> An extreme real world example that might support the idea of using a
> second
> thrust bearing/bearing plate:
>
> My tower is a Tri-Ex (Tashjian) LM-354HDSP crank-up/tilt-over. My mast is
> 30' (18' CM + 12' aluminum) with 6' in the tower. (Before anyone asks, yes
> I
> did analysis and the load I have on the tower is acceptable.) The Tri-Ex
> rotator plate sits on top of the braces. It is not held by anything other
> than gravity when the tower is vertical; I have stabilized it somewhat
> when
> the tower is horizontal by placing compression clamps on the tower legs
> immediately above the rotator shelf.
>
> When the tower is horizontal, there is a quite noticeable bow in the CM
> mast
> between the rotator (Yaesu 1000) and the thrust bearing at the top of the
> tower. I can imagine that this same bowing happens when the tower is
This is an indication that the top plate to rotator distance is not long
enough rather than too long, or the mast extends too far above the top
bearing and the mast needs to be a bit more rigid.(or a combination of all
three) You have 30-6=24:6 or a 4:1 mechanical advantage. IOW the force on
the mast above the top is multiplied by 4.
Other than for a single tri-bander I'd not consider mounting the rotator
closer than 10' from the top and preferable the same distance as the top
antenna avove the tower.
The Aluminum on top is a good idea, but I'd extend the CM tube farther down
into the tower and use one a bit more rigid.. This would add additional
weight, but the strength gained and reduced side load on the rotator would
be worth it..
> vertical and the stack is being pressed by the wind. A second bearing
> between the rotator and the top bearing should eliminate (or at least
> minimize) this lateral force on the rotator.
I'm running a lot of antenna area and mast weight combined that extends 30
feet above the top of the tower.It also extends nearly that far down into
the tower. Unless it's realy windy you can center the shaft with one hand at
the rotator. OTOH with even a little wind it takes a big strap wrench or
pipe wrench to keep them from turning
>
> I've already got the second shelf and second thrust bearing; I'll probably
> install it in the not too distant future.
>
> BTW, the Tri-Ex thrust bearing at the tower top has a grease zerk.
Whether to grease or not depends on the bearing design. The ROHN bearings
are split at the 90 degree point. That puts the weak point at 90 degrees to
the mast. (These are not ideal bearings for no load and a strong side force-
IOW centering only) Weight will shift the bearing load point down into the
race for the 90 degree force. Grease allows the load point to shift upward
with pressure which puts more load at the split point. Depending on the
bearing material many can withstand a substantial load at the split. OTOH
again depending on bearing design, grease can collect a great deal of dirt
which is abrasive. In the past I've had Silicone grease, used for flooding
connectors, leak out from under the tape or heat shrink and literally form a
dirt ball even at 100' above ground.
73
Roger (K8RI)
>
> Doug
> K4AC
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