MARB owners:
I use plain-old automotive grease on the inside surface of the upper collar
on the rotating base. That's what U.S. Tower told me to use. It's OK to lay
it on thick. I use a grease gun to squirt more grease between the collar and
tube when the old grease layer gets thin or rubs off (sometimes I spread it
around with a popsicle stick.) The grease is important for preventing
hangups and also prevents abrasion of the tube. It's conceivable that
rotating the tower against an unlubed collar could eventually cut right
through the steel and compromise the tube. U.S. Tower hinted about this
possibility. As mentioned, don't forget to lube the lower thrust bearing.
I've found that the single most important factor for preventing hangup
(besides greasing the collar) is making sure the pipe section that connects
the tower drive shaft with the rotor can move freely. It's a poor-man's
u-joint. It's there because it's not possible for the mast to be perfectly
concentric with the rotor -- it moves around too much in the upper collar
and the very long bottom tube isn't perfectly round and straight. Therefore,
a flexible joint is needed between the tower and rotor. The short pipe
section serves this purpose. If the rotor jaws are too high on the pipe
section (say, as high as the bolt connecting the pipe to the drive shaft),
then the pipe can't move far enough to provide the needed flex. There should
be two inches or more clearance between the bolt and the top of the rotor
jaws, more if possible. The same thing can happen if you tighten the
aircraft nut on the bolt too tight -- the pipe can't move. Just tighten the
nut until it rests against the pipe.
73, Dick WC1M
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