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Re: [TowerTalk] Recommendation for mast on bottom mount rotator

To: Bill Turner <>,
Subject: Re: [TowerTalk] Recommendation for mast on bottom mount rotator
From: Jim Lux <>
Date: Sun, 17 Sep 2006 09:41:30 -0700
List-post: <>
At 08:58 AM 9/17/2006, Bill Turner wrote:
>On Sat, 16 Sep 2006 09:12:10 -0700, Jim Lux <>
> >K8RI made a good point.. I assumed that you'd have another thrust
> >bearing at the bottom to hold the weight of the torque tube. If not,
> >then the entire weight of the tube will also be on your top thrust bearing.
>How would you get the weight evenly spread between the two thrust
>bearings? It would seem to me that one or the other would take it all
>and leave the other hanging, so to speak.

I would use some sort of spline coupling between torque tube and 
antenna mast.  Perhaps not an actual spline, but something that works 
similarly.  Depending on the amount of length variation, one of those 
elastomeric spider couplings might work very nicely. They allow a 
fair amount of angular misalignment, some axial play, but still 
transmit the power.  (They look sort of like a U-joint/Cardan joint, 
except the middle cross thing is flexible)

>Even if you shimmed up the
>unloaded one, how would you know you had the load spread evenly? I
>would think with gradual wear, heat expansion, etc, etc, the load
>would shift back and forth anyway. Better to just use one that is
>rated for the job and let it do it. As far as that goes, I could just
>use the rotator itself to carry the load, since it's rated at 400
>pounds vertical load.
that would work too.

Like many mechanical engineering problems, there's lots of ways to 
solve it, and the "right way" is probably more determined by the 
materials and manufacturing technology you have available.  I have a 
friend who truly believes in hydraulics for everything.  He'd 
probably suggest putting a hydraulic motor up top and a pump down 
below.  (A 5HP motor/pump is pretty small..a couple inches or three 
on a side.)  To that, I'd just ask how much kitty litter he's going 
to buy to absorb the inevitable leaks as they drip down the tower.


> >As an off the wall idea.. have you considered using wood?  You can
> >get long straight round poles.  I haven't done any calculations of
> >relative strength, but it might be a possibility.
>Hadn't thought of using wood. My gut feeling says not to, but it's
>worth thinking about.

Exactly.. or what about something like abs or pvc pipe.  You'd 
probably want the black abs stuff for UV resistance. PVC pipe seems 
to go pretty brittle with exposure to the elements (even the UV 
inhibited stuff), having had some practical experience in the last 
month ripping out irrigation and sprinkler lines that have been in 
the last 8 years. There are stranger things out there than can be imagined.

>Bill, W6WRT
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