I have thousands of cast aluminum "wheels" that have been press fit
tightly into steel tubing (5" diameter, 1/8" thick wall).  Removal is
proving to be difficult.  I need to salvage the tubing.
Would the application of heat or cold be most likely to facilitate
removal?  Include any suggestion as to how to apply the heat or cold,
keeping in mind that cost considerations will limit time per piece to
2 or 3 minutes.

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Request for Question Clarification by pafalafa-ga on 23 Nov 2004 12:24 PST

As the comment below indicates, aluminum is more sensitive to changes
in termperature than steel, in terms of expanding or contracting, so
that cooling the materials will shrink the aluminum more than the
steel, which should loosen things up.

As to HOW to cool them, it depends an awful lot on your site-specific
circumstances.  Are you in a cold place?  Can you turn off the heat
and open the windows?

Let us know the specifics of your circumstances, and perhaps we can assist you.

pafalafa-ga

Hello tgates, I have some experience in the type of project you ask
about. I have worked with steel tool joints on aluminum drill pipe. I
agree with what racecar says about the difference in coefficent of
thermal expansion between steel and aluminum. It may be that the
method suggested will work, but I think there is a better way. You can
heat the assembly and actually crush the aluminum. After the assembly
cools it should be easy to remove the aluminum wheel. There is an old
trick for removing brass or bronze bushings from a steel housing that
uses the same technique. You don't say what type of steel is involved
in the tubing. The temperature that you heat the assembly to might be
limited by the heat treatment of the steel and whether that is
important to maintain. I would think that you need to heat the
assembly to around 1000 °F to get the results required. I assume that
you are not interested in what happens to the aluminum since you only
mention the steel tubing. To heat the assemblies you have several
choices. The most simple would be to use a gas torch (propane,
oxyacetylene, etc.). However, this might not be the most efficient
with thousands of parts involved. I have used induction heating coils
for similar projects. This would be fast and efficient, but depending
on whether you obtain the equipment yourself or hire it done I don't
know how the cost would compare. It is also possible to put many
assemblies into a large furnace such as found at a heat treat
facility. No matter how the heat is produced you still have to handle
the parts again to actually remove the aluminum wheel. Whether this
process is cost effective versus starting from scratch with newly
purchased 5 inch O.D. x 1/8 wall mechanical tubing, I don't know.
In working with the aluminum drill pipe we had to be very careful. The
steel part had internal threads which were several thousandths smaller
diameter than the male threads on the aluminum. The idea being to heat
the steel and shrink it onto the aluminum tubing. We had several
failed trials because the aluminum heated up too much and crushed
resulting in a loose fit. This is why I think this process will work
for you.

Let me know if you have further questions, Redhoss

Answer was very useful.  Decided against using either heat or cold
because of time and cost constraints.  Using an impact method, which
is quick and effective (though noisy).  Thanks for the input. 
Sometimes it helps just to know what not to do.

The thermal expansion coefficient of aluminum near room temperature is
about twice that of steel (the exact values depend on the type of
aluminum and the type of steel, but for most types it's around 24 ppm
per deg C for aluminum and 13 ppm per deg C for steel).  Making your
parts cold will loosen them.  You could stick them in a freezer or dip
them in liquid nitrogen.

You can also use an inductive heater to heat the steel ring and the
aluminum wheels should fall out.  this is relatively easy to construct
and easy to use