[TowerTalk] [Bulk] Re: [Bulk] How to "lock down" wire rope clamps/clips?
Roger (K8RI) on TT
K8RI-on-TowerTalk at tm.net
Sun Nov 29 14:24:49 EST 2015
Thanks for the comments and corrections Grant.
I appreciate the work you have done and that what seems like it'd work,
doesn't always<:-)
73
Roger (K8RI)
On 11/29/2015 1:45 PM, Grant Saviers wrote:
>
> Comments in-line
>
> 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.
> Check 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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--
73
Roger (K8RI)
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