"Joe" wrote in message
news:Hzz1e.23138$Ue6.22152@trndny04...
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.

In short, the material doesn't expand in the path of least resistance, it
expands as a whole from the center. Sorta... Try bending a small tube.
It is harder to bend initially than a solid rod because you have to
"crimp" four walls of material versus just the two outside walls of a bar.
In a pulley, the inside ring cannot shrink into itself so the unit expands
outwards and "stretches" the outer portion of the ring rather than
compressing the molecules of the inner ring.

Good explanation.

Sorta...

I think. LOL