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Thread: Lets talk about Crankshafts--Machined, built up, etcetera

  1. #1
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    Default Lets talk about Crankshafts--Machined, built up, etcetera

    I have tried my hand at just about every kind of crankshaft construction I know of. Many successes, some that went terribly wrong, and some that I "made do". I have found that the absolute best material for a crankshaft machined from solid, is 1144 stress proof steel. Unfortunately, I am only able to buy it as round stock, so there is a lot of "hogging" to end up with a bar of flat material. It is a marvelous material, because as parts of it are machined away, internal stresses don't try and make the rest of it move or twist. I turn the material between centers, using a lathe dog to make sure it doesn't slip when I am machining it. This is an excellent way to make a crankshaft for a single cylinder engine with only one "crank throw" on it. There are numerous "how to" posts on the internet about machining a crankshaft from solid, so I'm not going to try and repeat it all here.
    I find however, that if you try and use this method to make a crankshaft for a double or triple cylinder engine, it is very difficult to keep things all moving "true", and you get into some very scary moments when turning the con rod journals with the tool stuck out a ridiculous amount.
    Many of the crankshafts I have made from "built up" pieces press fitted and Loctited together give satisfactory results, but there are pitfalls in this method as well. I traditionally use plain cold rolled steel for my crankshafts and crankshaft webs, and get good results. However, there is a caveat in doing this, and it has to do with "press fits". I have "on size" reamers, and reamers that are 0.0015" undersize. Cold rolled steel generally comes in .0005" undersize, so you don't really get a 0.0015" interference.--You get a 0.001" interference. I have always thought that .001" interference is not enough, so on many pressed together crankshafts I use "drill rod" because it comes in "on size" and you do get a full 0.0015" interference.
    --After building the most recent crankshaft in my "back to steam" thread, I am of the opinion that .0015" interference is dramatic overkill, especially if you are building to a design that has thin web plates (3/16" in my case".) I think that .001" interference would have been sufficient. The mere fact that you are pressing so hard to get pieces together sets up stresses in the main crankshaft that will cause it to move far out of alignment. When the pieces of the main crankshaft are cut out between the web plates, these stresses become less, but they don't completely go away.--I ALWAYS use Loctite when assembling a built up, pressed together crankshaft. The Loctite rep says that even though the majority of the Loctite will be pushed away by the press fit, enough remains at a microscopic level to strengthen the joint by at least 30% over not using Loctite at all.
    ---The problem I see with the crankshaft I just made, is that the web plates are too thin in relationship to the crankshaft diameter. There simply isn't enough material in a 3/16" plate to make a really sound inflexible joint with the mating crankshaft.
    ---In my opinion, if I was designing an engine from scratch with a built up crankshaft, I would try and make the thickness of the web plates at least to the same thickness as the crankshaft diameter.
    ---I "cross-pinned" the joints on my crankshaft with 1/16" diameter steel dowels, but with such a thin web plate I don't know if pinning the joints gives a more secure joint, or makes it weaker because you are removing 1/3 of the cross section of the web plate.
    --I am giving some thought to tapered reamers and tapered pins at the joints if I do this again, but I really have very little experience with tapered pins and reamers, especially with the small diameter (5/16" to 3/8" diameter) crankshafts I generally use.
    ----In my early days of crankshaft building, I silver soldered some built up crankshafts together, but the results were questionable. The joints were extremely strong, but the application of heat tended to pull the crankshafts "out of true" which sort of defeated the purpose.
    ---That is everything I have to share with you about crankshafts.---What do you guys find works best?---Brian
    Brian Rupnow

  2. #2
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    I would build an over-sized crankshaft out of whatever weldable material you want to use. Once your done with the rough crank, it will obviously not be straight, or true but as long as it's completely over-sized, it's just the basic outline of what you want.

    I would then chuck it up in a lathe and turn all of the outside diameters.. The bottoms of the ballancers, the main crank journals, and everything else that is relative to the center-line.

    I would then find a way to turn each off-center crank journal on the lathe so you can machine each one down to the desired size. As long as you measure the center point for each crank the same distance relative to the center line, then you'll have true crank journals once you cut them all down to the same size.


    So basically, weld up the general form of your crank shaft so that everything is plenty oversized so you can then machine everything down to desired dimensions. Kind of like pouring an over-sized cast and then machining all surfaces down to desired sizes and tolerances.
    When in doubt, doubt your doubt.
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  3. #3
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    Have you ever watched Forged in Fire a competition between knife makers? They give them some random scrap metal and they hand forge an incredible knife, sword, etc. I actually binge watched a half dozen shows because it was so interesting, and I HATE reality TV! I'll bet one of those guys could forge a real nice one piece crankshaft from minimal material, and heat treat it too.

  4. #4
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    Almost anything can be made to work on a model engine. If it would be high rpm chainsaw engine or something like that then shrink fitting would be way to go.

    Speaking of chainsaws: the aftermarket spares like needle bearings could be handy for model making, like:
    http://www.ebay.com/itm/Piston-Wrist...oAAOSwCQZZDfpI

  5. #5
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    This is how I would make a crank-shaft... Except instead of the original forge, I would weld it up. Everything else is easy to do, except maching down the off-center crank journals... Would have to build a special rig like they used for that

    When in doubt, doubt your doubt.
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  6. #6
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    I agree with all your comments about the various ways of making a C/S. I have found however, that machining a single cyl. C/S from a solid bar of 1144 is the most satisfying and, having the confidence that it is going to stand up in an IC engine.

    For steam engines that are run on air I have used Locktite (#680 with a slip fit) only and there have been no problems. On a 1" scale Traction Engine that is in the process of being built, I made the web section from solid and pinned the shaft (5/16") to it with 7/0 taper pins after Locktiting it together. Only then did I machine away the section of shaft between the webs.

    Geoff

  7. #7
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    3PhaseLightbulb--You are very quick with your answers and expertise. I have to ask--Have you ever done this?---Brian
    Brian Rupnow

  8. #8
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    Automotive forged cranks died out before 1980 I think, but over last 10 years, automotive cranks are more often forged.

    Cranks for the Marine Diesel/Electric propulsion packs up to 12 cylinder and 3 ~ 4 metre long ( 12 ~ 16 foot) are forged.
    These are made by starting with a bar, and progressively locally heating just the right length,
    for forging feature-by-feature in a special press. The bar is usually cooled before the next feature is heated.

    I often wondered if a skilled blacksmith could do that mini scale on an anvil,
    maybe against a partial closed die to form the blank journal diameter and the web walls.

  9. #9
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    Quote Originally Posted by brian Rupnow View Post
    3PhaseLightbulb--You are very quick with your answers and expertise. I have to ask--Have you ever done this?---Brian
    Yes I absolutely have not but would love to some day I would make mine out of mild steel

    EDIT: I'm now seeing the value in adding a 4th rotary table axis to my CNC BP.

    Yes, I think I need to get one of these CNC rotary tables

    Last edited by 3 Phase Lightbulb; 10-06-2017 at 08:41 PM.
    When in doubt, doubt your doubt.
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  10. #10
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    I really like working with 1144. It machines like 12L... almost but close enough.
    Best part is if you need to harden it you can fully harden it without having to draw it back. I think max hardness ends up at around 42 -48 RC. Something like that.

    But making a crank is a lot of hogging, especially if it's a big crank.

    JL.................

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