Don wrote:
"George E. Cawthon" wrote in message
...

Don wrote:

"Richard Cline" wrote in message
...


In article ,
"Greg O" wrote:



"Don" wrote in message
thlink.net...


Won't expanding the pulley make the hole smaller?



Nope, when you heat a ring shape, such as a pulley, the hole will get
larger.
Greg



Is that because the air in the hole expands more than the metal and
forces it to be bigger?


I wonder what scientific property allows one portion of the metal to
expand (the outside surface of the pulley) while the other part contracts
(the hole).

None. Once upon a time, I though the hole contracted as the piece was
warmed, but only because I am not a materials person and didn't really
visualize what would happen. But, someone explained the shape/size of the
control acts just like the piece of metal taken from the hole. As an
example, the diameter of a dime sized piece expands .004 inches from
temperature A to B, if you measure a dime size hole in a plate of the same
material, the hole diameter will expand .004 inches from temperature A to
B.

It also makes sense from a visualization of molecules. In a solid, the
molecules will be fixed at a specific distance from each other at a
specific temperature. As the temperature increase, the vibrate faster and
the distance between molecules increases. Visualize the molecules as a
single layer of marbles touching each other, then take a bunch of marbles
out so that a hole is formed, i.e, there is a ring of touching marbles
around the hole. As the item gets warmer the marbles push away from each
other and thus the hole gets bigger. The marbles around the hole can't
get closer to each other unless the molecules become colder and they can't
squeeze past each other into the hole because the repelling forces won't
let them.


So, are you saying the metal, upon expansion, acts like the hole is not
there, that the whole thing is solid?
Incidently, I know that heating metal up, like pulleys, works because I've
done it.



Yep. Doesn't matter whether is a hole or not, at
least theoretically for all practical purposes.