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[TowerTalk] chromemoly mast and wind load

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Subject: [TowerTalk] chromemoly mast and wind load
From: (Bob Bergman)
Date: Tue, 02 Sep 1997 19:28:44 -0600
I am just in the planning stages for next summers antenna project and
have a BUNCH of questions:

First of all, I currently own a Force 12 C4XL/d that is mounted 4 ft
above the top of a Rohn SSV tower.  The SSV houses 2 - thrust bearings,
one at the top and one 6 ft below the top. A Tailtwister is mounted 9 ft
below the top of the tower, turning a 2.0 inch OD carbon steel mast.

I have located a chromemoly (sp?) vendor in Denver that has 2.0 OD, .25
inch wall, spec 4130 chrome molly tubing for $7.12 per foot ( not
galvanized ). Maximum 24 ft.

Northern Colorado, 90MPH zone.

   Replacing my carbon steel mast with the CM.
   Fabricating an extension of carbon steel mast between the rotor and
first thrust bearing from the existing mast and a 24 inch piece of 2
inch ID sleeve material (.18 inch wall.) Set the CM mast in the
extension so as to allow the CM mast to pass through both thrust
bearings. This will put the top of the CM mast approx 17 ft above the
top thrust bearing (top of tower.)
   Install the Force 12 C4XL/d, (wind load 9.6, weight 65#, boom length
30 ft, mast torque 900 in/lb) at the top of the mast. Allowing height
for the truss cable, this should place it about 14 ft above the tower.
   Install a new Force 12 Magnum 520, (wind load 8.1, weight 57#, boom
length 36 ft, mast torque 700 in/lb) 10 ft below the C4XL.

   Whatever CM spec 4130 is, will it be strong enough to handle this
load with .25 inch wall or should I go stronger? .5 inch?

   Would it be better to place the 520 on the top, over the C4XL?

   Is there a different design that I should consider?

   Would I be pushing the limits of the Tailtwister?
   What is the optimum spacing (givin what I have to work with) for
these two yagis? With a lesser distance between yagis, the manufactured
mast extention at the rotor may not be necessary (top of mast 15 ft
above tower.) 

   Any suggestions and/or comments are welcome! This is what planning is
all about!!
Tnx & 73's 

   K0FR -- Bob

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