The moment at the tower top is what it is from mast and antenna wind
load above the top bearing. You can move around a bit how the mast
shares the loads with the top section of the tower structure but you
aren't going to reduce it with a center bearing. Also, bending
(deflection) is not bending moment.
There are several types of top bearings. With the tube version on US
Towers or a Rohn tapered top section essentially no moment can be
transferred to below the tube. Some thrust bearings with a ball bearing
single race can tilt a bit, but will be damaged if the bending moment
causes more than a few degrees deflection. For thrust bearings with two
races the bearing will prevent most tilt. If a spherical outer race
flanged pillow block is used, then more tilt deflection can happen and
thus more bending below the bearing. If a plastic top radial bearing is
used then how much bending is below the bearing depends on mostly the
thickness of the plastic. If a combined thrust and radial plastic
bearing is used, then there will be little bending below the bearing.
(i.e. 3" mast in 4" of UHMW and a 5.6" diameter 6061 thrust plate on the
mast).
Since the original question was about R45 top plate thickness, for any
sensible thickness the concern is more the bending of the plate vs the
weight it can carry. My "backyard engineering" guess is 3/16" is
probably ok, 1/4" likely ok, and 3/8" is overkill.
So for some top bearing types, you can slightly reduce the mast bending
inside the tower and a small amount outside with a center bearing. The
moment of inertia ("stiffness") of Rohn 25 is 30 times that of a typical
2" x 0.25" wall steel mast. Bending deflection for a point loaded beam
and a fixed base is proportional to the length cubed divided by the
moment of inertia. So there is 30x less deflection in a stick of R25 vs
a 2" x 0.25" wall mast for the same length and load, which is why the
adage "masts are no substitute for a tower" is true. Thus, there is
also very little contribution to the strength of a tower section from an
internal mast. Long internal masts may be nice for where the rotator
goes, but not much more.
A center bearing might slightly change the moment applied to the
rotator, but again that is usually not a concern. And yes, 6061-T6
aluminum masts, with about 1/3 the modulus of steel will bend more for
the same physical dimensions and forces.
There are handy calculator apps for
moments of inertia of pipe at
http://www.engineeringtoolbox.com/pipe-formulas-d_1335.html (helps
understand why bigger diameter is better than thicker walls)
And for simple beam deflections and stress (not for lattice towers) at
http://www.engineersedge.com/beam_calc_menu.shtml
The Rohn 25 mechanical data is at
http://static.dxengineering.com/global/images/chartsguides/r/rohn-25gdrawingc630625.pdf
caveat emptor etc. My comments are not to be used for designing structures.
Grant KZ1W
On 8/12/2014 10:59 AM, Doug Renwick wrote:
Here is my take. A blanket statement does not fit all situations. A
middle bearing does have merit. I cases where a long mast fits inside the
tower or there are stacked antennas on the same mast, a middle bearing will
stop the bending moment in a strong wind. And a bending moment will be
larger with an aluminum mast.
I put a middle bearing in my towers. Alignment has not been a problem. The
middle bearing is loosely fitted so there is some room for motion. It is
easier to have it there than try and install it later.
Doug
-----Original Message-----
I agree with Steve that a middle bearing is not a good idea as aligning
3 bearings is very difficult and a middle bearing serves no purpose at
rotator rpms.
However, a middle support plate with a removable u-bolt or clamp, that
can constrain the mast when you want to raise it to remove the rotator
for service is a big help. Then the mast stays in column and doesn't
overload/twist the top bearing.
I also dislike the commercial "thrust" bearings. A block of black UHMW
3" thick bored for the mast od will never need servicing. Or stack up
some HDPE kitchen cutting boards bored to fit, but they will need a coat
of paint to protect them from UV. A rotator sized for a 4L full size
40m beam bearings can easily handle a 500lb thrust load. btw, my 40m 3L
full size OWA weighs 350# plus 150# for the 3" x 0.25" wall alloy mast
(a must for that size antenna).
Grant KZ1W
On 8/12/2014 8:04 AM, K7LXC--- via TowerTalk wrote:
I'm working on getting the parts together for a Rohn 55g tower.
? The Rohn 55g Bearing plate sure is expensive. I see two other
possibilities.
1 - Use the less expensive beacon plate and drill a hole for the
mast/bearing.
2 - Use a rotor plate for the same purpose.
I plan on having a top bearing, an intermediate bearing then the
rotor.
I have no idea what the thickness of the Rohn bearing plate is as
compared to a beacon or rotor plate. I'm hoping to put up a full size 4
element 40m beam some day, so the system will need to handle a good
300+lbs.
Hiya, Rich --
The beacon plate is FB if you don't mind drilling it.
As for an intermediate bearing, I'd discourage it. The plates are
not
precision items and you'll just introduce bind into the system. Since all
you're trying to do is capture the mast, put the rotator plate in the
middle
and skip the bearing - cheaper too. And the upper TB carries all the
weight anyway so a middle bearing doesn't provide any structural help.
Cheers & GL,
Steve K7LXC
TOWER TECH
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