On 12/9/16 8:53 AM, charlie@thegallos.com wrote:
<snip>
Aluminum is 22 ppm/degree C
Steel is 12
UHMW PE is about 100
<snip>
SO let's take an example of a 2" diameter mast. going from -40C to 60C
is 100 degrees, and probably wider than the range most ham towers will
see. So that's a change in diameter of, worst case 100*100 ppm or 1%
Your 2" hole that just clears at -40 will be 2.02" at +60C. I don't
think you're going to have a big problem with that 10 mil extra clearance.
If you have a 20 foot mast sticking up above the top, and 5 feet in the
tower, that 0.02" slop will turn into 0.1" at the very top of the mast.
Don't forget that your MAST is going to expand 12*100 ppm too, so it'll be
a bit less than that
Yeah, I was worst-casing it, assuming that the mast was made of Invar<grin>
Nylatron 66 is at 80ppm, go with MD grade it is 50ppm
I think the filled materials would be better choices in general, but I
think from a practical standpoint, you could probably use almost ANY
hunk of plastic you find sitting around.
I'd choose on the basis of pleasantness of fabrication - which mitigates
against glass filled, because the glass will cut up your tooling,
although if you're just using a hole saw in the garage, it's probably
not a big deal.
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