Piston Rings

[erikjan]

oldnorton (Norm) I really do not know if these rings weep air when cold. My latest loco (almost brand new one could say) can only very difficult pushed along the track, when out of steam and in mid gear. I did however made a joint; by simply cut them with a razorblade. I know that not the best way, but it works without any problems.

rrmrd66 (Malcolm). I do agree with you; our rings are not suitable for industrial usage. But as you stated; our pressures are not of the same kind as hydraulic pressures normally seen in industry. My new loco has a steam chest pressure gauge; even with a few wagons up hill this hardly comes above 3 bar. (Half of the boiler pressure)

Kind regards

Erik-Jan

[Roger]

Feb 11, 2018 12:12:17 GMT erikjan said:

oldnorton (Norm) I really do not know if these rings weep air when cold. My latest loco (almost brand new one could say) can only very difficult pushed along the track, wenn out of steam and in mid gear. I did however made a joint; by simply cut them with a razorblade. I know that not the best way, but it works without any problems.

This confirms what others have done and found to work, ie slitting PTFE rings with a blade, but the rings mentioned by others have been thinner and more flexible than the ones you showed.

The amount of thermal expansion in PTFE is significant, so it's hard to see how any rigid ring is going to work when both hot and cold.

Some people have made solid PTFE rings that seal well when hot, but leak then cold. Getting the precise size right appears to be difficult, some using an oven to heat the cylinder and then fitting the ring to that.

Using Fluorosint instead of PTFE reduces the problems significantly because the coefficient of thermal expansion is much lower, in fact I believe that it's similar to aluminium. It's still a lot more than Cast Iron, so there would need to be a bigger gap when cold which is going to leak to some degree.

[j1949]

Dear All,
Many thanks for all your erudite replies and advice. I have not yet removed the pistons to look at the wear but will do so idc. 
For info: my Simplex was made by me, a complete amateur, on a Myford M type and the boiler in a iron wheelbarrow with lots of bricks. Following Martin Evans instructions and great help from LBSC's books.
We have had tremendous fun on a ground level track in the garden. The radius on one curve is 17 feet where the gauge is widened to 5 1/4".  The trucks running on prewar cast iron wheels which have a much wider tread than modern castings.
The cylinders and pistons are cast iron. We run at 60psi which gives ample power and is easier for friends to control.
Many thanks
Iain

[joanlluch]

Feb 11, 2018 11:52:58 GMT rrmrd66 said:

Hi Everybody

As I said above, "what do I know"?

Malcolm

I am having a hard time at understanding why people don't just use standard seals from industrial hydraulics such as this one:




This seems to me the easier option and you can just replace the seal -as any part subjected to wear- if it is ever required.

You just need to design your piston with a split groove to install it as per the manufacturer recommendation:

SplitPistonSeal by joan lluch, on Flickr

Joan

[Roger]

Feb 11, 2018 18:51:35 GMT joanlluch said:

Feb 11, 2018 11:52:58 GMT rrmrd66 said:

Hi Everybody

As I said above, "what do I know"?

Malcolm

I am having a hard time at understanding why people don't just use standard seals from industrial hydraulics such as this one:




This seems to me the easier option and you can just replace the seal -as any part subjected to wear- if it is ever required.

You just need to design your piston with a split groove to install it as per the manufacturer recommendation:

SplitPistonSeal by joan lluch, on Flickr

Joan

Hi Joan,
I don't think this solution addresses the issues with expansion when it gets hot. If it seals when it's cold, I think it will seize when it gets hot. If it seals when it's hot, I think it will leak when it's cold. The ring that Erik-Jan made looks thinner than this, yet that needed slitting to get it to work, just like the other thin section PTFE rings needed.

[joanlluch]

Hi Roger, You gave me that same exact answer long ago. But experience shows that they seal in all circumstances. I believe they just expand inwards when hot or they absorb their own expansion through elasticity, just as welded continuous rail does. It's a thermoplastic after all, not a hardened metal, and they are sprung with a rubber o-ring underneath. My comment was about why people would not want to use, or even try, something that is readily available. On the other hand, if concerned about excessive seal thickness or rigidity for the working pressure of the locomotive, then just use a thinner version of the seal.

Joan

[61962]

Feb 11, 2018 11:52:58 GMT rrmrd66 said:

Would still appreciate one of the seasoned members advising me if my one piece gunmetal pistons and gunmetal cylinder bores with one clupert ring is a good or bad idea and why they say so.

All my engines have two cast iron rings, regardless of whether the cylinders are iron or bronze. When I built my A4 I had soft packing in the cylinders and in the first three years of intensive running I must have repacked them three or four times. When I came to do them again the bronze bores were barrel shaped and needed reboring, so against advice I fitted new bronze pistons with three iron rings. After three years they were opened up and I found the bores were unworn detectable to my measuring equipment and glassy smooth. The rings were well worn, something like 10 thou at the opposite side to the gaps, but still working well. This loco is now 40 years old and still runs well on the second set of rings since it was last overhauled in 1985.

Eddie

[Roger]

Feb 11, 2018 22:21:42 GMT joanlluch said:

Hi Roger, You gave me that same exact answer long ago. But experience shows that they seal in all circumstances. I believe they just expand inwards when hot or they absorb their own expansion through elasticity, just as welded continuous rail does. It's a thermoplastic after all, not a hardened metal, and they are sprung with a rubber o-ring underneath. My comment was about why people would not want to use, or even try, something that is readily available. On the other hand, if concerned about excessive seal thickness or rigidity for the working pressure of the locomotive, then just use a thinner version of the seal.

Joan

Hi Joan,
I wan't aware that this had successfully been tried. Do you know of locomotives that use them?

I don't think there's any particular resistance to trying something like that, but there has to be a realistic chance of success for them to bother. Not many people are that interested in development of new ideas, they would prefer to just do what they know works. There's also the matter of availability. Some of the more exotic seal materials as shown in catalogues, but when you go to order them, you find that they are specials that have to be ordered in quantity.

[rrmrd66]

Good Morning to you all.

Sorry to continue to bang on about this, but as you will have realised this is a subject I thought I knew a lot about but the deeper I dig I find I don't!

Eddie. Thanks very much for this information. Cast Iron piston rings (x2) in bronze pistons running in bronze bores can give long life. Exactly what I wanted to know.

Joan. Good point. I sometimes wonder why it is necessary to reinvent items. Unless I am mistaken you have highlighted what was always referred to as a "Shamban" seal, after the US manufacturer, but now part of the Trelleborg empire. All seal manufacturers do a similar style.

This type of seal is specifically made for the industrial light/medium duty hydraulic cylinder market and incidentally have been remarkably successful.
However, this specific seal needs a split piston design which I don't have.
Roger asks has anybody actually had long term success with this type of seal in a low pressure steam application? Any feed back on this?

I have already purchased two Clupet piston rings 1.625" dia.x 0.125" length x 0.0625 radial thickness (one spare, one to fit) and naturally I am keen to use them. They were £50+ the pair!

The original build articles from Don Young in LLAS are not particularly helpful although the drawings do refer to a gunmetal cylinders and pistons. The same applies to Reeves,from whom I purchased the castings in a job lot, although I seem to think they recommend a Viton O ring, which Pete (Doubletop) also recommends.

The piston casting is nominally 1.73" dia x 0.75" length
The finished size is 1.625" dia x 0.625" length
One seal groove is shown. Finished dimension: 0.25" length x 0.234 radial depth, thus I have sufficient room to fit two Clupet rings.

Thanks for reading

Cheers

Malcolm

[weary]

Hello Malcolm,

Where did you source your Clupet rings? Or was it from the Clupet Piston Ring & Gage Co., in Maryport Cumbria? I ask as I am looking to source some metric rings. The Maryport source only appears to manufacture Imperial.

Regards,
Phil

[joanlluch]

Hi Roger and Malcom.

To answer your question about long term success of these seals. Short answer is Yes. In Catalonia and Spain model engineers adopt two kind of approaches, both of them based on off-the-shelf items.

- First approach is using metal rings -possibly cast iron- that are sold as spare parts for small IC engines such as the ones in chainsaws or in electric power generators.
- Second approach is using hydraulic seals as the one I discussed above.

Choice depends on personal preference and whether you want to have a split piston.

There's at least one known locomotive that operates every Sunday pulling a long string of passenger cars for several hours in a club near Valencia (that's Spain) that uses these kind of hydraulic seals. However, that's a non-superheated locomotive. I have never heard on the social networks and local FB groups about these seals being ever replaced, but I am going to ask to the owner of the locomotive to be absolutely sure.

Joan

[Roger]

Feb 12, 2018 10:11:29 GMT joanlluch said:

Hi Roger and Malcom.

To answer your question about long term success of these seals. Short answer is Yes. In Catalonia and Spain model engineers adopt two kind of approaches, both of them based on off-the-shelf items.

- First approach is using metal rings -possibly cast iron- that are sold as spare parts for small IC engines such as the ones in chainsaws or in electric power generators.
- Second approach is using hydraulic seals as the one I discussed above.

Choice depends on personal preference and whether you want to have a split piston.

There's at least one known locomotive that operates every Sunday pulling a long string of passenger cars for several hours in a club near Valencia (that's Spain) that uses these kind of hydraulic seals. However, that's a non-superheated locomotive. I have never heard on the social networks and local FB groups about these seals being ever replaced, but I am going to ask to the owner of the locomotive to be absolutely sure.

Joan

Hi Joan,
That's interesting, it will be good to get some feedback and also to find out if anyone uses them with Superheated steam.

In reality, it didn't take long to turn up a couple of cylinders and slit them, so I would probably stick with what I've designed anyway. I prefer the idea of accommodating the expansion at the gaps rather than in stresses and movement in the material itself. To me is seems to be more technically sound. I also like experimenting!

[rrmrd66]

Feb 12, 2018 9:33:51 GMT weary said:

Hello Malcolm,

Where did you source your Clupet rings? Or was it from the Clupet Piston Ring & Gage Co., in Maryport Cumbria? I ask as I am looking to source some metric rings. The Maryport source only appears to manufacture Imperial.

Regards,
Phil

Hello Phil.

I got them at the 2017 Midlands ME Exhibition from Live Steam Models trade stand. See:

www.livesteammodels.co.uk/

Got a funny feeling that they only do imperial sizes and that you have  a long wait for anything non standard.

Good luck

Malcolm

[uuu]

When I spoke to the original Clupet chap, years ago, he advised against fitting two of his rings into one groove. He was quite firm on the point, but didn't go into why.

Wilf

[smallbrother]

Feb 11, 2018 13:00:26 GMT j1949 said:

Dear All,
Many thanks for all your erudite replies and advice. I have not yet removed the pistons to look at the wear but will do so idc. 
For info: my Simplex was made by me, a complete amateur, on a Myford M type and the boiler in a iron wheelbarrow with lots of bricks. Following Martin Evans instructions and great help from LBSC's books.
We have had tremendous fun on a ground level track in the garden. The radius on one curve is 17 feet where the gauge is widened to 5 1/4".  The trucks running on prewar cast iron wheels which have a much wider tread than modern castings.
The cylinders and pistons are cast iron. We run at 60psi which gives ample power and is easier for friends to control.
Many thanks
Iain

Hi Iain,

A few years back I bought an old Polly 1. It did go, but not brilliantly so.

One of the jobs I did was to replace the rubber piston rings. The original, and I am fairly sure they had been there all along, were flush with the piston surfaces. The new ones were a bit of a struggle to get back in the cylinders but she has run nicely ever since. There were a number of minor issues that contributed to the poor running so I can't say how much difference the new rings made when compared with the other jobs done.

However, they took no making, no cutting, no adjusting and cost very little.

Clearly they work as she goes very well now.

Pete.

[jon38r80]

Quote;

"My Simplex could well get the Viton treatment on the next major stripdown. I'm currently working on a Steam Indicator for small locos based on my "Dynobox" hardware (ME article). I'm using the Simplex as a testbed and work to date has suggested that the CI rings are blowing by to some extent.


Early results, don't read too much into"

it would be very interesting to have a similar performance graph for your engine with Viton rings for a measured comparison. Would it be possible to run it on your "Dynobox" rig?
Measured comparison is always more convincing to the Doubting Thomases  than description or feelings.

I don't make model steam engines but find this sort of investigation fascinating, just as much as Joan's investigation of gas fired boiler design is intriguing.

[Roger]

Quote:
"...work to date has suggested that the CI rings are blowing by to some extent."

It's hard to see how conventional 'gapped' rings wouldn't leak. The idea is to minimise leakage, not to eliminate it. Multiple rings with gaps set away from each other create a labyrinth that massively reduces leakage.

The size of the gap is clearly important, and this is why it's difficult to make PTFE rings with gaps. My design takes the Gap out of the equation when it comes to forming a seal. If the ends and underside of a gap are closed by other rings, the size of the gap is not important.

Clupet rings also avoid the gap, so you would expect them to seal better than conventional rings.

[steamer5]

Hi,
Like a lot of threads here, this one is evolving into a really interesting one! I cant offer any constructive info, other than I like the idea of the clupet rings, the piston castings I have are done in such away that one end is set up so the rings can be chopped off the end so I'm sitting on the fence at the moment on which way to jump (as I've been sitting on the fence for a number of years its getting a little uncomfortable!). However awhile back there was an article in ME on using clupet rings, the gent figured that the rings as supplied were to thick & had clupet company make some special's made with a thinner wall, if anybody is interested I'll try & hunt out which issue.

Cheers Kerrin

[rrmrd66]

Hi Roger

You wrote

"This is my solution for using PTFE for piston rings. Originally I was going to peg them like I did for the piston valves, but thought they probably wouldn't rotate in the bores so I didn't bother."

Yes you are right as long as the rings do not rotate and the gaps meet and create a leak path.

In the back of my mind is something from the past in that it is not good practice to have two o rings on the inside of the piston head as you can get pressure build up caused by the pumping action of the piston and subsequent damage to the static seals.

Maybe not a concern at our relatively low pressures?

Hello Wilf.

It is almost certainly due to the fact that the wall of the piston ring groove and its clearance to the piston ring is an integral part of the seal system. Hence if you try to put two rings in one groove this would not be the case.

I stand to be corrected by those who know more about this than me but isn't it standard practice with ic engines to only ever have one groove with one piston ring?

Cheers

Malcolm

[smallbrother]

So guys, why are you going to all this effort and expense?

What do you gain that a simple rubber ring doesn't provide?

Pete.