Thermal expansion question ?
Thermal expansion question ?
(OP)
Situation, if one has a 1 inch diameter x 6 inch long solid brass bar and a ¼ diameter hole was drilled in the center of the solid brass bar, now if the brass bar was heated to 1000 F would the ¼ hole expand or contract?
Thanks,
Dave
Thanks,
Dave





RE: Thermal expansion question ?
Metals, in general, expand when heated. Therefore, without doing any detailed analsysis, I'd vote that the "hole" would contract.
Patricia Lougheed
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RE: Thermal expansion question ?
The question to ask yourself is "What would happen to the material that used to be in the hole?"
Or from a more practical perspective, when you want to assemble two parts with an interference fit what do you do? You heat up the part with the hole so that the hole gets bigger and fits over the part without the hole.
RE: Thermal expansion question ?
The hole expands.
corus
RE: Thermal expansion question ?
that's why you can heat a washer and put it over a bar, then find that it won't come off when it's cool.
RE: Thermal expansion question ?
Much appreciated,
Dave
RE: Thermal expansion question ?
\\Thus, in this case - up to the point of melting "into" a copper hole - ain't no black hole here! - when heated, the rod expands = the OD gets bigger, the rod gets longer, and the hole thing inside the rod gets smaller.
RE: Thermal expansion question ?
A circular disk with no hole. Heat it up. EVERY POINT ON THE DISK GETS FARTHER AWAY FROM THE CENTER.
Take the circular disk, cut out a hole (with an imaginary zero kerf). Heat up the resulting two pieces. The "slug" that became the hole gets bigger. The hole that the slug came from has to get bigger too. If the two pieces are left "co-axial" there will be no gap. Forming a hole is not some sort of mystical quantum process. It won't cause a discontinuity in the behavior of the material.
RE: Thermal expansion question ?
Solid brasses have a published linear thermal expansion coefficient from 68 to 572oF, α, of about 11.3×10-6 F-1.
Assuming isotropy, every linear dimension expands in the same proportion upon heating causing the hole to expand too.
Again, assuming isotropy, the volume expansion coefficient β = 3α.
RE: Thermal expansion question ?
If there is no "center" of the disk as in a hollowed disk, then there is no out opposing outward pressure, and so (a smaller) part of the ring DOES move inward. Its just that a smaller part of each hollow concentric ring is moving inward than is moving outward.
RE: Thermal expansion question ?
RE: Thermal expansion question ?
Imagine removing all but a narrow annulus next to the circumference of the hole. Apply temperature. Since the linear expansion along the circumference of the annulus is positive, the annulus must grow in diameter to accommodate the new circumference, so the hole grows. Look at the next annulus, same thing.
Note however, since its circumference is longer, the second annulus has a larger diameter growth relative to the width of this annulus, so there's no "push" from the outer parts of the washer.
Note also, that growth in width of the annulus from expansion is miniscule compared to the growth in diameter from the circumference increasing, so, again, nothing pushing to make the hole smaller.
TTFN
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RE: Thermal expansion question ?
I strongly urge anyone who did not instantly know the correct answer to seriously think through the problem until it becomes obvious that the hole grows. Many real world problems involve exactly the principles discussed in this thread. By understanding how thermal expansion works in this simple example will make seemingly complex problems you encounter later in life become simple.