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  1. #1
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    Default Reducing brass thermal conductivity - practical ideas?

    I have made a small decorative knob (35x20x20) to control the air flow into a wood heater. This is to replace a wooden knob. I am concerned that the brass knob will get a lot hotter than the wooden one (which got hot). I have a thought that I could seat the knob on a ceramic washer (no idea where to find one) to stop the thermal conductivity from the steel finger the knob mounts to.

    Making a wood knob is out of the question at the moment as the request was for brass to replace a wooden knob.

    Any ideas on a practical way to reduce the heat transfer to this brass knob?

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  3. #2
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    Default Heat transfer

    How is the knob mounted?
    If it is held by a screw or bolt the heat will migrate through the mounting. Maybe a ceramic sleeve through the knob together with the washer will help.

    Roger

  4. #3
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    Maybe a threaded Bakelite insert.

  5. #4
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    A washer of any kind will not work as the heat will still be conducted by the metal connection mechanism that connects the brass know to the (presumably steel) control lever.

    One possibility would be to turn up a true nylon or teflon connection mechanism. These materials will handle 250ºC and be much easier to find and make but won't be as effective as ceramic.

    But of you can do this you might as make a Wooden knob.

  6. #5
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    Agree with the comments about heat conductivity. Perhaps you could design a knob that can be turned by a C spanner or T handle screwdriver or something along those lines.
    Cheers,
    Rod

  7. #6
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    Ueee is offline Blacksmith, Cabinetmaker, Machinist, Messmaker
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    Rather than a knob you cold always wind yourself a coil style handle.....

    Ew
    1915 17"x50" LeBlond heavy duty Lathe, 24" Queen city shaper, 1970's G Vernier FV.3.TO Universal Mill, 1958 Blohm HFS 6 surface grinder, 1942 Rivett 715 Lathe, 14"x40" Antrac Lathe, Startrite H225 Bandsaw, 1949 Hercus Camelback Drill press, 1947 Holbrook C10 Lathe.

  8. #7
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    Conductive heat transfer is but one method of heat transfer.
    The distance of the knob to the heat source will dictate how much radiant heat that the knob receives.

    In the end, the result it is likely to be a combination of both radiant and conduction heat transfer.
    Modification to take both into account may well be required.

    I would do a test.

    No doubt a brass knob will look terrific , but I suspect the brass will absorb radiant heat more efficiently than a wood or ceramic knob.

    Grahame

  9. #8
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    Seeing as this is the knob adjusting the air damper, could it be placed so that it gets some air cooling.. and maybe some cooling fins on the shaft. Teflon coupling.

    I think a highly polished finish would reflect radiant heat.. anything but black.

    Ray

  10. #9
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    Thanks for all the suggestions. I like the bakelite, ceramic and teflon ideas. Unfortunately none are available to me here today. What I did find however was a nylon bolt at my local "nuts and bolts" store. I figure it conducts heat less efficiently than the stainless bolt I had in place. The thermal conductivity of nylon is the same as teflon. The melting point is significantly lower however (200-350 deg C). Hopefully it does not get soft. I will take an infrared thermometer with me to check the heat..

  11. #10
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    One thing that might be locally available and will limit conductive heating from whatever the knob is mounted to would be a fibre washer, a'la fibre tap washer. Fairly easy to find in a variety of size at engineering supplies, electronics stores, bolt or car part places etc.

    You could possibly use a couple with larger polished brass washers sandwiched between to dissipate heat in stages as it passed to the knob. Not sure of the overall heat range, but it might keep you going till you can get some ceramic buttons to replace them. Can't think of anything that will be viable and offer a slower heat buildup into the knob than a stainless screw, but one with a small head isolated from the mounting face with a fibre washer/ceramic button would limit the area exposed to high temps, and hence the amount of heat being transmitted. With this approach, you could use an oversize mounting hole in the surface you are screwing through, with small washers/buttons as packers limiting contact between the item you are fixing to and the screw shank.

    I suspect that the nylon screw will have a short life as they aren't particularly strong and don't have a lot of shear strength.
    I used to be an engineer, I'm not an engineer any more, but on the really good days I can remember when I was.

  12. #11
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    Quote Originally Posted by variant22 View Post
    Unfortunately none are available to me here today.
    The only thing I came up with in the "available here today" category was a spark plug or parts there of.

    Stuart

  13. #12
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    You want to make a knob with a thin brass shell outside glued onto a wooden core. That gives you the best of both worlds - the good looks of brass with the heat insulating properties of wood.

  14. #13
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    How about one of those old brass door knobs that are hollow? Fill it with something like epoxy or plastibond (check temp range). Mould in a thread, or just drill & tap later.

    PS: I was once told that araldite would take whatever temp it was cured at. Cure it at 250C and that's the temp it's good for. Presumably within limits.

  15. #14
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    if thermal and knob is screwed to steel I would try a ceramic sleeve insert?...certainly not metal to metal.

    if it is radiant only a shield from infra red rays will stop it from getting hot.

    heat in the near surrounding air eg next to glass window in door would also contribute the thermal gains into the knob

    when human hands touch metal the heat gains/losses into or from the skin is very quick as compared to timber at same temperature condition...which is reason why they use timber

  16. #15
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    I would try Ewans suggestion of a wound spring type of handle. Years ago I worked at a minesite/ concentrating plant, and on afternoon and night shift many wood heaters were made by the blokes. the "go to" handle was a welders chipping hammer handle which was a stock item in the warehouse. The steel wire has a much lower conductivity than brass, and being wound, it cools much better than a solid item via convection air currents, and even if at the same temperature as brass, the heat transfer would be less, (but not as low as wood), and so would not feel as hot.
    Rob

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