Don wrote:
"Richard Cline" wrote in message
...

In article ,
"Greg O" wrote:


"Don" wrote in message
link.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.