In a message dated 97-12-01 14:01:01 EST, madamson@compuserve.com writes:=
<< Another illustration of this is making molds for cast iron. Molds are=
made
<<larger>> than the actual piece desired to accomodate the shrinkage tha=
t
occurs when the part cools. The exception is if you want a void in the
part. The plug for the hole is made <<smaller>> so that shrinkage
results
in a void of the proper size.
=
Regards,
=
Michael.
>>
Tom Green wrote:
>> I can't stand it any more!! I've tried not to comment, but here it
goes. The bores should also get smaller as they get cooler. For instanc=
e,
in order to put a new ring gear on a flywheel, does one heat it or put it=
in
the freezer? The obvious choice is that it is heated to a very high
temperature to increase its circumference. This allows it to be put onto=
the
flywheel where it cools. The decrease in circumference allows it to stay=
on
the flywheel. This anology should also hold true for the circumference o=
f
a
cylinder bore.>>
You feel frustrated! =
The illustration you used is not valid because you are dealing with a
narrow ring of material that does expand when heated; the inside and
outside diameter does get larger due to the shape of the part. The very
important difference is that you are not dealing with a large mass of the=
material. Also your ring gear is not cast iron so you are able to heat i=
t
with a torch..
If your analogy held true, then an engine block with pistons of the same
material, with the same coefficient of expansion, should never sieze when=
overheated or even get difficult to turn over. As the pistons expand so
would the bore, no problem: wrong!
As an aside, the design of durable cast iron parts is an exacting science=
that we as B owners are reminded of from time to time. A <brittle> metal=
,
cast iron is very intolerant of webs and thin areas, say between valve
seats.
Next someone is going to cite the heating of a siezed bolt to release it
from a casting. It works except it shouldn't. The heated bolt should
expand and the hole in the casting, according to your theory should get
larger. It doesn't. Actually the cast iron barely gets warm. The trick =
is
that you have two different materials with different properties. =
Take a torch and try to cut a piece of cast iron. You won't even to be
able to get the area where the torch is playing cherry red so that the
oxygen can oxidize the material out of the cut line. Cast iron has a hig=
h
rate of thermal transfer making it ideal for engine blocks, cookware and=
cast iron stoves. =
What happens is that the heated steel bolt expands rapidly breaking the
bond on the thread line. The heat from the bolt is dissapated quickly by=
the cast iron and when the wrench is applied the bolt can be removed.
That's it. My final sally into this area. =
Regards,
Michael.
=
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