Author Topic: What piston rings material is best?  (Read 5514 times)

Offline geoff5269

  • Full Member
  • ****
  • Posts: 116
What piston rings material is best?
« on: February 15, 2016, 05:01:51 PM »
My first steam engines had graphite yarn as a sealer for the piston which is ok but it does ware out quickly, next I tried O-rings as this is often recommended but I found when the engine was left for a short time it would become sticky so this didn't seem right. I've had better luck with a Nylon type of material I found from an old film processing machine so I went one step further and bought some PTFE rods which seemed good until I brought my engine indoors where it is warmer and the next day it was stiff to turn, when I took it out side again it was ok. I have now tested and found PTFE expands 3 times as much as the Nylon so that's not going to work. I dabbled with cast iron but I couldn't get a good seal and making rings around 16mm is a bit wasteful as cast iron bars are much bigger. I hope some one will say what they have found successful.
Geoff

Offline crueby

  • Full Member
  • *****
  • Posts: 8863
  • Rochester NY
Re: What piston rings material is best?
« Reply #1 on: February 15, 2016, 05:17:05 PM »
I use o rings made of Viton, they hold up very well, do not get sticky, give a very good seal. Got mine in bags of 50 or 100 from amazon, just a few bucks a bag. They come in a whole range of sizes.

Offline Florian Eberhard

  • Full Member
  • ****
  • Posts: 432
Re: What piston rings material is best?
« Reply #2 on: February 24, 2016, 06:37:11 PM »
Hi Geoff

I have been using teflon rings supported by a o-ring (nitrile or whatever) recently and they work really good, becuase teflon has a very low coefficient of friction.
My Stuart boiler feed pump is running on a teflon ring and I believe it is only because of the teflon that it is able to move that slow. 

Cheers Florain

Offline Ramon

  • Full Member
  • ****
  • Posts: 1744
  • Suffolk in the UK
Re: What piston rings material is best?
« Reply #3 on: February 24, 2016, 08:14:44 PM »
Hi Geoff,

You don't say but are you running on steam or air?

I have had very good success using square section PTFE impregnated yarn in the engines I have made but they only ever run on air. I keep them indoors and the have extremely good piston seals but with very low, almost not existent friction. How good it would be running on steam however I have no idea. I think you can purchase it from Blackgates in the UK.

The piston is usually made 2 thou down on diameter with the groove 2-3 thou wider and 2 - 3 thou shallower than the yarn nominal dimension and it is cut with a scarf joint with the cut vertical. A small taper in the bore will help the initial insertion but once it's in the bore it quickly finds it's own level and beds in quickly.

I built my Double Diagonal many years back and it still as good as the day it was fitted. (If I close the stop valve and turn it over backwards the seal is such that pressure generated will turn the engine over a couple of revs).


Hope that helps - Ramon
"I ain't here for the long time but I am here for a good time"
(a very apt phrase - thanks to a well meaning MEM friend)

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 5129
  • Barrie, Ontario Canada
Re: What piston rings material is best?
« Reply #4 on: February 25, 2016, 01:07:23 AM »
Geoff--Try a Viton o-ring. Standard butyl rubber o-rings will become sticky. Viton won't.---Brian

Offline derekwarner

  • Full Member
  • ****
  • Posts: 498
  • Wollongong ...... Australia
Re: What piston rings material is best?
« Reply #5 on: February 25, 2016, 01:44:43 AM »
Guys......

1. The following is from another WEB page some weeks back.....
2. It also reinforces the comment from Florian on an optimal material for dynamic sealing with model steam applications.....

Derek
_______________________________________________________________________________
One of the more important things is to understand that any form of synthetic piston ring is pressure energised...so therefore piston seal groove geometry and tolerancing to the cylinder bore is critical

 Lets discount NBR/Nitrile as it is not suitable for applications approaching 95 to 100 degrees C.......[totally disregard any manufacturers specification of up to 105 degrees C as this is only for static applications]

H-NBR is suitable for applications up to 140 degrees C, however this hydrogenated material is not readily available in standard sized o-ring sections

 Considering that most of our model steam plant will produce steam at approaching 135 degrees C, we need to look for materials that will cope at these temperatures

 Silicone [MVQ] is suitable for temperature applications to 200 degrees C, however has uniquely high  physical levels of expansion at elevated temperature, and so the piston seal cavity dimensions are applicable for this material over any other elastomer. A number of principal seal manufactures suggest that MVQ o-rings only be used in static applications

 Viton [FPM] is also suitable for temperature applications to 200 degrees C, is also totally suitable for use with mineral oils and is physically dimensionally stable with steam and mineral oils

 One of the disadvantages of Viton is the low level of abrasion resistance and tear strength, therefore piston seal cavity and cylinder barrel bores need to be manufactured to optimum/minimal degrees of surface roughness  [umRa] - Viton o-ring selections are readily available, however usually at the lower 70 Shore hardness

 Teflon [PTFE] piston sealing elements are use extensively in industrial applications with great results, however this elastomer requires higher system working pressures to achieve pressure energisation, and are commonly used in conjunction with o-ring energisers. Teflon elastomer has ZERO memory and seals as used in industry are one piece construction and require expansion and resizing cone type tools to correctly install these elements. Teflon elastomer is also totally suitable for use with mineral oils and is physically dimensionally stable with steam and mineral oils

 In a perfect world, and if it were dimensionally possible.....a wound Teflon string over a Viton o-ring energiser would be the near perfect piston seal or and gland seals for model steam..........

 Derek..........

 
 

Derek Warner - Honorary Secretary [Retired]
Illawarra Live Steamers Co-op - Australia
www.ils.org.au

Offline geoff5269

  • Full Member
  • ****
  • Posts: 116
Re: What piston rings material is best?
« Reply #6 on: February 25, 2016, 09:20:32 AM »
Thanks Derek & Florian for your detailed replies, it seems whatever you use there's no substitute foe a very accurately made cylinder and piston.
I have just got some Viton rings and will have another try with these. I mostly run the engines on air but at low pressures so self sealing probably does not come in to play. I remember Florian's thread for piston rings I might try that next time.
Geoff

Offline K.B.C

  • Full Member
  • ****
  • Posts: 164
Re: What piston rings material is best?
« Reply #7 on: February 26, 2016, 10:56:53 AM »
Geoff,
I stopped using Viton "O" rings after I read the following (https://en.wikipedia.org/wiki/Hydrofluoric_acid ) especially when you scroll down and read " Health and Safety "

For the past 15 years I have used only Silicone "O" rings in steam cylinders, infact Suppliers specifically state that Viton "O" rings are for pump rams and glands only.

I have a copy of the Silicone "O" ring groove dimensions if you would like a copy P.M. me and I shall mail it to you.
George.
Your never too old to learn.

Offline crueby

  • Full Member
  • *****
  • Posts: 8863
  • Rochester NY
Re: What piston rings material is best?
« Reply #8 on: February 26, 2016, 12:37:59 PM »
Geoff,
I stopped using Viton "O" rings after I read the following (https://en.wikipedia.org/wiki/Hydrofluoric_acid ) especially when you scroll down and read " Health and Safety "

For the past 15 years I have used only Silicone "O" rings in steam cylinders, infact Suppliers specifically state that Viton "O" rings are for pump rams and glands only.

I have a copy of the Silicone "O" ring groove dimensions if you would like a copy P.M. me and I shall mail it to you.
George.

I don't get the connection of the acid to Viton o-rings. The rings are not acidic. Can you explain it further?

Offline Jasonb

  • Full Member
  • *****
  • Posts: 6598
  • Surrey, UK
Re: What piston rings material is best?
« Reply #9 on: February 26, 2016, 01:31:22 PM »
when they get very hot or burnt they turn to hydrofluric acid.

From the link above

Hydrogen fluoride is generated upon combustion of many fluorine-containing compounds such as products containing Viton and polytetrafluoroethylene

Having said that the last three steam/air engines I have made all have a Vitron ring and perform well. The rign should not be used to compensate for a poor bore but to enhance the seal, all these engines will have run without rings, gaskets or gland packing when being tested


Offline K.B.C

  • Full Member
  • ****
  • Posts: 164
Re: What piston rings material is best?
« Reply #10 on: February 26, 2016, 04:39:42 PM »
Hi Jason,

I know that the Viton rings work and work well in steam cylinders but I saw the Wikipedia article some years ago and when I read the Health and Safety bit I decided to use Silicone exclusively.

I have a skin problem that means I have to wear Nitrile gloves with an old pair of fingerless cycling mits for protection otherwise I wouldn't be able to do any machining at all, so as you can imagine I still have a pair of Lily white hands and clean fingernails.
The Health and Safety warning really put the wind up me when I saw the state of the hands so I am careful in protecting mine.

For all the difference in price for Silione against Viton I decided not to take a chance and I don't know how long Viton rings last but the Silicone ones have been in a D10 for the last 10 years and on inspection don't need replacing.

George.
« Last Edit: February 26, 2016, 04:46:17 PM by K.B.C »
Your never too old to learn.

Offline Malevthi

  • Full Member
  • ****
  • Posts: 14
    • BloOOo Forum
Re: What piston rings material is best?
« Reply #11 on: March 31, 2016, 07:09:14 PM »
Hello,
Here is no ring but teflon cup :


<a href="http://www.youtube.com/watch?v=e8hhLliAe9o" target="_blank">http://www.youtube.com/watch?v=e8hhLliAe9o</a>

Offline steamer

  • Global Moderator
  • Full Member
  • *****
  • Posts: 10316
  • Central Massachusetts, USA
Re: What piston rings material is best?
« Reply #12 on: March 31, 2016, 08:44:14 PM »
Teflon Ramsbottom rings work well,   I have them on my steamboat running 165 psig saturated steam and they have been fine for 3 seasons.....I wouldn't run superheat if I were you though.

Do a search and you'll find this question has been asked before....

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