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  • Briggs engine, cam failure and cure

    Had first core engine failure at 160 hours, camshaft broke in the middle of the span, probably a function of high lift, high ratio rockers and stiffer springs, continuous 4000rpm for two hours before it went.

    Doing research, turns out Precision Cams in the USA reinforce theirs with a steel rod down the middle; measured up the parts (see attached diagram) and the best route appears to be to bore the centre 11mm and insert a silver-steel rod down the middle, retained using Loctite.

    This route is simplest I can think of, you can buy 11mm ground Silver Steel (comparable to Drill Rod) on eBay in short lengths from model engine suppliers, an ordinary 11mm diameter drill will go 115mm or so while still keeping enough to grip it in a drill chuck.

    According to measurements the bore is 10.4mm so running it through to 11mm should be easy enough, as it is iron it shouldn't grab as easily as drilling steel out small amounts.

    To lock the rod in place intend using Loctite 620; already discovered as have another Briggs pilot that the cam gear on the crank tends to wear the key and fret on the crank, a drop of this stuff left to cure locked the gear solid on the crank, preventing another potential failure long-term. Temp range is up to 200C or 392F for cheap alternatives, 230C or 446F for Loctite; I'll go for Loctite...;-)

    Only issue now is how to remove the ball carrier sleeve, may be best to bore right through and use longer rod so it can be gently punched out and replaced

    Cheers

    Kev

    Attached files briggs-3864rev-s-camshaft-close-up.pdf (17.1 KB)
    G-KEVA
    BMAA 5696

    "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

    R.J. Mitchell :- Designer of the Supermarine Spitfire

  • #2
    Briggs engine, cam failure and cure

    Kev,

    As the rod will be going in to a blind end, do you intend to put a flute along the rod to allow the air out? If you don't you may have problems trying to get the rod right to the end of the hole.

    Best regards,
    XL's forever! Well, one of them anyway. It's all I can afford, not to mention the Raven and the Mini-Max. Oh, and I almost forgot the Spectrum as well :-)

    Comment


    • #3
      Briggs engine, cam failure and cure

      Had a 4am epiphany Bob

      Am going to get full length bored 11mm, then insert 11mm silver steel rod all the way to the spring seat; this allows easy removal of the ball and sleeve at the other end, or even can be left in place if the lathe operator is careful

      cheers

      Kev
      G-KEVA
      BMAA 5696

      "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

      R.J. Mitchell :- Designer of the Supermarine Spitfire

      Comment


      • #4
        Briggs engine, cam failure and cure

        Kev,

        At this rate you'll be having your own complete camshaft made out of whatever material you think will give the best performance and longevity, with high tensile strength to prevent future snapping. Perhaps a billet has your name on it, who knows? :-)

        Best regards,

        P.S. note the new signature under the line?
        XL's forever! Well, one of them anyway. It's all I can afford, not to mention the Raven and the Mini-Max. Oh, and I almost forgot the Spectrum as well :-)

        Comment


        • #5
          Briggs engine, cam failure and cure

          You can buy billet steel cams Bob, but Al Hodge is making me one with a gentler profile to produce power closer to my cruise revs

          Looking forward to that ;-)

          Kev
          G-KEVA
          BMAA 5696

          "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

          R.J. Mitchell :- Designer of the Supermarine Spitfire

          Comment


          • #6
            Briggs engine, cam failure and cure

            It may well move the crack in the cam to where the bore gets bigger for the spring , with a rod in.

            One common problem with some rocker gear is both putting a high lift cam in together with lightened rockers and competition springs. What can happen is the rockers are given a lot more work than the standard items and the flex results in less control of what the valve is experiencing so you don't get all the cam at the valve plus a greater chance of breakage.

            I would have thought you don't want a power cam but more a torque cam, in road terms a rally cam because you have basically a constant speed motor.

            In the good old days we sometimes found the plot went faster if you removed the competitive springs and run with sports ones presumably as we were not losing power trying to bend all the bits.

            You sound as if you are working towards this with your new cam.

            Just a thought
            Mick Broom
            Member 909
            Shadow G-MWTN

            Comment


            • #7
              Briggs engine, cam failure and cure

              Spot on Mick, you're one of the bloke I pay keen attention to.

              The risk is the stock motor on Page 20 of the Briggs Workshop manual is rated at up to 4250rpm, when used in certain applications that don't use the inbuilt mechanical governor, I believe this is generator or welding sets.

              So technically as Al's motors have peak power at 5800rpm the valve train is the first thing you upgrade, but ironically loading the cam to a higher level; the tuning kit includes two packing washers to stack under each valve spring, I might remove one of each to reduce the loading on the cam

              The substitute cam makes peak at 5000rpm, Brigg's stock cam peaks torque at 2600 and hp at 3600-3800 and falls off from there, but I don't want to use the stock cam as it has even more holes in it for the decompressor rig.

              Having got 160 hours out of her bear in mind I was partly proving the unit to see what would break it, and have done quite a few climbs to 6500-8500 at full power to prove it wasn't time limited, so easing the pressure on the valvegear might be just the ticket, one outfit in the Netherland has over 500 hours on theirs pushing paratrikes.

              Cheers

              Kev
              G-KEVA
              BMAA 5696

              "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

              R.J. Mitchell :- Designer of the Supermarine Spitfire

              Comment


              • #8
                Briggs engine, cam failure and cure

                Check on the initial acceleration of the tappet/follower off the Cam ramp as it opens. I don't know it the Cams are standard and re-ground, but if they are, it is not uncommon for the opening ramp to be too steep, giving greatly increased acceleration and stress. Taking the standard cam as being reasonable, compare opening acceleration with the high lift cams; DTIs and degree disc to plot lift/opening is sufficient.
                Also in a normally tuned engine with the correct valve gear, there is some degree of asymmetry in the valve opening in running, in that at higher speeds the tappet is not in contact with the cam exactly after peak lift - it 'ski' jumps off the peak - I've measured it on racing engines back in the day. For this reason, the highest stresses on the camshaft are usually at low speed and highest peak wear too, where the cam takes fuller pressure an stress reversals from the spring.
                'Tuftriding a Cam will double its 'strength' http://www.keighleylabs.co.uk/heat-treatment-html/tufftride-ferritic-nitrocarburising/ We used to Tuftride nearly ever ferrous part in racing engines - often standard parts, but now subject to 4 times the power, with no failures. Tuftriding can be done to finished components, whereas Nitriding is risky.
                The Silver steel rod is a good idea, but could benefit from light knurling, or at least the surface emerying, to give the Locktite better grip. I suppose oil flow down the centre is not a factor.
                Hope these few thought help.

                Comment


                • #9
                  Briggs engine, cam failure and cure

                  Make sure any heat treatment is done before Loctiting in the rod ;-)
                  Is the camshaft rough finished? If so, a good polish might slow crack development.
                  It's torsional stresses that are causing the problem, I assume.
                  Dave

                  Edit: Just realised you'd posted a drawing.
                  So the crack propagated from the section change at the root of the second cam.
                  I'd have expected it to go at the internal bore change from 11.5 to 10.4 - though it may be more chamfered than your drawing.
                  If you stiffen the 10.4 bore, you might just shift the crack to there.
                  Would be really nice to have a stepped rod that penetrates up to the gear...
                  How tight is the machining at the cam roots?

                  Comment


                  • #10
                    Briggs engine, cam failure and cure

                    Dave Smith wrote: Make sure any heat treatment is done before Loctiting in the rod ;-)
                    Is the camshaft rough finished? If so, a good polish might slow crack development.
                    It's torsional stresses that are causing the problem, I assume.
                    Dave

                    Edit: Just realised you'd posted a drawing.
                    So the crack propagated from the section change at the root of the second cam.
                    I'd have expected it to go at the internal bore change from 11.5 to 10.4 - though it may be more chamfered than your drawing.
                    If you stiffen the 10.4 bore, you might just shift the crack to there.
                    Would be really nice to have a stepped rod that penetrates up to the gear...
                    How tight is the machining at the cam roots?
                    Thanks Dave

                    Researching Loctite 620 high temp shows it has a performance graph for gap-filling up to 0.38mm; that means a clear through of 11mm will still give some support in the 11.5mm region, as the max diameter would be 11+0.76mm. I need to keep the 11.5-11mm step as the spring seating washer rests on it, although if the rod length is carefully measured and cut the spring/washer can sit on that.

                    Initial plan is to insert the Silver Steel rod horizontally as this would avoid hydraulic-locking if fluid gets ahead of the tight fitting internal reinforcing rod, blathering the rod as it slides into the bore, press home hard then clean out the bore where the spring sits to avoid clogging or restricting it. 24 hours in a warm place and install

                    It's going to strengthen the cam at the mid span point anyway.

                    The other change intended is the cam failure looks like a combination of high lift cam, higher ratio rockers and stiffer springs, as the first two improve power the stiffer springs, which allow up to 5800 rpm or more, seem superfluous at my 4000 cruise and 4250 max in a climb. I'm removing one of the two 1/16" packing washers that the high rev kit provides for under the valve springs. They are steel with a grooved surface, perhaps to limit spring rotation wearing the alloy head?

                    At an early stage the valve lift was so great the caps crushed the springs on the valve stem seals, inlet side, it is recommended to remove the seals altogether as oil burn is tiny even without them.

                    Cheers

                    Kev

                    Attached files
                    G-KEVA
                    BMAA 5696

                    "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

                    R.J. Mitchell :- Designer of the Supermarine Spitfire

                    Comment


                    • #11
                      Briggs engine, cam failure and cure

                      The location of the fracture and its relationship to the section geometry suggest that it is a bending failure rather than torsional. This is based on the observation that torsional load peaks would be the same over the length between the drive (RH end in the drawing) and the opposite end cam. Torsional load would be expected to cause a failure at the location of minimum moment of area. Bending loads will be greatest at the midpoint of the distributed stiffness between the supports (bearings).

                      The surface of the original forging (in the photo) is rough as a badgers ar*e - each of the pits in that surface is an initiator for a crack. There is much to be gained in fatigue life by machining the areas between the cams with large blend radii and a decent finish (Ra 0.8 or better).
                      Pete T.

                      "A closed mouth gathers no feet".

                      Comment


                      • #12
                        Briggs engine, cam failure and cure

                        All true, but always aim to make mods and fixes accessible to the 'average' tech minded bod; at 62 years of age will be 66 before another four years of flying about passes by, to achieve 160 hours of testing (that's when it failed). So whilst polishing the cam is a great idea of yours, I want to find out whether the simple expedient of drilling and stuffing the core with high tensile 800N/mm rod will do the trick as this is something anyone can do with a B&Q pillar drill, 11mm drill bit and a bottle of Loctite 620. It's also what Precision Cams do in the USA and they're specialists.

                        Annoyed at this bad luck as Parazoom with similar specs have 500+ hours, however the habit of proving the motor by long distance continuous high-ish rpm might have been too ambitious.

                        It's fate; the under panels in a certain area of the wing Dacron contaminated with Synthetic oil can be torn easily by hand after the prang; it's normal UK grade stuff, so maybe the prang saved something worse? If I'd landed safely, washed the thing and repaired the motor maybe that would have failed, so a new wing entire on order

                        Thanks Peter

                        Kev
                        G-KEVA
                        BMAA 5696

                        "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

                        R.J. Mitchell :- Designer of the Supermarine Spitfire

                        Comment


                        • #13
                          Briggs engine, cam failure and cure

                          Probably talking out of the wrong end again as I do not know the Briggs, but as Peter indicated, the failure does look rather as if it could be due to bending/flexing of the cam shaft.
                          Back in the day I used to have similar cam shaft problems on my racing Ford 3 bearing AngleBox engines... Problem was due to the cast aluminium cam support pillars "stretching" under the cyclic valve loads... (similar thing used to happen with the hollow crankshaft and cast main bearing caps......... and the somewhat "fragile" con rod bearing caps).

                          Comment


                          • #14
                            Briggs engine, cam failure and cure

                            You have to be careful in polishing. You can actually make things weaker, because existing surface hardening will follow the ups and downs of the surface, so that polishing will cut off the peaks. If you do polish, for max toughness the item should be shot peened.
                            Back in the day, Cosworth and Holbay used to have steel rocker posts and steel bearing caps. We used to put steel backing plates on the main bearing caps on Mini engines too.

                            Comment


                            • #15
                              Briggs engine, cam failure and cure

                              Ted, sounds like we are both "of a certain age".... to resolve my problems I copied Cosworth and always fitted "home made chunky" steel rocker pillars, machining flats on the main bearing caps, and bolting "massive chunks of steel" (weight was not quite so much of a killer as on aircraft) across with a few thou "pinch" to the cap.... similarl smaller version on the rod BE caps.. what a %** to get balanced..... the rods etc were polished.... partly to reduce drag and then as you point out.... "shot at!!!!

                              Sorry all...... a bit off topic, but nice to remember "the good ole days". I shall now retire back into my normal drunken stupor:-)

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