Metals choice for up scaling strength

Discussion in 'General Engine Discussion' started by Naiveambition, Sep 15, 2018.

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  1. Sep 15, 2018 #1

    Naiveambition

    Naiveambition

    Naiveambition

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    I'm building a Kerzel at double scale and my guess it size wise I will end up about 1/4 scale of other hit and misses. I think with the added compression and so forth, if I should upgrade?. Original version calls for alum. I would prefer steel but don't know a suitable one to pick . Will have bronze bearings though. 6 inches long x 3/4 roughly

    Second part is cylinder head. Cast iron? Steel? Initial thought is 1214. Any thoughts. 1x3 is this peice

    And thirdly is the crank. What type of steel is best suited for ease of machineing and very good strength and not require grinding. Although if greatly better I may look into a temp setup. Will end up about 1 inch thick but 9 inches long
     
  2. Sep 16, 2018 #2

    Cogsy

    Cogsy

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    1144 stressproof is very good for machinability and reduces the chances of distortion from release of internal stresses. Should be perfect for a crank. I don't know what piece/s you're talking about changing from aluminium to steel, but aluminium is quite a strong material with many full-sized engines made almost entirely from it. My guess is most of the materials called out in the original plans would likely be fine at double scale. Certainly I can't see any issues with aluminium for the cylinder head.
     
  3. Sep 16, 2018 #3

    Naiveambition

    Naiveambition

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    Thanks for the reply, The con rod being alum I'm sure would be fine as you state, more of a personal liking to a mill engine style shape in steel.
    I've been searching for my supply's and I swear every part I need x2 in size is a 100$ give or take. Prelim estimates ar around 4-500$ for all needed stock. Stress proof is cheap under 2" but but climbs fast.
    So might resort to flat bars for some vs rounds.
    Given the pricing dilemma I'm pondering changing most of the brass to stainless and polish
    So here's what I think so far
    Body is alum
    Piston and liner is cast iron
    Con rod is 12l14
    Tool steel for cam
    Crank will be built up using stainless for webs and 1144 for rods

    The governor and it components are brass, at this size it gets very $$$$
    So am thinking of switching to stainless for all. Am wandering if the weight difference between the two if it would create a abnormal running.
     
  4. Sep 17, 2018 #4

    Naiveambition

    Naiveambition

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    Doing some research I came across a issue of putting stainless and alum together. Evidently the electrons travel between each other, with the metals acting like a cathode, and anode. Does this only count when parts are touching, or just close to each other. I plan to use stainless for the governor system and pushrod, valve rocker, and valves in a steel head.
     
  5. Sep 17, 2018 #5

    WOB

    WOB

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    The con rod is better made from 2024 alum. Much lighter, more than strong enough and has good bearing qualities so that no bronze bushings are needed.

    Before machining cast iron piston from round stock, stress relieve the blanks at 1200 F for 1 hr. per inch of thickness. Otherwise, you may well find that the piston is no longer round after hollowing it out for the con rod.

    WOB
     
  6. Sep 18, 2018 #6

    Naiveambition

    Naiveambition

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    Thanks for the tips, I've not built a motor to this size so learning as I go With weight savings and some easier machining , and polishing I think 2024 alum will work out great. The price does vary quite a bit between sellers though.
    The only part I need to narrow down now is the head. With the heat, compression etc. would their be a case for thermal expansion to the point of causing leaks using certain steels.
    Also looking to possibly change weights to a ball vs flat disks 11$ gets me the 1" balls and 90$ ish for the 2 inch bar to make them.
     
  7. Sep 18, 2018 #7

    redryder

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    Just a quick note..... Scaling up the entire project by 2 times will not change the compression ratio. Best of luck with your build.
     
  8. Sep 19, 2018 #8

    comstock-friend

    comstock-friend

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    Regarding strength, as the dimensions double, areas are going up by the square, volumes (and mass) by the cube. So forces due to mass are cubed, but the areas to resist stress are only squared. Still the design is probably hell for stout, seeing that the original was likely scaled down from full size so the scale up from the model is still favorable from the square/cubed law standpoint.
     
  9. Sep 19, 2018 #9

    WOB

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    There is no need whatever to make the piston from cast iron. 6061 or 2024 alum with cast iron rings running in the cast iron cylinder is an ideal combination. Much lighter making vibration much less. I also disagree with the choice of brass for the wrist pin. 3/16" steel drill rod floating in a 0.188 reamed hole ( piston and rod) will last much longer. It doesn't even have to be hardened. Make it several thou under the piston dia., polish the ends and it doesn't need end caps.

    WOB
     

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