[TowerTalk] Rohn 25 & Mast length

Jim Thomson jim.thom at telus.net
Wed Apr 19 12:22:54 EDT 2017

Date: Tue, 18 Apr 2017 17:47:43 -0700
From: Grant Saviers <grants2 at pacbell.net>
To: jim at audiosystemsgroup.com, towertalk at 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

##  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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