For a couple more "cents worth."
The top cable on my prior 89' snapped several years ago. There was nothing
to indicate it was going to happen. Our best guess is that the earthquake
(Oct '89) probably damaged the top pulley, as the cable snapped just before
the Feb NA Sprint. At the time of the quake, the tower was about 2/3 up.
I had just raised it to full up and did my usual final check. I noticed the
top cable begin to vibrate, reached for the down switch - too late. Bang. No
--> BANG. About 3 seconds later, it is all over. One welded cross brace came
loose and hit me in the chest. Parts of the bolts holding the pulleys were
thrown about 100' into the street (never did find some of the parts). The
lower cables did not have any chance at all of containing the falling upper
section. Not too much antenna damage, but the rotator shelf was driven 18"
farther into the tower and the upper thrust bearing being was tight. The
tower was totaled.
So, what I do now...........the pulleys seem to be the key. Afterall,
all of the force is concentrated on the pulley bearings. That is a lot
of force and heat for a small bearing to take on a regular basis. Have
changed cables on other towers since then and every time there is at least
one pulley that needs replacement (the bearing is broken). I have painted a
bright red stripe on every pulley and watch them as the tower goes up to be
sure they are rotating. This will give a little indication that something is
not working right. If a pulley freezes, the cable will stretch only so far.
Before anyone thinks about changing the cables and/or pulleys, please
remember that a tower is a giant meat cleaver. If you think a steel bar
or wood 2x4 will stop it, re-think what is holding the steel bar or 2x4: the
welds on the cross bracing. The welds will break, just like mine did, and
there was no bar across them adding to the force. Your arm going through
the bracing won't stop it, either.
Changing cables and pulleys is a limb and life threatening situation. The
danger is waiting to happen. Gravity will not give you a break. The safest
way is to have the tower horizontal. This is not fool-proof, either.
Please use more than common sense. As Steve (K7LXC) said, there are no
factory certified people to do this kind of job. There are maybe a few who
understand the dangers involved and feel confident that the risk is
minimized by their safety precautions. Let them do it and stand by with a
telephone (and probably a good attorney - ugh).
Have a good day and 73,
Force 12 Antennas and Systems
(Home Page http://www.QTH.com/force12 )
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