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Resurfacing vintage crankshafts and nitrided component surfaces

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  • powerandpassion
    Never Be Afraid to Ask

    Resurfacing vintage crankshafts and nitrided component surfaces

    If you need to rebuild a Napier Dagger for your Hawker Hector or Ha140 for your Ki61 Tony you are unlikely to find crankshafts at your local pizza shop. I have been to many pizza shops, and I know that this is true! Techniques for plasma spraying have evolved in the last 30 years that, for the innocent, seem to offer possibilities for the resurfacing of unobtainable or money black-holing components such as crankshafts. Before you grizzle faced aerospace pros spit out your coffee, roll eyes and reach for the well thumbed Explanations for Idiots Book (I-Book) let me mouse whisper that some of these old, obscure crankshafts were not nitrided, so pg 1 of the I-Book that says nitrided Merlin crankshafts can't be done won't win you a beer, yet...

    Then I think, so what if it's nitrided?! Not out of disrespect, but all the flim flam gears and gear shafts in the Napier Dagger will certainly be nitrided, and worn, so it would be good to find a technique that would allow resurfacing of these, because no waiting at the letterbox will ever deliver a new Dagger supercharger planetary gear shaft, but you might trawl a worn one out of the North Sea. What would happen if you actually machined off the nitriding, then built up a new coating of the right metal, then nitrided it again?

    Given all the development of new coating techniques and technologies, when was the last time somebody looked at applying these to the refurbishment of seventy year old components? So I am interested in why it can't be done. There are a number of relatively recent techniques, HVOF, for example, that I would like to explore.

    Now I have some sacrificial Armstrong Siddeley Cheetah engines and components that I would be willing to run cruel experiments on, to test some of these new technologies, and see what happens. If you are willing to help me define these tests, I will happily do them, just to see what happens
    If a Cheetah engine crankshaft can be resurfaced, for example, using HVOF, and the engine subject to a 1930's era Endurance Test, that demonstrates that it might be fit for purpose. Or fit for porpoise, if it doesn't work, to be sunk back into the sea...
  • Miclittle
    Rank 5 Registered User

    #2
    No Armstrong Siddeley Cheetah or parts are sacrificial to me, I'm desperately trying to get these parts please don't destroy them! Those parts are getting scarce, like all vintage aircraft engines, even bad parts are great for static display. Why not use a period commercial vehicle crankshaft instead for your "cruel" experiments?

    Comment

    • Avro Avian
      Rank 5 Registered User

      #3
      P&P, I think the biggest problem you are going to have is getting someone to sign off a repair scheme. For engine rebuilds and repairs, one usually has to rely on the manufacturer's data. If you had a number of identical engines and or components to repair, it would be feasible, but one offs, not so.
      For example, there is a large amount of data (ie repair schemes) for the Rolls Royce Merlin series of engines, published by Rolls Royce and also other individuals/companies. This is helped by the relatively large number of these engines still in operation. Unfortunately, the threat of legal liability makes approval of repair schemes that much harder.

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      • powerandpassion
        Never Be Afraid to Ask

        #4
        Gosh, an Oxford only has two! I promise not to diminish the stock of engines. Any decent experiment will require a test and control, so one engine, entirely unaltered, and one engine with a key element altered, running side by side. In the first test, both crankshafts ground down, at minimum, within spec, control on original oversize main bearing, test on new oversize main bearing, to determine if new main bearing OK. Then, pushing, crankshaft built up to standard size using HVOF on test, new standard size main bearing, see what happens. Probably bisect built up crankshaft after test to examine bond between old and new material, so only one crankshaft lost. To gain might be a method of revitalising all your Cheetahs, turning scrap into gold. Why Cheetah? Because after doing it on some old diesel cranks subject to greater forces, you still won't believe it until you see it. And then we can tippy toe into something terribly rare. I bet that the method will work for engines designed for copper lead main bearing type pressures, but not silver indium bearings, so pre 1943 types, that need something like this to ever have a hope of getting up, interesting once off type engines...

        Comment

        • powerandpassion
          Never Be Afraid to Ask

          #5
          AA, I understand this issue. It is dismal and holds back so much that could be inspiring in our own heritage. It might be easier to do a Japanese engine, with nothing but science to rely on in determining what is safe and insurable. The point to consider is that 70 year old repair schemes do not encompass 2016 technology. More, the confident practitioners of the art, who wrote the original repair stuff, have moved to the great golf course of the sky, so no one is left to put a nappy around this stuff. So back to the basics of test and control, scientific data, proof, disproof, progress and God forbid, fun! I am really interested in the why and the why not. Some of the crankshaft resurfacing is used in industrial applications that exceed the service pressures and service exposures of things like Cheetahs, so the Godess of science says 'seek and Ye shall prosper'.

          Comment

          • Vega ECM
            Rank 5 Registered User

            #6
            I've spent a good bit of time trying to make HVOF coated flying parts work, and they do;- But as ever, the devil is in the detail;- in particular the surface finish.

            You need a surface which is just so, not too rough but not too smooth either. This is because you need to leave just the right number of microscopic pits so as to load the surface with oil. So brush up on your Ra, Rb, Rc, Rz, Rmz etc

            In the good old days of Chrome plate, so long as you applied 15-40 microns then the surface micro-cracking 'mud crazing ' was inherently right for oil loading on the surface. It's not the same with HVOF you have to make sure you hit the right numbers with the post finishing operations. No two components seem to be the same so there's always a bit of a process development faf to go through.
            Last edited by Vega ECM; 12th November 2016, 14:56.

            Comment

            • Robbiesmurf
              Rank 5 Registered User

              #7
              Hat's off P&P. I admire those who are willing to try new techniques with a little help from lateral thinking.
              I wish you success.

              Comment

              • TonyT
                Rank 5 Registered User

                #8
                The point to consider is that 70 year old repair schemes do not encompass 2016 technology. More, the confident practitioners of the art, who wrote the original repair stuff, have moved to the great golf course of the sky
                I agree, unfortunately your sticking point is always going to be the authorities and also the type certificate holder / manufacturer. It wouldn't surprise me if they called for several of the items you are wanting to reuse being tested to destruction / cut up.
                Last edited by TonyT; 12th November 2016, 19:57.

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                • powerandpassion
                  Never Be Afraid to Ask

                  #9
                  Thanks Robbiesmurf. Will try and report back what happens. Will take a few years....

                  Comment

                  • powerandpassion
                    Never Be Afraid to Ask

                    #10
                    Originally posted by Vega ECM View Post
                    I've spent a good bit of time trying to make HVOF coated flying parts work, and they do;- But as ever, the devil is in the detail;- in particular the surface finish. So brush up on your Ra, Rb, Rc, Rz, Rmz etc
                    Vega ECM, I am a lost lamb in the woods on this, but want to learn.
                    Can you confirm that HVOF is accepted in the serious side of the game.

                    Um, what is Ra-Rmz?

                    The particular interest is vintage crankshafts, most of them nickel chromium alloy. The back of the cereal packet says that bonding between substrate and coating was the first issue of concern. Can you confirm that 2016 HVOF has resolved this ? Is it necessary to machine to original surface to allow HVOF to fix better?
                    Does impregnation of oil and other contaminants in the crankshaft metal require some pretreatment to remove?

                    What are your thoughts on HVOF applied to nitrided parts?

                    In terms of oil holding capacity and finish, I assume this means that 2016 HVOF results in a working finish, ie no machining afterwards?

                    So many questions! I would be immensely grateful if you would guide the experimentation on the Cheetah crankshaft, all in good time...

                    Comment

                    • Vega ECM
                      Rank 5 Registered User

                      #11
                      Originally posted by powerandpassion View Post
                      Vega ECM, I am a lost lamb in the woods on this, but want to learn.
                      Can you confirm that HVOF is accepted in the serious side of the game.

                      Um, what is Ra-Rmz?

                      The particular interest is vintage crankshafts, most of them nickel chromium alloy. The back of the cereal packet says that bonding between substrate and coating was the first issue of concern. Can you confirm that 2016 HVOF has resolved this ? Is it necessary to machine to original surface to allow HVOF to fix better?
                      Does impregnation of oil and other contaminants in the crankshaft metal require some pretreatment to remove?

                      What are your thoughts on HVOF applied to nitrided parts?

                      In terms of oil holding capacity and finish, I assume this means that 2016 HVOF results in a working finish, ie no machining afterwards?

                      So many questions! I would be immensely grateful if you would guide the experimentation on the Cheetah crankshaft, all in good time...
                      Let's take it one question at a time;-

                      Can you confirm that HVOF is accepted in the serious side of the game?.- Yes there's lots of HVOF used on military and commercial aircraft parts. It's replacing Chrome plate which is now regarded in Europe as another nail in planet Earth coffin.

                      what is Ra-Rmz?;- These are the engineering terms used to describe surface finish. Try google whacking "surface metrology" as there's far too much to this subject to type here.

                      HVOF Coating Nickel based steels or nitrided surfaces?;- No problem, it's done routinely

                      Does HVOF need post application finishing?;- Yes after coating it's rough, really rough (about 180 grit) so needs to ground back and polished. It's largely this step which decides how long the surface will last in service, so this where you need the to apply the science of surface metrology. There's a good level of experience building up with this process but I'm still seeing quite a bit of trial and error to get the final surface just right.

                      Oil surface loading? HVOF is very hard and will act as an abrasive in a application where you have a sliding bearing contact . So on say a cam lobe contact surface it's very critical that you hold lubricant within surface pores created (more left by actually) during the finish grind and polish.

                      Good luck and happy to help
                      Last edited by Vega ECM; 13th November 2016, 09:45.

                      Comment

                      • powerandpassion
                        Never Be Afraid to Ask

                        #12
                        Thank you! Very interesting stuff. Very encouraging that HVOF is being used. Will take a few months to set up the test, strip down the Cheetahs and look at all the crankshafts, bearings. Will pop back up with this in 2017. Very, very interesting....

                        Comment

                        • powerandpassion
                          Never Be Afraid to Ask

                          #13
                          Here we go, some sacrificial lambs lined up. Cheetah Fits and Clearances courtesy of ANAM Archives Moorabbin.
                          Found a local HVOF aerospace workshop, so, in theory, it's all possible. Just have to get engines dug up from paddocks working.

                          Been reading books and thinking - what is supposed to supply the ideal surface for the lubricant : the bearing or the crank journal (surely not?)
                          Don't you want the sacrificial bearing to wear some nice grooves rather than the journal? Reading something about the theory of white metal bearings where the more resistant parts of the matrix stand up like goosebumps while the softer metal wears away, allowing the oil to do its sinewy dance through the bearing metal and around the journal.

                          Or do we want pickup on both surfaces ?
                          Why do journals wear anyway? Surely the bearing is the softer metal? Bits of aluminium crankcase have to be softer than the journal. Is it dirt in the oil? I get the sands of Libya being ingested through open carbs. If it's little bits of hardened steel from wearing roller bearings why don't aircraft engines have magnetic sump plugs or traps?

                          There's a bloke I know that hand pumps oil before start until he gets a pressure reading off the crank before starting. Never blown an engine. Then there's another bloke I heard about that paid big bucks for a Merlin, got his mates around, opened some beers and kicked the thing over to full revs, soon laying a leg outside the block. Most damaged bearings and journals seem to be oil starved rather than anything else, dirty oil, poor pressure, blocked orifices, too much beer.

                          I think the journal, subject to the oscillating pressure via the conrod, wibble-wobbles through the clearance between bearing and journal, while retained by the bearing cap. There is more pressure in the middle of a V12 crankshaft and less at the front and back, but better oil supply at the end it is fed in. For a radial main bearing, the wibble wobble must be more extreme. The V12 middle journal does more work than the outer ones, more wibble-wobble. I imagine oil is squeezed around rather than oil pump pressure fed around a journal : the pump gets it there but the squeeze comes via the conrod and the wibble wobble. The journal is a kind of lobe in a lobe pump, but the lobe is imperceptible unless you shrink to ant size and get sucked into the oil pump and carry a bright torch. I am thinking out loud here, and drinking absinthe....

                          So smooth journal or not? I am thinking the original journal was very smooth.

                          Merlins took DTD109 in 1940, what has happened to aero engine oil since then that gives better Merlin engine life? Crikey, I had better look up Cheetah oil, maybe they were designed around castor oil...
                          Attached Files

                          Comment

                          • Dev One
                            Rank 5 Registered User

                            #14
                            And you have not mentioned the cooling or viscosity effect of the oil.........maybe that will come with the castor oil research?
                            Keith

                            Comment

                            • TonyT
                              Rank 5 Registered User

                              #15
                              I still cannot get over why you would choose to possibly screw up what are getting rare engines, why do you not simply find a knackered lycoming and use that, their cylinders are nitrided, which incidentally you cannot renitride, however you can chrome plate them to restore them, you just need to make sure you get the right rings for it as the standard ones will strip it..

                              https://www.google.co.uk/url?sa=t&rc...hl6PBfD0JJCVdA

                              Comment

                              • powerandpassion
                                Never Be Afraid to Ask

                                #16
                                Originally posted by Dev One View Post
                                And you have not mentioned the cooling or viscosity effect of the oil.........maybe that will come with the castor oil research?
                                Keith
                                I thought the introduction of the term wibble-wobble into rheological discourse was enough for one day.
                                I am sure temperature and related viscosity are great influences. I seem to recall stories of groundcrew heating oil over a gas burner in winter to allow a quick fill and start for scrambles. I think the Soviets just lit fires under their engines.

                                What is proposed is test and control - one Standard engine and one with a HVOF crankshaft, run simultaneously with identical oil and fuel, identical atmosphere and identical operator incompetence. What I would love to find is an original AS engine test chart for Cheetahs, to understand duration, revs, fuels and oils used in original acceptance tests. Run the same test, can't argue with that.

                                Comment

                                • powerandpassion
                                  Never Be Afraid to Ask

                                  #17
                                  Originally posted by TonyT View Post
                                  I still cannot get over why you would choose to possibly screw up what are getting rare engines, why do you not simply find a knackered lycoming and use that, their cylinders are nitrided, which incidentally you cannot renitride, however you can chrome plate them to restore them, you just need to make sure you get the right rings for it as the standard ones will strip it..
                                  Lycoming will run different metaullurgy and crank pressures are different and many 1930's cranks are not nitrided. I can only half stuff up because one engine will run stock as Control.
                                  So I wish to demonstrate that a 2016 process, not within the OEMs 1930's repair scheme, that will work with 1930's British metallurgy on a crankshaft subject to X pressure.
                                  if it works, THEN I might try the process on an excruciatingly rare crankshaft that is worn beyond limits, like a Napier Dagger or Bristol Jupiter crankshaft trawled out of the sea.


                                  I guess I want to create a scientific dataset to engage in a conversation with a regulator using pertinent data to find a pathway over the next decade for keeping rare aeroplanes flying.
                                  So, once your rare Cheetah crankshaft is worn beyond limits, beyond plating, what do you do? I think ALL of these engines were worn back then anyway. No mint Cheetahs laying around. The real problem is that some of the beautiful vintage stuff flying today can't in the rapidly approaching future, unless some modern techniques are introduced to keep them safely flying for decades to come.

                                  Stop grumbling and rebuild an Anson airframe and wings, and I might show you how to keep the engines going for 50 years. Might have to cast some new pots though, and the magneto coils have probably turned to chewing gum too. Few bits of silicon aluminium in there turning back into bauxite as well. Hoffman roller bearings unavailable. Maybe you are right ! Leave it alone ! Too hard. Siddeley was a grumpy old coot! It wasn't a fabulous engine. Why invest? To learn.

                                  Comment

                                  • powerandpassion
                                    Never Be Afraid to Ask

                                    #18
                                    Gordon waxes

                                    Now you got me fired up!

                                    On the front engine you will notice no exhaust gear. How it came. Corks where the exhaust rocker arms came out. Apparently they were used in the 60's as air compressors on diesel locomotives. Diesel accessory drive connected to prop shaft, so the Cheetah was driven through its own shaft. All accessories stripped of back of Cheetah, air inducted through carb, into inlet, out to exhaust manifold which had a one way valve on the end. Some inefficiency, but if you follow the Otto Cycle and firing order it worked, obviously better than the original air compressor on the loco. Lots of swept air, cheap war surplus engine. So I did not feck this engine, it was fully fecked over last century. I will try and unfeck it. Most of these engines come out of paddocks where they have been shet upon by birds and used by farmers for shotgun practice. They are junk already. As Gordon Ramsay would say, get fecking real!

                                    Comment

                                    • TonyT
                                      Rank 5 Registered User

                                      #19
                                      So, once your rare Cheetah crankshaft is worn beyond limits, beyond plating, what do you do?

                                      you manufacture new ones


                                      and I might show you how to keep the engines going for 50 years.
                                      I am licensed you know and hold 6.3 ( ALL piston engines)

                                      Comment

                                      • powerandpassion
                                        Never Be Afraid to Ask

                                        #20
                                        Tony, I apologize for my Ramsayesque outbursts. I have been watching a few episodes and Mr Ramsay is a lurid exponent of that art what used to get your mouth washed out with soap. I lost my cool and the dignity of the forum has suffered for it. The forum is not a kitchen !

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