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

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Subject: Re: [TowerTalk] Recommendation for mast on bottom mount rotator
From: "K8RI on TowerTalk" <>
Date: Sun, 17 Sep 2006 17:52:09 -0400
List-post: <>
>From: "Jim Lux" <>

> 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.
This isn't a great deal different, if any, than a thrust bearing on top with 
the rotator on the bottom as the rotator serves as a weight supporting 
thrust bearing. I would think for bottom mounted rotators they should be 
mounted independently of the tower and if the rotator is designed to support 
a load such as the PST-61, then let it do so.  With the rotator mounted 
independently the tower becomes a fixture to hold the rest upright. It no 
longer has a twisting moment and only has to support its own weight. BUT for 
all but a few ham installations these are not really significant issues.

The typical installation of a tri-bander, or VHF/UHF antennas at 40, 50 or 
even 60 feet are a different breed of animal than the 100 foot plus 
installations with multiple antennas and arrays even when in high wind 
and/or ice prone areas. The amount of engineering, concrete, and money goes 
up in a hurry with height and the antenna load.

> I would use some sort of spline coupling between torque tube and
> antenna mast.  Perhaps not an actual spline, but something that works

I think Jim's idea of the spline has some real possibilities.  If each shaft 
were fitted with a spline (male one end and female the other) it could 
eliminate a number of problems. Yes, it'd take fair amount of prep work, but 
the more I think about it the better I like it. Water is going to get in so 
some form of elimination would be needed and possible rust/corrosion inside 
the mast sections would needed to be taken into consideration.

> 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)

With a spline joint at the end of each 20 to 24 foot mast it would make 
assembly much easier.  Bearings or
bearing blocks could be used at each end to keep the masts centered. They 
would also allow for about any combination of top and bottom support.  It 
would require extra rotator/thrust bearing supports, but the bearings could 
easily be a couple of oil soaked hard wood blocks except for the ones that 
are load bearing.  One point I really like is the ability to take out and 
install individual 20 to 24 foot sections with my existing gin pole and no 
more having to weld sections together inside the tower.

>>Even if you shimmed up the
>>unloaded one, how would you know you had the load spread evenly? I

As with the top thrust bearing and bottom rotator... the answer is a simple: 
You don't know.

>>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.

Probably the best and most simple in the long run as long as the weight is 
within the design limits of the rotator.

> 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.)

Aircraft landing gear pump

>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.

Think of it this way. If the pipes are properly oriented he'll never have to 
worry about rust.  Maybe this'd be a good approach for those near salt 
water? OTOH it could make tower climbing more than a bit interesting.<:-))

> Pneumatics?

Having worked in industry for many years in earlier careers I have learned 
to detest pneumatics and particularly the sound of pneumatic power tools 
although I do use them quite a bit. Pneumatics are easier to use and much 
more reliable than hydraulics in many applications, BUT they do have some 
short comings beyond being noisy. Position control does require pneumatic 
motors rather than pistons and these things require oil. Actually most of 
them require an oil mist in the air.  Pneumatic systems out in the elements 
are subject to freezing which means adding an air dryer to the system along 
with the previous mentioned oiler. That means a well maintained pneumatic 
system is likely to be messy.
>> >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.

I think the wood would be strong enough, but probably far too springy.

Roger Halstead (K8RI and ARRL 40 year Life Member)
N833R - World's oldest Debonair CD-2
> 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
>>TowerTalk mailing list
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