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

[Grant Saviers <grants2@pacbell.net>]

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




On 7/20/2016 13:17 PM, Roger (K8RI) on TT wrote:
> A heavy duty rotator with a large antenna should be mounted well down 
> into the tower. Mounted at the top, in the tapered section would 
> likely put very high stress at that point.
> 73
>
> Roger (K8RI)
>
> On 7/20/2016 Wednesday 10:56 AM, Bill Tippett wrote:
> > > What’s the strongest rotor that will fit (or can be made to fit) 
> > > inside a
> > 25G?
> >
> > If you can find one, a KLM 1500HD (no longer available) will fit into a
> > tapered top section of Rohn 25.  I know because I just tried the one 
> > I have
> > in 25G.  You have to mount it in a 25AG series top section at the 
> > ~16" open
> > window without Z-bracing.
> >
> > Abbreviated specs:
> >
> > Diameter: 8"
> > Height with top mast clamp: 16.73"
> > Weight with top mast clamp: 28 pounds
> > Rotating Torque: 1,550 inch-pounds
> > Stall Torque: 1,740 inch-pounds
> > Brake: 10,500 inch-pounds
> > 360 degree rotation time: approx 60 secs (48V 1A motor)
> >
> > It's like a small prop pitch (another alternative) with no brake 
> > mechanism
> > other than the motor/gearbox.  The data sheet says it's good for 32.6 
> > sq ft
> > at 90 MPH when mounted inside the tower.  Mine survived regular 100 MPH
> > winds and two exceeding 140 MPH in Colorado (but derate by ~15% for wind
> > pressure at 5000' elevation).  I had a KLM 5L 20 and KLM 3L 40 
> > mounted on
> > it.
> >
> > I *might* consider selling it but will not give it away (e.g. I believe
> > K7NV's prop pitches are ~$3000).  Make me an offer I can't refuse if
> > interested  :-)  I have the manual and can scan the complete 
> > specifications
> > page if interested.
> >
> > 73,  Bill  W4ZV
> > _______________________________________________
> >
> >
> >
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