viton o rings

[chuck foster]

hello guys

i am thinking about using viton o rings in my next gas engine model. up to now i use cast iron rings or no rings at all, but i would like to try the viton rings.

my way of thinking is if i cut the grooves too deep the rings will not seal and if i cut them too shallow there will be allot of ring drag.
so with that said my question is, is there a formula to figure out how wide and deep to cut the ring grooves, or like most things i will have to experiment
and figure it out the hard way???

thanks for looking and thanks for any help you may provide

chuck

 

[Blue_Rock]

Hi Chuck

Courtest of Brian, see about 1/3 of the way down this page http://www.homemodelenginemachinist.com/index.php?topic=8388.195 for O-ring groove specs

 

[Ken I]

Chuck,
I would be a little leery of using a standard fit and go for less.

Fisrtly understand that an "O" ring behaves like a fluid - the preload seals the ring - any further applied pressure will add to the hydrostatic forces within the "O" ring - increacing its ability to seal (it will also try to extrude the "O" ring out through any clearance - this being a fairly common failure mode).

As long as the ring has almost any preload it will work.

However when used in an IC if you get leakage you will also get hot comubtion gasses passing the "O" ring which will kill it in no time (as in the Challenger disaster). By similar token your cylinder walls will need a fine cross hatched "hone" finish which carries lube well.

You also want to keep the preload down because of rpm's and friction heat which will also destroy the "O" ring.

I have used "O" rings to seal up to 16000 psi (as a dynamic seal in pipe bursting testing) without problems. I used to run an old Ingersol Rand reciprocating compressor that ran at 230°C - Viton "O" rings (static seals) survived this - nothing else would.

In my model engines I like to keep the preload down to 1/2 to 1/3 the normal interference - but this on air engines not IC.

I recalculate the groove width by assuming the ring compresses to an oval (semi-circular ends + flats - not an ellipse) and give a thou or two extra - too much width clearance can also cause problems - such as the ring rolling and twisting. Be sure to work off actual rather than nominal section sizes a 1/8" "O" ring is 0.139" section, metrics are normally size).

This just my 2c worth of thinking on the subject - let's wait and see someone chime in who's actually run "O" rings on an IC motor.

Ken

 

[Blogwitch]

One thing that needs to be mentioned every time that Viton is being discussed.

At higher temperatures, if ever the Viton is taken to a stage of burning, it gives off Hydrofluoric acid, which if in contact with the skin can lead to curling up of your toes (death).

http://en.wikipedia.org/wiki/Hydrofluoric_acid

This was borne out when firemen went to automobile fire accidents, they started to lose flesh from their bodies because the viton o-rings being used in such places as water pumps etc were being burnt and producing this nasty acid, and of course, they were moving it about and causing it to become airborne with their water jets.

So please, do be aware of the hazards that can be generated if you start to heat them up above their designed limits, or even if your car catches fire.
It might be a lot higher temperature than you are working with, but some people have no common sense and could try lighting one, in which case .........


John

 

[Wrist Pin]

"in which case..." would be Darwin's theory of evolution, (devolution), in action...

 

[gbritnell]

Hi Chuck,
If you have been using iron rings up till now why switch. The amount of fiddling around trying to get O rings to seal but not drag seems like a bigger pain to me. This subject has been thoroughly covered in the past. There are two different camps on the use of O rings, each one supporting their use. I personally have tried them and switched to iron rings and will never go back.
gbritnell

 

[lensman57]

hello guys

i am thinking about using viton o rings in my next gas engine model. up to now i use cast iron rings or no rings at all, but i would like to try the viton rings.

my way of thinking is if i cut the grooves too deep the rings will not seal and if i cut them too shallow there will be allot of ring drag.
so with that said my question is, is there a formula to figure out how wide and deep to cut the ring grooves, or like most things i will have to experiment
and figure it out the hard way???

thanks for looking and thanks for any help you may provide

chuck

Hi Chuck,
I posted a very similar question a few days ago re the Viton rings for a Webster engine, reading the replies to your post I think I shall have to start learning how to make cast iron rings from scratch.
Regards,

A.G

 

[Ken I]

Re prior post - whilst I love "O" rings for most applications, I must admit that for an IC (I haven't built one yet) it really wouldn't cross my mind to use an "O" ring - I would stick to cast iron.

So I'm with George on this one.

2c

Ken

 

[doc1955]

Do yourself a favor and stick with iron get your self some good nodular iron I have never tried o rings and from what I've hear from others probably won't. So I have to agree with George on this.

 

[popnrattle]

I am of the Viton o-ring camp. They are rugged and don't wear the cylinder, Easy to replace, readily available(I suppose any reputable hardware store would have them) and cheap($0.79) each. You only need 1. If the ring groove dimensions are correct you only have a slight drag and hardly any after break-in. I will admit here that I have not used cast-iron ring in miniature engines and don't yet see a reason to.
The video is of an engine with an abused o-ring. Later, Rick.

[ame]http://www.youtube.com/watch?v=IAezrZPANZU[/ame]

 

[chuck foster]

thanks for the replies guys.
i ordered cast iron rings from otto engine works and they arrived here in 4 days.
i figured since i have had no problems with cast rings in the past i would stick with what work's for me.
thank you rick for the video, that engine sure run's good.

chuck

 

[Engine maker]

I built Jerry Howell's Farm Boy sn 003 with the rubber "O" ring. It's been running at the shows all day long since I built it in March 2009 with the same "O" ring. Things you need to do are first the bore needs to be as smooth as you can get it, maybe even polished, there's no such thing as too smooth. Next you have to keep the "O" ring lubricated! If not you'll wear it out. Next you need to keep the heat down. This works great on a hit and miss engine that's coasting most of the time. Mine never gets above 94 degrees even with no water in the hopper.

Now for the size. The engine has a cast iron bore that's 1.00" dia and uses a 1 inch Dia. .093" "O" ring. The ring grove in the .998 inch dia. piston is .105" wide by .110" deep. So you see there is clearance all around. My guess is that the ring swells just a bit when the fuel and heat hit it.

The "O" ring works so good that I went back and reworked some of my other engines to use these rings. Also is works just as good with the Viton or Bura-N ring. Here's a video of mine after 3 years of running.

[ame]http://www.youtube.com/watch?v=Djai9h7El9c[/ame]

Jim

 

[popnrattle]

Beautiful engine that runs great. Just love the sound it makes. Great job! Thanks for sharing. Later, RT.