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  1. #1
    bb218 is offline Aluminum
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    Default Thermal expansion

    Question,
    If I have a 5" diameter steel slug with a 3" hole in it the hole diameter will increase with a rise in temp. What if the hole diameter is 1/4 inch, seems to me as the metal expanded the hole would get smaller.
    Would a 5" diameter ring with a 4.9 diameter hole grow the same as one with 1" diameter hole? Mike

  2. #2
    huntinguy is offline Cast Iron
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    Default

    Quote Originally Posted by bb218 View Post
    Question,
    If I have a 5" diameter steel slug with a 3" hole in it the hole diameter will increase with a rise in temp. What if the hole diameter is 1/4 inch, seems to me as the metal expanded the hole would get smaller.
    Would a 5" diameter ring with a 4.9 diameter hole grow the same as one with 1" diameter hole? Mike
    1: Nope, heat causes molecules to loosen their bonds hence, thermal expansion.

    2: If you mean the same "rate" then, if the materials are the same yes but not the same amount because the bores are not the same size to start.

    Thermal expansion per each material is a constant. I do think that the wall thickness may make a difference. Been a long time since I did any of that type of calculation. The rate of expansion is also part of the thickness (size) of the expanding material. So the .1 difference in the two bores would mean that the total amount of expansion would be different.

  3. #3
    Troup is offline Titanium
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    Default

    Quote Originally Posted by bb218 View Post

    If I have a 5" diameter steel slug with a 3" hole in it the hole diameter will increase with a rise in temp.
    What if the hole diameter is 1/4 inch, seems to me as the metal expanded the hole would get smaller.
    I think you're probably right - it's an interesting question, if you're correct I wonder what the 'crossover point' would be.

    I'm not sure how these questions can be answered without resorting to the indignity of a physical experiment.

    I suspect for a really accurate answer you might have to know the temperature of the workpiece when the OD was turned, and (if different) when the hole was bored.

    I guess the ring will adopt a 'minimum nett stress' configuration at each temperature, with (at raised temperatures) some compressive stress around the bore and tensile stress around the rim, so I guess answering it would require Mohr's circle together with differential calculus, and the latter is definitely something I leave to others....

  4. #4
    Forrest Addy is online now Diamond
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    Default

    Don't overthink this problem. A material in thermal equalibrium expands/contracts at all points in proportion. The 5" OD will expand X for a given temperature increase. The 3" ID will expand 3/5 X but all parts of the material have to be at the same temperature.

  5. #5
    Close Work is offline Cast Iron
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    Default

    Thermal expansion will result in the 1/4" hole in a 5" diameter growing along with the rest of the part. Think about the material right in the area of the 1/4" hole and realize that it grows. One nice feature of the thermal conductivity of materials being a finite number is that if you heat the 5" diameter the expansion of the material near the OD expands more than the material near the 1/4" hole and the expansion of this material acts to pull on the material near the center which is still at a lower temperature. You can use this to advantage when you need to remove a pressed in pin or bearing or other part you don't want to get too hot. Quickly heating (big torch or electric band heater) can open the bore without over heating the fragile part. You can also use liquid nitrogen to chill the part in the middle while heating the OD to remove very tight parts or shrink fitted components. Thermal expansion is given in terms of inch/inch-F meaning so many micro inches per inch of dimension per degree. This means that an aluminum rod 1 inch long will grow say 13 microinches (0.000,013") for each increase in temperature of 1 degree F. If the aluminum rod is 100 inches long it will increase in length by 1300 microinches or 0.0013" for each degree on temperature increase.

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