Jim Lux wrote:
>
>Cutting boards (the white ones) are usually High Density PolyEthylene
>(HDPE) or UHMW (Ultra High Molecular Weight) PE. You can look up the
>material properties at somewhere like http://www.matweb.com/ or any of
>a zillion plastics suppliers.
>
>Real low friction (aka poor man's teflon), fairly good dielectric
>properties (somewhat lossy at HF and higher, compared to some other
>plastics) (It's great for HV DC insulators)
>
Old cutting boards are also good for dipole center pieces, custom end
insulators, gamma arm supports and many similar applications.
In these islands we can't speak much about long-term effects of
sunlight, but HDPE/UHMW does have the advantage of being a continuous
solid material, so it doesn't flake and de-laminate like fibreglass and
other composites.
>The real problem with using it is that the melting point is quite low,
>which makes for trouble machining, and if it's going to sit outside
>with a static load because it will ooze.
>
The ooze or "creep" problem can be minimized by making most of the
bearing out of metal, and using the plastic only as a facing for the
bearing surfaces; and also by taking care to spread the total load over
the largest possible area.
However, as W0UN pointed out a few weeks ago, if you make the whole
bearing out of a big solid chunk of UHMW, that is enough to spread the
load. For most TowerTalk-type applications, you aren't worried about
precision pointing - even if a small amount of play does develop over
the years, who cares?
Moonbounce is where precision pointing really does matter... and even
there, plastic bearing faces have been used very successfully. W5UN used
teflon very successfully for the bearing surfaces of his "Mighty Big
Array" - the one that was rotated by a motorized truck chassis on a
circular track. Teflon is notorious for creeping under load, but W5UN
took care to spread the loads over very large bearing faces. Inspired by
Dave's description in QST, I used pipe flanges faced with 1/8-in teflon
sheet for my own much smaller array. It had extremely unbalanced
sideways loading, but it worked fine with no evidence of creep or play
developing over 10+ years.
--
73 from Ian G/GM3SEK
_______________________________________________
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Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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