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Re: [TowerTalk] TB3 Postmortem

To: <towertalk@contesting.com>, "K9MA" <k9ma@sdellington.us>
Subject: Re: [TowerTalk] TB3 Postmortem
From: "Bob Shohet, KQ2M" <kq2m@kq2m.com>
Date: Sun, 5 May 2019 12:50:54 -0400
List-post: <mailto:towertalk@contesting.com>
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
>

-- 
Scott  K9MA

k9ma@sdellington.us

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