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.
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.
You might want to worry a bit about torsional resonance in response
to wind loads, but I have no idea whether it would be a problem.
>I would drill and pin the couplers to keep them from coming loose. The
>antenna is a four element quad, about 65 pounds.
I'd make sure that the bottom coupler (torque tube to rotator) is the
weakest one.. You'd rather have the one the fails be the one at the
bottom, rather than the one at the top.
Jim, W6RMK
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