[TowerTalk] Rohn 25 & Mast length

Grant Saviers grants2 at pacbell.net
Thu Apr 20 00:51:41 EDT 2017


You make some good points and we agree on a lot, but the Leeson analysis 
re spreading the moment only works in some cases, IMO.

As a counter example, consider the HDX589 (I think you have a 689 with 
the same top section?)  As you note at the top there is a tube sleeve 
that constrains the 2" od mast over about 10" and thus precludes the 
bending moment to be transmitted to below that sleeve.  Almost all of 
the moment goes into the two welded plates holding that sleeve since 
there is very little clearance sleeve to 2" mast.  It's like making a 
lever and welding the lever to the pivot.  So for this tower design all 
of the moment is exerted top and bottom of the sleeve, no matter where 
the rotator is mounted. As one post noted, the taper top R25AG has legs 
bent in and the sleeve only held at the point, no wonder these legs 
bend.  The other 25AG1/2/3/5 point tops all have tube bracing top and 
bottom of the 15 to 24" of tube (which share the moment) and won't pass 
any mast moment on to the rotator with a mast that is close fitting to 
the tube ID.

How much a top bearing permits the mast moment to be shared with the 
rotator depends a lot on the bearing construction.  If the bearing acts 
as a pivot with unconstained angular motion (at least over a several 
degrees) then the lever arm analysis I think is correct re spreading the 
moment over a longer section of tower when the mast length inside the 
tower is increased.  Whether that spreading is needed within the other 
factors considered re tower loading is doubtful to me.  Otherwise, tower 
manufacturers would specify top bearing construction and rotator 
locations (which UST did for the 589 with welded in place structures!) 
and PE's would specify these as well.  I also note the BAS25G 16" very 
short section combo top plate and rotator plate has about 14" between 
the two plates.

I think a top bearing such as the new DX Eng design won't tilt much from 
what is shown in the cutaway drawing.  The Rohn TB I had many years ago 
had little angular stiffness.  The pillow block bearings some folks use 
with spherical od outer races have no angular stiffness by design.  
OTOH, one set of grub screws in a single plane to hold a mast smaller OD 
than the bearing ID is probably going to permit (some, a lot?) the 
moment to transfer - ala DXE, Rohn, Yaesu.  My 4" thick polymer closely 
fitting (0.030")  radial bearing is pretty stiff angularly, but probably 
not as much as the UST589/689 sleeve.

Leeson gives a one brief example of moment spreading but does not 
provide a thorough analysis. (who am I to argue with him hi hi).  I 
think other factors as well as the bearing construction play 
significantly into that analysis.

Grant KZ1W


On 4/19/2017 9:22 AM, Jim Thomson wrote:
> ##  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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