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Thread: ZRA Set Screw FAIL

  1. #21
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    Lightbulb Just fer Grins...

    Something like this could eliminate some cost and problems. Just a "Quickie" I whipped up, so it could/would need a bit of development, of course! Might not work at all?


  2. #22
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    And ANOTHER THING!!!!!!

    Pinch bolts.... Gawd they're TERRIBLE!!!!!!!

    Essentially, the Series 1 screw is a pinch bolt.

    When they are made, the screw hole (threaded or not) is perpendicular to the flat(s) that the head and nut tighten against.

    When you torque the screw, the gap is closed and the hole in one half is NO LONGER perpendicular to the flat on the head end!!!!

    So as you turn the screw under increasing tensile stress, it is being bent around, like you would do when you're trying to break off a piece!

    There are different ways to solve this, but they all cost more to manufacture.

    Every time I see images of those hyper-expensive designer chairs with unobtanium and carbon fiber everywhere, I wonder if they cut corners on these fastener applications, too, like all the other wheelchair manufacturers???

  3. #23
    Quote Originally Posted by a la carte View Post
    SCI, did/do you work for this wheelchair manufacturer?
    Nope. If I did, I'd have done everything I could to prevent them from using those screws in the components being discussed here.

    Quote Originally Posted by a la carte View Post
    The military and SAE are two big organizations who have had a need and the resources to develop specs for screw materials (among many other things). When saying "Mil Spec," you need to quote which spec!
    I would need to quote the spec if I were an engineer and knew better or if I was billing for my services. I've replaced many of my fasteners on my ZRc with fasteners comparable to the one in the drawing below...



  4. #24
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    Doing things the "best way" (engineering-wise), as might be done on spacecraft and high-end racin' cars, is usually prohibitively expensive. So, if you find that whatever material MC's screws are made of is suitable for your needs at an affordable price, Go For It!

    I feel fortunate for having the abilities to design and build my own solutions, which eliminates most of the cost of the "best ways," and then make my own unilateral decision as to whether I want/need to for any given project I'm working on.

    I'm not a myopic propeller-head who insists that things can only get done one way! That's why I can't work with most other engineers (just like everyone else - LOL).

    Cheers!

  5. #25
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    Quote Originally Posted by SCI_OTR View Post
    I've replaced many of my fasteners on my ZRc with fasteners comparable to the one in the drawing below...

    Note the Mil Spec (MS) and Nat'l Aerospace Std. (NAS) specs in the drawing's title block.

    Among the list of the screw's complete dimensions, properties, etc. in the referenced specs is the min. Yield Strength of 30,000 psi and ultimate Tensile Strength of 80,000 psi.

    So this gives you numbers to compare against SAE Grade 5 or 8 specs, for example. FWIW to you.

    By the way, the socket dimensions required are .188 to .190, which is why the (typically undersized and soft) 3/16" (.1875) wrenches strip out so easily.

    Usually, the OEM screws, as you have found, are of a much lower grade (cheaper) material and OEM suppliers will almost never admit the specs of their materials - their sales and engineering folks just say, "It's Aerospace Grade" or "Military Spec", etc... I know this from working for a company that did just that! I left after 6 months because of it.

    A few customers complained about the poor quality of fasteners, plating, etc., but the owners didn't care. Higher grade stuff would eat into their profit margin and it was very few complaints that they actually heard, after all. Who knows how many customers "jumped ship" because of the poor quality? No way to quantify that. They were and are in business for a very long time. Says a lot about what people really want to buy, despite what they complain about!

  6. #26
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    Screw Sources & Prices...

    I see McMaster Carr gets $9.51 for a pack of 5 ($1.90 each).

    Check out Aircraft Spruce as an excellent alternative source!

    Same screw is only 82 cents, and you can order any quantity.

  7. #27
    Sorry to dig up an old thread but having this problem and even more confused about what screw and nut I should get after reading the discussion! Can anyone clarify? Planning to try and replace all the dodgy fixings on my ZRA once I have my new Helium so I'm trying to learn but I'm no mechanic!

  8. #28
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    Chas
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    I am a person with mild/moderate hexaparesis (impaired movement in 4 limbs, head, & torso) caused by RRMS w/TM C7&T7 incomplete.

    "I know you think you understand what you thought I said, but what I don't think you realize is that what you heard is not what I meant."
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