[TowerTalk] Rohn TB-3 Warning

[David O. Hachadorian]

On Mon, 20 Oct 1997 11:44:43 -0500 "Gary R. Hosler"<grhosler@imation.com>
writes:
>
>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

Although Gary was doing something different, his experience
lends credence to the old advice of letting the rotator take
the vertical load, and making the thrust bearing bolts just
snug enough to take up any lateral slack.

Dave Hachadorian, K6LL
K6LL@juno.com
Yuma, AZ


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