Piston Rings

[j1949]

I have run a Simplex for over 20 years. The piston rings were just twisted twine and must be worn. Does any one have experience of using nitrile rings as I think I might try them or perhaps I should fit cast iron rings? Any thoughts please.

[maninshed]

You could try p.t.f.e rings, activated by an O ring sitting under the p.t.f.e ring. Or just the p.t.f.e ring, machined then scarfed and fitted to piston.

[joanlluch]

If you decide to use reinforced PTFE with o-ring underneath you can look at the ones from Trelleborg. They can be purchased online from here:

www.seals-shop.com/eu/

This is the link in that store to the product that I use:

www.seals-shop.com/eu/en/piston-seals/turcon-glyd-ring-t-piston

Unfortunately not all the sizes are available, particularly imperial sized ones, but they are totally reliable in non superheated locos, and presumably very good in non-extremely-superheated ones. Of course you need to find a set that matches your cylinder diameter.

Joan

[Roger]

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.

The idea is to avoid the issue of needing a large gap in the ring by blocking the passage of steam with a ring both under and next to each gap.

There's not enough natural spring in the rings to form the initial seal, so there's an 'O' ring under each one. There's very little pressure on the 'O' ring, just enough to make contact. Once it's under pressure, the steam under the rings will do that job.

To date this is has only been tested on air, but I don't see any issues with this running with steam.

Piston ring assembly exploded by Roger Froud, on Flickr



Piston assembly by Roger Froud, on Flickr

[ettingtonliam]

Feb 8, 2018 12:25:08 GMT j1949 said:

I have run a Simplex for over 20 years. The piston rings were just twisted twine and must be worn. Does any one have experience of using nitrile rings as I think I might try them or perhaps I should fit cast iron rings? Any thoughts please.

Only think of cast iron rings if its got cast iron cylinders. Don't try it with gummetal cylinders. You'll need to careful about drying things out and oiling up after running when you are putting the loco away, especially if its for more than a few days.

If its run 20 years on the old style graphite yarn packing, why not just repack it?

[cjfield93]

Having witnessed a friend make a set of piston rings using nothing more than 5mm PTFE square rope and leaving the accommodating groves in the piston heads just shy of the full depth, when they were pushed in the PTFE rope trimmed itself ever so slightly in the process and giving a snug fit in the bores. This loco was used successfully for a season on the GL5 circuit and gave excellent repeated performances and on some impressive loads, before the loco was sold onto pastures new.

This convinced me to go for the same approach when I made a new set of piston heads for the 13xx the other week.

DSC_0614 (2) by Chris Field, on Flickr

[uuu]

We have a Pumphouse visitor who has fitted cast iron rings on aluminium pistons in gunmetal cylinders. It works! So far. Time will tell.

Wilf

[Roger]

Feb 9, 2018 18:01:27 GMT cjfield93 said:

Having witnessed a friend make a set of piston rings using nothing more than 5mm PTFE square rope and leaving the accommodating groves in the piston heads just shy of the full depth, when they were pushed in the PTFE rope trimmed itself ever so slightly in the process and giving a snug fit in the bores. This loco was used successfully for a season on the GL5 circuit and gave excellent repeated performances and on some impressive loads, before the loco was sold onto pastures new.

This convinced me to go for the same approach when I made a new set of piston heads for the 13xx the other week.

DSC_0614 (2) by Chris Field, on Flickr

This is interesting. I think it works because there are voids in the braiding that allow it to accommodate the significant thermal expansion. I'd be interested to know if it blows by when cold though.

[rrmrd66]

Good Morning everybody.

Do you know the old saying about ask three experts a question and they will come up with three completely different answers.

And so it seems in Model steam engine engineering.

For 30+ years I worked in the hydraulic seal/mechanical engineering industry. For 25 years I was fortunate to be the MD of a company of 35 people, until retirement.

The seals were mainly of various elastomers, reinforced with appropriate fabrics, intended for use in high pressure (160-400bar) dynamic industrial hydraulics applications (steel, mining, earth moving machinery etc)

Our USP was that we looked closely at the application parameters (speed,pressure,temperature,hydraulic media,limits and fits and surface finish to name but a few).

We then recommended, in our experience, what would be best suited for that particular application. Normally with satisfactory results. If the factory did not have a suitable item we designed it and made it ourselves.

Amongst the product range of one of our suppliers was the product that from now on ladies and gentlemen you will please refer to as braided packing, not square rope!

Mainly a static sealing material for valve spindles etc but also used in dynamic applications, both rotary and reciprocating.

Indeed many of the NCB's mine hydraulic roof support three throw reciprocating ram pumps (with 95% water/5% oil hydraulic fluid) were sealed by braided packing sets in the most severe of applications (400bar pressure, mean rod speed of 15m/sec). Running times of 30000 hours were achieved. So we must have done something right.

Imagine my incredulity when I read that j1949 (first post in this thread) has achieved 20 years service from "twisted twine". Really? On a piston! Sealing steam!

My first thought would be impossible.

But there you are. What do I know?

Cheers

Malcolm

PS Roger. Your elegant design for your coaxial ptfe + O ring energised piston seal. I trust that the dynamic sealing ring, shown as a white material,is a modified "filled" ptfe and not virgin material.
But there again...............

[Doug]

Having fought with piston seals for a few years now I am going back to twisted graphite yarn, it just works. I have tried nitrile orings, silicon orings, ptfe and graphite string the only one that worked well was the graphite although because I used string and not twisted yarn it washed out in the end. I now have good quality yarn so that’s going in to replace the PTFE which works well cold but seizures occur when under high heat loads.

[Roger]

Feb 10, 2018 8:35:41 GMT rrmrd66 said:

Good Morning everybody.

Do you know the old saying about ask three experts a question and they will come up with three completely different answers.

And so it seems in Model steam engine engineering.

For 30+ years I worked in the hydraulic seal/mechanical engineering industry. For 25 years I was fortunate to be the MD of a company of 35 people, until retirement.

The seals were mainly of various elastomers, reinforced with appropriate fabrics, intended for use in high pressure (160-400bar) dynamic industrial hydraulics applications (steel, mining, earth moving machinery etc)

Our USP was that we looked closely at the application parameters (speed,pressure,temperature,hydraulic media,limits and fits and surface finish to name but a few).

We then recommended, in our experience, what would be best suited for that particular application. Normally with satisfactory results. If the factory did not have a suitable item we designed it and made it ourselves.

Amongst the product range of one of our suppliers was the product that from now on ladies and gentlemen you will please refer to as braided packing, not square rope!

Mainly a static sealing material for valve spindles etc but also used in dynamic applications, both rotary and reciprocating.

Indeed many of the NCB's mine hydraulic roof support three throw reciprocating ram pumps (with 95% water/5% oil hydraulic fluid) were sealed by braided packing sets in the most severe of applications (400bar pressure, mean rod speed of 15m/sec). Running times of 30000 hours were achieved. So we must have done something right.

Imagine my incredulity when I read that j1949 (first post in this thread) has achieved 20 years service from "twisted twine". Really? On a piston! Sealing steam!

My first thought would be impossible.

But there you are. What do I know?

Cheers

Malcolm

PS Roger. Your elegant design for your coaxial ptfe + O ring energised piston seal. I trust that the dynamic sealing ring, shown as a white material,is a modified "filled" ptfe and not virgin material.
But there again...............

Hi Malcolm,
The outer ring material is a graphite loaded one, but I think virgin PTFE would be ok too. I think the problem most designs using PTFE have are due to trying to use them as if they were metal rings with a controlled final gap that ends up virtually closed to maintain a seal. If you do that, you risk seizure when hot, and leaks when cold. That's why I've used a large gap that never closes because it's not part of the sealing regime.

[rrmrd66]

Hi Roger

Can"t wait to see if it works under steam conditions.

BTW. Are there any engines that you know of that are running cast iron piston rings (1 5/8"" clupert) in gunmetal bores?

"Hard" journals work ok in "soft" bores. Why not hard piston rings in "soft" cylinders?

Any body else care to chip in?

Cheers

Malcolm

[uuu]

The cast iron rings I mentioned earlier, in gunmetal cylinders, are of the Clupet type. Home made, I believe.

Wilf

Edit: 1 9/16" bore

[mbrown]

I've never been able to work out how Clupet rings are manufactured.... If someone on this forum knows the secret, especially for Clupet rings in our sizes, it would be great if you could share it.

Malcolm

[simplyloco]

Basically a square spiral spring. Ancient thread here: modeleng.proboards.com/thread/1127/piston-ring-supplier

And a reference here: not mine!
Gents,

Thank you for the comments. I’ve done a search and found that the firm in Cumbria is:-

Clupet Piston Ring & Gauge Co
Factory B, Solway Trading Estate, Maryport, Cumbria CA15 8NF
Tel: [phone]

I also found a comment on a website said that if you want off-the shelf rings from Clupet, ‘….they’ll send them to you next post with a bill. Good, trusting folk’.

I think they’re a simple, elegant solution to the challenge of producing a parallel ring, with no through-gap, that be sprung over a piston, without needing highly sophisticated production methods.

[Roger]

Feb 10, 2018 17:04:17 GMT mbrown said:

I've never been able to work out how Clupet rings are manufactured.... If someone on this forum knows the secret, especially for Clupet rings in our sizes, it would be great if you could share it.

Malcolm

Hi Malcolm,
This has been discussed here before, I'm sure you can find it using the search. The key stage seems to be that after slitting, the sides are forced over each other in the direction that would close the gap. In doing that, you put a 'set' on the ring where the two halves meet, and when the rings are returned to their desired positions, there is now a spring force trying to close the gap.
At least, that's my understanding of it.

[erikjan]

I've used Teflon (home made) rings in my locomotives.



The Teflon U-shaped ring. This was turned from solid Teflon and sits with a side play of 0.2mm in the grove of the piston. In the depth there is a clearance of approx. 0.4 mm.




They have a sliding fit in the cylinder bore. Expansion due to heat and steam will press them further against the bore; but they also have the possibility to buckle the thin 'legs' of the U shape, if the expansion would be to much so that they wouldn't fit in the grove in the piston.






When the engine is in steam, steam pressure will also get under the ring and will press the ring against the cylinder bore. I have these types of rings successful in service in my 3½" gauge and 5" and 7¼" gauge locomotives for many years now.




Regards,

Erik-Jan

[doubletop]

Feb 8, 2018 12:25:08 GMT j1949 said:

I have run a Simplex for over 20 years. The piston rings were just twisted twine and must be worn. Does any one have experience of using nitrile rings as I think I might try them or perhaps I should fit cast iron rings? Any thoughts please.

I am running my Dart with twin Viton O rings, following the lead of one of our members who has run his 7.25" NZR single Fairlea with them for years. He has done hundreds of hours of passenger hauling without any problems at all. As my other posts on here will show my Dart runs pretty well.

The other factor that made me go the Viton route was the number of successful small I/C engines on other forums that had been built with Viton rings. They seem to start just about first time as they had good compression from the start when my efforts homemade CI rings on the same engines hadn't been so succesfull.

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 this.



Pete

[oldnorton]

Feb 10, 2018 21:05:33 GMT erikjan said:

The Teflon U-shaped ring. This was turned from solid Teflon and sits with a side play of 0.2mm in the grove of the piston. In the depth there is a clearance of approx. 0.4 mm.

The 'U' shape is a clever way of overcoming the high thermal expansion of PTFE - I like it. I guess it works in a similar way to the square section rope (sorry, packing :-) ) with a void to absorb the expansion.

These rings 'might' weep cold air under an air test once used a few times - what do you find Erik-Jan? But the first leak of steam will expand them and then get underneath to seal. By comparison, that is the benefit of the design using an o-ring under the PTFE seal as the o-ring always pushes the PTFE back against the bore.

But these 'U' rings would be easy to quickly knock up, and there is no joint!

Norm

[rrmrd66]

Hi Everybody

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

Erikjan has actual good running experience with virgin ptfe with no filling material (which would certainly improve all mechanical properties). Roger is also going to use a ptfe based filled material.

Note that these designs rely on a "split" piston design. A "groove" type housing, like an O ring would use, would not allow a large radial cross section seal to be stretched in to the piston groove.

All my experience from industrial hydraulics says no. But at these very low pressures (6bar max?) the material is working well within its elastic limit (I guess) and so permanent deformation and extrusion is not an issue.

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.

Cheers

Malcolm