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QDV - Piston seal leaking

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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QDV - Piston seal leaking

Unread postAuthor: Warthog » Thu Jun 15, 2017 12:29 am

I have built a test 20 mm test device with QDV. And it leaks badly.

My piston is turned from a polyamide / silon rod. The diameter of the piston is 19,75 mm and it slides perfectly in the pipe (which is made of aluminum). The lands have diameter 16 mm and I use 17 x 1,5 o-rings as free-floating seal. When I pressurize the tank (ca 0,14 litre) to 5 bars, it takes about 5-7 seconds for that extra air to escape. With 16x2,5 o-rings, the seal is very good, but friction is much higher and the valve does not perform very well.

Should I just use o-rings with larger outside diameter, or is there some other trick to QDV design?
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Re: QDV - Piston seal leaking

Unread postAuthor: jackssmirkingrevenge » Thu Jun 15, 2017 5:31 pm

Even with careful design and o-ring dimension selection you will always have significant friction at that diameter and reasonable pressure.

One way to counter it is to have the barrel sealing seat to be slightly smaller diameter to give the piston a bias towards opening that would be counteracted by the piston in practice.

First step is to pressurize the system and with the muzzle pointing vertically upwards, start hanging weights in small increments (with at least a minute between each weight) and see how much weight you need before you start overcoming the friction and the piston starts moving.

Let's say it needs 5 lbs before it starts to move.

You have a 0.75" Piston at 75 psi. At full pressure there is 33 lbs pushing each o-ring.

Let's go for a bias of 70% of the friction weight to add a safety factor.

What I want therefore is 3.5 lbs less of force towards the barrel. I can achieve this by reducing the seat diameter of the barrel o-ring to 0.71". This means that at the moment of firing, instead of 5 lbs of force to move the piston, I will only need to apply 1.5 lbs of force.

It works, but it's a fine line to tread...
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Re: QDV - Piston seal leaking

Unread postAuthor: Warthog » Thu Jun 15, 2017 11:00 pm

I am not using a "normal" piston valve which opens by venting the pilot volume. My design is basically this: https://www.spudfiles.com/spud_wiki/ind ... _parts.JPG

Yesterday I experimented with o-rings a bit and ended up with o-rings whose outside diameter is a full mm more than barrel inside diameter. The seal is pretty good and the force needed to open the valve is still reasonable.
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Re: QDV - Piston seal leaking

Unread postAuthor: jackssmirkingrevenge » Fri Jun 16, 2017 10:50 am

Perhaps a diagram will be clearer, this is what I was suggesting. Top is a normal QDV, bottom is a slighly unbalanced one:

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Re: QDV - Piston seal leaking

Unread postAuthor: Warthog » Tue Jun 20, 2017 1:00 pm

Thank you, now it is perfectly clear. It might be a bit more difficult to build, but very suitable for one new design, that I have in mind for this summer.
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