Hi Tom,
For the sections inside the tower, I would install a thrust bearing above
the first mast joint to take the bending stress off the joints. From here
down to the rotor, then, you would have mostly vertical compression and
torque.
I would use a sleeve of larger tubing, 2.5" OD and 2" ID by about 6" long to
slip over the joint. Then, I would join the sleeve and mast sections using
large, 5/8" diameter stainless clevis pins (http://www.mcmaster.com). I
would use one pin on each side of the joint.
It's important to provide a tight fit for the pins to prevent them from
banging around and wallowing out their holes over time. I would drill the
pin holes with the same size drill as the pin such that they have to be
lightly tapped into place. This, combined with the large size pin (more
bearing area) and external sleeve (as opposed to a smaller sized internal
one) should help preserve the most strength for the fluctuating torque loads
better over time. The long mast drive will also give you some 'spring' and
will reduce shock torque loads on your rotor.
You might be able to use the MC-1 mast coupler from IIx Equipment (
http://www5.interaccess.com/iixeqpt/ii00009.htm ) as a ready made sleeve,
then add the clevis pins.
Hope this helps,
--...MARK_N1LO...--
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