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  • It's Sunday night. I hurt my back yesterday, so I'm not up for much machining (until tomorrow at least.) I've spent today thinking about reversing gear, doing a bit of layout work, and yacking with my internet friends.--When I built this engine, I put Viton O-rings on the pistons. One ring per cylinder. All of the other "steam-engines" I've built, about a dozen, never had any rings at all---Just two or three very shallow grooves around their circumference for oil retention. They all spin very freely, and I never noticed any of them having a problem with pressure "blowing by" the pistons. With this engine I just finished, I was expecting some binding as all the pieces were firmly bolted together, so my reasoning was that if I leave a little more clearance between the steel piston and the brass cylinder, there would be less chance of the piston scoring the inside of the cylinder. The Viton ring would ensure no leakage of air, even though the piston wasn't as tight a fit as on my earlier "steam engines". A 1/16" cross section Viton ring is actually about 0.070" in cross section. I made the groove in the piston 0.060" deep, which is about .003" more than I do on my internal combustion engines. Everything works, just as I had planned. What I didn't plan, is the amount of drag that Viton ring would create. I know this engine should run easily on about 5 psi.---it doesn't. I have ran it for about 6 hours now, and it isn't going to wear in anymore than it has already. 10 psi is about as low as I can turn my air regulator without the engine stalling out. And---When the engine sets for a while, not running, the Viton takes a "set" and is a real pig to get the crankshaft rotating again. Soon as the crankshaft rotates even a little bit that "set" goes away, but it never goes totally away. I'm not going to change things now, but I just thought I would post this as information.---Brian
    Brian Rupnow

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    • Oiled? Greased? O-rings like lubrication...

      Watching with great interest...

      Pete
      1973 SB 10K .
      BenchMaster mill.

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      • Heavily lubricated with oil. Viton does a very admirable job of sealing the piston so that nothing gets past it, but it does drag. That's probably why I could never get my hit and miss i.c. engines to perform the way I wanted to.
        Brian Rupnow

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        • Hmmmm, is it just Viton or have you tried other o-ring materials? I've never used o-rings in that application and wonder if there's another material that would give less drag... but something still easy to find and use...

          Here's hoping you feel better soon and looking forward to more "installments".

          Pete
          1973 SB 10K .
          BenchMaster mill.

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          • I have a selection of different Viton O-rings. I use them on i.c. engines because of their high tolerance for heat. I have never used any other kind.
            Brian Rupnow

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            • Originally posted by brian Rupnow View Post
              Heavily lubricated with oil. Viton does a very admirable job of sealing the piston so that nothing gets past it, but it does drag. That's probably why I could never get my hit and miss i.c. engines to perform the way I wanted to.
              Imagine if the engine was running on steam, that would strip that oil in short order. I read that you had poor results when attempting metal rings but many small engine builders have made them successfully. It might be worth re-exploring that some day. I remember a excellent article in strictly-IC magazine on it. Alternately, what about just buying rings intended for one of the RC aircraft engines, KB for example and standardizing your builds on those rings. Or.... just stick with the viton o rings and accept the degrading of resulting performance.

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              • I got up before the chickens this morning and snuck (that is the Canadian form of sneaked) down to my shop and finished the jig for milling my reversing plates. At one end, the links can be set up and adjusted to all be exactly the same length. (the one shown doesn't have the 1/8" rod running between the brass end pieces.) The other end will come to life and hold a reversing plate later today.---And yes, after I had the plate firmly bolted to the shaft, I set it up in the lathe and took a 0.015" clean up pass on the face of the plate. This plate was a piece of repurposed scrap, so there are a few holes in it that don't mean anything.
                Brian Rupnow

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                • Originally posted by Sparky_NY View Post
                  Imagine if the engine was running on steam, that would strip that oil in short order. I read that you had poor results when attempting metal rings but many small engine builders have made them successfully. It might be worth re-exploring that some day. I remember a excellent article in strictly-IC magazine on it. Alternately, what about just buying rings intended for one of the RC aircraft engines, KB for example and standardizing your builds on those rings. Or.... just stick with the viton o rings and accept the degrading of resulting performance.
                  Sparky--Much of what I do here is a voyage of discovery. I post all of my results so others can learn from what I've done. Unless the Viton rings cause a problem which prevents the engine from running, I'll just leave them. All it ultimately means is that I lose the right to brag about how MY engine will run on .0002 psi of air pressure. It makes little difference to me if it runs on .0002 psi or 10 psi. If I built another air/steam engine, I wouldn't use the Viton rings---or---I'd make the ring groove .065 deep in the piston instead of .060" deep to see if that would cut the drag even farther without losing the ability to seal really well.
                  Brian Rupnow

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                  • Maybe you could put a smaller o-ring on the piston and the "stretch" would lower the protrusion and reduce the drag.
                    Great post!
                    Bill
                    I cut it off twice and it's still too short!

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                    • Most times, with the steam engines I've built, I use a good fit piston to cylinder (.001-002) under on piston. (on up to the 1.50 dia bore)
                      Then I use cotton string as the packing. The groove is machined so that I get 3-4 wraps of the string. I'm using common .030-.040 Dia. cotton.
                      The advantage is low drag, and it retains the oil, provides plenty of seal.
                      I'd bet that you could replace the current O-ring with cotton, and see an improvement at low pressures, then you can retain your bragging rights, OD.

                      Sid

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                      • Sid--You are very probably right. If I find that I can't live with the Viton, I will try the oiled string.---Brian
                        Brian Rupnow

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                        • the way an O ring seals isn't (at least wasn't for) me intuitive. Its not outward pressure or a tight squeeze between cylinder and bottom of groove, its how it bunches up. Its an engineered thing, the size of the groove, and there is suppose to be some play in the width and its the ring bunching up at one end of the groove that provides the sealing. if you google it, there is all kinds of good information from manufacturers on sizing the groove. Kind of like to get the seal you need to do X, and the drag will be what it will be.

                          Why not iron rings? There's nothing in making them you wouldn't be able to do.
                          .

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                          • McGyver--I have a bad history with cast iron rings. I've tried to make them about four times, and after four failures said "Screw it!!!" As far as the depth and width of grooves for a 1/16" cross section o-ring---0.093" is fine for width (and also happens to be the width of my parting off tool). The depth of cut is also what determines the amount of "squash" on the ring when you fit the piston into the cylinder. On all of my i.c. engines, I made the grooves 0.057" deep. On this one I made the groove depth 0.060" deep. Note that a 1/16" o-ring is actually 0.070" in cross section.
                            Brian Rupnow

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                            • String - how century before last. Square section graphite.
                              You did give the o-ring rolling room ?

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                              • One picture of set-up and one of main slot milled. Lots and lots of breath holding and butt clenching going on!!!

                                Brian Rupnow

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