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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