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

  1. #11

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    In the built up cranks that go into my drag bike engine, I don't turn the pins or the crank parts but push them apart and rebuild them with different bearings, and what I do to keep them indexed is drill down the side of the pressed in crankpin and put a roll pin that intersects the crankpin and the web, then weld on top of it.
    If it goes out of index after that, it probably bent rather than slipped and is toast anyway.

  2. #12
    Join Date
    Mar 2008
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    Barrie, Ontario, Canada
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    Something very important that I forgot to mention in my first post---I never drill and ream the crankshaft webs as separate pieces. I stack all of them together and either use a machinists clamp or sometimes even a bit of weld to hold them together as if they were one single entity. That guarantees that you are not building in a bias for misalignment of the crankshaft. Also on the sections of crankshaft or conrod journal that assembles with the web plates, I make them about 1/2" longer than they will end up at and using 280 grit sanding strips I put a slight and gradual taper in that 1/2" that is going to be trimmed later. This ensures that the shaft doesn't start in crooked when pressing into the web plates. The extra length is trimmed on the bandsaw later. Since I use 638 Loctite on all of my pressed together crankshaft joints, I don't recommend using an abrasive wheel in an air tool for this trimming, because the heat it generates can weaken the Loctited joints nearest to where you are cutting
    Brian Rupnow

  3. #13
    Join Date
    Jan 2004
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    Missouri
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    26,230

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    Having made a couple pressed/shrunk single crankshafts, it seems as if you still need to machine them for alignment. The advantage is in not hogging off so much material, and the resulting chance to use a grinder for finishing (which I have not done).

    I was not impressed with the alignment, even though it was a 3/8" web, and I took a lot of trouble and care to get it aligned.

    Others must agree, since I saw a set of pictures from the Doxford plant in the UK. They assembled their marine diesel crankshafts from huge webs and "pins", but then machined them. Of course, they wanted a proper finish and so forth, but the process would also tend to ensure alignment.
    1601

    Keep eye on ball.
    Hashim Khan

  4. #14
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    Mar 2005
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    Toronto
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    just my opinion, for model sized work, turning from the solid is always best. You get everything perfectly aligned, round and to dimension with a proper finish, don't have to worry about the high accuracy required of press fits or shrink fits or splitting a web. Multi cylinder or single, same deal. The exception might be the simplest crank - the 1 web variety.

    When you get into larger engines and from the solid isn't practical, built up makes sense. If any haven't seen it, Peter Cowie has done a fantastic job of documenting a real man sized project, the Elliot Bay Triple. Here's is his really good discussion on crank shaft options - read this to understand built up crank options and considerations. Read the whole site to see what a determined home shop guy can do. A keyed shrink fit would be one good option, but for small stuff....make it from the solid

    http://www.steamboat.com.au/EB%20Tri...crankshaft.htm

    I'll put this up again, photo of the Stuart triple crank I made. Counterweights added after the crank was turned (and milled)

    .

  5. #15
    Join Date
    Dec 2015
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    Chilliwack, BC, Canada
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    That's a nice neat job Mcgyver. How did you bond or otherwise lock the counterweights into position?

    A buddy of mine used to build up new crankshafts for his and a few other Fox .36X combat engines. Think "2HP+ in the palm of your hand". RPM's of these was up around 18K. He built up the main journal and the web but used dowel pins pushed into a hole for the crankpin. I recall he used something really crazy like .002 on even this small a size. But he said that when he tired less it allowed the pin to come loose or cock off to the side or some other failure. I can't recall what he used for the actual crank but I'd guess it was something like this 1144 or perhaps something tougher and harder. The originals used to shear off at the square cut crankshaft inlet port. Which is why his ports were reamed axially with a round nose end mill and the port cut using a method that maintained the round and smoothed transition. I know that "tough and spring like" was way more important than "hard" in this case. I was still surprised that the pin didn't cause the crank web to split though. The hole for the pin being really close to the OD of the web.

  6. #16
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    Mar 2005
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    Toronto
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    Quote Originally Posted by BCRider View Post
    That's a nice neat job Mcgyver. How did you bond or otherwise lock the counterweights into position?
    .
    thanks. The thought was glue the counterweights on, their function is almost entirely aesthetic on such a small engine at steam rpms. Off the mill the fit was so nice I just sort of finger pressed them on, and there they are, 10 years later.

    Peter Cowie gives an account of splitting the web - the strength of material around the press or shrink fit is absolutely the main factor on how much force you get acting on the pin.
    Last edited by Mcgyver; 10-07-2017 at 02:45 PM.
    .

  7. #17
    Join Date
    Mar 2010
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    I have always used solid 1144 for model crankshafts and have had good results. I have made 1 throw, 2 throw, and 4 throw shafts with no problems once fitted in the engines and running. All were aero engines that peaked at 7200 to 13000 RPM. Sorry I don't have any pics.

    The only built-up crank was for Hodgson's 9 cylinder radial and it has a single throw made from 5 pieces pinned together with taper pins. If I had to make another, I would make it from 2 pieces turned from the solid with only one clamp joint on one end of the rod journal. It would take a big piece of 4140 and leave a big pile of chips, but it would be stronger and easier to achieve good alignment.

    RWO

  8. #18
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    Aug 2007
    Location
    Custer WA
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    447

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    Quote Originally Posted by MrFluffy View Post
    In the built up cranks that go into my drag bike engine, I don't turn the pins or the crank parts but push them apart and rebuild them with different bearings
    I picked up a 1983 Suzuki GS1100E with a roller bearing crank like you are describing for a winter project. I would imagine your application is the ultimate test for a built up crank.

  9. #19
    Join Date
    Jul 2001
    Location
    Green Bay, WI
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    3,173

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    I have made cranks out of 4140 tool steel without problems.
    I use a jig to machine it. It really is quite easy and fast to do using a aluminum fixture to hold the throw positions

    Rich
    I have tried for the past hour to upload pictures here, but have given up . The HSM website is not conducive to this unfortunately
    So I have it with Photobucket images . ???





    <a href="http://s273.photobucket.com/user/StationarySteam/media/Shop%20tales/Panther%20Pup/PA080315_zps6nzqe0s5.jpg.html" target="_blank"><img src="http://i273.photobucket.com/albums/jj220/StationarySteam/Shop%20tales/Panther%20Pup/PA080315_zps6nzqe0s5.jpg" border="0" alt=" photo PA080315_zps6nzqe0s5.jpg"/></a>

  10. #20
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    Jul 2001
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    Green Bay, WI
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    I think it is self explanatory , but questions are welcomed.
    This is a very stable way of handling crank construction.
    Rich





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