"Joe Gwinn" wrote in message
...

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

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.