Forwarding from KZ1W:
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From: Grant Saviers <grants2@pacbell.net>
Date: Wed, May 28, 2014 at 9:56 PM
Subject: cable clamps on old Phillystran
To: topband@contesting.com
Cc: tree@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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