I wanted to share a problem that I encountered yesterday that might save
someone else some grief. I have a TB-3 thrust bearing installed at the top
of my Rohn 25G tower, and another TB-3 installed on a modified accessory
shelf about 5 feet below the top one. I was in the process of raising the
mast (20 ft CM .250 wall) to allow the installation of the rotor. Not
having a better way to raise the mast those last few feet, I used a trailer
axle clamp (two flat plates and 3" U bolts) which was attached to the mast.
I attached a boat winch to the side of the tower with the line running up
to a pulley, then down to another pulley attached to the axle clamp, and
then up a few feet to a side leg. I would crank in some pre-load, release
the thrust bearing bolts, and crank up the mast about 10-12" (travel
limited by the length of the axle clamp U bolts hitting the "Z" braces).
At this point I would cinch down the TB-3 bolts securing the mast, take
tension off the winch line and move the axle clamp back down for another
pass. Slow and tedious but I haven't figured out a better way at this
point. During one of the cycles I attempted to secure the mast with the
TB-3 bolts when one of them sheared off right at the TB-3 body.
ARRRGGGHHH! At this point I couldn't raise or lower the mast. I climbed
down and returned with a cordless drill and bolt extractors. No dice! Had
to drill out the entire bolt to free it up. Back in the basement (where
the Minnesota wind chill isn't a factor) I examined the TB-3 to see if I
could determine the failure mode. It appears that the bolt was simply sub
standard and failed. As a side note when I received the bearing two of the
bolts were NC thread and one was NF! I substituted a grade 8 NC for the
incorrect bolt. Although the bolt in question had failed for no particular
reason, another failure mode was noted. The action of tightening down the
mast locking bolts against the CM mast material had caused the ends of the
bolts to flare slightly. In other words if you were to back out the bolts
to a point that the flared ends would begin to enter the body fo the TB-3,
the enlarged ends would begin to gall and seize. This was even the case
for the hardened grade 8 bolt (although not to the same degree). A word to
the wise, be VERY careful not to back out the mast locking bolts any
further than necessary to free the mast. I have a pair of VERY sore and
tired legs from far to many trips (40-50 in the past week and a half) up
100 feet of Rohn 25. Those crank up/tilt over towers are looking better
all the time.
de Gary W0AW
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