[TowerTalk] Thrust Bearing Problem...maybe

[Stan Griffiths]

>I may be in a oinch here...not sure yet but I'm going to assume that I
>am.
>
>I went to turn my 15/20 stack last night and noticed that the rotor was
>not moving.  I had my daughter activate the brake and I could hear it
>popping...so the wedge is working (T2X).  I then went up the tower and
>had my helper attempt to turn the antennas, the rotor growled with no
>antenna motion...I then grabbed the 20m boom and gave a big pull.  The
>antennas then took off.
>
>The problem seems to be some type of binding in the thrust bearing.  As
>the antennas rotated, I could feel a considerable catch....I'm assumming
>in the bearing...that would momentarily bog the rotor down.  It would
>then pop through the catch.
>
>Right now, the bottom antenna physically is sitting on the bearing
>collar.  Maybe the race has been damage with all of the wind...I have no
>idea.
>
>My next step will be to bump the bottom antenna off of the bearing and
>loosen the bearing to see if it spins freely. If not.....what do I do?
>
>My option is to "build" some type of bearing in-place on the tower or
>remove 2 large mono-banders plus a big mast.(which I would like to
>avoid).
>
>Any suggestions??  I've heard of a wooden bearing....is that something I
>could do??
>
>Any input would be appreciated. 
>
>Greg
>K9IG

Hi Greg,

You did not say if this was a Rohn thrust bearing or not.  If it is a TB3,
there is a potential problem with these that can cause exactly what you are
experiencing.

If you had the bearing (Rohn TB3) in your hands, you could see the hex head
(Allen) set screw on the inside that plugs up the hole where the ball
bearings are insterted and removed during assembly and disassembly of the
the thrust bearing.  This hex set screw "floats" in the hole and is not
tightened since doing that would clamp one ball tight in the race and cause
the TB3 to lock up tight.  When the mast is inserted in the TB3, it prevents
the hex set screw from coming out and spilling bearings.  The problem is
that there is a range of one entire turn or so over which this hex set screw
can turn freely from clamping down on a ball bearing and bumping up against
the mast.  If it gets too tight, it will catch the balls as they go by,
which is what it sounds like yours might be doing.  If it gets too loose, a
ball can drop into the hole it leaves in the wall of the bearing race and
this, too, causes rough operation of the TB3 and it can lock up.

The cure for this is a little modification on the TB3 before installation.
(I know this advice is a little late.  Sorry, Greg, better late than never.)
What I have done is drill and tap a hole in the aluminum cast TB3 part and
install another small set screw to hold the large one in the right place
such that the balls run smoothly in the race.  If the large hex set screw
can't get too tight or too loose, it should work just fine.  Without this
modification, nothing keeps the large hex set screw in the right spot except
a little thread friction and a lot of luck.  You may have run out of both of
those.

Obviously, you have to take your antenna system apart to make this change if
you decide it needs to be done.

This is one of those times that an ounce of prevention is worth a ton of cure.

No, Rohn did not tell me to do this.  I figured it out all by myself . . .
:-)  So it is not in the Rohn book and I am not a registered PE.  Use this
advice at your own risk.  (Personally, I think the risk is in NOT using this
advice . . . but then that is just MY opinion . . . )

Hope this helps.

The wooden bearing you mentioned is the Rohn AB and is nothing more than a
short piece of hardwood 2x4 with a 2 inch hole in it and drilled to mount on
top of a 25AG4 flat top section.  It only serves to keep a two inch mast
centered laterally in the top section and provides no vertical support of
the mast or antennas.  I don't think this is what you want.  Also, being
wood, they will eventually rot and split just like all wood left out in the
weather.  They ARE treated so they last quite a few years, however . . .

Stan  w7ni@teleport.com


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