How to determine piston/0-ring size

[compressor man]

Ok, the title did not do a good job of explaining my question but I had to fit something into that little box! I am wondering how one determines how large to make a piston for a steam engine bore if you want to incorporate an o-ring in the piston. Here is an example that should make things clear.

Lets say the plans call for a 1/2" bore with a solid brass piston. I want to use a piston/o-ring combination instead. How large should my piston be and how in the world do I determine the diameter of the o-ring and how deep should the groove in the piston be for it? I have tried this in the past a few times by just eyeballing it and never really had good results. Is there a rule of thumb that I do not know about that would be helpful?

Chris

 

[miglincit]

You could ask Google..: calculating o ring groove dimensions

Thomas

 

[Brian Rupnow]

A pretty safe bet is to open the cylinder out a full 0.500". Turn the piston to 0.498 to 0.499". For a 1/16" cross section o-ring, cut the slot in the piston to 0.056" to 0.058" deep x .094" wide. O-ring sizes are "nominal". A 1/16" cross section o-ring is actually 0.070" cross section. When you buy the o-ring ask for a 1/16" cross section o-ring x 1/2" outside diameter.

 

[Wizard69]

As suggested above there are all sorts of online calculators out there.

I can't speak much to engine building but have a whole bunch of experiences with o-ring usage at work. One thing we have run into is that if you try to hard to avoid compression of ring you will end up with the ring sometimes not fitting tight enough as there are variances in cross sectional size even for the same manufacture. Frankly it can be tricky to get the right fit for a specific usage.

This is where experience counts a lot and I would suggest taking Brians R's suggestions above. I'm not sure if he has ever implemented a piston with a nylon backing ring for the O-Ring but that is often a helpful way to enhance performance.

 

[BobsModels]

Chris

I have limited experience with O – rings in a steam engine cylinder. I built live steam locomotives and in doing so built a number of pump cylinders using O-rings. I always built to the drawing specification which matched the standard O –ring tables. At the time I thought the piston pushed really hard ie too much wall force. Having moved on from that hobby after 30 years I now build small model hit & miss engines. The following information was gleaned on my first attempt at using an O-Ring for that application. The engine built has over 400 hours on it and still has solid compression. I am about to rebuild a 2” scale Case Traction Engine piston(1.625 OD) / cylinder. It has a huge amount of blow by the rings and steam engines do not like to run well if steam is applied to both sides of the piston at the same time – kind of a brake! I am going to use a Vitron O-ring in the rebuild and experiment with wall force. The following is an excerpt from a post on O-rings in a Hit and Miss engine, but might be useful. I included a PDF of the information as applied to the cylinder and piston.

I suggest a Vitron O ring 1/16 cross section, you can get them at McMaster, you need to buy them by the 100.

I tried an O ring for the first time on a 1/4 scale Gade. I had the opportunity to actually take apart two Gade's that use O rings built by two different people (both of whom made many of them for sale). What I found was a consistency of dimensions. About six months ago I pulled apart a York with an O ring and found the same dimension to within .002 of the my original analysis.

The two Gade's had a wall pressure or compression of the O-Ring of .0045, and .0048. The York was .0062. Let me define wall pressure as I use it. It is the total amount of compression the O-Ring is under when squeezed by the wall of the cylinder and the groove in the piston.

First I bore and hone the cylinder, measure it and you now have the cylinder ID. I can now calculate the OD of the groove. Assuming a 1/16 O ring sized for your basic piston ID, I would recommend the 1/16 for low friction, the equation is (using a .875 nominal O-Ring):

OD of the groove = Cylinder ID - ((.070 *2) - .005))

So for my Gade the Cylinder Id was .8743:

.8743 - ((.070 * 2) - .005) = .7393

Wall compression = .8743 - .7393 - .140 = -.005 (ie compressed)

I also lap the piston so when one end of the cylinder is blocked the piston stays and when you remove the block it drops through.

So in my scenario the groove is independent of Piston Diameter but dependent on Cylinder ID.

One other Dimension consistency, all the observed groove widths were .080 to .084. One of the builders of the Gade's mentioned to me, as well as someone at NAMES that you did not want the O-Ring to roll as in a normal O - Ring application. You just wanted enough room for it to expand when crushed and a little more for expansion when the fuel ignited. Again just some addition information I gathered.

My Gade and the other two run at shows all day, stopping for fuel only.

Good Luck

Bob

View attachment O Ring Data updated.pdf

 

[Brian Rupnow]

Never used a back-up ring--It isn't necessary on small engines, either gas or steam.

 

[compressor man]

Wow, thanks for the replies guys!

Chris

 

[minh-thanh]

thanks for useful information !

 

[mcostello]

There is another style of O ring available that might work, no one has mentioned it, so it might be new territory. It is called an "X" ring. The cross section is an "X", might be worth exploring.

 

[ninefinger]

IMHO the best guide for o-ring selection is the Parker o-ring handbook, freely available from their website and now its a bit more interactive as well.
Here is the link to the section on reciprocating o-ring design / selection. http://promo.parker.com/promotionsite/oring-ehandbook/us/ehome/ci.Reciprocating-O%E2%80%93Ring-Design-Guide,EN.EN

The guide makes it easy to pick the right o-ring size and design the groove the right width and depth, specifies the corresponding piston size and tolerances - for applications where you want the fluid to stay on one side and not get to the other side.

All that said, depending on your application (i.e. small steam engine) there may be too much friction going by the design guide (i.e. too much o-ring compression). At that point you would then need to experiment - possibly making the groove deeper in the piston to reduce compression.

Mike

 

[compressor man]

Cannot thank you guys enough. This is what I was looking for.

Chris

 

[vederstein]

Reference the Parker O-Ring bible:

https://www.google.com/url?sa=t&rct...ok.pdf&usg=AFQjCNEfcyse9NcxmvBQMMI4QOBdWwQFXQ