[TowerTalk] Recommendation for mast on bottom mount rotator

[Jim Lux]

At 08:58 AM 9/16/2006, Jim Lux wrote:
>At 08:24 AM 9/16/2006, Bill Turner wrote:
> >I'm going to move my rotator to the bottom of my 80 foot tower and
> >would like recommendations for the mast. I will support it at the top
> >with a thrust bearing, and at least one, possibly two, non-thrust
> >bearings part way down to prevent side to side movement.
> >
> >My question is what would be a good mast material? It would only have
> >to support it's own weight and I'm wondering if 2 inch water pipe
> >would be suitable? How about 2 inch EMT? The upper mast section which
> >actually holds the antenna is high-strength steel.
>
>
>So the idea is that the drive shaft only has to carry the torque load
>(and its own weight)?
>You've got a thrust bearing at the top to take the downforce of the
>antenna and its mast.


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.


>You've got two questions to answer:
>1) How big does the tube have to be to not fail
>2) How much torsional rigidity do you need?
>
>Both of these are optimized by a large diameter, thin walled, tube
>(here we are, back at beer cans). So, given that both water pipe and
>EMT are basically priced by the weight of the steel in them, for the
>same dollars, the EMT (being thinner walled) might be a better
>choice.  You could even go to something like 4" EMT and probably be
>cheaper than 2" water pipe or fence rail.

Another thing that occurs to me, though, is that EMT isn't designed 
to take pressure (like water pipe), so the weld that makes it a tube 
might not be as consistent or strong.  I just went out and looked at 
some 4" EMT laying in the yard, and the weld bead looks fairly 
consistent, but, EMT is kind of a brittle alloy (if you smash it 
flat, it tends to crack at the bends).  Water pipe tends to be a bit 
more ductile, and because it has to hold pressure, the welds are a 
bit more consistent.

OTOH, your torque loads may be SO far below the failure loads that 
it's almost immaterial.

Aluminum thin wall irrigation pipe is another possibility.  Fairly 
stiff in torsion, sometimes available inexpensively in odd lengths 
(someone drives a tractor over the end of the crop row and mashes the 
last 3 ft of the pipe... or it slides off the trailer and dings the end, etc.)

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.


Jim W6RMK