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Thread: Does heating ever make a hole smaller?

  1. #1
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    Default Does heating ever make a hole smaller?

    From the "Heating up a rusted joint?" thread. http://bbs.homeshopmachinist.net/thr...a-rusted-joint

    Most responses so far are saying to heat he manifold (outside), but some have recommended heating the pipe.

    I clearly remember our engineering science teacher posing the following question. If you heat a piece of steel with a hole in it, will the hole get larger or smaller? His answer was that the hole gets smaller because the steel expands.

    Clearly there are cases when this is not true. For example, when heating the inner ring of a bearing to expand it. But there isn't much material there, so the whole ring can expand easily.

    What if there is much more material and the outer extremes cannot expand to keep up with the hotter part? The circumference is larger as you move away from the hole, so it might still absorb any increase in size without growing the same amount. It could also warp. Would the hole ever get smaller, or was the teacher wrong in this case?

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    Thermal expansion.... every dimension increases, even the ID of the hole

    Joe B

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    If you heat locally in a large cold part, the hole may get smaller because material expands, but cannot expand outward. it either has to "upset", or the hole gets smaller.

    If you heat the entire part, I see no way for the hole to get smaller.
    1601

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    Hashim Khan

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    Yes, if the whole ( simplistic) body is evenly heated, all its dimensions get bigger including holes:
    http://hyperphysics.phy-astr.gsu.edu...thexp2.html#c1

    It the heating is concentrated close to the hole, we would have to be careful of staying under the stress limits of the cast iron.
    And how would we know except by analysis or experience of testing?

    I dimly remember the "Gun Barrel" problem when there was a certain hoop thickness that would minimize expansion of the bore,
    with the differential temperature radially.
    but can't find it in the books here or on a quick search.

  5. #5
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    Quote Originally Posted by pinstripe View Post
    From the "Heating up a rusted joint?" thread. http://bbs.homeshopmachinist.net/thr...a-rusted-joint

    Most responses so far are saying to heat he manifold (outside), but some have recommended heating the pipe.

    I clearly remember our engineering science teacher posing the following question. If you heat a piece of steel with a hole in it, will the hole get larger or smaller? His answer was that the hole gets smaller because the steel expands.

    Clearly there are cases when this is not true. For example, when heating the inner ring of a bearing to expand it. But there isn't much material there, so the whole ring can expand easily.

    What if there is much more material and the outer extremes cannot expand to keep up with the hotter part? The circumference is larger as you move away from the hole, so it might still absorb any increase in size without growing the same amount. It could also warp. Would the hole ever get smaller, or was the teacher wrong in this case?
    Reminds me of a science text book my daughter brought home with 'experiments' in it. It was written by a company my wife worked for. Virtually every reason for the experimental results given were completely wrong! Why America is loosing ground. Writer probably had one of those Alternate Science courses in school.

  6. #6
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    If you heat a ring of steel/cast iron/brass/aluminum it will grow larger..period !
    Simple Proof ? watch how they install "tires" on railroad locomotives ( shrink fit)
    https://en.wikipedia.org/wiki/Railway_tire
    BUT:
    WHEN the exterior of the ring is restrained ( say in a bore much larger mass and not heated --as much) the interior diameter will get smaller as yield points are achieved . And , even if heated the same , but the materials are different as mentioned in the next sentance. Say you are heating a brass or aluminum bushing that is inside a steel ring (larger bushing ) the ID of the Brass or ALuminum will get smaller because the expansion rate ( .00000999 to .000012 ) is greater than steel (0000063) and the yield strength is less than steel so the bushing ID gets smaller, and if the conditions are right ( yield /interference etc) the brass/aluminum will fall out when cooled. This is because the yield of the material is not greater than the restriction it sees ( steel is 60K and brass at 10 K ) The brass wants to expand but it has no strength to overcome the force needed to push the steel to a larger OD and because it has to grow due to thermal forces, it grows longer and smaller

    Rich

  7. #7
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    Your engineering science teacher is what I like to call WRONG. At least in the general case. He/she is not the first and won't be the last.

    If a part with holes is more or less uniformly heated, then the whole part will expand and that includes the sides of the holes. Think of a square frame made with four lengths of the same material. If it is heated, then all four of them will get longer and the dimensions of the inside opening will become larger. Now think about a ring of the same material and heat it. The ring will get longer which means it will have a longer circumference. Larger circumference equals larger diameter. Again, the internal hole gets larger, not smaller. But remember, I said if it is more or less uniformly heated: all parts of the square or ring are at about the same temperature. It does not make any difference if you add additional rings around the original one, each one with it's ID in contact with the OD of the smaller one. All of them will expand when heated and the original, internal hole in the original ring will still expand. This is true for two rings, ten rings, or a hundred rings: the hole always gets larger. Now, if all those rings are welded or otherwise made into a single piece of metal, the, now imaginary concentric rings will still all expand upon being heated and the hole will still get larger. In short, it works for solid parts with small holes too.

    So holes get larger when the part that they are in is heated, at least when it is uniformly heated.

    BUT, what if you have a fairly large part and you heat only the inside surface of a hole. Imagine a piece of steel that is 12 inches in diameter with a 1 inch diameter hole in the center. And a heat source is inserted in that hole. Then, what will happen. The bulk of the part is still at the original, cooler temperature. But the metal near the surface of the hole is hotter. Yes, there will be stress. But what will happen to the size of the hole? I have not done this experiment nor have I read of anyone else doing it. But it seems just possible that the hole may get smaller. At least until the temperature equalizes. This would be an interesting experiment.
    Paul A.

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  8. #8
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    Quote Originally Posted by Paul Alciatore View Post
    BUT, what if you have a fairly large part and you heat only the inside surface of a hole. Imagine a piece of steel that is 12 inches in diameter with a 1 inch diameter hole in the center. And a heat source is inserted in that hole. Then, what will happen. The bulk of the part is still at the original, cooler temperature. But the metal near the surface of the hole is hotter. Yes, there will be stress. But what will happen to the size of the hole? I have not done this experiment nor have I read of anyone else doing it. But it seems just possible that the hole may get smaller. At least until the temperature equalizes. This would be an interesting experiment.
    I haven't tried it either, but the material in proximity to the hole that is being heated will expand. The hole might shrink. The material might get
    thicker in that region. That piece might warp while trying to accommodate the expansion. Or maybe even some combination those.

  9. #9
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    Thanks for all your responses. I spent a little bit of time reading more about this, probably should have done that before I posted the question . The word "uniformly" keeps popping up on most pages. I don't think anyone disagrees that the hole gets larger when a part is uniformly heated.

    There is an explanation on this page that I haven't seen elsewhere:
    https://qedinsight.wordpress.com/201...the-same-size/

    It's in a comment from Teng Guo. I won't post the whole reply here, so here is a very condensed version.

    ...when we heat the ring, indeed the atoms ‘want’ to space out in all directions. But there are favored and unfavored directions... If the hole was to shrink upon heating, these atoms would be placed closer towards the hole center. However, we know as we get closer to the center, the circumference of the hole decreases, which means a reduction of spacing between atoms circumferentially. This is contrary to what the atoms would ‘like’ to behave and thus I call it unfavored direction.
    So if Teng is right, then a smaller hole is the most unlikely result. The atoms would rather move in any another direction (up/down) if they cannot move out.
    Last edited by pinstripe; 08-08-2017 at 03:51 AM.

  10. #10
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    Default

    Quote Originally Posted by pinstripe View Post
    It's in a comment from Teng Guo.

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