Indeed my US Tower has a long tube at the top for the radial bearing,
but it also has a machined bearing sitting on top of the tube for holding
the mast. I'm not sure if it is a ball bearing or a roller bearing. I guess
that is their way of instantiating the TB-3.
Mike K9MK
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Grant
Saviers
Sent: Sunday, May 05, 2019 10:39 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] TB3 Postmortem
A great list of why a bored plastic plate as a sleeve radial bearing is
better than the so called "thrust" bearings. Note that US Tower
crank-ups use a piece of galvanized tubing as a radial bearing at the
tower top.
Grant KZ1W
On 5/5/2019 9:50 AM, Bob Shohet, KQ2M wrote:
> Hi Scott,
>
> Your post brings up some important points.
>
> First, when using any thrust bearing (TB), it is important to realize that
the TB may not be machined perfectly so that the angle of the holes for the
setscrews may be slightly different, so when tightened, can cause the
antenna mast in the rotator to be slightly out of alignment when compared to
vertical. This can become a problem especially with a heavy antenna and a
heavy mast!
>
> Second, often the clamps on the rotator shelf (on which the TB is mounted)
or on the top flat tower section are also slightly out of alignment when
compared to vertical so that the combination forces the mast holding the
antenna to NOT be vertical. This can make is easier for the antenna and
rotator to bind in the TB and constrict rotation. You would notice this
even more on very windy days or when there is ice which adds additional
weight to the entirety of what is being rotated and adds to the forces
fighting each other. There is only so much binding that the rotator can
overcome and often it will help to degrade the ball bearings and the motor
much more quickly than would otherwise occur.
>
> Third, most people make the mistake of tightening the mast clamps in the
rotator before rotating the antenna to see if there is anything if
misaligned and if the antenna can rotate freely. This ensures that the TB
is most likely to bind, especially if it is on a sidemount which may be
misaligned in and of itself.
>
> So there are at least four types of potential misalignments that can
account for binding of rotation over and above any existing issues with the
rotator or the TB.
>
> I have had a lot of experiences with these various binding forces. It is
quite amazing how the complex combinations of the slightest differences in
misalignments of fractions of an inch here and there can be the difference
between an antenna that freely rotates with a happy rotator and thrust
bearing vs an antenna that frequently binds with a worn TB and damaged
rotator.
>
> I???m inclined to think that turning the antenna every so often and
leaving in different positions should help to spread the wear and deter the
formation of any ???grooves???. But a better alternative is to make sure
that the rotator that you are using is significantly higher rated than what
you are attempting to turn and that everything is aligned as close to
perfectly level as possible with the tightening of all bolts and clamps done
in several stages with rotation observed in between to ensure that further
tightening is not causing any problems while the person doing the tower work
is still on the tower and observing everything from a few feet away before
finally making the final tightening. In some cases I have had to leave
extra ???play??? in the TB screws to adjust for the misalignments using the
TB more as a ring to hold the mast in place as opposed to being tight.
>
> It also helps to have the antenna balanced for weight right at the point
where it is clamped to the mast. All of my yagis are perfectly balanced for
weight
> at the boom to mast clamp because to do otherwise not only makes it harder
and less safe to work on them but also because it puts additional stress on
the TB and the rotator even while fixed in place and may cause them to wear
unevenly and fail more easily.
>
> 73
>
> Bob KQ2M
>
>
> From: K9MA
> Sent: Sunday, May 05, 2019 11:05 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] TB3 Postmortem
>
> True, but the rotator distributes the load among a much larger number of
> ball bearings. The shape of the races in the TB3 make it clear it was
> designed primarily for radial loads, not axial. Also, an "in tower"
> rotator can be removed without taking the whole antenna system down.
>
> Another thought occurred to me: With the thrust bearing bolts tightened
> down, the system is overconstrained. That means that very high stresses
> could occur if the two bearings systems aren't perfectly true as they
> fight each other.
>
> 73,
> Scott K9MA
>
>
> On 5/5/2019 06:46, mike repinski via TowerTalk wrote:
>> Then the rotator gets the little marks. Can't win.
>>
>>
>> -----Original Message-----
>> From: K9MA <k9ma@sdellington.us>
>> To: towertalk <towertalk@contesting.com>
>> Sent: Sat, May 4, 2019 9:39 pm
>> Subject: [TowerTalk] TB3 Postmortem
>>
>> After nearly 30 years of holding up my TH7 and D40, the thrust bearing
>> was pretty rough. The T2X never had any trouble turning it, but I took
>> it apart to see what was going on. I found the surfaces of the two
>> aluminum pieces deeply indented by the ball bearings. It's scrap metal
>> now, but I wonder if the long periods I left the antenna in one position
>> might have made things worse. Would it have helped to just turn it once
>> a week or so? Or is such failure inevitable? After all, once the
>> bearings create the slightest indentation, they're going to always
>> settle into those positions, making them worse. I suppose one could
>> periodically take it down and smooth out the surfaces, but who is going
>> to bother with that? It lasted almost 30 years, so I got my money's
worth.
>>
>> Another possibility: Would it be better to leave the bearing set screws
>> slightly loose, so the rotator takes the vertical load?
>>
>> 73,
>>
>> Scott K9MA
>>
>
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