The Teflon Ring Saga

[sshire]

My PMR 1/4 horse runs very nicely on air. As Rough and Tumble was approaching, Philjoe and I had been exchanging PMs about running it on steam.
According to PM Research, my horizontal boiler will run the engine with no load (let's say a sewing machine) so, I didn't even test it at home knowing that the big boiler at Rough and Tumble would supply plenty of steam.
The 1/4 horse was connected, displacement oiler filled and the engine refused to turn. Total lockup.
After thinking about this, Phil and I came to the conclusion that the Teflon rings (supplied with the engine) had expanded.
A few points.
1. The cylinder bore and the piston were carefully machined to the drawing dimensions. Multiple measurements by my QC Dept (me) showed +- 0 from the drawings. The bore had been honed with Flexhones and then lapped with Timesaver. (Before measuring). Bottom line: the parts were correct.
2. When fitting the Teflon strips in the ring grooves, I had left no gap at the ends.

The next morning, after the engine had cooled overnight, I removed the rings and when Phil had the boiler up, the engine ran quite well. So, the issue was ring expansion.

This morning, at home, I delved into this.


According to the Thermal Expansion Data (below) Teflon at 212° F. will show a linear expansion of approximately 3%.



The piston's (approx) 2" diameter (circumference of 6.283") should expand by .1884" to  6.471"

This means that the diameter has increased to 2.059". No matter how much one tries, a piston that is .059 larger than the bore isn't going to move.

I didn't bother to calculate the cubic expansion, but, if Marv has nothing better to do, here's that table.



The real question is this, if I remake the rings and allow a gap between the ends of the Teflon larger than the 3% expansion, can this work?

[GailinNM]

A couple of things Stan.

First off your 3% expansion figure is starting from a temperature of -328° F. In the temperature range from room temperature to steam operating temperature the coefficient of expansion is about 7×10 to the -5th inch per inch per degree F. Assuming 170° F temperature rise your linear expansion will only be about 1.2%, or in your case an end gap of about 0.038 inches.

There are several alternatives to a square cut and gap to accomplish this. If no one else chimes in I can do a couple of quick sketches later in the day and post them.

Most important is to allow adequate clearance in the width of the ring groove and the ID of the ring groove so the Teflon never fills the groove from expansion. Extra clearance in both of these dimensions is no problem as the pressure of the steam will force the ring to seal on the cylinder wall from pressure on the ID of the ring and also will seal to the bottom of the ring groove from pressure on top of the ring. For this to be effective the width of the ring needs to be about twice the radial radial thickness of the ring.
Gail in NM

[fcheslop]

Hi Stan, Iv seen some information on a German 16mm loco site and on this site http://www.vapeuretmodelesavapeur.com/telechargements4/index.html
Sorry it may not be a lot of help but theres some interesting builds
cheers
frazer

[Steamer5]

Hi Stan,
 I've an article I was given on making teflon piston rings for steam loco's. I'll hunt it it & let you know what info is in it. I had a hunt back on you build log & at 1.5" bore would be about a mid size 5" loco. The guy that gave me the article I can no longer contact so will be unable to post its contents. However if memory serves a gent in South Austrailia came up with the "how to" & it works well by all accounts. Hope you aren't in a rush as I havent come across the article for awhile so it may take some searching to find it.

Cheers Kerrin

[sshire]

Guys
Thanks for the help.
I now see Gail's point. I machined the grooves as I do everything else: a perfect slip fit. Well, that is obviously too narrow. Easy enough to remove metal and widen and deepen the groove.

Kerrin
No rush at all as I won't be running under steam until next year.

Frazer
Your link is on this evening's reading list.

[Jasonb]

Just a thought Stan, have you got any cylinder drains on the engine, could be the condensating steam on the cold engine hydrolic locked it but when running without rings the condensate could get back past the piston allowing the engine to start.

[sshire]

Jason
That was one of the possibilities that were discussed. After looking at the rings, which were very abraded, they seemed to be the culprit. A draincock is on the list to be done along with widening and deepening the ring grooves.

[Steamer5]

Hi Stan,
 The hunt for the article wasn't as hard as I thought!
Turns out its for pistons valve rings, which would get a harder time than a piston ring due to sliding over gaps!

If I understand it these rings are for 1" scale as required!!
Ensure the bore is parallel & a lapped & smooth bore

1, The piston valve as shown have bolt on ends that trap the ring.
2, the rings are to be a max thickness of 0.093", 3/32" or 2.4 mm you take your choice, scale as required on diameter, to avoid binding at operating temp
3, the ID to be 0.003 GREATER than the piston ring "slot"
4, he suggest that you make a mandrel at the 3 thou over size the land size as in 3 above
5, turn you PTFE rod to 5 to 10 thou OVER the bore size (many due to the difficulty in measuring it as it "moves")
6, bore to be a tight fit to the mandrel ...not sure what is meant by this would guess you wouldn't want it to tight as it might contract
7, part off over size
8,turn down to very neat fit in the bore, needs to be push fit using your thumbs with a little resistance
9, return to mandrel & face to width, he suggest a +0.001 fit, as made they expand inwards
Its suggested to use light lube oil on assembly, & you don't drag ash in when coasting!

According to the article a club member at the time of writting, some 15 years back, had run a loco 3 times a month for 2 1/2 years when the valve was removed they couldn't find any issue.

This is an abridge version on how to do it! The gentleman who came up with the how to is from Adelaide South Australia, sounds like at the time his health wasn't good. Maybe somebody on the forum is from the area we can get an update & maybe an "OK" to post the whole article & associated drawing

Hope this is clear & of use

Cheers Kerrin

[sshire]

Kerrin
Thanks for that. There are some useful bits in the description.
The "rings" for the ¼ horse come from PMR as Teflon strips to be cut to length for the circumference.
I've got an email out to PMR about this issue. I'll report when I get a reply.

[b.lindsey]

Stan, I am interested to see what the PMR folks have to say. I had assumed the Teflon rings for their two larger engines were already pre-formed as they are for their smaller kits. I am now wondering if those that run the smaller engines on steam (with the pre-formed rings) have had any similar issues. Happy your "issue" was only a temporary one though and that it ran fine on air!

Bill

[sshire]

The "rings" are strips of Teflon, not preformed rings.
Awaiting a reply.

[steamer]

Stan,  Machine them as over lapping rings,   That will give them room to expand, while sealing the gap.

I made the rings in my launch engine this way and I run 160 psi steam....and they worked very well



It looks hard, but it's not that bad to make....think about it for a minute.....   I can type more tonight...

Dave

[steamer]

These rings were machined from solid on the lathe, and the slots were cut on the mill after.

Dave

[Dave Otto]

Dave,

After you cut the slots did you put the ring on a mandrill and squeeze it shut then turn the OD to your cylinder size? Or did you not worry about the ring being slightly out of round when compressed?

I'm thinking pretty seriously about a Teflon ring in the Goulds pump and this thread has given me lots to think about and learn.

Thanks,
Dave

[Bobsmodels]

Hi

When I was rebuilding a 3/4 inch steam locomotive the valve spool and its rings were shot.  I wanted to use Teflon as an experiment.  A friend whose son worked for a company that rebuilt steam engines and at that time had just finished some paddle wheel engines that ran on the Mississippi said they used glass impregnated Teflon.  He gave me some to try.  The ring was made as Steamer shows except there was an overlap then the slots cut.  You needed to slit between the two slots.  This stuff is really tough.  That 3/4 was redone in the early 80's and saw lots of miles.  I also put the rings on the driver pistons.  I just acquired that engine and pulled a spool out - looked like they were new yet.

Unfortunately this stuff is real expensive, 1" x 1' - $43 from McMaster. 

If you ever get a chance to get some - try it for your steamers

Bob

Bob

[steamer]

Dave,

After you cut the slots did you put the ring on a mandrill and squeeze it shut then turn the OD to your cylinder size? Or did you not worry about the ring being slightly out of round when compressed?

I'm thinking pretty seriously about a Teflon ring in the Goulds pump and this thread has given me lots to think about and learn.

Thanks,
Dave

Dave, I added to the OD enough to give it about half a lap worth of expansion and then cut the slots.   I didn't worry about it being out of round.   It's Teflon, so not very stiff, so steam pressure just pushed it right out and it sealed just fine.   

I've gotten 3 seasons on them, and they still work pretty well

Dave

Dave

[steamer]

I cut mine so that the slots were JUST connected at the corner, then finished the cut with a razor blade.....worked fine

I sized the ring so that when in the cylinder, the gap was half closed up.   That let the Teflon expand under temperature.

Dave

[steamer]

Thinking about it some more Dave, my HP piston is 1.5" and my LP is 3" so the rings are pretty flexible.   On a small bore...like under 1"....maybe not so much, so you might want to mandrel turn the ring to final size after on a small engine.....jus make sure you have at least half the lap open when you do so that the ring can expand

Dave

[Ian S C]

The Teflon rings that I know of are made of about 1/4"/6 mm strip of 1 mm Teflon sheet, the ends are cut in a step so that quite a large end ways gap can be left.  These rings are on the piston in a car shock absorber, I don't know how warm that might get on a long trip.
Ian S C

[Bearcar1]

Good day to you Stan….. I always have enjoyed your work and we have exchanged emails on several occasions. This discussion of PTFE rings is quite educating and eye opening. Have you considered the use of graphites yarn instead? If so, what are your feelings on doing so? It is my feeling that PTFE is a good candidate for running these engines on compressed air, put I prefer (Im of the old school) to use graphites yarn when running in a wet environment such as live steam. It seems to me to be a lot less of a hassle but perhaps I have just been lucky so far.


BC1
Jim

[sshire]

Jim
I have graphite string packing, twisted PTFE pipe tape (which is used on a few of the Rider Ericsson pumping engines), Teflon packing string which is on my Rider.
If the Teflon doesn't work out (though, I suspect it will with better clearance in the groove depth and width) I'm not above trying anything.

[Tennessee Whiskey]

Vern, at this point in time, should one possibly entertain the idea of dabbling into the cast iron ring arena? I do say old boy,  one must cross that moat at one point in time,  don't you agree?  Some Sunsetter thinking 

Cuz

[sshire]

Cletus
Old school. 

[paul gough]

Maybe 'Rulon' might be an alternative to PFTE, I have been told Aster use it in some of their gauge one locos, as a first stop try; www.polymer plastics.com for some info. Paul Gough.

[steamer]

Maybe 'Rulon' might be an alternative to PFTE, I have been told Aster use it in some of their gauge one locos, as a first stop try; www.polymer plastics.com for some info. Paul Gough.

Would be a good choice....just expensive.

Dave