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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