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Post by suctionhose on Feb 22, 2020 21:06:50 GMT
On 5"g models, people have successfully used orings in piston rod glands for many years. I am one of those people. I'm building a larger engine currently - a traction engine - and wanted to improve the application of the oring to accommodate some lateral movement of the piston rod possibly caused by boiler expansion, running clearances or some other unplanned misalignment. In the course of racking my brain to 'invent something new' I discovered that two loco's in the club already have this arrangement and have had so for decades! Though the gland shown in the pictures below is untested at this stage, based on evidence from the loco's I mentioned, I'm prepared to say the concept is very reliable. As this was a personal indulgence really - gland packing would have done the job - the external appearance when assembled remains that of a traditional gland. I thought it might be of interest....    |
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dscott
Elder Statesman
Posts: 2,440
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Post by dscott on Feb 22, 2020 23:06:04 GMT
Yes indeed everything of interest as I spend most waking hours working on various projects in the workshop or at our local track that someone put 3.7 miles from our home.
Keith Wilson has put them on his Bulldog Dukedog but there being so few built and running feedback is very rare.
Alan Crosfield may have used them on his model. Ask via the Leyland Club.
Lovely work and a superb finish you have on your cast iron.
Regards.
David and Lily still warming up after Club Running.
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jma1009
Elder Statesman
Posts: 5,917
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Post by jma1009 on Feb 22, 2020 23:44:05 GMT
Hi Ross,
I am very old fashioned in my views. I don't like the propensity of an 'O' ring to apply a 'squeegee' action to the oil on the rod, which if of stainless is prone to cause the rod to score.
I have replaced lots of piston and valve rods of stainless over the years. Phos Bronze and stainless is not an ideal match in miniature - they got it better in fullsize, and perhaps that is the way to go?
Cheers,
Julian
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Post by suctionhose on Feb 23, 2020 1:13:12 GMT
Julian,
The rod is 12mm dia Chrome Bar in this case - not stainless. Further more it only touches the oring and short land in the carrier (yes bronze) and remains clear of all else.
External lubrication of oring by oiling piston rod is essential to long life of orings in this service. Cylinder lubrication alone is not sufficient in my experience.
Of stainless piston rods, I agree, not an ideal material. That said, with lubrication as above, a polished rod and accurate alignment, the stainless rods I have, have done a million trillion cycles without scoring.
On the traction engines, larger components and continuous high revolutions led me to choose chrome for the reasons you mention.
Ross
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Post by David on Feb 23, 2020 8:21:04 GMT
If the "short land in the carrier" is the bit circles in red here, why does that need to touch the piston rod? Can't that opening be a bit larger while still giving the o-ring the support it needs? Or will that be worse and allow that part move relative to the rod causing more and uneven wear?  |
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Post by suctionhose on Feb 23, 2020 8:41:03 GMT
Your circled detail has contact with the rod so if the rod shifts to the side the oring carrier is forced to remain concencentric about the rod.
Under pressure, the oring carrier is forced back to seal, metal to metal, against the cast iron retainer.
Any leakage passed the metal to metal seal is vented to atmosphere via the relief and clearance about the rod so pressure cannot build up to counter the pressure holding those faces together.
However, if the rod wants to move to the side, the metal to metal seal is expected to slide as the rod bears against the circled contact point thereby sparing the oring of off centre squeeze from the rod.
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Post by suctionhose on Feb 23, 2020 8:47:28 GMT
DS Scott mentions a couple of names of people using orings (?).
The innovation here is not "oring" it is the "floating".
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Post by 92220 on Feb 23, 2020 9:04:11 GMT
Hi David.
What materials are the yellow and blue 'collars' and the cylinder casting? The Blue collar has a shoulder to be pressed up against but I am assuming the yellow collar is able to slide in it's housing. The material of the yellow o-ring collar, must have equal or smaller coefficient of expansion than the cylinder materiel or it will expand when heated and lock up, i.e. a bronze collar in an iron cylinder, would lock up; an iron collar in a bronze cylinder would not. Also, producing a close slide fit is going to quite a challenge, to make the o-ring collar without the gap being too big and allowing steam to flow around the outside. Depending on the size of the collars, a face sealing o-ring would get over this problem - if there is room to fit it. (edit): This is assuming all the materials are such that you don't get thermal expansion locking it all up and ending up with no 'float'.
Bob.
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stevep
Elder Statesman
Posts: 1,073
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Post by stevep on Feb 23, 2020 9:18:56 GMT
I used Keith Wilson’s floating O ring design on my Stanier, and never had any problem.
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Post by David on Feb 23, 2020 9:46:55 GMT
Luckily Ross is making it ;) |
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Post by suctionhose on Feb 23, 2020 11:16:27 GMT
I used Keith Wilson’s floating O ring design on my Stanier, and never had any problem. Quite possibly the credit belongs to Keith Wilson. The late Bill Richards of our club got the credit here. Looks like a case of "Great minds think alike and the rest of us catch on eventually..."! With respect to 92220's query: the materials are bronze and cast iron which I think shows up in the picture. The bronze 'oring holder' or 'carrier' has clearance all round and 5thou end float so no seizing is expected. The steam pressure will equalize on all sides of it but steam pushing on the oring will drive the carrier outwards causing it to seal on the back face. Once the seal is made, opposing pressure bleeds off to atmosphere keeping the seal faces together. Sorry too late tonight to do diagrams and an early start for work tomorrow. |
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tony9f
Seasoned Member
Posts: 114
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Post by tony9f on Feb 23, 2020 14:25:03 GMT
A very similar set up to the metallic type packings as made by Britimp and United States in as much that while maintaining a seal they would also float to stay concentric to the piston rod.
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Post by Roger on Feb 23, 2020 15:32:39 GMT
On 5"g models, people have successfully used orings in piston rod glands for many years. I am one of those people. I'm building a larger engine currently - a traction engine - and wanted to improve the application of the oring to accommodate some lateral movement of the piston rod possibly caused by boiler expansion, running clearances or some other unplanned misalignment. In the course of racking my brain to 'invent something new' I discovered that two loco's in the club already have this arrangement and have had so for decades! Though the gland shown in the pictures below is untested at this stage, based on evidence from the loco's I mentioned, I'm prepared to say the concept is very reliable. As this was a personal indulgence really - gland packing would have done the job - the external appearance when assembled remains that of a traditional gland. I thought it might be of interest.... Hi Ross, Maybe I'm missing something? You're using a metal to metal face seal on the 'O' ring carrier then. What stops the steam from going around the outside? For it to be able to move sideways but not leak, it must be a very close axial fit in the pocket? Too tight and it won't allow the 'O' ring carrier to move sideways, and too much clearance and the 'O' ring won't do anything? |
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stevep
Elder Statesman
Posts: 1,073
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Post by stevep on Feb 23, 2020 18:31:15 GMT
In Keith Wilson’s design, the O ring presses around the rod, and also on the rear face of the recess - providing a seal in both directions.
The floating collar that holds the O ring is retained in the recess by the dummy gland.
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Post by Roger on Feb 23, 2020 19:31:01 GMT
In Keith Wilson’s design, the O ring presses around the rod, and also on the rear face of the recess - providing a seal in both directions. The floating collar that holds the O ring is retained in the recess by the dummy gland. Ah, I see. That's not how this is modelled though, the 'O' ring is clearly gripping only on the outside. You're not supposed to contain them in more than one plane, that's not how they're designed to be used. You can't use any of the usual data about compression if you do it that way because the elasticity is compromised by containing it in two planes. I could see a second 'O' ring being arranged to seal in the axial direction though, that would work. |
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Post by suctionhose on Feb 23, 2020 20:46:05 GMT
To look more closely at the arrangements, the following sketch exagerates the clearances. The trick is that a pressure imbalance is achieved causing the face seal to seat in the same way a slide valve does. Roger, the oring is only contained by nip on ID and OD. The axial space for it is to the standard groove dimensions.  |
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Post by Roger on Feb 23, 2020 21:02:41 GMT
To look more closely at the arrangements, the following sketch exagerates the clearances. The trick is that a pressure imbalance is achieved causing the face seal to seat in the same way a slide valve does. Roger, the oring is only contained by nip on ID and OD. The axial space for it is to the standard groove dimensions. Hi Ross, Thanks for making that clear. On the original model I couldn't see any clearance so I couldn't see how it could float. I see how that works now, using a metal to metal seal. It does make me wonder if a large section 'O' ring in the same size pocket wouldn't do just as good a job? Anyway, it's a pleasing design that achieves what you set out to achieve. |
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Post by Oily Rag on Feb 23, 2020 21:12:56 GMT
Maybe a good situation for quad rings rather an a o ring.link
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Post by Roger on Feb 23, 2020 21:46:46 GMT
Out of interest, where do you intend to feed the oil on this design? You mentioned before that you need to oil them when they have 'O' rings.
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Post by suctionhose on Feb 23, 2020 21:56:02 GMT
Out of interest, where do you intend to feed the oil on this design? You mentioned before that you need to oil them when they have 'O' rings. Using a oil pot dripping onto the rod or more likely feeding a groove inside the gland retainer itself. I may cut some nicks in the oring carrier to help oil pass through the region of shaft contact. Once oil gathers in the oring groove it lingers there and is adequately maintained by general rounds with the oil can every couple of hours. |
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