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Post by 92220 on Feb 24, 2020 9:21:49 GMT
Hi Ross.
Like Roger, I couldn't see the clearance either, so couldn't see how it worked. Now it is obvious, and looks good. I have to take my cylinders off, sometime, to fit the piston rings. I might look at whether a similar rod sealing system would fit on mine.
Bob.
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Post by suctionhose on Feb 24, 2020 10:31:29 GMT
Hi Ross. Like Roger, I couldn't see the clearance either, so couldn't see how it worked. Now it is obvious, and looks good. I have to take my cylinders off, sometime, to fit the piston rings. I might look at whether a similar rod sealing system would fit on mine. Bob. It would be quite compact and prototypical in appearance(?). I do like the PTFE cones specified by...(was it Michael Breeze?) and shown here www.youtube.com/watch?v=VbBN81ivr14I'm going to try those on my throttle shaft... |
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Post by ettingtonliam on Feb 24, 2020 11:43:36 GMT
I was going to fit O rings to my piston rod and valve rod guides, but now Julian has warned us off with predictions of rod scoring, I'll just stick to
soft packing.
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Post by Roger on Feb 24, 2020 11:49:58 GMT
This is how I decided to make the rod seal for the Cylinder rod. It's just a PTFE sleeve with a thick flange and two 'O' rings that provide the function of applying the pressure on the rod and also sealing the path around the outside. The though is that the thin PTFE is flexible and will pinch the rod where the 'O' rings are. It's a trivial matter to make a new one when it wears out. The gland retainer has a shoulder so that the PTFE part isn't under any axial compression.  Piston rod gland by The train Man, on Flickr |
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Post by Roger on Feb 24, 2020 11:54:40 GMT
I was going to fit O rings to my piston rod and valve rod guides, but now Julian has warned us off with predictions of rod scoring, I'll just stick to soft packing. Ross has used 'O' rings for years without issues, as long as you provide a little lubrication by way of some sort of small reservoir. Each of these methods has its drawbacks. Personally, I prefer a design where you don't need to adjust it to retain a seal. 'O' rings cost next to nothing. There's nothing to stop you from putting a couple of extra loose ones on the piston rod so you can cut the old one off and stretch the new one over the retainer so you don't have to disconnect the piston rod from the crosshead. |
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Post by 92220 on Feb 24, 2020 19:40:53 GMT
Hi Ross.
As far as stainless steel rods go, I don't see why they can't be used. Scoring isn't a problem as long as lubrication is present as Roger points out. We have a local engineering company that makes hydraulic cylinders of all sizes, entirely of 316 stainless steel. This enables them to be used on board ships with no corrosion problems, and 316 stainless cannot be hardened, so stainless hydraulic cylinder rods on a stacker truck, on board an aircraft carrier, or freighter, obviously can't have rod scoring as a problem. The odd droplet of oil, every so often, should be sufficient. The rod should also get lubrication from the steam oil in the steam to the cylinders.
Bob.
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Post by suctionhose on Feb 24, 2020 20:10:08 GMT
I was going to fit O rings to my piston rod and valve rod guides, but now Julian has warned us off with predictions of rod scoring, I'll just stick to soft packing. Gland packing should last a very long time once settled in. I agree in principle that it is probably the best material for the job. This is just one of those intriguing design challenges that are hard to resist! |
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Gary L
Elder Statesman
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Post by Gary L on Feb 25, 2020 1:09:21 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 Many thanks for that insight Julian, it completely explains a case of mysterious scoring I've had with no obvious misalignment etc. Gary |
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Post by Roger on Feb 25, 2020 8:07:37 GMT
Hi Ross. As far as stainless steel rods go, I don't see why they can't be used. Scoring isn't a problem as long as lubrication is present as Roger points out. We have a local engineering company that makes hydraulic cylinders of all sizes, entirely of 316 stainless steel. This enables them to be used on board ships with no corrosion problems, and 316 stainless cannot be hardened, so stainless hydraulic cylinder rods on a stacker truck, on board an aircraft carrier, or freighter, obviously can't have rod scoring as a problem. The odd droplet of oil, every so often, should be sufficient. The rod should also get lubrication from the steam oil in the steam to the cylinders. Bob. Hi Bob, I very much doubt if the actual bearing surfaces are 316 to 316 in that application, I imagine there's a shell of some kind of bearing material between the two. |
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Post by Deleted on Feb 25, 2020 10:46:54 GMT
I have followed Don's design for the gland packing, using 'O' rings as being discussed but of course in a more simple format. Time will tell how well they seal..  Pete |
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Post by John Baguley on Feb 25, 2020 12:29:46 GMT
I think you will find that the scoring on piston rods mentioned by Julian is actually caused by the piston rod being incorrectly supported by the fixed gland nut or similar. The rod should be supported by the crosshead at one end and the piston at the other. There should be no physical contact between the rod and the gland nut or between the piston rod and the bore in the cylinder end cover. The only contact should be with the seal.
As Julian says though, stainless steel and hard phosphor bronze are not a good combination.
I suppose the notion of having a close running fit between the rod and the gland nut goes back to LBSC's days when builders were told to ream the gland nut to a good fit on the rod. That's why it was stressed that the thread on the gland nut had to be concentric with the bore in the gland otherwise the rod would lock up when the gland nut was tightened.
Jim Ewins described a design for floating rod seals for his 2-10-0 'Loadstar' back in 1982 so that may be one of the earliest uses of them? (apart from on full size locos)
John
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Gary L
Elder Statesman
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Post by Gary L on Feb 25, 2020 17:11:44 GMT
I think you will find that the scoring on piston rods mentioned by Julian is actually caused by the piston rod being incorrectly supported by the fixed gland nut or similar. The rod should be supported by the crosshead at one end and the piston at the other. There should be no physical contact between the rod and the gland nut or between the piston rod and the bore in the cylinder end cover. The only contact should be with the seal.
As Julian says though, stainless steel and hard phosphor bronze are not a good combination.
I suppose the notion of having a close running fit between the rod and the gland nut goes back to LBSC's days when builders were told to ream the gland nut to a good fit on the rod. That's why it was stressed that the thread on the gland nut had to be concentric with the bore in the gland otherwise the rod would lock up when the gland nut was tightened.
Jim Ewins described a design for floating rod seals for his 2-10-0 'Loadstar' back in 1982 so that may be one of the earliest uses of them? (apart from on full size locos)
John
I'm sure that's all true- an O-ring isn't going to score a stainless rod unless there is metallic contact somewhere. But in my case, any contact was very light verging on undetectable, at least with the engine cold. I think that the traditional 'reamed' fit is mostly to blame, allied with the difficulty of maintaining concentricity in the gland nut. But even with metallic contact, you would expect to see 'shiny' wear, not scoring with the stainless visibly' picking up'. I think that is where Julian's observation is most pertinent; a combination of bad lubrication due to the O-ring wiping it off, and a bad combination of metals using the traditional phos-br gland nut. -Gary |
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Post by Roger on Feb 25, 2020 17:35:32 GMT
Gary makes a very pertinent point about the difficulty in retaining a concentric hole in a gland nut which is screwed into a thread in the cylinder cover. Unless you carefully screw cut both of these items, the chances of getting these concentric is pretty remote. I'm sure some people get lucky, but using taps and dies for this is very hit and miss. It's easy to see why using two studs to retain the gland is a much better proposition.
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Post by Deleted on Feb 25, 2020 17:56:41 GMT
Gary makes a very pertinent point about the difficulty in retaining a concentric hole in a gland nut which is screwed into a thread in the cylinder cover. Unless you carefully screw cut both of these items, the chances of getting these concentric is pretty remote. I'm sure some people get lucky, but using taps and dies for this is very hit and miss. It's easy to see why using two studs to retain the gland is a much better proposition. it's a good point Roger... note on the drawing that I posted that there isn't any thread for the piston gland. The gland itself is a drift fit into the gland housing bore and the 'O' ring which sits in a recess in the gland is held in place by a simple flat disc. As John (baggo) stated, there shouldn't be any contact between rod and gland. Same for yours and Ross I think? Pete |
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Post by 92220 on Feb 25, 2020 19:00:04 GMT
Hi Roger.
I have to admit I misread the website. The rods are stainless but not 316. They are F51 stainless which is a Duplex stainless.I still maintain there is no reason for not using 316 stainless, or any stainless rod, for piston rods. In any case there is almost no chance of getting a stainless to stainless sliding contact, as stainless wouldn't normally be used for anything apart from the rods, and a bronze bush in the rear cylinder cover would be quite normal procedure. In fact I have used that combination on my loco......316 stainless rods, mild steel rear cylinder covers, with a bronze bush between the seal and the front face of the cover. So where would the rod be subject to scoring?
Bob.
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Post by andyhigham on Feb 25, 2020 19:17:10 GMT
I think you will find that the scoring on piston rods mentioned by Julian is actually caused by the piston rod being incorrectly supported by the fixed gland nut or similar. The rod should be supported by the crosshead at one end and the piston at the other. There should be no physical contact between the rod and the gland nut or between the piston rod and the bore in the cylinder end cover. The only contact should be with the seal.
As Julian says though, stainless steel and hard phosphor bronze are not a good combination.
I suppose the notion of having a close running fit between the rod and the gland nut goes back to LBSC's days when builders were told to ream the gland nut to a good fit on the rod. That's why it was stressed that the thread on the gland nut had to be concentric with the bore in the gland otherwise the rod would lock up when the gland nut was tightened.
Jim Ewins described a design for floating rod seals for his 2-10-0 'Loadstar' back in 1982 so that may be one of the earliest uses of them? (apart from on full size locos)
John
I beg to differ. The piston rod is supported by the crosshead and the bore of the cylinder cover. The piston is only there to hold the piston rings |
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stevep
Elder Statesman
Posts: 1,073
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Post by stevep on Feb 25, 2020 19:29:48 GMT
I think you will find that the scoring on piston rods mentioned by Julian is actually caused by the piston rod being incorrectly supported by the fixed gland nut or similar. The rod should be supported by the crosshead at one end and the piston at the other. There should be no physical contact between the rod and the gland nut or between the piston rod and the bore in the cylinder end cover. The only contact should be with the seal.
As Julian says though, stainless steel and hard phosphor bronze are not a good combination.
I suppose the notion of having a close running fit between the rod and the gland nut goes back to LBSC's days when builders were told to ream the gland nut to a good fit on the rod. That's why it was stressed that the thread on the gland nut had to be concentric with the bore in the gland otherwise the rod would lock up when the gland nut was tightened.
Jim Ewins described a design for floating rod seals for his 2-10-0 'Loadstar' back in 1982 so that may be one of the earliest uses of them? (apart from on full size locos)
John
I beg to differ. The piston rod is supported by the crosshead and the bore of the cylinder cover. The piston is only there to hold the piston rings I tend to agree with you Andy, but I think the crucial bit is that the gland should not be a close fit on the piston rod. As Roger points out, getting it set up so that everything is concentric with a threaded gland is next to impossible, so when the gland is tightened, it rubs on the piston rod. I think the point about the floating gland is that it can move around, whilst maintaining a steamtight seal against the rod. |
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Post by ettingtonliam on Feb 25, 2020 19:30:14 GMT
I'm not sure about that. There is a tendency for large horizontal cylinders (full size) to wear more on the bottom, so this must bed due to the piston, because wear caused by the rings would be uniform all the way round.
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stevep
Elder Statesman
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Post by stevep on Feb 25, 2020 19:32:08 GMT
I'm not sure about that. There is a tendency for large horizontal cylinders (full size) to wear more on the bottom, so this must bed due to the piston, because wear caused by the rings would be uniform all the way round. But surely, if the hole in the rear cover wears, the piston will then drop - causing wear on the bottom of the cylinder. |
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Post by ettingtonliam on Feb 25, 2020 19:41:08 GMT
True, but theres a lot more surface area on the piston than there is on a little bitty rod.
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