The entire leverage, or tipping moment (not the straight lateral force)
is applied between the thrust bearing (or sleeve and the bottom of the
rotator.) If the rotator is mounted at the junction of the next
section down, the pivoting force is the multiple of the arm. The
straight lateral force is the same, but the leverage is greatly
reduced. Just use the ratio of the arm above the pivot point to that
below the pivot point. For a short arm (rotator at the top of the tower
that leverage may be greater than the entire lateral wind load.
The greater the antenna wind load the greater the leverage. It's much
the same as using a mast to raise the antenna above the top of the
tower.which increases the arm.
http://www.rohnnet.com/rohn-25g-tower Click on 25G and download the
specifications.
Revision G = 90 MPH, Revision F= 70 MPH
The "maximum wind load for a 35', 25G in the lowest wind area is 22.6 sq
ft Exposure B Revision G for exposure C it is only 16.6 sq ft.
At 60 feet the loads are reduced to 20.3 and 15.2.
These figures are for 25G tower installed as per ROHNs instructions
Many of us have used 25G in excess of the ratings with no problems, but
most of us do so with the knowledge that our insurance "may" not cover
the installation.
With more violent weather and huge numbers of claims, insurance
companies may not be nearly as forgiving as in the past. You can also
expect your premiums to go up with a major claim, or a number of small
claims. Ask around. Anyone on here have their insurance canceled, or
major increase in premiums after a claim, or just an increase in claims
from your area?
73
Roger (K8RI)
On 7/20/2016 Wednesday 10:58 PM, Grant Saviers wrote:
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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73
Roger (K8RI)
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