Need the opposite of a relief valve 3

[TheTick]

I need a valve that functions in a manner that is opposite of a relief valve: a valve that closes when pressure gets above a setpoint.

The application is at the outlet of an air cylinder that is to slow movement of an arm if the actuator lifting the arm fails. The outlet of the air cylinder is ported through two parallel openings, a small orifice and a larger opening. If the load is "dropped", I want the larger opening to close so that the air cylinder drops slowly.

Another possibility: a check valve that allows a certain amount of low pressure leakage before it is forced shut.

 

[hydtools]

Perhaps a velocity fuse that will close the large port when flow exceeds some value and forces the flow out the small orifice.

http://www.hydraulicsystem.com/pdf_...ries AVF) - Valves,Check Valves,Pneumatic.pdf

Ted

 

[TheTick]

That almost does it. Really the same problem frame around velocity instead of pressure

The smallest one is still a bit large, though, and may not respond fast enough. Maybe if I use a larger cylinder...

 

[PeterIgg]

Is using a universal 3/2 pilot spring valve too simplistic? Loss of pressure to the pilot causes the valve to change state under spring force....?

 

[itascot]

TheTick,
You could try a safety excess flow check valve.
Ciao,

 

[ione]

What about an automatic valve operated by a pressure switch?

 

[TheTick]

The current plan is for a pressure switch and a solenoid valve. That is preferred, because then the switch can also signal to shut off the test fixture.

I still need to spar with the old school guys who want something 100% mechanical. At least this is getting me out to learn a few new things.

I can almost picture a device that could work, like a swing check or diaphragm valve with a spring to counter light pressure. Not actually finding such an animal.

 

[itascot]

TheTick,

Check out this link

http://www.metalgoodsmfg.com/excess.htm,

a purely mechanical device (quite symilar to your description). You can couple this up with a pressure switch to signal line failure.

Ciao

 

[ione]

TheTick,

The idea of itascot is pretty good. Take a look at the link below (specific for air).

http://www.rego-europe.de/upload/pdf/flow-control/05-excess-flow-valves.pdf

No need to design a new item, just evaluate the flow rate corresponding to your pressure threshold in order to have an idea of the most suitable item for your application.

 

[hydtools]

By using a veloctiy fuse or excess flow valve you could possibly eliminate the secondary small orifice port. The excess flow valves all have a small through flow when closed.

Ted

 

[TheTick]

I thought a regulator might do the trick. If I had an orifice to keep normal exhaust pressure around 1 to 2 psig and a regulator that would keep downstream pressure at that level regardless of what is happening upstream. The question is if a regulator can act fast enough.

 

[itascot]

I am unfamiliar with any ehaust flow regulator that is "quite that precise & repetitive" I have been working with actuators for the last 26 years & never considered flow regulators to be precise to 1 or 2 psig....more precise in time taken to exhaust a cylinder volume (this also with a tolerance).
In answer to your question on reaction time, need to know what size cylinder & what pressures are involved.
Ciao,

 

[MikeHalloran]

You'll go out of your mind trying to tune the system so the load doesn't bounce on the trapped air when it's dropped.

This sort of system works much better when the damping fluid is a liquid.



Mike Halloran
Pembroke Pines, FL, USA

 

[itascot]

MikeHalloran is perfectly right, air is compressible....oil is not.
The simple solution is as previous postings have suggested.
Ciao,

 

[unclesyd]

As you state you are looking for a relative small device you should take a look at the offerings by Lee. They may have what you need as single piece or as a unit.
They were good to work with.

http://www.theleeco.com/LEEWEB2.NSF/AeroStart!OpenPage