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[TowerTalk] aerospace 7075-T6 material: Unobtanium

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Subject: [TowerTalk] aerospace 7075-T6 material: Unobtanium
From: (Steve Jackson)
Date: Thu, 18 Mar 1999 11:58:16 -0500

Good thoughts!  Thnaks... such questions help me keep an 'open mind' about
this whole project.  I sure am learning a LOT about metallurgy and physics.
 And they say ham radio is boring ... ha!

The Glen Martin 'Voyager Hazer' is a steel cradle that rides up the side of
the tower on 2 tracks.  The rotor/bearing cradle that rides in these tracks
goes to the top of the tower, then bolts to the tower itself (not via the
tracks) and thus transfers all the torque to the tower.  I am using Rohn
55G, guyed with 5/16" cable, at 75' and 50'.  The 80' tower should be good
for 36 sq. ft. loading at 90 MPH.  (I live in a 70 MPH zone)

I am planning on 3 antennas, physically 'stacked' but not electrically.
The big antenna, a 36' boom 10-15-20-40 at 10 sq. ft. windload, is at the
bottom only a foot or so above the thrust bearing.  12' above it is a
12-17-30 antenna, 4 sq. ft.; in between is a 6 meter beam (small in
comparison) at 2 sq. ft. windload.  Total windload is 16 sq. ft, not that
bad for 3 beams giving coverage from 40 through 6 meters!

At 90 MPH, the combined forces of these 3 beams, plus the mast load itself,
gives me a requirement for 42,773 PSI yield mast material.  With only a 10
MPH wind increase to 100 MPH, the yield requirement goes up to 52,803 PSI
... almost 20% more!  6061-T6 alloy mast, 2" OD with a 1/2" wall specs out
at 42,000 PSI.  So, you see my dilemma.

The track system has a weight limitation of 225 lbs.  My rotor weighs 42
lbs., and the antennas weigh 145 lbs. combined.  So, that "leaves" only 38
lbs. for the mast weight.  That is why I must use aluminum and not steel.

73 Steve KZ1X/4

* * *

At 09:34 AM 3/18/99 -0500, you wrote:
>OK Steve, but that brings up the question about the strong mast...
>If you are expecting the Hazer to hold stacked antennas in 90mph winds,
>you may be expecting too much.  Remmeber that the lever arm will be trying
>to yank the Hazer off the tower.  Is it rated for that, or is its
>wind load rating expecting the force to be at the top of the Hazer,
>not several feet higher?  And if you are NOT planning on stacking antennas,
>why do you need a heavy duty mast?  Even a cheap mast will hold a big
>antenna if the load is right at the bearing.
>Just thoughts.
>Terry Zivney, N4TZ/9

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