Author Topic: The Teflon Ring Saga  (Read 8859 times)

Offline sshire

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The Teflon Ring Saga
« on: August 16, 2015, 01:26:32 PM »
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?
Best,
Stan

Offline GailinNM

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Re: The Teflon Ring Saga
« Reply #1 on: August 16, 2015, 03:47:52 PM »
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
I would like to be the kind of person my dog thinks I am.

fcheslop

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Re: The Teflon Ring Saga
« Reply #2 on: August 16, 2015, 04:50:50 PM »
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

Offline Steamer5

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Re: The Teflon Ring Saga
« Reply #3 on: August 16, 2015, 05:02:11 PM »
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
Get excited and make something!

Offline sshire

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Re: The Teflon Ring Saga
« Reply #4 on: August 16, 2015, 05:17:21 PM »
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.



Best,
Stan

Offline Jasonb

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Re: The Teflon Ring Saga
« Reply #5 on: August 16, 2015, 06:33:43 PM »
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.

Offline sshire

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Re: The Teflon Ring Saga
« Reply #6 on: August 16, 2015, 07:16:07 PM »
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.
Best,
Stan

Offline Steamer5

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Re: The Teflon Ring Saga
« Reply #7 on: August 17, 2015, 12:39:36 PM »
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
Get excited and make something!

Offline sshire

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Re: The Teflon Ring Saga
« Reply #8 on: August 17, 2015, 12:51:44 PM »
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.
Best,
Stan

Offline b.lindsey

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Re: The Teflon Ring Saga
« Reply #9 on: August 17, 2015, 01:29:58 PM »
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

Offline sshire

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Re: The Teflon Ring Saga
« Reply #10 on: August 17, 2015, 03:15:54 PM »
The "rings" are strips of Teflon, not preformed rings.
Awaiting a reply.

Best,
Stan

Offline steamer

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Re: The Teflon Ring Saga
« Reply #11 on: August 17, 2015, 03:31:48 PM »
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
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline steamer

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Re: The Teflon Ring Saga
« Reply #12 on: August 17, 2015, 03:34:15 PM »
These rings were machined from solid on the lathe, and the slots were cut on the mill after.

Dave
"Mister M'Andrew, don't you think steam spoils romance at sea?"
Damned ijjit!

Offline Dave Otto

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Re: The Teflon Ring Saga
« Reply #13 on: August 17, 2015, 11:08:43 PM »
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

Offline Bobsmodels

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Re: The Teflon Ring Saga
« Reply #14 on: August 18, 2015, 01:22:25 AM »
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

 

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