Several years ago I had to replace all the insulators in the bottom set of
guys on my Rohn 25 series-fed 160 m vertical due to a lightning hit that
pulverised every single insulator at the bottom guy level. Originally, all
the guy insulators were attached using three u-bolt cable clamps at each
termination plus serving the strands, per the Rohn book. With the
replacement insulators I used Big Grip pre-forms instead of cable clamps.
As I recall I, purchased the Big Grips from Texas Towers. They arrived with
no written instructions whatever, but it appeared pretty obvious how they
went together so I installed the new insulators with no difficulty.
Now, I have just recently read for the first time on a couple of websites
that I ran across, that with the type 502 insulators I used, you are
supposed to begin wrapping the grips at the SECOND set of paint marks, while
with metal thimbles you begin at the first set of marks. When I assembled
mine, I began the wrap at the first set of paint marks, which seemed the
obvious way to do it, and the assembly appeared to have gone together
perfectly.
I would like to hear from others how you attached these insulators using Big
Grips. Does anyone know the reason why it would be recommended to begin at
the second set of marks instead of at the first?
It has been 8 or 10 years since I installed those insulators, and I noticed
just a couple of weeks ago that one of the insulators is freshly cracked,
and the bottom part of the insulator that makes up the eyelet where the wire
threads through is missing. The part of the insulator that takes the
compressive strain remains intact, so I don't feel an urgent need to
replace it immediately, but I plan to do so as soon as I can gather up the
materials. I am wondering if beginning the wrap too close to the insulator
might have led to the failure by causing the wire to exert excessive
pressure on the end of the insulator. I see no lightning zorch marks on the
broken insulator, nor is there any evidence of metal residue from a bullet.
It would seem that beginning the wrap at the second set of marks would
leave short, spiraled sections of wire between the wrapping and the
u-shaped section of wire that loops through the insulator, which would have
less strength than straight unkinked wire, plus the pre-forms would be
gripping a shorter length of wire. Supposedly, utility pole guy grips are
unsatisfactory for tower use because they have slightly less length than the
ones designed for tower use. Wouldn't beginning at the second marks defeat
the advantage of using the Big Grips?
I also read on one website that with Big Grips the guy tension should be
precisely set at the recommended 10 percent breaking strength of the guy
wire, in this case 3/16" EHS, which would come out to 400 lbs if I recall
correctly. Not having a reliable tension gauge, I eyeballed mine,
tightening each set of guys until each cable felt tight without excessive
sag.
Maintaining the tension precisely at 10% would require readjusting the
tension at least twice a year, since I have noticed that during the coldest
weather my guy wires tense up as the steel contracts, and they noticeably
loosen up during hot weather, as indicated by the amount of sag and the feel
of the cable. I could see how too much tension could pose a danger of
slippage, but what kind of failure could be caused by tensioning them at a
little less than 10%?
Also, does the manufacturer of Big Grips publish any written instructions?
This tower has been up for over 25 years, and broken guy insulators has been
the only failure I have ever had.
Don k4kyv
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