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[TowerTalk] Biggest rotor that will fit in a Rohn 25G

To: <towertalk@contesting.com>
Subject: [TowerTalk] Biggest rotor that will fit in a Rohn 25G
From: "Jim Thomson" <jim.thom@telus.net>
Date: Thu, 21 Jul 2016 04:57:10 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Date: Wed, 20 Jul 2016 19:58:01 -0700
From: Grant Saviers <grants2@pacbell.net>
To: "Roger (K8RI) on TT" <K8RI-on-TowerTalk@tm.net>,
towertalk@contesting.com
Subject: Re: [TowerTalk] Biggest rotor that will fit in a Rohn 25G

The assertion is often made that a longer mast inside the tower 
strengthens the tower.  I think not.

So perhaps those with civil/structural engineering skills greater than 
my novice level could help me to understand why lowering the rotator 
lowers the tower stresses.  My analysis follows.

For a perpendicular wind load to the tower and antenna, and a given 
antenna height above the tower,  the lateral force exerted on the tower 
is the same whether there is 1' or 100' of mast inside the tower.

Given that a large antenna should be close to the top of R25, the added 
moment from the mast applied to the tower top is essentially independent 
of the mast below the top, when the top section has a mast sleeve such 
as tapered R25 or big UST crank ups.  Thus the amount of side load on 
the rotator is also minimal on towers with those tubular mast tops since 
the mast can only move a fraction of a degree inside that tube.

Some posters claim that a mast "strengthens" the tower,  but the ratio 
of the bending moment of inertia is large from 2" x 0.25wall mast to a 
10' stick of R25.  The moment of inertia for R25 per factory spec sheet 
is 15.3 in^4 and for 2"od x 0.25 wall is 0.54 in^4.  In other words the 
R25 tower is almost 30 times stiffer in resisting bending than the 2" 
mast.  Hence the mast makes an insignificant contribution to the 
strength of the tower.  (see Leeson's analysis of sleeved elements in 
Physical Design of Yagi Antennas)

A stock "thrust bearing" (Yaesu, Rohn) may have enough slop to allow the 
mast to pivot at the top plate quite a bit, at least the Rohn one I used 
(never again!) could let the mast achieve scary angles when the rotator 
was removed.  So in this case there is some moment transferred to the 
rotator  bearings and mounting plate, but the tower resists those 
bending loads 30x better than the mast.

A lattice tower is engineered to have high stiffness due to the large 
"diameter" (distance to the neutral axis) and low weight by using small 
tube legs and welded bracing to distribute the loads among the legs.  
While it is beyond my skills to compute the stress distribution for 10' 
of R25 loaded as a cantilever, IMO it will withstand a lot more force 
before it fails than a 10' length of 2" diameter mast.

Grant KZ1W

##  the mast wont strengthen any tower.  If you look in leesons book,
He sez the bending moment  on the tower is  normal bending moment you
would have at the bottom of the mast, as it enters the top bearing. 
But he then converts the bending moment in ft lbs...to inch lbs, by multiplying
x 12.   Then he take the bending moment in inch lbs.... and simply divides by 
the 
distance, in inches,   between the  top bearing..and what ever is below it, 
like a 2nd
bearing...or a rotor. 

##  IE:   2500 ft  lbs  X  12 =   30,000 inch lbs.   Say the rotor is down 4 ft 
below top of tower. 
Its then 30,000 / 48  inchs  =   625 ft lbs.  ( you can also just use 2500 / 4 
= 625 ft lbs)
Assume the wind is from the north.   The side force experienced at the south 
side of the top bearing
is now 625 lbs.   The side force experienced at the north side of the rotor is 
also 625 lbs. 

##  If you now instead insert  the mast  10 ft into the tower,  its now becomes 
2500 /10 =  250
lbs  on the side of the top bearing..and also the rotor.   So yes, putting the 
mast lower into the tower
will help a lot. 

##  heres  where that concept wont work.   Put the mast 10 feet into the tower, 
but now use a 2nd thrust
bearing, say 4 ft below tower top.   IE: top bearing, then 2nd bearing down 4 
ft, then the rotor down 10 ft. 

##  all the forces are between the 2 x bearings..... = 625 lbs   The  portion 
of the mast below the 2nd bearing.... between
2nd bearing and rotor, plays no part in the results. That portion of the mast 
could be made of way lesser strength
since all it has to handle is the torque of the array.  

##  If any of you have leesons book, its explained at the top of  page 7-14

Jim   VE7RF



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