O-ring for piston ring

[Vietti]

Thanks Ken- very rational reply. Will be interesting if someone comments on how the dual ring Chinese engines work, they do seem to coast OK

John

 

[Steamchick]

Ken. Thanks for clear and sensible explanations. I now understand why my water pump wears o-rings at a horrible rate. Too tight! I made it based on static o-rings in a carburettor. So not understanding the sliding o-ring dynamics dropped a boo-boo. I'll sort it now, and change from silicon to viton.
Thanks K2

 

[guy]

looks like you need a Teflon backup ring. I worked in a hydraulic repair shop. You need to widened you o-ring groove the thickness of the backup ring. if you can't find the right size you can make one. the backup ring goes down stream from the pressure. this should keep your o-ring from extruding out. Back-up ring - Wikipedia

 

[ninefinger]

I've seen alot of discussion on o-rings for pistons, but usually the discussion is around "slow running" engines - hit'n'miss types. What are peoples experiences with faster running engines? My thoughts are to try o-rings on my 9 cylinder build by making a set of custom pistoms for it, but its expected to rev to about 6000 rpm (max). (22.2mm / 0.875" bore x 24mm stroke)

 

[MIKE4444]

Hi what advantage does one have by using 4 mm or a 3 mm copper pipe for a small steam engine, Tank size, 38 mm dia , by 90 mm long, copper tube , with two side pistons , around Piston size 10 mm dia , refer to my Dribbler that i am constructing, refer to ATT .
thank you MIKE

 

[Ken I]

Ninefinger - Download the Parker-Hannifin "Bible" here :-

https://www.parker.com/literature/Section V.pdf

More information there than you can shake a stick at.

One of the more curious things pertinent to your question is that low reciprocating velocities are bad for "O" rings - like less than one foot per minute - whereas higher velocities produces less friction - due to the dynamic "wedging" effect of the oil film.
Obviously if lube failure occurs at high speeds (you are talking peak velocity of 6.8m/sec.) then the "O" ring will burn.

Personally never tried them at such velocities. It would be nice to hear from others that have.

As to Vietti's comment on dual ringed Chinese engines - If the gap is unvented, then I suspect they are running them with almost zero interference and relying on the ring to seal dynamically. So in between cycle transitions it effectively "leaks". This would explain why they "coast".
If that is the case ring life might be quite short.

Regards, Ken

 

[Vietti]

Ken,

Just curious but why do you predict short life for a loosely fitted 0 ring?

I wish someone with one of these dual ringed Chinese engines would tell us more about how the 0 rings are fitted and is the gap vented.

 

[guy]

you should look at this pdf. it is from parker seal and o-rings. look at paragraph 3.2orings. this should need to know about orings

 

[Ken I]

Vietti, Using no preload is referred to as floating / dynamic in the Parker-Hannifin bible. As pointed out by Guy above.
That's fine for hydraulic where some small leakage during the stroke in either direction is permissible - particularly at slow or stationary speed / low pressure transitions.
The problem with using that approach on an IC engine, is that hot blowby may eventually damage the seals and once it starts will fail rapidly.
I have no personal experience of this so it would be nice to hear from those who have.
Like the "Challenger" as long as the "O" rings seal - then the hot gasses do not get to them - but once they start to leak........
Regards, Ken

 

[Steamchick]

Re: Boiler query, post#45 on the dribbler boiler:- 3mm copper pipe for your little boiler I reckon. 90 mm long: bore needs to be >1/80th of the length: = >1.13mm. So 3mm is fine. - And more surface area for heat exchange.
K2

 

[mrehmus]

Here is the Tom Stuart article on O-Rings as found in Issue # 15 of Model Engine Builder magazine. He used them in his big model Fairbanks-Morse engine for 7 years with no problems. Please use this information for your use only and do not spread it around.

I have been using O-Rings as piston rings in my Hit and Miss engines for the past 8 years with few problems. An interesting feature of O-Ring use is the very low friction when the engine is coasting and yet the immediate seal-ing of the O-Ring to the cylinder wall during a firing cycle. O-Rings do a good job on slow–speed engines. They will probably not survive in a high–speed engine and would quickly convert themselves into a melted plastic ring.
You can modify your engine to use O-Rings with minimal effort (new piston and polishing the cylinder walls) and will probably see an immediate improvement in performance. The improvement I mention is more revolutions between a 'hit' and very little exhaust smoke. Of course O-Rings are easier to 'make' than traditional cast iron rings and you only have to use one.
I've had good success as long as I followed a few simple rules about their application.
Unlike 'normal' O-Ring applications, the dimensions of the piston ring groove are such that the O-Ring is not under compression in the static state and does not effect a aero-pressure seal. It takes pressure above the ring to cause it to press against the sides of the cylinder and the bottom of the ring groove.
For the example shown on the Drawing Page 15, a typical 1.5" bore cylinder in this hit and miss engine example will have 0.002" clearance between the piston and the cylinder. The O-Ring with a 1.5" OD and a 5/32" circular cross-sec-tion is made of Buna-N material. The piston ring groove is cut to provide 0.005" clearance top and bottom and 0.005" clearance on the back wall of the groove.
If we need a math formula for the groove it is:
Groove inner diameter = Cylinder I.D. – ((2 x O-Ring circular cross section) + 0.010).
With the values we have, that would be 1.5–((2x0.15625)+0.010) or 1.5-((0.3125) + 0.010) or 1.5" –0.3225" = 1.1775".
Cylinder wall finish should be as smooth as you can make it. Somewhere between 2 (mirror) and 32 (smooth shaft) micro inches is a good value. Some people actually recycle old hydraulic cylinders since they have chrome-plated inner walls. That isn't necessary for this application but a good smooth surface will help the life of the O-Ring.
During use, you want to have a light coat of motor oil on the cylinder walls. During a full day of running, I oil the engine about every hour.
Editor's Note:
Because we cannot adequately show surface finish in print, I use a BAR S-22 Microfinish Comparator which is a cali-brated assortment of different finishes all cast into a single rectangle about the size of a candy bar. The finishes range from mirror to very rough (in machining terms). In use, you visually compare your surface finish to the samples and se-lect the closest finish on the Comparator.
These are available from machine tool stores.

 

[Vietti]

Just a thought, you might be able to cut the 0 ring groove in an existing ringed piston. Some think the lower the better to get the 0 ring further from the heat of combustion. Another plus is that an engine that has been running with ci rings will generally have a polished bore.

The Hoglet was designed for 0 rings and builders report long life and the 0 rings seem to survive the relatively high rpms the Hoglet can run at. My last two engines rev up pretty good and so far no problems with the 0 rings. One is a Silver Bullet and the other is a Kittiwake.

I have been building model engines for 40 years and have made numerous successful rings. I only tried 0 rings in the last two months to see if they would be superior to what I had in my last two conventionally ringed engines. I think they run better with the O ring. I plan to use them in a hit and miss engine I'm beginning to build.

Try them, you might like them!