On Thu, 28 Aug 2008 14:46:23 -0700, Roy Lewallen
wrote:
Jeff Liebermann wrote:
. . .
You don't want to heat the outside. That will cause the outer tube to
expand, which will increase its compression on the inner tube. You
want to cool both tubes causing both to contract. If you can remove
an end cap, pour in some ice water.
. . .
If you heat a ring of material, both the OD and ID increase in
proportion. This loosens, not tightens, its hold on an inner object.
I did this with an aluminum torus way back in college daze. You're
mostly correct. The O.D. does most of the radial expanding, but there
may be a small amount of expansion in the I.D. depending on the
compressibility of the material. If the material is soft, the I.D.
will shrink slightly. If the material is hard and incompressible,
then the I.D. will expand slightly. This illustrates it fairly well
(for incompressible materials):
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/thexp2.html
What I'm counting on is NOT the change in radius cause by a change in
temperature. I'm looking for a slight change in length, which with a
taper, has a somewhat similar effect on releasing the jam than a
radial change. In that case, the inner taper shrinks, while the outer
taper lengthens.
I recall from freshman physics that if you heat two concentric rings
made of the same material (having a positive coefficient of expansion),
all dimensions increase by exactly the same proportion. A little
geometry shows that any gap, even microscopic, between them will
therefore also increase proportionally. Cooling both objects of course
has the opposite effect, reducing the size of any gap and tightening the
fit.
Apparently, I'm not the only one having problems with the concept.
See:
http://www.eng-tips.com/viewthread.cfm?qid=99646
Heating the outer one and cooling the inner one, as Ed did, gives
you the best of both, increasing the ID of the outer tube and decreasing
the OD of the inner tube.
Maybe. If the ID of the outer tube does not change with temperature,
then heating the outer tube is a waste of effort. If the material is
sufficiently incompressible, then it will expand slightly as you
suggest. I'm not sure how fiberglass reacts as it depends heavily on
the glass weave pattern and composition. If the pole bends, it may
have some compressibility.
Drivel: If the surface roughness is more than the heated or cooled
radial expansion of the tubing, it may still remain stuck.
Roy Lewallen, W7EL
--
Jeff Liebermann
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