small servo operated air tight valve

[digiex-chris]

I need to make some type of valve to hold small air pressure (less than 5 psi) and be operable by a standard size RC servo. Having serious trouble coming up with ideas that aren’t high drag. I could use a cam-lock type clamp on a piece of tubing, but I dislike crimping the tubing, it kind of stays partially flattened when open. It's going to turn on or off the crank case pulses going to a walbro carb's fuel pump.

Any ideas?

 

[digiex-chris]

Of course it works this way. Wrack my brain for a few days, come up empty. Post the question to the forums, the ideas start flooding in.

I think maybe something like a needle valve, a conical needle going into an accurate hole, pushed in by the servo instead of with threads.

 

[sssfox]

I saw a valve once that a guy was using to supply oil to the exhaust to make smoke. It was three sheets of some type of plastic. The outer two were tied together and had the supply and output lines tied to them. The center plate had a hole in it. When the hole lined up with the two lines, the oil flowed. When it didn't line up, the flow stopped.

I saw a picture of a similar one that was the same principal, but it was a disc between two plates and a square block with a hole that matched the disc. The hole went through the edge of the disc and the block. The whole unit was sealed, I guess to prevent leaks. I don't remember what it was designed to be used for. Maybe it was a fuel cut-off.

 

[Noitoen]

Peristaltic pumps work on the tube pinching process and the tube lasts a long time. It's just the matter of choosing the right one. Silicone model engine fuel line would do the trick.

 

[digiex-chris]

the problem with silicone is it breaks down in the presence of petroleum fuels.

I like the idea of the 3 sheets! I suppose a disc between 2 sheets with a slot in it would work too, just turn the disc. The trouble with either of those is making it air tight.

 

[sssfox]

It depends on what you are trying to accomplish as to how air tight it needs to be. 5 psi isn't much to overcome.

How about if you used three plates, the middle one thick enough for a slot.
The input and output would be on one of the outer plates and a light spring or O-ring, almost as large as the plate with the slot, to push the plate with the slot against the input/output plate.

You could also use clips to squeeze the plates together.

A little grease could initially prevent it from leaking and the fuel in the exhaust would keep it going.

I keep seeing a better answer in the back of my mind, but it won't come out.

 

[solarenergyadventures]

Greetings Everyone,
It sounds like what you need is a retract valve for retractable landing gear on RC airplanes! It is a simple piston spool valve just like on a steam engine, but it uses rubber o-rings for sealing. Naturally they move easily enough for a servo, even a micro servo, to push back and forth. I hope this helps.
Cheers,
Chris

 

[sssfox]

That's what has been trying to find it's way out of the back of my mind.

Thanks Chris.

 

[RICHARDDV]

I need to make some type of valve to hold small air pressure (less than 5 psi) and be operable by a standard size RC servo. Having serious trouble coming up with ideas that aren’t high drag. I could use a cam-lock type clamp on a piece of tubing, but I dislike crimping the tubing, it kind of stays partially flattened when open. It's going to turn on or off the crank case pulses going to a walbro carb's fuel pump.

Any ideas?

try using the small duck bill valve out of a homelite chain saw fuel tank. Its silicone about 5/16 long and 1/8 dia. Its used to control fuel pump action with crankcase pulses.

 

[peatrich]

Would the kind of tap used on a small motorcycle/scooter petrol tank work? They operate by rotating the lever through 90 deg which would be easily achieved by a decent RC plane servo.

 

[digiex-chris]

Greetings Everyone,
It sounds like what you need is a retract valve for retractable landing gear on RC airplanes! It is a simple piston spool valve just like on a steam engine, but it uses rubber o-rings for sealing. Naturally they move easily enough for a servo, even a micro servo, to push back and forth. I hope this helps.
Cheers,
Chris

that's a great idea! How do they use o-rings to keep the rotating part sealing the input from the output? Do you have any pictures that would help?

 

[digiex-chris]

try using the small duck bill valve out of a homelite chain saw fuel tank. Its silicone about 5/16 long and 1/8 dia. Its used to control fuel pump action with crankcase pulses.

Sounds like exactly what I want except it seems to be a check valve.

 

[sssfox]

I have a couple around here someplace, but I have no idea where, exactly.

From what I remember, they works similar to a cylindrical steam engine valve. There is a spool inside that has O-rings on each end. With the spool at one end, it provides pressure to one end of the air cylinder and when the spool is at the other end, it provides pressure to the other end of the cylinder.

There is also a rotary version, but I have never taken one of those apart. I think Robart makes both types, but it's been a while since I've dealt with them.

 

[mu38&Bg#]

http://shop.dubro.com/products/prod...t_number=255/101.0.1.1.5161.5217.0.0.0?pp=12&

Greg

 

[solarenergyadventures]

Hi everyone!
The piston type retract valve was what I had in mind. The piston slides back and forth like a steam engine piston valve. Like this.... http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXET30&P=7

Cheers!
Chris

 

[mike4517]

Why not use a carburetor from model engine.

 

[digiex-chris]

thanks all, I'll borrow the retract design. Seems perfect.

Model engine carbs aren't air tight valves, they're actually pretty leaky. They also don't close down perfectly, they stay open a bit for idle (not really a valve). They're also about as expensive as buying a whole new model engine. I'm looking for a design I can make myself.

 

[mike4517]

I use these for fuel taps a servo will operate them easy
http://www.ebay.co.uk/itm/1-4-BSP-F...Compressors&hash=item19cf8d7824#ht_3328wt_699