I would think that in a small tower like a 25G, a 2" steel mast captive
in ea section (thrust bearings) would add noticeable rigidity to that
portion of the tower.
I can understand where a mast would add little to none on larger towers.
After installing the 45G and climbing it many times, it felt much more
secure than the properly guyed 25G.
73, Roger (K8RI)
On 4/19/2017 12:22 PM, Jim Thomson wrote:
Date: Tue, 18 Apr 2017 17:47:43 -0700
From: Grant Saviers <grants2@pacbell.net>
To: jim@audiosystemsgroup.com, towertalk@contesting.com
Subject: Re: [TowerTalk] Rohn 25 & Mast length
So beyond distributing the mast load over some amount of tower length,
more mast in this common example doesn't make the tower significantly
stronger. The proxy I used for the minimum amount of mast inside is the
top plate to fixed rotor bracket as provided from the factory and that
is about 3' to 4' in many towers.
Re wind loads, I think the mast strength calculators all treat the top
of the tower as a fixed hard attachment and perform the appropriate
cantilever beam calculations on the exposed mast. Thus they ignore
whatever is going on inside the tower. I'm not aware of any that model a
mast with two supports (top bearing and rotator), but then what would
the proper models for those pivot points?
There have been prior posts re this topic as well. Of course a real
structural engineer may have more to say or correct me where I have erred.
Grant KZ1W
## What you folks are all missing is.....with more mast inside the tower, you
REDUCE the SIDE LOADING at BOTH the top bearing AND the rotor. This is clearly
explained in Leesons book, on page 7-14..which is in chapter 7... Masts +
Rotators.
## IE, if the wind is coming FROM the north..and yagi pointed north, the side
loading
will be at the south end of the top bearing plate, and at the north end of the
rotor plate.
## If you move the rotor from 4 ft inside the tower.... down to say 8 ft
inside the tower,
you will have reduced the side loading at both the top bearing plate and rotor
plate by one half.
## now if you install a 2nd bearing, down 4 ft inside the tower, the side
loading is now between
the top bearing..and 2nd bearing. The rotor is still down 8 ft inside the
tower. In this case,
using two bearings, the rotor can be located any where below the 2nd bearing,
and you will
have no change in the side loading between the two bearings. There will be NO
side loading
at all on the rotor..when 2 bearings are used above the rotor.
## the side loading is between the top bearing.....and whatever is the 1st
thing below it, which may
be either a rotor, or a 2nd bearing.
## On top tower sections that are not very strong, installing the rotor down
6 ft or more is a real
advantage, esp if there is a lot of mast above the tower, or wind loading on
the yagi is high, or both.
## grant is correct. The mast inside the tower does not add to the strength
of the tower, 3% at most,
and even less, if bigger face width towers are used, like 45, 55, 65.
## re mast calculators, they assume the top of the tower is a fixed
attachment. So even if your shiny
new 3 inch CM mast, with its .25 inch wall thickness, and sticking 20 ft
above the top of the tower,
with the remaining 4 ft inside the tower, with rotor down 4 ft, looks good on
the mast calculator, you
also have to factor in the strength of the top 4 ft of the tower. With huge
windloads, esp way up the
mast, the side forces at the top bearing plate and rotor plate will be massive.
If the top 4 ft of tower
is not strong to begin with, it will fold on you, even with guy wires at the
top of the tower. It will still
fold on you, with guy wires down 4 ft from top of tower. Ditto with guys down
2 ft from top of tower.
## Most crank ups have heavy duty top sections. UST uses both a welded top
bearing plate, and a 2nd
welded plate, just a few inches down from top plate. Then a welded tube
collar, between the two plates.
Then the mast is positively captivated between the 2 plates. Then either a
2nd bearing, or a rotor can be installed
4 ft down inside the tower. Then another plate can be installed 6 ft inside
the tower..with a rotor installed.
## I used the top bearing, and a 2nd bearing down 4 ft... then the prop pitch
down 6 ft. Then a 20 ft length
of 2 inch OD x .375 wall CM mast. 6 ft inside the tower, and 14 ft above
the tower.
## For the OP, who just wants a mid sized tri-bander 4 to 5 ft above the
tower, a 10 ft length of just about anything
will suffice, with the rotor down 6 to 5 ft. 4 ft down and 6 ft above
tower..with yagi at 4 to 5 ft above tower will also
work.
## The mast estimator / calculator on DXE site, uses the UBC-97 Exposure D
spec..which is very stringent imo, beyond overkill.
The UBC-97 Exposure D spec also requires you to input a height. Their mast
calculator is based on 100 ft tall tower, with mast
above that.
Jim VE7RF
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73
Roger (K8RI)
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