Bad lack of understanding or poor wording -- Bending moment is the result of
applied force and geometry-- NOT material.
Bill--W4BSG
-----Original Message-----
From: Doug Renwick
Sent: Tuesday, August 12, 2014 12:59 PM
To: 'Grant Saviers' ; towertalk@contesting.com ; rthorne@rthorne.net ; 'Don
' ; 'Charlie Gallo' ; 'Gary Jones'
Subject: Re: [TowerTalk] [Bulk] Re: Top Bearing Plate for 55G vs Rotor Plate
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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