Topband: Fwd: cable clamps on old Phillystran

[Grant Saviers]

Ice is an interesting question.  I'll speculate that it doesn't matter 
much since the sheath is pretty flexible and the Kevlar has a small 
sensitivity to moisture.  The Kevlar demonstrated very high crush 
strength, I think about the same as its tensile strength, so to me that 
is not a concern.  However, if the sheath is degraded to expose Kevlar 
to UV then it is a whole different ballgame.  I also think it takes a 
bit of faith that the factory recommended plastic endcaps on current 
production Philly keep all moisture out.

Grant KZ1W

On 5/29/2014 7:45 AM, Carl wrote:
> What about ice forming inside the sheath from those breaks?
>
> Carl
> KM1H
>
>
> ----- Original Message ----- From: "Tree" <tree at kkn.net>
> To: "160" <topband at contesting.com>
> Sent: Thursday, May 29, 2014 9:48 AM
> Subject: Topband: Fwd: cable clamps on old Phillystran
>
>
>> Forwarding from KZ1W:
>>
>> ---------- Forwarded message ----------
>> From: Grant Saviers <grants2 at pacbell.net>
>> Date: Wed, May 28, 2014 at 9:56 PM
>> Subject: cable clamps on old Phillystran
>> To: topband at contesting.com
>> Cc: tree at kkn.net
>>
>>
>>
>> Per prior topband posts and discussions about this topic, I've 
>> concluded a
>> round of testing of cable clamps on parallel strand (old style)
>> Phillystran.  Here is the Conclusions and Summary I wrote:
>>
>> A hydraulic jack H frame press was modified to provide tension in 
>> excess of
>> 20,000 lbs.  Tension was applied to a 4 foot long 5/8” od parallel 
>> strand
>> (old) Phillystran cable terminated with four 5/8” cable clamps and ¾”
>> thimbles at both ends.  Clamp nuts were torqued to specific values 
>> and the
>> holding capacity of the cable assembly was measured over periods of 
>> weeks.
>>
>> There is significant creep of the plastic sheath from the cable clamp
>> forces between the clamp and the Kevlar core.  In the first test 
>> sequence,
>> the residual torque of the clamp nuts reduced by 65% in 21 days. 
>> Subsequent
>> tightening of the clamp nuts showed smaller sequential reductions of
>> residual torques.  Five cycles of tightening were demonstrated as 
>> necessary
>> over a period of weeks to achieve sufficient residual torque of the 
>> clamp
>> nuts.
>>
>> A conclusion at 66 days since initial assembly was that four 5/8” 
>> wire rope
>> clamps, torqued in 5 cycles over weeks to a 50 ft-lb value, will 
>> support a
>> long term tension without significant slippage at the desired holding
>> strength of 6600 lbs, about 25% of the cable rated strength. After 
>> removal
>> of the cable sheath, there was no visible damage to the Kevlar core 
>> at the
>> clamps or at the thimble.   It is speculated that a slightly higher 
>> torque
>> value than 50 ft-lbs would improve the slip strength.  Adding a 5th 
>> clamp
>> would further improve the slip strength.
>>
>> The core around the thimble showed evidence of small differential 
>> slippage
>> of fibers.  The test sequence was such that the fibers could slip 
>> against
>> each other as tension and clamp nut torques were increased 
>> sequentially. Thus,
>> the test process was not the same as tightening the clamps and then
>> installing the guy.  However, the Phillystran tested is to be used at 
>> 25%
>> of its rated strength, so the risk seems minimal in this case. Note that
>> wire rope is expected to hold at least 80% of rated strength when 
>> properly
>> terminated with cable clamps, and is not sequentially pre-tensioned when
>> put into service.  Whatever unequal forces exist in the individual wire
>> strands around the thimble are equalized in some manner.
>>
>> From this testing, it seems unlikely that parallel strand Phillystran 
>> can
>> be reliably terminated with cable clamps at more than 1/3 of rated 
>> breaking
>> strength.  The simplified conclusion is that the cable will slip 
>> unless the
>> clamp has extruded out most of the plastic sheath in the clamping area.
>>
>> The planned tower has maximum pretension in the guys of 600 lbs.  
>> Thus, the
>> average long term tension is substantially below the measured slip value
>> produced in these tests, so it seems unlikely that the preload 
>> tension will
>> cause slip over a period of years.
>>
>> Since the plastic sheath was breached by at least one clamp, water will
>> intrude into the core.  Moisture does slightly reduce the strength of 
>> the
>> Kevlar fiber.  The clamp fully covers the split area so degradation from
>> sunlight seems unlikely, although UV degradation is a major concern with
>> Kevlar.
>>
>> Although the tests were successful in achieving a stable termination at
>> 6600 lb tension,  doubts remain in my mind about the long term 
>> reliability
>> of using cable clamps.  Hence, I plan to assemble and test a Phillystran
>> cable terminated with the standard Crosby Spelter sockets for 5/8” wire
>> rope using epoxy potting.
>>
>>
>> A pdf of the full report is available, contact me offline for a copy.
>>
>> Grant KZ1W
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