Comments in-line marked **
On 11/27/2015 17:54 PM, Roger (K8RI) on TT wrote:
My thoughts:
When I see the specs on clamping wire rope, phillystran and EHS, I
have to wonder if they are talking about using one clamp, or three as
is suggested for the Phillystran boom truss?
** Here is what one antenna manufacturer has in the manuals: SteppIR has
4 clips each end of 1200 Philly on the Monstir boom truss and 4 clips on
each element truss end. My 4L steppir has no element truss and has 4
clips each end for the boom truss.
**Here is a warning from the Monstir manual "Do NOT completely tighten
the wire clips until you have double checked that the lengths are
correct, because once the clips are fully tightened the jacket is
deformed so much it will expose the Kevlar to sunlight if the clip is moved"
**This is exactly what I found in my testing of clips on 26,500# rated
"old style" Philly. Result - four clips can't develop any more than 30%
of rated breaking strength of that Philly. I suspect current stranded
Philly will do better since the jacket adheres to the Kevlar.
I believe The jacket thickness of the little "stuff" is a much
smaller % of the cable thickness than it is for the 4,000# and 6,000#
strength (and larger) cables. If that is correct, then the saddle
clamp use would be much more damaging on the larger cables than on the
small stuff used for a boom truss..
** Incorrect, as the diameter increases, and the thickness of the jacket
is about the same, it follows the ratio of jacket to core decreases.
This is easily seen looking at a cable end.
** Here are of my measurements for 4 sizes of Philly I have. The 26.5k#
Philly is "old style", straight Kevlar fibers and a jacket that easily
slips off. All the others are recent production with the jacket that
strongly adheres to the Kevlar (why grips work). In new Philly, Kevlar
is spiral wound in bundles like 7x19 wire rope, 7 bundles of fibers, but
since the fiber diameter is so small and each bundle size increases with
Philly rating, it is "7 by a lot".
Philly od Kevlar od jacket T diameter ratio (% jacket)
1200 0.18 0.09 0.045 50%
2100 0.23 0.13 0.050 43%
6700 038 0.27 0.050 26%
26.5K 0.60 0.48 0.060 20%
(dimensions in inches)
It makes engineering sense that Philly only needs a jacket thick enough
to withstand long term UV exposure, so they apparently determined about
0.050" thick jackets will do that. Also, I note that the smaller sizes
are a bit uneven in the centering of the Kevlar in the jacket, probably
due to manufacturing tolerances. So, the measurements are best estimate
averages. For the 1200 and 26.5k I stripped off the jacket to measure
its average thickness. For the "7 by a lot" Kevlar I used the average of
the peak and valley diameters of the Kevlar winding lay as the Kevlar
diameter.
Three clamps on phillystran, do not need to be tightened near as much
as one. Three clamps tightened to snug will likely give more holding
power than one tightened to destroying the Phillystran jacket.
**incorrect, read the Crosby pdf, it takes multiple clips to develop the
holding force on steel wire rope. Each clip is torqued the same. The
tables recommends the number vs wire rope size and also the nut torque
value. There is also the recommendation to add one more clip if life
safety is involved. The steppir 4 clips is 2x what Crosby recommends
for wire rope. I would suggest that clips that clamp Philly should be
tightened to about 70% of the wire rope value, about the ratio of the
modulus of Kevlar vs steel. My testing of 26k Philly took 5 cycles of
clip tightening over 30 days to establish a stable torque value -i.e.
the clip cutting through the jacket to the Kevlar, but then 4 clips only
had 30% of breaking strength when they slipped.
**My bottom line remains, use 2100 Philly or larger and grips. Clips on
1200 likely have variable results. For instance, 4 clips develop
holding with a short contact of the saddle with the jacket and
significant distortion of the cable under the u-bolt. A grip develops
holding with 25" of contact on 2100 Philly. Exactly how multiple clips
share the load is a mystery to me. Something has to move or stretch for
each clip to contribute, and I have not found a reference that explains
the mechanics of that. Anybody have a link to a engineering analysis??
Anyone up to testing this configuration. They also reduce the "creep"
of the Kevlar core inside the jacket.
**I did extensive testing of four clips on 26.5K old Philly. There is
negligible movement of the Kevlar core, once the clip is actually
tight. Kevlar inherently is a low creep material.
** some day I will test clips and some other clamp ideas I have seen for
1200 Philly.
Grant KZ1W
I've seen tests run with figures, but I couldn't find a listing
anywhere as to the number of clamps. Most I've seen listed only used
one saddle clamp.
I much prefer wire rope over EHS. I admit to being prejudiced against
EHS. Yes, it's strong for its size, but I've found working with it to
be a royal PITA. Wire rope is more flexible and much easier to work
with. As far as that goes, I much prefer Phillystran over either, but
installed with the Big Grips.
When we figure the loads on the tower, we take into consideration, the
angle and resting tension of the guys. Don't forget the weight of the
guys. A horizontal guy would impart half its weight to the tower
base, A vertical guy would impart 100% of its weight to the tower
base. Neither angle is practical, but the least weight imparted to
the tower is 50% of the guy's weight. Using 1/4" EHS on a 120' tower
would add substantial weight to the tower base. OTOH that weight is a
fraction of the load imparted to the base by the guy tension.
The longest booms I've used were 42 feet of 3" on a 5L 20 meter
monobander and a 6L 15 meter monobander,. The boom truss on these
could be held with one hand, or more precisely, two fingers IF the
truss anchor points were high enough to provide the proper offset.
Yes, the really big antennas do have a substantial load on the truss,
but often the excess truss load comes from the truss offset being too
low, as in 3 feet for a 40 or 50 foot boom. I use 3 feet for the
lightweight 7L C3i 6-meter Yagi with a 29'6" boom.
Ice in the area? You might better raise the support a few feet
instead of beefing up everything else.
73
Roger (K8RI)
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