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Thread: Material choice question

  1. #1
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    Default Material choice question

    I'm hoping you guys can give me some ideas on materials choice. Sorry this is long. Some background:

    I'm working on a bracket on my recumbent trike. About 100 lbs fully loaded and my 175 lbs. There is a spacer between two pieces of metal on the Sturmey Archer drum brake assembly. It's round, measures 10 mm in diameter and length, and is made of hard plastic or black nylon. It has a stainless steel machine screw going through it to hold two pieces of metal plate in their relative positions. The spacer deforms under pressure and the whole assembly flexes when braking.

    I don't like the flex. Especially going downhill at 25 mph. I emailed the manufacturer and they indicated I could stiffen it up with a piece of metal spacer. However, they were pretty nonchalant about the plastic. I think it is because they normally install disc brake systems and the drums were a special request. So my confidence in them on this one issue is not very high.

    So my question. I've fabricated a steel spacer. It's pretty heavy. How will aluminum perform with cyclic compression? And is the failure mode deformation or cracking. Or no way to know as a practical matter? I'm inclined to go with steel but it is "just" a spacer. But it is the brakes. I'd love to save the 1-2 ounces in weight.

    Any thoughts appreciated.

    Regards

    Tom

  2. #2
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    Default Re: Material choice question

    You realize where you are asking, right? Of course you should use Silicon Bronze.

  3. #3
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    Default Re: Material choice question

    10mm dia is "heavy" in steel? I'd go with alum, but I'm not so nitpicky as to say 10mm steel is "heavy". I used to do this material choosing stuff for a living.

  4. #4
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    Default Re: Material choice question

    So use 6061-T6 aluminum, it'll be fine.
    "For a successful technology, reality must take precedence over public relations,
    for nature cannot be fooled."

    Richard Feynman

  5. #5
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    Default Re: Material choice question

    According to the calculator here: https://www.steelexpress.co.uk/steel...alculator.html

    A steel round bar 10mm in diameter and 10 mm long would weight 0.01 lb. That said, I would think that aluminum would be fine.

  6. #6
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    Default Re: Material choice question

    Use of titanium will allay your concerns about cyclic loading

  7. #7
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    Default Re: Material choice question

    Make an aluminum one and carry the steel one as a spare....

    R
    Sleep with one eye open.

  8. #8
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    Default Re: Material choice question

    7075 T-6 Aluminum

  9. #9
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    Default Re: Material choice question

    You are looking at about 6 vs 2 grams, steel vs aluminum, fwiw, are you that much of a weight weenie?
    Steve Martinsen

  10. #10
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    Default Re: Material choice question

    Ha....Crap! I misread the little digital scale. Dropped it on there and read 1.8 oz. I ran with it. I just double checked. s really. 0.18 oz. Early onset.

    But...I did find some big aluminum ferrules that will work great.

    Thanks all and sorry bother.

    I did, however, save 0.5 grams.

    20180516_192058.jpg
    Last edited by bluedog225; 05-16-2018 at 07:23 PM.

  11. #11
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    Default Re: Material choice question

    Quote Originally Posted by Ron Williamson View Post
    Make an aluminum one and carry the steel one as a spare....

    R
    I am constantly gobsmacked at the brilliance of Canahoodians <G>
    David G
    Harbor Woodworks
    http://www.harborwoodworking.com/boat.html

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  12. #12
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    Default Re: Material choice question

    6061 aluminum should be fine, it's high strength. You usually don't need steel unless hardness is an issue, however it is 3X as stiff as aluminum. If you used steel, you'd want it to be stainless, however stainless steel has even worse heat transfer than carbon steel, so if the part gets hot, aluminum may be better. One AR10 rifle maker went from an aluminum barrel nut to steel for better heat dissipation.
    When you can take the pebble from my hand, it will be time for you to leave.

  13. #13
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    Default Re: Material choice question

    Use the steel and skip the cheeseburger.

  14. #14
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    Indian Land, SC, USA
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    Default Re: Material choice question

    Quote Originally Posted by bluedog225 View Post
    Ha....Crap! I misread the little digital scale. Dropped it on there and read 1.8 oz. I ran with it. I just double checked. s really. 0.18 oz. Early onset.

    But...I did find some big aluminum ferrules that will work great.

    Thanks all and sorry bother.

    I did, however, save 0.5 grams.

    20180516_192058.jpg
    Tom, if the part on the left is your original spacer, I can imagine why you were concerned about flex / failure.



    Rick

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