WTC O-ring size ?

[Sonnich]

Hi Guys.

I would like to try make my own WTC
I have a PVC pipe with 100mm inner diameter, and I want to try to use 3d printed endcaps.
I tried to design something in freecad in 3d for my printer for the first time, and I think endcaps is the perfect subject to practice on, as they are fairly simple....still a steep learing curve when starting from scratch :-)
The test turned out fine.

But.....what size O-rings should I buy for a my 100mm inner diameter tube ? And how thik do they need to be ?

And how deep should I make the groove in the WTC cap assuming it is also a perfect fit at 100mm outer diameter ?

Should I use 2 o-rings pr. endcap ?
I want to try the ater pump and bladder system, which will increase pressure in the whole WTC when taiking in water.

Can I use Viton O-rings , or only silicone ?

[Guest]

3mm cross section should be adequate, unless your tubing is very poor quality.

O-rings are measured on the inside diameter, so you add on twice the cross sectional thickness for overall diameter. Generally you want the o-ring to have a small amount of stretch when in the groove, about 1-2% of the o-ring internal diameter, and no more than 5%. You should aim for a compression of about 10-15% o-ring thickness to get a decent seal, and more than that and you'll find removal a struggle. A single o-ring should suffice, and silicone or nitrile are both fine, silicone tends to be a bit softer than most nitrile compounds and resists ozone degradation better. Viton is overkill but will work.

The groove for your o-ring should be square, and it helps to make the slot wider than the o-ring, to allow it to roll a bit as you remove and replace the cap, as well as to permit the extrusion of the seal as it squeezes up.

So to put some meat on the bones.

100mm cylinder bore, with 3mm thick o-ring. Subtract 6mm from 100mm, 94mm, we want a little snap on our o-ring ideally about 2%, so subtract another couple of mm giving us a 92mm diameter o-ring, although 90-93mm will suffice.

The groove on your endcap, assuming it fits in the cylinder with minimal clearance should be between 2.55-2.7mm deep and I would go for a 4mm groove width.

[Guest]

Additional tip, adding a lip to the endcap makes it easier to get the caps off. Or fit a tyre valve, and pump in a shot of air for easy release.

[Sonnich]

Thanks a bunch, that is awesome. Really great info, thanks. Will order some rings, and play with CAD i bit more

[thor]

Andy's information is spot on. I have been able to use 2mm o-rings with success on 3" polycarbonate tubing, but your grooves must be perfectly sized and cut. Sometimes it takes a bit of trial and error to get right. I use Buna o-rings with no issues.

[Sonnich]

Thanks for the tips Thor.


I made a new endcap, but making it the exact size as the inner diameter is probably a bit tight..... I think it could seal the WTC even without an o-ring installed

Made the groove 4mm wide, and square.

Ordered o-rings on ebay.