[TowerTalk] 2 inch Chrome Moly mast needed

Fri Apr 8 19:05:59 EDT 2016

I've put a link upmany times to the system I had.  The mast was 2" 
structural steel.  I don't know the specs, but I knew at the time I had 
it substantially overloaded. It was an experiment. There was the TH5 at 
2' above the top of the tower which is not much load, but then there was 
the 7L C3i 6 meter Yagi  About 12 to 15' above the TH5 and a 14' cross 
boom with  a pair of 12L C3i  at the boom ends and a pair of 11L C3i 440 
antennas inboard on the same cross boon at somewhere around 28 - 30' 
above the top of the tower.

We are not in a high wind area, but many times I saw those top antennas 
moving in an arc that O though was so severe that they would never survive.

Surprisingly that mast never failed, but it beat the crap out of those 
12L 2 meter beams and destroyed a TB3 a and a TB4.  There is no way we 
could get a crane into position to lift the mast and antennas.  They 
couldn't get down our street, they'd never make the turn, into our 
driveway, our driveway wouldn't support the machine, and because of the 
trees and the house, they'd never be able to reach the tower.  If the 
mast had bent, I would have lost all the antennas.

  So after lowering and raising the whole works twice to replace the two 
thrust bearings about about 2 to 3 years apart AND doing the same to 
replace the 5  28' pigtails when one rotator failed I decided to put the 
3L  40M3 with the 7L C3i 6-meter Yagi about 12' above that, if I can 
find a local climber.

I could rotate the head on the milling machine and bore out the opening 
in the tower top plate to take a 3" mast and bearings, but at this 
point, it's hardly worth the effort.

73

Roger  (K8RI

On 4/8/2016 Friday 9:56 AM, Jim Thomson wrote:
> Date: Thu, 7 Apr 2016 21:39:52 -0400
> From: <john at kk9a.com>
> To: <towertalk at contesting.com>
> Subject: Re: [TowerTalk] 2 inch Chrome Moly mast needed
>
> My suggestion was to consider a 3" x 1/4 wall 1026 instead of a 2" Chrome
> Moly mast. I suspect that 3" 1026 is stronger than 2" 4130 (I don't have a
> mast calculator handy) and it is easier to climb. I purchased a 3" 1026 DOM
> from steel company and had it galvanized.  My costs were significantly less
> Array Solutions plus did not want their odd 2 7/8 size.  At that time DX
> Engineering did not sell masts and no one else sold a 3" mast so making one
> was my only option.
>
> John KK9A
>
> ##ARRL has a free, dead on accurate  mast calculator.   You can plug in any OD,
> any wall thickness, and any yield strength .  Then insert the yagis at any point above the
> top of the tower, windload on each..and it spits out the survival speed in mph.
>
> ##  fun to play with, esp when you compare stuff like 1.9 OD  x sched 80, 35 ksi
> pipe vs  2.0 OD  x .25  4130 CM.... or  2.0 OD  x .375 wall  4130 CM.   Yikes, the
> results are more than an  eye opener.
>
> ##  yes, I did on the software, just what  you used.   I compared 2.875 x .25
> 87 ksi  mast,  made from 1026 DOM, as sold by  array solutions.....  VS   2.0 OD  x  .25 wall 4130  CM
> and the difference is astounding, and then some!    Even though the  2.875 mast is ‘only’  87 ksi, its
> one HELLUVA lot stronger  VS  2.0 OD  x .25 wall  4130 CM... (107 ksi).
>
> ##  also tried  3.0 OD  x .25 1026 DOM  ( 87 ksi)   VS   2.0 od x .25 4130 CM  (107 ksi)  and of course
> the difference is even more.   Its also a helluva lot stronger than the 2.0  x .375  4130 CM  mast I  use myself.
>
> ##  Im stuck with a 2.0 O mast, with the HDX-689 tower, since the top of tower + mating tube /plate assy  directlybelow the
> tower + both thrust bearings are all set up for 2.0 OD.  The sleeve tube between  top plate and  2nd plate, that is a few inches
> below the top is all welded.    I bought the tower used.... and would highly NOT  recommend  choosing a 2.0 OD mast  to
> anyone.  Whoever standardized the 2.0 OD mast  was fubar.     Plan B was I bought a 20 ft long x 2.0 OD  x .375 wall
> 4130 CM mast... 107 ksi.    It weighs a whopping 140 lbs, with 14 ft of it above the tower.  two trust bearings  used, peer brand,
> plus the UST sleeve tube assy, plus the K7NV small PP that hangs below the rotor plate.   It’s the best I can do.  My 1st
> choice would be 3.0 x .25  4130 CM..since it weighs the same as my 2.0  X  .375 mast..and cost the same.... yet the 3.0
> mast is a magnitude stronger.
>
> ##  the strongest mast  I heard used was by Blackie, W6HLH, back in the mid 70s.  3.0 OD   X  .375 wall,   4130 CM,
> 115 ksi.     The highest I have seen  4130 CM  heat treated to is 125 ksi.   115 ksi  was quite common, and also 120 ksi.
> In recent years,  the heat treated CM has gone down in ksi.  Most are   105-107 ksi range.
>
> ##  Local engineer   would only accept a 2.0 x .375 wall CM  mast..barely.   What else I have seen used is
> 2.5 OD  x .375  4130 CM.
>
> Jim   VE7RF
>     
>
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-- 

73

Roger (K8RI)

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