Tom -- Yup. It's holding more heat.
The common aluminum casting alloy 356 has anywhere from 2-1/2 to 3 times
better thermal conductivity (meaning 2-1/2 to 3 times more heat conducted
out of the housing in a given time) than the magnesium casting alloy AZ63A
(6% Aluminum, 3% zinc; I'm guessing it's the alloy in the QC housing). This
could explain most of difference in observed temperatures.
If you're measuring 290F at the cover that means the pinion bearings are
running even hotter. Suggests careful selection of the lube to insure that
it's a good synthetic that can handle the high temps. Cast magnesium does
have a tendency to lose yield strength to a greater degree than cast
aluminum as it approaches 400F. This would most likely be observed as a
loss in the quality of the bearing press fits at the pinion.
My references today are the 1991 edition of the Materials Engineering Annual
Materials Selector (Penton Publishing) and Designing with Magnesium
published 1947 by the American Magnesium Corporation (an Alcoa Subsidiary)
I have other metallurgical reference books in my library.
Ed Weldon
----- Original Message -----
From: "tom sarda" <sardatech@yahoo.com>
To: <land-speed@autox.team.net>
Sent: Saturday, June 14, 2008 6:24 AM
Subject: [Land-speed] Magnesium
> Hey, List,
> Does magnesium transfer heat differently than aluminum? The question
arises as a result of work I have done on six of the cars I have worked on
that run at the local short track. I have rebuilt the Quick change rear
ends. Five of the rears are aluminum and one is magnesium. I have measured
the temps on each after racing as soon as the return to the pit area...as
soon as i can get to them. The temps are measured at the bell(on the side)
and at the rear cover where the spur gears are housed. I expect to see the
temps to be higher at the cover than the bells. The aluminum rear ends run
anywhere from 206 to 225 at the bell and 225 to 244 at the cover. The
magnesium runs 270 at the bell and 290 at the cover. (Track temp last night
was 118). Is the mag rear end holding more heat than the aluminum?
> the lubricants vary from team to team with Mobil 75/90 the most popular,
Royal purple and Gibbs 75/110 ars also used. The mag rear uses Gibbs lube.
Went to it after last weeks temps were 10 degrees higher at the cover.
> Any one have an idea about aluminum versus magnesium? Thanks in advance
for your thoughts.
> Tom
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