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[TowerTalk] chromemoly mast and wind load

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Subject: [TowerTalk] chromemoly mast and wind load
From: (
Date: Wed, 3 Sep 1997 16:03:37 -0400 (EDT)
In a message dated 97-09-03 06:19:50 EDT, (Bob Bergman)

>  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.)

     Sounds like a fun and rewarding project.

     I'm not sure why you need the extension. The only reason I can think of
is if your rotator is way down the tower. Putting couplers/extensions on
round mast material is a moderately tricky proposition. There's usually some
cumulative slack in the system that allows it to swing back and forth in the
wind and elongate any bolt holes. It will eventually fail. 

>     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?
      The TOWER TECH MARC Program is a Mast, Antenna and Rotator Calculator.
Running your numbers through it shows that there will be approximately 61,000
in.lbs. of bending moment on the mast. It recommends a mast with 115,000 psi
yield strength and 0.250 inch wall. 

     The MARC Program is available from TOWER TECH for $9.95.

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

     I don't know if I would mount two 20M antennas that close together.
>     Would I be pushing the limits of the Tailtwister?

    According to MARC, this system will produce an Effective Moment of almost
3000 ft./lbs. which is within the 3400 ft./lbs. capacity of the T2X.

    I also have a reprint of my 3-part series on rotators that appeared in my
Up The Tower column in CQ Contest magazine. It's available for an SASE with 2
stamps from TOWER TECH.
>     What is the optimum spacing (givin what I have to work with) for
>  these two yagis? 

     I suggest you talk to the manufacturer.

73 and GL,  Steve  K7LXC

    TOWER TECH -- professional tower supplies for amateurs

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