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  • Aluminium/steel galvanic corrosion

    Discussion in 'The main mechanical design forum' started by Trace, Mar 8, 2016.

    1. Trace

      Trace New Member

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      Hello all,

      This is my first post, therefore I sincerely apologise if this topic is in the wrong place, or if this question as been answered a thousand times before (I searched, didn't find anything, so I'm hoping I won't get banned from this forum straight away :)

      Here's my problem:

      I am designing a mechanical system, which consists of two parts that can rotate about each other.
      Part A will feature a small but wide shaft, on to which a bearing will be fit.
      The outside of the bearing will then be inserted in part B, and the two parts will be held together by bolting a cap on top of the shaft.
      Therefore, part B will be pressed between part A and the cap.

      Each part will be made from aluminium (currently 6061 alloy, but that's not definitive) and shrouded in carbon fiber, except on the contact area, which will not be covered in CFRP.
      The bearing will be low-profile (i.e., a wide (80mm) and thin (13-15mm) bearing).

      My problem is galvanic corrosion between the aluminium shaft and the bearing.
      How do I prevent corrosion?
      My guess would be to anodise the alu shaft, but does that pose a problem when fitting the bearing?
      Should I anodise and then seal the shaft? If it is sealed, will it bond well to epoxy resin?
      Would an aluminium bearing be more suitable (the load the bearing will take is about 2000N (static))?

      To reduce lateral play between the two parts and keep friction to a minimum, my idea was to put PTFE gaskets between the parts. Can PTFE be "squished" a little to reduce play and account for wear? Is another material more suited, or should I just let the aluminium parts slide against each other?

      I hope this is clear, if not, I'll include a drawing.

      Thank you all very much for your help.
       
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    3. robertjeffery

      robertjeffery Active Member

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      Galvanic corrosion only generally occurs when water and an additional component like salt or chemicals are present. what I am basically asking is what environment is this component working?

      also think of examples where aluminium and steel are often combined with little issues, and how do they get over them. engines and transmissions are generally aluminium, but their are plenty of things bolted to them made out of steel. eg the bolts.

      If anodising is sufficient enough to reduce the corrosion between the components. coating thickness of "1.8 μm to 25 μm (0.00007" to 0.001")" Wikipedia. on a 50mm shaft if you where to allow a tolerance of 49.9 +- .05 (shaft) and allowed .01-.025 for the coating, that will be allow for a sliding fit between bearing and shaft, then Loctite the bearing on. This will do two things, it will create a barrier between the steel and the aluminium. Also when or if the bearing needs to be removed, the Loctite can simply be heated till it fails and then the components can be easily separated. rather then attempting to press out the aluminium shaft which is likely to cause damage to the softer shaft.

      The final question you ask is about the sliding surfaces, this is dependant on the load applied, eg bearing pressure. if the load is small enough then aluminium on aluminium will be suffice, with maybe a little white grease. If the load is sizable then maybe look at some form of hardened and ground thrust washers keyed or glued onto the corresponding surfaces and then either place a thrust bearing or DU thrust washer between them. And just a warning, if you place a plastic washer between the two aluminium surfaces, the plastic washer will stay relatively intact and the aluminium will wear. I don't know the scientific explanation for this, just observed in real world applications.
       
    4. Trace

      Trace New Member

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      Thank you robertjeffery, these are excellent comments.
      I love the idea of loctite!!!
      The system, which is an artificial limb (leg) will be used in cold conditions (snow).
      There will be no salt, so corrosion should be minimal. I will stick with your idea, make a thin anodising layer and loctite the bearing.
      It is usually preferable to have an interference fit on one side, and a loose fit on the other.
      Is it a problem if I loctite both ends (inner and outer ring)? Or should I have one side of the bearing as a loose fit?

      The lateral loads will be small, so aluminium on aluminium should suffice, but I am worried about excessive play, especially in cold weather (thermal contraction).
      Back to the drawing board :)

      Thanks again for your answers, it is really helpful :)

      Have a great day!

      Trace
       

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