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

  1. #1
    Associate Engineer
    Join Date
    Dec 2014
    Posts
    1

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

    Hi. I can't find answers searching for this subject.
    I have a mechanical design where I have two clamped rotating discs that are rotated manually, so this is about ergonomics to some degree. I am trying to achieve that 'hydraulic' feeling of the volume knob on a stereo but I am limited to sliding surfaces; actual hydraulic design (fluid flowing through an orifice) is not feasible for me.
    I have tried numerous combinations of lubricants and materials and although it feels oily-smooth when I rotate it, 24 hours later I have to overcome an unacceptable of static friction to get it started, then it rotates smoothly again even if I stop. I have not timed how long it takes to seize up but it is not while I am twisting the joint back and forth.

    My best result for the oily-feel is using a heavy silicone grease on the either acetal or ptfe disks. The other side of the grease is a fairly shiny smooth anodized surface. It is a sandwich around a static base plate so there are two slip-disks clamping in opposition to each other on a common wall. May nen khi I will try to illustrate the stack-up from one end to the other:
    -thick metal disk (1/4" x 3" diameter), this spins with the center bolt, which clamps to nothing except the far end of the stack, another stiff metal wall.
    -super-glue
    -acetal or ptfe disk (15 mil) (etched on one side)
    -very thick silicone grease (Molykote)
    -shiny thick metal wall (~1/4") that is anodized on both sides
    -very thick silicone grease (Molykote)
    -acetal or ptfe disk (15 mil) (etched one one side)
    -super-glue
    -heavy washer (30 mil steel). This is to distribute the o-ring force evenly across the plastic disks
    -buna o-ring for the spring force (it's a size dash 214 for what it's worth)
    -solid backing for the o-ring

    A large bolt running through the stack to clamp it together.
    The whole stack is hard-clamped to a fixed gap-width using a stack of shim washers around the center bolt. The size and durometer of the o-ring determines the clamping force. I change the shim thicknesses to adjust the clamping force. I have a nice medium stiffness when rotating and it has a good glide. But it seizes up after sitting for a while. I am assuming that the grease is being squeezed out somehow and is being re-distributed after overcoming the initial sticking, but that may be a completely wrong assumption.

    I seemed to have solved this problem once but I wasn't able to determine what has changed, something subtle I'm sure. Am I designing everything right but something is not right in the build?
    Any ideas or references to other sites would be greatly appreciated. There must be a developed art to doing this kind of controlled action.
    thanks
    Matt
    Last edited by davidhiep86; 01-08-2015 at 03:01 AM.

  2. #2
    Lead Engineer RWOLFEJR's Avatar
    Join Date
    Mar 2011
    Location
    Rochester Pennsylvania
    Posts
    396
    Consider eliminating the grease and o-ring. Substitute a wave spring washer or light bellville for your tension. The nylon or similar running on smooth hard metal will slip wonderfully without grease. The grease will get sticky and collect crap. Hard smooth flat metal.... hard nylon... hard smooth flat metal and tuck a wave spring or the like between two of the steel plates or the backing or whatever to keep tension on the stack.

    Good luck,
    Bob

  3. #3
    Lead Engineer
    Join Date
    Aug 2013
    Location
    Houston TX USA
    Posts
    420
    I did a bit of experimenting with two flat plates and some molycote silicone grease I have and at this point I am not seeing any effects from sustained periods between tests. I think that your o-ring loading may be a bit of your problem. In that respect, there is really no need to apply any loading to your plates because they will actually resist being separated due to the adhesiveness of the silicone grease.

    Because I find in my testing that I can achieve a very satisfactory damped motion with only two surfaces separated by the silicone I would, also, recommend that you elimiinate your second interim metal disk and load the rotating disc with a very soft wave spring against the opposite unlubricated face of your rotating disk.

    What you are trying to take advantage of in your assembly is the silicone grease viscosity and surface adhesion to the two conact disc faces, which, does not require any contact loading between the two disc surfaces and the enclosed silicone grease to be effective.

    With regard to your o-ring disc loading, due to an extensive background with dynamic o-ring seals, I can tell you that o-rings really do not make good loading springs because even a few thousands change in their x-section compression can result in significant load changes)

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