroke to reach the intended force on the sample.
The end pistons seem to be the design challenge. Flexing. Cylinder
analysed by "hoop stress" no problem.

Also - adjusting it so you can arrive at different forces...
Because you need to explore the shape of the fatigue "S-N curve".

That's why took thought of servo-hydraulic.
With "catalog" equipment you could quickly get started.
Connect it up and off you go.
You'd probably use a pressure transducer and servo valves, wth digital
logic linking them. In reality. As the equipment would be already
there and familiar.

[Richard Smith]

I've drawn the servo-hydraulic drive system.
Mechanical logic though.
Drawn as best I can.
Sorry about any offence to familiar conventions.

http://www.weldsmith.co.uk/tech/fatgres/210511_fatgt_hyd_inner/210520_fatgt_servo-hyd_drive.pdf

There's two "bobbin" valves.

The big one dumps the main power system oil to the tank.
It opens when the cylinder pressure exceeds the reference pressure.
That cylinder oil comes through a check-valve, so once pressurised and
vlave open, it stays open.
Until...
The other "bobbin" valve opens when the cylinder pressure is so low it
cannot overcome a spring - that opening dumping the ex cylinder
pressure holding open the main bobbin valve against the reference
pressure - that reference pressure reopening the main valve.

All this mechanism could be on one "pallet" - reference system and
power system adjacent, with one pipe to the test-rig atking
back-and-forth flows.

[Richard Smith]

Sorry that's "reference pressure re-closes the main valve" - to start the next cycle.

It would be good if this system could cycle very quickly and the bigger the pump, the faster it cycles.

[Richard Smith]

there's other errors.
Reference pressure must hold main dump valve closed, until cylinder pressure exceeds refernce pressure.
Sure there will be other errors.
Got to head off now. Thanks for everything
Rich S