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  • How to achieve an oily-smooth sliding surface that does not sieze up later?

    Discussion in 'The main mechanical design forum' started by mdcarson, Dec 12, 2014.

    1. mdcarson

      mdcarson New Member

      Dec 2014
      Likes Received:
      Sorry, this was supposed to be a thanks but I got lost in the details. So..thanks for all the replies!
      I singled out K.I.S.S because he/she pointed out the separate issues of stiction/friction vs viscous drag. Once I realized I was not addressing each one individually I changed the design to do so. I can't machine the tolerances I need so I used a small HDPE shim-ring to gap the two sliding surfaces, and filled the remaining un-filled gap with stiff grease. Viscous drag now stays constant because grease isn't squeezed out, and friction is provided separately by the shim size and material. I think what I was calling 'stiction' was not the usual hysteresis between static and kinetic friction (because that should be evident immediately), but the transition from a friction-based resistance to a viscous-based resistance when I first attempt to turn it for the first time, which is huge once the grease has been statically compressed for a day or so (not sure exactly what was going on; some kind of vacuum-weld effect?).

      I'm still using a compressed spring to clamp it together. The sliding surface of the shim is highly compressed with the spring so it (should) stay grease-free and does not enter viscous-drag behavior. The impact of the actual 'stiction' of the plastic shim-ring is minimal because I didn't need a lot of friction anyway (and I'm using a slick plastic). At this point I am just dialing in the parameters, but overall it did not change 'feel' between the initial build and after sitting for about 10 days. I have to do more builds for repeat-ability but this feels like a more proper design approach.
      Thanks again!
    3. K.I.S.S.

      K.I.S.S. Well-Known Member

      May 2014
      Likes Received:
      Hi mdcarson,
      Sorry, perhaps you misunderstood my last post, or quite probably I expressed myself incorrectly - I wasn't implying that you should say thank you to all who attempt to assist, some topics can develop very long threads..
      I was merely trying to say thanks for the feedback, which I'm sure everyone appreciates. Sometimes forum members provide advice and the original poster never bothers to respond, which I'm sure is a bit irksome for lots of members, not to mention rude.
      Have a great 2015 :)

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