viton o-rings

[grapegro]

Can anyone please give me the most worthwhile data on tolerances on the grove to use a viton o-ring for compression ring in a petrol engine. O-ring size is 27mm id and 2.5mm section. Thank you all for an enjoyable and informative forum.
This information should be able to give me the answer to evasive compression in my Atkinson Cycle engine.

 

[artist334]

Search "parker o-ring handbook" from google. The pdf has pretty much all there is to know about o-rings.

 

[Omnimill]

Search "parker o-ring handbook" from google. The pdf has pretty much all there is to know about o-rings.

Agreed, all the data you need as I remember although it's a large document.

This may help as well.

http://www.qualityseals.com/pdfs/groovedesignandoringinstallation.pdf

 

[bazmak]

Note that the tolerances are critical and must be adhered to
The design allows for the O ring to roll and not to compress
to form a good seal without wear to the O ring and therefore
wear and rapid deterioration,Regards barry

 

[abby]

I am not sure what Barry means by "roll and not compress" , they work by being compressed .
Here is an indication of what is required to fit o'rings in a high pressure hydraulic or pneumatic system , you must bear in mind that these systems will have sufficient power to overcome any friction due to the o'rings.
http://everyoring.com/pages/Oring_grooves.html
Viton will expand with increase in temperature as will any of the PTFE based rings and they are not "frictionless" as when used as a bearing bush.
In small models , particularly steam engines , I have found it necessary to reduce the amount of "squeeze" , I hone the bore until the piston with rings can move freely without the slip-stick phenomenon common in small installations.
Any type of ring will wear or degrade - eventually , but o'rings , even those made from exotic compounds are cheap enough not to worry too much.

 

[grapegro]

Thanks for the input to my query. Looks like a bit of study coming up. Thank you to you all. Norm

 

[Brian Rupnow]

Grapegro--I have built eight i.c. engines, and used a single Viton o-ring on all of them. I seen your post when it first went up, but didn't respond because I work in British Imperial not metric. Since it seems that you have not received an answer to your question this is what I know from experience. First, you only require one Viton ring. Two are not necessary, and add too much drag to the piston. The cylinder on my most recent engine was 1" (25.4mm) bore. The piston was made .0015" (.038mm) smaller in diameter so it had an outer diameter of 0.998" (25.36mm). The viton o-ring that I use has a NOMINAL cross section diameter of 1/16" (1.59mm). However, in reality the o-ring has a cross sectional diameter of 0.070" (1.78mm). I make the groove in my piston .093" (2.36mm) wide, x .057" (1.45mm) deep. This works very well, allowing the proper clearances to seal the piston effectively but still let it slide freely enough in the cylinder without binding. I run an approximate 30:1 mix of Naptha gas with two cycle engine oil to run my engines on, in order to supply lubrication for the ring. It is a good idea to lap the cylinder bore with 600 grit lapping paste to get a very smooth finish. If you don't have a very good "almost mirror" finish on the inside of the cylinder it will tear up the Viton o-ring very quickly.---Brian

 

[grapegro]

Thanks for the practical remarks, more what I was after. One big problem is converting imperial to metric and visa versa. Looks like it be a case of back to the drawing board. Thank you Brian

 

[bazmak]

Maybe im getting old and my memory is not what it was or you are all talking on a diifferent wavelength.I assume a viton O ring is a silicon rubber.If my memory serves me correctly,then i was taught that the O ring when fitted in a groove to a piston in a steam locomotive was not sqeezed to form a seal but
deaigned to roll in the groove to overcome wear in one spot.Can anyboby put me right or confirm this Regards barry

 

[robmort]

.......I assume a viton O ring is a silicon rubber.If my memory serves me correctly,then i was taught that the O ring when fitted in a groove to a piston in a steam locomotive was not sqeezed to form a seal but
deaigned to roll in the groove to overcome wear in one spot...........

Sorry, you are wrong on all counts. Look in the literature quoted above.

Rob

 

[Brian Rupnow]

Barry--They do not roll in the groove. They do compress a certain amount when the piston is installed into the cylinder. The reason that the piston groove has to be wider than the ring diameter is so that when the ring compresses between the bottom of the piston groove and the cylinder, it gets "fatter" in the other plane and needs the room to "grow' a bit in that direction. However, you really don't want the groove so wide that the ring slides back and forth in it, as that will quickly wear out the ring.---Brian

DuPont introduced Viton in 1957 as the first commercial fluoroelastomer. Other companies also make fluorocarbon-based synthetic rubbers. The generic name for this type of synthetic rubber is FKM.

Read more : http://www.ehow.com/facts_7429216_viton-material_.html

 

[bazmak]

Hi,maybe i am on the wrong track,but by roll i dont mean that it rolls about
The O ring is compressed slightly to form the seal.If it is static in the groove
it seals/wears in a single line touching the cylinder.If it rolls its wears evenly
over the complete surface. Example.Fit the O ring over a piece of dowel as a tight fit and move it down (as like fitting a condom) and it will roll down the dowel.This i was told by very experience people and have never doubted it
Makes sense to me,but how do we test this theory in a live stean
m engine.Please keep your thoughts coming. Regards barry

 

[Swifty]

Barry, I understand what you are describing, but the groove for the O ring would have to be wide enough to allow for the travel of the piston, I think that it would not be practical in use.

Paul.

 

[Cogsy]

They are designed to be static rather than roll in the groove and you're right, they do wear in a single contact line. O-rings are also available in square sections rather than round, and for applications requiring low friction and good sealing they are also made in an X cross section (normally referred to as X-rings).

 

[bazmak]

OK i bow to pere pressure,when i used O rings i did work to the required tight tolerances and did believe what i had been told.I can now accept this not so
even if it makes sense

 

[abby]

X-rings , commercial name "Quadra lip" quite expensive but work very well