The TB3 did last for almost 30 years.
73,
Scott K9MA
On 5/5/2019 22:38, Grant Saviers wrote:
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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Scott K9MA
k9ma@sdellington.us
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