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OT sliding bushing without stiction

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  • OT sliding bushing without stiction

    I need some advice from the mechanical inclined people here, as I only understand things that spark.

    We're supporting a university project which is run by electrical engineers, who have even less knowledge of mechanical stuff. The system itself is comparable to an electric version of a hydraulic cylinder. The 40mm steel rod is riding in a glasfiber/epoxy bushing with a PTFE coating, but is suffering from stiction, messing up the control strategy. They added transformer oil to lubricate it and alleviate the stiction, as that is what they have laying around.....

    I don't have any numbers yet for the surface loading of the bushing and surface roughness of the rod. But it's a turned finish and probably rougher than the 3um Ra recommended by the bushing data sheet.

    Apart from adding a dithering signal to the position to solve this, are there better suitable materials for the bushing? I'd think something like PTFE loaded acetal or plain PTFE, but this is way out of my comfort zone.

    Would polishing the rod make the stiction better or worse?

    These are technology demonstrators and are not intended for mass production, so it doesn't need to be a best cost solution.
    The current size of the bushing is about 40mm X 300mm.

    Any ideas?
    Igor

  • #2
    I've heard that one solution is to always have the shaft rotating, that makes the friction alway past static into kinetic - Not sure if the complications of this merits it.

    Or try an air bearing like they have on end mill grinders.

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    • #3
      The rod itself is a linear motor, and thus wired. So no rotating.

      The system also doesn't have pressurised air, and power consumption will skyrocket when adding a compressor.

      Thanks for your thoughts,
      Igor

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      • #4
        Yes, polishing the rod will cause it to slide easier.(double entendre)
        But if you look at an actual hydraulic cylinder rod you'll notice that it is chromed and highly polished. That is the finnish you're looking for.
        On any close fit bearing, like a rod bearing, any imperfections on the rod will cause problems with the intended action of getting the rod to slide smoothly.
        I would reccomend getting a piece of polished hyraulic rod.

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        • #5
          The rod is actually a stack of steel discs with aluminium shims to isolate the magnetic phases. This will make it hard to use a hydraulic rod.
          They tried coating it earlier with something, but the baking of the coating burned the isolation on the coils....

          Would chrome plating also adhere well to the aluminium shims, or would it break off there and make a big mess of things?

          Igor

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          • #6
            There is a graphite impregnated teflon that could be used to make the bushings that is significantly slicker than other bushings.
            Byron Boucher
            Burnet, TX

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            • #7
              Originally posted by ikdor
              The rod is actually a stack of steel discs with aluminium shims to isolate the magnetic phases. This will make it hard to use a hydraulic rod.
              They tried coating it earlier with something, but the baking of the coating burned the isolation on the coils....

              Would chrome plating also adhere well to the aluminium shims, or would it break off there and make a big mess of things?

              Igor
              Why not shrink a very thin liner over the rod?
              Peter - novice home machinist, modern motorcycle enthusiast.

              Denford Viceroy 280 Synchro (11 x 24)
              Herbert 0V adapted to R8 by 'Sir John'.
              Monarch 10EE 1942

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              • #8
                there are bushings called fiberglide. they are ptfe with some fibres on steel and advertise a friction coeficient of as low as 0.02 and 210 n/mm. but since you have a ptfe coating already?

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                • #9
                  have you looked at Thomson Ball Bushings? http://www.thomsonlinear.com/website...g_bearings.php
                  gvasale

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                  • #10
                    The rod is actually a stack of steel discs with aluminium shims to isolate the magnetic phases.

                    "This" I feel is the issue. This series of alternating (?, not stated but inferred) discs is in effect a series of edges (as opposed to what a solid hydraulic rod would be, for example, a "single" smooth surface) and would, logically cause a lot of stiction.
                    How accurate are those discs to each of the same type but also to the other type?
                    The only way off the top of my head I can see to get them all as smooth as possible would be to set them up in the order they are being used on a "dummy" central shaft, some how apply a clamping force on the ends (clamping the stack of discs tightly) and then turning them to the finished size you need...as there will likely be some very very small amount of taper you would need to reassemble the final version in the same disc order (so number them first ?)

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                    • #11
                      The stack of alternating discs is clamped and then turned on the lathe as one unit, making it reasonably smooth. But nowhere near a chrome plated hydraulic rod. The stack could be polished afterwards.
                      I wonder what it looks like though when it heats up, as the aluminium shims would expand more and might protrude slightly between the steel discs. I'll do a back of the envelope on that.

                      Shrinking a liner over it sounds interesting. Radial space is at a minimum though, so it would be really exciting getting a thin liner over the rod without binding halfway down......
                      Now that I think about it; as the assembly is not rotating, a slit liner would even be possible. No exciting shrinking actions then.

                      The ball bushings will take up too much space, they will displace magnets around the bushing, reducing the stroke.

                      Thanks for the tips on the other materials, I'll have a look at them tonight.

                      Igor

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                      • #12
                        Any way you could use a ball bushing rolling on a guide rod inside the stack? Clearance on the outside of the stack instead of your current sliding contact.

                        Can't quite visualize your geometry so maybe not helpful.

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                        • #13
                          There are oils that are formulated to help reduce or eliminate stiction, some of them work.

                          I have had good luck with a Kendall product made for use in limited slip differentials. Started using it on a Lodge & Shipley lathe I have with reground, hard ways. The carriage would move smooth as silk in increments of 1/4"- 6', but it was nearly impossible to move it .001", even by bumping. Started using that oil and even .0001 became possible. Can't remember the name of it, if you were interested I would get that for you?

                          Dave

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                          • #14
                            Assemble the rod and have it centerless ground. It would be better if all the parts can't rotate individually. Like all glued together.

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                            • #15
                              Dave, I'm definitely interested, if they can't use it because of compatibility issues it would still be a good bit of trivia to know. (especially now that I have all the parts in house for my submicron poor mans DRO)

                              With regards to the centreless grinding, that might be a good one. The discs won't rotate as the stack is permanently clamped with a few tonnes.

                              I'll shoot a video of the application next time I'm there

                              Thanks,
                              Igor

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