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[TowerTalk] chromemoly mast and wind load

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Subject: [TowerTalk] chromemoly mast and wind load
From: (Tyler Stewart)
Date: Wed, 03 Sep 1997 04:45:41 +0100
Bob Bergman wrote:
> 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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Doesnt the C4xl already have 20 meters?  If so, you are going to have
serious problems with those beams being so close together and resonant
to each other.

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