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How does one fasten a rotating joint that does not self-tighten or self-loosen?

I'm trying to make a "slider crank" mechanism (for cheap) where all the movement is parallel to the ground. There are no huge forces at play, hoping to push a slider of a couple hundred grams ~15cm/6". Not my diagram, but essentially this:

slider crank mechanism

There are plenty of photos online, but I'm unclear how to robustly make joints (looks like they're labeled B and C in the diagram above). I've tried something like this:

bolt with washers and nut

but with the continuous rotation the nut eventually loosens or tightens.

Accepting that any pieces of hardware-store materials can be modified to suit, what is a good way to fasten these joints?

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    I'm voting to close this question as off-topic because it has nothing to do with home improvement.
    – Daniel Griscom
    May 11, 2018 at 20:21
  • @DanielGriscom this is a project for installation in my home where the central concern is common fasteners. Do you feel this is more appropriate on a different StackExchange?
    – user2152081
    May 11, 2018 at 21:29
  • @DanielGriscom -- would this be a good candidate for migrating to Engineering.SE?
    – ThreePhaseEel
    May 11, 2018 at 22:31
  • I was just notified that this question received the "Popular Question" badge, so closing this without moving it to a more appropriate home seems like over-moderation. Perhaps more meta feedback, but closing well formed, answerable questions as off topic without an "on topic" SE feels destructive, where the upside is extremely subjective.
    – user2152081
    Jul 30, 2020 at 16:55

3 Answers 3

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The bolt/nut configuration you show requires high friction against the joined members to maintain tightness.

You need independent locking of the nut. The most common method is to use two nuts. They are tightened against each other, rather than the captured articulating part.

An alternative is to use a nut locking compound (a sort of nut glue), and not fully tightening the nut, but the removable type will probably also eventually loosen, and the permanent type prevents easy disassembly.

I would also add a washer between the articulating arms or rotating parts.

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  • Two nuts is the way to go. Other solutions will be more prone to loosening over time
    – Eli Iser
    May 11, 2018 at 13:21
  • Can you clarify where the two nuts would go? Exactly the same as the picture but with a second nut on the outside of the existing nut?
    – user2152081
    May 11, 2018 at 14:10
  • Yes. The inner nut is not quite tight against the washer, and is held with a wrench as the outer nut is tightened against it.
    – bib
    May 11, 2018 at 15:01
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Is it possible to drill the hole to a larger diameter than the bolt and insert a metal sleeve that is slightly longer than the thickness of the 2 arms and tighten against the sleeve. This will act as a replaceable bushing and should reduce wear.

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Instead of a bolt and nut design, just a short length of your aluminum rod with holes drilled for cotter pins, and washers.

Or a bolt and a castle nut with a hole drilled in the bolt for a cotter pin.

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