> ===== Chris Burger, ZS6EZ Writes:
> There's a lot of talk on the Reflector of mast bending and
> overturning moments, but none of the obvious solution. I use twin
> thrust bearings on all my towers. The twin bearings are spaced about
> 500 mm (20 inches) apart, and carry all the weight and overturning
> moment of the antennas. The rotators then carry no weight and no
> lateral force. They only supply torque.
> I have a set of bolt-on steps on each mast, to enable me to get to
> the top antennas if needed.
> Because of the absence of any lateral play in the mast, alignment is
> pretty critical. However, it's a small price to pay. I have the
> ability to remove rotators without moving the mast, as there's a
> short piece of stub mast between the rotator and the main mast. The
> joint can be removed, after which the rotator comes out without any
> hassle. The mast remains supported by the bearings.
I feel anyone with a long mast who DOESN'T use this two bearing technique
is foolishly asking for trouble. A tower is not cheap anyway so spend the
extra bucks for safety. There is absolutely NO WAY I'd want to wrestle
with my 20 foot 160 pound mast without that extra thrust bearing ... yikes!
> A new idea that's being incorporated in my new station's design is
> to use two concentric masts. The inner mast has a concentric
> rotator, while the outer mast has a motorcycle drive sprocket to an
> offset rotator. This way, you can have an independently rotatable
> top antenna without much additional cost. The rest of the stack is
> turned through the motorcycle chain. On the one tower, I intend
> putting a spotting tribander and 6 m beam above the 40/80 beams in
> this way. To claim originality would be unfair; this idea is the
> result of a brief visit to IQ4A.
Hmmm, interesting concept. I'm curious about the bearings and lubrication
method to keep it all working.
73 de Glenn, N3BDA (please open Gate 2!)