Dyneema and Not Dying
Index
02
What
Is
Dyneema (or UHMWPE or HMPE or Spectra)?
03 Before You Read More: Where BARE
Amsteel Sucks
06 More on Munters and Carabiners
07 The Heat Conductivity Advantage
08 Knots, Not-Knots, High-Friction
Sleeves, Splices, and Heat Again
10
Construction Stretch and Maximum Force
13
Appearances Can Be Deceiving
This
document*
summarizes my experiences using bare (uncovered) and polyester-covered
Dyneema for short rappels and ascents. Initially, I wanted to test thin
high-modulus cord for emergency raps down a snowfield, using minimal
equipment, such as a Munter on a carabiner and hasty harrness.
However, I was stunned by the number of negative reponses I received,
all based purely on the low melting point of Dyneema (145 °C (293 °F))
and some odd views on energy transfer. I had done many tests rappelling
on bare Dyneema, and I had never found the rope to heat anywhere as much as polyester- or nylon-sheathed ropes.
So I thought I would dig into the matter more. I’ll
discuss some of the issues limiting use. I've been stuck at home
with kidney stones for about 2 months, and had lots of time for
testing.
To summarize some of the most important
points:
1) BARE Dyneema has a very low surface coefficient of
friction (CoF) [REF 01],
probably 0.2-0.3x that of polyester or nylon. The low CoF makes it
less likely to heat up in a rappel “device” (which will typically just be a carabiner), but creates serious
issues when tying knots. However, there are partial solutions for
knotting, and the low CoF isn’t as serious a rap issue as you might
think. As we’ll see, some rappel “devices” depend little on the
traditional surface CoF
for braking power, and more on energy lost to deformation of the
rope. Such energy loss mainly stays in the rope, and contributes less to device heating.
2) Dyneema has a radial thermal conductivity
(perpendicular to the fibers in the rope) just slightly higher than
nylon or polyester, so it takes a long time for heat to diffuse
“into” or “out of” a rope. HOWEVER, the axial conductivity (parallel
to the fibers and rope) is very high, 15-50x polyester or nylon [REF 02]. That means that heat that does manage to
diffuse into the rope spreads out and diffuses away (axially) much
more quickly than in nylon or polyester.
Before you give me a lecture about the low melting point of Dyneema, check the video linked below. Yes, the video progresses through the word slides slowly, but I’m hoping you will either fast forward, or actually read. For equivalent test conditions and rope diameters, in a friction heating test, the polyester fails 4x faster than the Dyneema. The output from this test is mainly for a thermal model, but it does give a visceral understanding.
I’ve
repeated this test with 8.5mm
polyester rope in a 1” nylon sling; that failed 2 times as
fast as the Dyneema, but the geometry is not identical and not as
useful for model testing. I've actually done 8 of these fail tests;
they are pretty consistent.
I test
new techniques and materials many, many times before I use them in
serious situations. The ceiling of my stairwell is reinforced, with
several clip-in points; I can do 10’ of hanging rap, or ascent, at
home. I test strength of ropes and knots using: 2 trees in my
backyard; an industrial pull scale; a come-along ratchet; and 97%
efficiency pulleys for mechanical advantage. In some cases I can
test the “strength” of high modulus materials with practice falls. When I feel fairly
confident, I go to a test area (below, 5 miles from my house), and
try the methods and materials there on 100’ of rap. Such tests are
not guarantees everything I’ve tried is safe, especially for someone
who doesn’t like to heed the cautions below. Use this
information at your own risk.
But
I still get lectures with the following scenario. Somehow the rapeller is going
to build up tremendous heat in the carabiner as the Munter slides over it, maybe due to an extremely
long rappel with a heavy loads. Then when s/he stops, that heat will suddenly be transferred to the Dyneema. The
scenario seems to be supported by a silly youtube video where a Dyneema sling
was placed in an oven set above the melting point of the sling. In a real rappel,
heat sources are not infinite, like an oven; the heat source will consist of a few
cm of aluminum carabiner used for the munter, and they will lose heat rapidly from radiation and convection
So let's be absurd and assume that the carabiner somehow gets 337F above the highest temperature I've seen (113F just below the rope on a real 100' rap with bare Dyneema), for a total of 450F. In the video below I simulate the biner with a 1/2" aluminum tube, packed with aluminum spacers and tightly mashed foil (to give the same thermal mass/cm as a fat biner). There is a thermocouple that just penetrates the bottom of the tube. I'll use a torch to get several cm of aluminum above 450F and then drop a weighted dyneema (Amsteel) sling onto the hot spot.
02
What Is Dyneema (or UHMWPE or HMPE or Spectra)?
Dyneema
is a trademark for ultra-high molecular weight polyethylene
(UHMWPE); that particular trademark is owned by DSM
Dyneema corporation. Spectra is another type of UHMWPE,
trademarked by Honeywell. UHMWPE is also referred to as high
modulus polyethylene (HMPE), for the very large tensile modulus,
compared to nylon and polyester fibers. Chemically, UHMWPE is much
like the polyethylene used in plastic bags and bottles; but
instead of having short, unoriented molecular chains, it has very
long individual molecules, arranged in parallel. UHMWPE is also
characterized by a very low surface
coefficient of friction (CoF), high axial heat conductivity, and
low melting point (~148 C) compared to nylon and polyester. UHMWPE
loses strength above 70 C, more so than polyester and dry
nylon. However, the mechanical/thermal properties of UHMWPE
typically mean it may
not reach the melting or failure point any faster than other
synthetic fibers in rappel and ascent. The video at the beginning
of this document demonstrates this point.
This
document mainly discusses one variant of Dyneema: Amsteel BLUE, by
Samson Rope. My experience is with BARE Amsteel in diameters of 7/64" and 1/8” (2.8 and
3.2 mm, intended for emergency raps or pull line), 3/16” and 1/4”
(6mm). (The quoted diameters of Amsteel may be slightly misleading,
as the rope narrows considerably under slight tension.)
I have also tested the 1/4” (6mm) polyester-sheathed
Amsteel, as well as an 8mm Bluewater Canyon Pro, a
polyester-sheathed rope with Dyneema core that is NOT Amsteel Blue.
I have tested New England Ropes Spyder line (polyester-covered
Dyneema) in 3.8mm (1900 lb) and 4.8mm (2500 lb) for prusik loops. The thinnest Amsteel is an
8-strand, single braid cord; all others are 12-strand single-braid.
The BARE ropes are soft and highly flexible (picture below). This softness is due to
a very low lay angle in the braid, meant to maximize the strength per weight. Those
properties make BARE Amsteel rope less suited for situations where
it may snag on rough rock.
Dyneema
braided rope, similar to Amsteel,
IS used for rappels in some European rescue groups [REF
03],
with special cautions. The longest recorded rap on BARE Dyneema rope
is 760 METERS. ALL RAPS are done with Munters and supermunters;
devices specifically for rappels, such as ATCs, are not allowed,
because of the heating issue (Munters get part of braking force from
rope-rope contact). In rope rescue systems, the low CoF means much
less rope drag; a carabiner can get over 60% efficiency with
Dyneema, vs ~53% for nylon or polyester.
Since rescues may require hundreds of meters of rope, the
low weight of Dyneema (per unit strength) is a significant
advantage. However,
the high modulus means that short falls can generate large forces,
so energy dissipaters are used in-line.
The material doesn’t absorb water, and is very resistant to
damage from UV light, which is advantageous when material must be
left out for days.
The EFC
(French) have experimented with a different kind of Dyneema, made by
Beal, for canyoneering [REF
04].
They use 5 mm cord and have found ways to use devices such
as figure 8s and Piranhas with extra wraps, and have even used
ascenders on this thin cord. The Beal cord is 100% Dyneema, with a
tightly-woven sheath. Such
a construction will not have the snagging and abrasion problems of
Amsteel. EFC are well aware of the low melting point, and report an
average device temperature of 40C (104F), with an observed maximum
of 72C (158F) on a 100 METER rap. These are far above the
temperatures I have seen with BARE Amsteel, but then again, they do
longer raps, and use “real” rappel devices (not munters). I will not
discuss this use further.
03
Before You Read More: Where BARE Amsteel
Sucks
First,
Dyneema is static (though Amsteel is, in practice, nowhere as stiff as the reported modulus would imply; see
“construction stretch” below), and is definitely not for belaying.
Because of the very high moduli, one can generate large forces in a
short fall. Amsteel can be used for a haul
rope, or a pull line (many canyoneers use it to pull fiddlesticks). But if you really want a
light pull line, consider arborist throw lines. Like all thin cords,
1/8” Amsteel is tangly, and the
very-loosely-woven braid tends to catch on rough rocks when not
tensioned. When it catches, it abrades faster than you might
imagine. It makes a
very poor handline, because it is slick and very thin for its
strength. BARE Amsteel is
not good for canyoneers, as a general rope; because:
1) of the abrasion and catching issue;
2) it is hard to knot securely; and 3)
it cannot be used with "normal" rap devices without extra
wraps and a chance of over-heating. Unsheathed Dyneema would be
mainly for escape situations; 200’ of 1/8”Amsteel Blue weighs one
pound and has a test strength (static) of 2500 lbs. I find people
are more likely to take emergency gear if it is light.
I have
rapped a lot on this stuff using Munters (“German” position) and
supermunters; however,
the speed of a rap on a Munter is harder to control , than a rap
with (e.g.) a Piranha on a polyester-, technora-, or nylon-sheathed
rope. Ironically, the large, round-cross-section HMS carabiner is
least suitable for a munter on thin Dyneema. A small D-shaped biner,
such as Grivel’s
Plume dual wire-gate, is far more suitable for thin bare Dyneema; the
tight turns cause the rope at the bottom of the munter to run over
itself in opposite directions, yielding another rope-rope contact and
also providing more “internal friction.”
With a large HMS, your speed is either slow and perhaps
bouncy (supermunter) or near-catastrophic (Austrian-position regular Munter on thin
cord). You MUST test
with the type of carabiner you will use. With a
dual wire-gate, I can quickly put in an extra wrap, before
I plan to go down an overhanging section; but that option is tough
with screwgates.
If you feel good with the supermunter on you carabiner, I highly
recommend you use that knot; it does NOT twist the rope, and provides a
safer experience, and generally you won't have to worry about
adding extra twists. If you intend to use a urethane-treated
rope, such as Amsteel, it is wise to run the entire rope thuogh a
Munter several times to remove the outermost urethane; this reduces
friction, but avoids surprises later.
Secondly, I weigh 148 lbs in my clothes. Don’t do anything on the thin stuff until you have tested it with your weight. If you weigh 250 lbs, try 9/64” Amsteel (at least) instead of 1/8”, and go for the supermunter instead of the Munter on thin cord. The longest I have rapped with BARE Dyneema is 100’double-strand, pushing as hard as I could. Typically the “device” (Munter on biner) is < 25F above ambient at the very end of the rap, right beneath the Munter. The most extreme delta T I have witnessed on a real double-strand rap was 41 F with 7/64"; the actual rap was about 90 feet, and was accomplished with a single Munter on a plume carabiner in 20 seconds. The T was measured with a NIST-traceable, very thin thermocouple placed on the aluminum right below the munter as soon as I finished (the dyneema itself remained fairly cool). The very highest delta I have seen was 54F, for a simulated rap on 100' of a double-strand supermunter with bare 7/64" Amsteel on an Attache carabiner. For the simulation, the carabiner was attached to a tree, my wife acted as the brake hand, and the Amsteel was attached to the back of my harness. I tried as hard as I could to "run" down the steet, pulling the amsteel through the supermunter on the biner, and pulled 100' (200' in double strand) in about 30 seconds. From tests pulling against an industrial scale, I know that I can generate > 100 lbs of force when leaning 45 degrees into the "run," on normal pavement. However, this is about twice the speed I can normally rap on a supermunter, so the friction energy/time simulates a 200 lb person taking a minute to rap 100' double-strand. So this was actually a fair test. However, if your ambient is 100F, that would put you just below the maximum recommended working T for Amsteel; but the measured T is dominated by the carabiner T, which is significantly above the Amsteel T at the end of the test. In short, if you are thinking of trying this stuff for a rap, I recommend first doing a hanging rap from a tree over a relatively short distance. The supermunter may surprise you with how slow it is on this stuff.
The
surface CoF of Dyneema and other synthetic rope fibers, is defined
by the familiar Coulomb Law. The surface Cof
is the proportionality
coefficient µ in
F
= µN
Where F is the force of friction opposing the motion, for a surface sliding over another, and N is the force normal to the surface. This is a reasonable approximation when N is small. Ideally, for synthetic fibers, µ is obtained in experiments with small forces and small fibers or bundles of fibers to avoid deformation, and one distinguishes between static and kinetic friction. This is the friction right on the interface, most likely (e.g.) to heat a rappel device as rope moves over it. In reality, µ is often measured for thick ropes, with substantial forces on a capstan, and terms proportional to rope dimensions add to F. The available literature has widely varied values for µ; values reported by rope manufactures generally come from the supplier of the fibers, and do not reflect the effects of incorporating the fiber into a soft braid versus a tightly-woven mantle. The CoF is also supposed to be dependent on temperature [REF 05], but my tests for nylon up to 150 C have failed to show this. Ideally, the reported values would describe the other surface in contact with the synthetic fibers – e.g. more synthetic fibers, steel, or aluminum; often that information is lacking. Dyneema Corp., Samson ropes, and many other sources give fiber-on-fiber ranges of 0.05-0.07 for Dyneema, and 0.15-0.25 for nylon and polyester. For braided, thick ropes on rough metallic surfaces, the static CoF may be 2x the kinetic CoF, and the values for polyester (Dacron) and nylon ropes vary widely, from similar low values (0.15) up to ~0.7 [REF 06].
Below is the CoF for Dyneema for yarns (fiber bundles) ON METAL, compared with other polymers. The low value (0.04) is less than 1/4th the CoF of polyester (1/5th that of nylon), according to DSM, the manufacturer. These are possibly the most relevant CoF, for (e.g.) movement over a carabiner. However, we will see that application to real life is not so straight-forward.
Here’s what I hear a lot: “Yeah, well I’ve used rap devices that got really hot! They would melt through Dyneema!” Well, your device got hot because the sheath on your rope – which overwhelmingly controls the heat production when rappelling on a ATC , figure 8 or Piranha – was made of material with a much higher surface CoF. Nylon, polyester, or Technora have surface CoF roughly 3x-6x that of Dyneema. It is irrelevant that the core of your rope may have been made of Dyneema. I have rapped 100’ with the same biner, same hour, same place, on: 1) bare 6mm Dyneema (8300 lb static test); and 2) 6mm polyester-covered Dyneema (4900 lb static test). With the same knot, the rap was faster for (1), and the biner was about 20-25F above ambient at the end (tested by NIST-traceable thermocouple). For (2), the biner gave me 2nd degree burns, suggesting the biner was ~100F above the ambient.
Dyneema
does lose strength as temperature rises (REF
12). By 75 C, the tensile strength has dropped to 1/2 the 25 C
value; by 100 C, it is 1/3 the 25 C value. That's actually a bit
better than nylon in humid air, but such nylon will continue with low
strength (perhaps 10% of the 25 C value) up to just below the melting
point 250 C, whereas Dyneema melts at just 148 C. But
remember, Dyneema is much less likely to reach higher
temperatures.
06 More on Munters and Carabiners
I've done hundreds of raps and many pull tests on Munters, so this advice is not given lightly. It is something of a hobby to test every carabiner I have with unlikely cordage (I've even rapped off a quicklink).
The
choice of carabiner is extremely important for Munters on BARE
Amsteel. Below is the
brake factor (ratio of force on anchor side, to hand force needed
for steady descent on brake side).
The test used 3/16”
Amsteel on a “German” position Munter (brake hand UP for extra
friction). I used 3 types of carabiners,
4 trials each and the uncertainty in the brake factor is about +/-
1. The results mean that much more brake force must be supplied by
the hand of the rappeller, for a
traditional round-cross-section HMS carabiner, than for the other
two. Most folks learn that the big, round-cross-section
HMS biner is best for a Munter; and that may be true with
polyester or nylon ropes, but it's dead
wrong with thin Amsteel. Folks also tend to learn the
Austrian hand position as well (probably because instructors like to
ramble on about unwinding screwgates).
Note that
if you were to calculate the brake factor from the capstan equation,
using the known CoF for Dyneema, you would get a value of ~2, which
is obviously in discordance with measured values of 8-15. While CoF
strongly affects the heating of the device, it doesn’t limit how the
rope dissipates energy by other means.
There are other
mechanism of energy dissipation that do not involve surface
friction, yet
still slow down the transit of rope; these are ambiguously
described in the literature as “internal friction” or “viscous
friction” [REF
07]. Bending and distortion
dissipate energy, and in modern
rap device, outweigh
the energy loss to surface friction
[REF
08],
and these results show the same is true for Munters, especially with
Dyneema, which has a low CoF.
Since this dissipation is generated within the rope, and the
radial heat conductivity of the rope is very low, a
rappel device won’t absorb much of this energy before the
rope moves off. The
surface CoF is often measured with as light a load as possible, on
single fibers or small fiber bundles, to avoid deforming the fibers. In reality, most Dyneema
ropes are very deformable; they tend to flatten out when pulled over
a carabiner. In my own tests, it takes about 16 lbs
of force to pull a 10 lb weight 180° over an
aluminum carabiner, using 3/16” Amsteel; it takes 20-24 lbs
to pull with an 8.5 mm polyester rope. Clearly the Dyneema is less
frictional, but the formal surface
CoF suggest much lower force than does the experiment. (The capstan
equation is clearly inappropriate here, as the rope diameter is not
small with respect to the carabiner thickness; nonetheless, the
capstan equation does get used for analysis of Munters [REF 07].) Note that for 180
degree turns over the rounded portions, the three biners above give
the same efficiency; it's only with larger turns that the irregular
cross section makes a difference.
For 1/8"
cord, a single Munter WITH EXTRA WRAP does well with a
small carabiner. An extra wind around the biner spine helps
to control the descent, and use the hand in UP position to maximize
force. My experience
with “twisting” of double strand:
it isn’t a BIG issue with 100’ rap with this STATIC braided
rope, even with Munter. It does happen, and one must be careful to
let the end of the rope untwist before bagging it. Supemunter
doesn’t twist rope. DO NOT use a simple Munter with Dyneema and a
large HMS-style carabiner with oval cross-section; you will simply
go too fast. Use a
supermunter on large burly biners. Yes
it’s slow, and bouncy, but on the plus side, you won’t die. I've
tested the 7/64" (2.8mm, 1600 lb test) only with a double-strand
supermunter on the Attache carabiner, as well as a tiny (3")
mettolius wiregate, and only in my in-house set-up on drops of 10';
that arrangement was surprisingly smooth, and I could control a
hanging rap with a few fingers... but again, I weigh 148 lbs. The is
a clear advantage in using a larger carabiner: there is more metal
and surface area to dissipate and adsorb any heat buildup.
07
The Heat Conductivity Advantage
The plot
below contrasts the temperature rise for Dyneema and polyester
ropes, when insulated bundles of the two rope types were placed
against an insulated can containing water at ~200F. (Oriented so the
rope fibers were parallel to the diameter of the can.) The
temperature was measured ~3.5 cm away from the can, inside the rope.
In this type of geometry, the time required for the T “front” to
reach a particular point x, goes as x2 . If an increment
of heat is added to a Dyneema rope, within 5-10 seconds, it has
blurred out (axially) about 5 mm on each side of the initial point
of addition, greatly diluting the temperature rise. In the polyester
rope, the equivalent blurring would be about 1mm, so a lesser volume
receives heat per unit time. While polyester rope is about 15%
denser than Dyneema rope, it has just 2/3 the heat capacity per unit
mass, so the same amount of added heat leads to a greater
temperature increase per unit volume in polyester, below
the glass point. However, above the glass point of
polyester, the heat capacities are comparable.
[Sophisticated
in-kitchen experiment, results plotted above. Boiling water was
placed in insulated can, insulated rope bundles were attached to
can with conductive tape.]
08
Knots, Not-Knots, High Friction Sleeves, Heat Again, and Splices
If you are just using BARE Amsteel for an emergency 2-strand rap, you don’t need any “knot” except the Munter or supermunter. But there are times when you need to provide a clippable end, as when using the cord to haul. I'll discuss splicing at the end of this section.
It is
pretty common to have “trustworthy” knots fail at 35% of the rated
strength, when Dyneema is tied to Dyneema
[Ref
09]. It is very useful to
witness a Dyneema knot failure in real time, because on-line videos
rarely show the force of breaking, and may be slowed down. In my two
pull tests on Dyneema-Dyneema unions, the knot failed by having a
tail pull through. The failure, once started, was very fast; if you
think you are going to tie longer tails to give you some more time,
think again. Be wary
of videos that show tails pulling through slowly; the machine that
provides the pulling may not be keeping up with the sudden rope
surplus, thus the tension is suddenly at a plateau; this gives the
illusion that longer tails will save the day. In reality you might
get just a few more seconds. Lastly, here is another good reason not
to tie Amsteel to Amsteel: once the knots are weighted with just 150
lbs, they are extremely hard to untie.
Thin
Dyneema, when paired with thicker nylon ropes, performs pretty well
in some knots. Here is
a figure eight on a bight with 1/8” Dyneema paired with 8 mm nylon;
the application is a pull cord for 8mm rope, when used with a
carabiner block. I took the pair up to 1400 lbs,
with no sign of failure. On left I pulled both ropes; on right, just
Dyneema was pulled. I’m
sure I could have gone farther, but I am really not interested in
using more than body weight on a pull cord.
Many climbing sites state that one should never use an EDK to pair ropes with different diameters, but these sites are typically thin on explanation or guidelines. I paired 1/8” (3.2 mm) Amsteel to 8 mm nylon, with two EDKs (the same short piece of Amsteel as in the previous test). This knot failed rather spectacularly (pictured below), when I brought the knots to 1100** lbs tension the 4th time. In the 2nd EDK (on the right), I had allowed Amsteel to cross Amsteel at nearly a right angle; effectively, one piece of Amsteel sawed through the other one. I see now that it is easy to put lop-sided (higher) stress on the smaller cord in an EDK. But 1100 lbs is not too bad for a rap, where the thinner rope would mainly be used as a pull cord under body weight. This is clearly a case where more (the 2nd EDK) was not "better.". Since the nylon rope was not stressed in the right-side EDK, this may have been an unrealistically harsh test. As tied and weighted, these were essentially water knots. It is notable that in DMM's tests on Dyneema slings gave a water not failing at less than 24% of the full strength, considerably worse than the results of this test (failure at 44% of 1/8" Amsteel rated strength).
Another
solution to the “knotability” issue is
to sheathe the Dyneema in a short section of nylon.
For the example below, I sheathed
3/16” Amsteel Blue in 9/16” thin tubular webbing (available
from Fish Products). Then I tied an overhand on a bight directly on
the sheathed portion.
WARNING
WARNING WARNING The following discussion describes sewing of
weight-bearing materials. This activity can be very dangerous,
especially if you are inexperienced. If you have the least doubt
about your sewing ability with strong threads, immediately stop
reading.
You
can also sew a loop termination with many stitches of 300 lb-test
fishing line. I first sew two strands of 1/8” Amsteel together, near
the loop, with 15-20 penetrations of each. Then I slip a short piece
of ½” tubular webbing sheath (ClimbSpec, 1000 lb test) over
the two cords below the loop, and sew about 15-20 more penetrations
through each cord, bringing the nylon tube into the stitching. Then I dip the whole end
in Seam Grip urethane, which has been thinned with Cotol
240. If you wanted an end that could be used for a girth hitch, you
wouldn’t urethane coat the loop; and you would make a larger loop,
and use a shorter sewn/tubing-covered section.
I have
pulled this type of termination, on 1/8" Amsteel, to 1400 lbs
without a sign of failure.
If you must connect the ends of two strands terminated as above (e.g. for a long save-you-butt rap), the first option is to hitch the two eyes, with a girth-hitch (what sailors call a cow hitch), but that might get tiresome with 100’ of rope. You might consider tying an EDK just below the terminations (LEFT side in photo above). You will reduce the strength of the double-strand to ~1700 lbs, and the knot will benearly impssible to untie after loading. EDKs are supposedly prone to inversion; I if I try to invert the EDK by hand, it hits the wide terminations and stops. That would probably be the point of failure in a pull tests, where the rope would simply break. My preference would be to connect the loops with a screwgate or dual-gate biner.
And of
course, you can buy sheathed Dyneema; some canyoneering ropes have
polyester or polyester/Technora sheath (e.g. old Canyon Pro and
Canyon Pro DS). The 2011 test in Heaps Canyon, on the heating of
ATCs while rappelling, actually used Canyon Pro DS. I’ve seen posts
on canyoneering blogs where it is claimed that the Technora is for
heat resistance; the manufacturer (Bluewater) claims the Technora is
for cut resistance. Apparently there was concern "on the
street" that heat from the rap device could melt the Dyneema core.
However, the radial conductivity of polyester is so low, and the
cooling of a rap device by radiation and convection is so fast, that
it is unlikely the Dyneema core will ever see significant T from a
hot rap device.
To
illustrate this point(video linked below), I pre-heated an ATC to
~96C (204F), placed Canyon Pro rope into the device, and measured
the temperature just inside the sheath. Just 1 mm in, T never
exceeded 36C (96F). Frankly, I'd worry more about a polyester
sheath being above the the glass point, where it might weaken,
contract, and then pull apart, exposing the core. At least one
manufacturer has recommended that ATCs not be used on
polyester-sheathed Dyneema, because of core slippage. Core slippage
has occurred on Polyester-sheathed, Dyneema (non-Amsteel) core
ropes, but the cause is not really clear. Apart from that caution,
these 8mm+ canyoneering ropes are meant to be used with “normal” rap
devices.
There
is also Samson-manufactured cord with a polyester sheath on Amsteel.
The ¼” (about 6mm OD) sheathed material is 4900 lb
breaking strength and weighs 1.6 lbs
for 100’. I’ve actually prusiked up this stuff, and it will hold a
light person with some versions
of real ascenders; again, I weigh 148 lbs
in my clothes, so adjust accordingly.
I’ve tested this stuff in the field, right after testing ¼”
bare Dyneema; it is MUCH more frictional, and requires an entirely
different rap device. The Mammut Nano
Eight is more suitable for this 6mm polyester-sheathed Dyneema. But
beware, you will *almost certainly* need to wrap the 6mm cord once
over the sidearm to be in control of the rap.
Eye splices are common in Amsteel. Joel Hartter's thesis shows that just burying tail resulted in 95% of
rope strength; but the amsteel rope was not weighted and unweighted many times. Simple
buried rope splices must be sewn if rope will be weighted and
unweighted. Most will want to "lock" the splice by using a Brummel,
which uses two penetrations of the rope before burying the
end, one through each of the strands on opposite sides of the eye.
The strength of the Brummel alone, without the unburied tail, is
greater than the strength of a water knot on Amsteel. There are two
ways to make the brummel:
1)
simple method requires access to both ends of the rope, and allows one
to slip a sheath over the eye, or add a metal thimble. The
method first requires that there be a penetration of the short end
through the long end, then a penetration od the long end through the
short end. This method is a pain
with long ropes; make VERY sure the rope has been flaked to full length
and is
completely free of snarls before you make the second penetration.
2)
the more complex process can be donw with access to just one end of the
rope, but requires pulling the eye through on second penetration and
de-inverting the rope, or reuires
"inverting" a section of the rope before the second penetration, and
de-inverting later. This method can't be used if there is any extra
material
incorporated in eye (such as a sheath or metal loop thimble). Practice this on scrap rope first, because you
will likely screw it up at least once.
A
long splice is actually simpler, and can be used when making prusik
loops. The long splice should definitely be used with a Brummel-type
dual penetration, and will end up turning about 12" of
single-width amsteel into 12" inches of doubled amsteel, shortening the
loop by 6". I always sew long splices.
There is one
caution about polyester-sheathed ropes with non-polyester cores.
Polyester can shrink a LOT when heated above the glass point, and that
shrinkage may enhance core separation. The figure below shows the
sheath removed from an 8.5 mm all-polyester rope. At top is the sheath
after I tried to stretch it by hand;I couldn't get more than a few %
stretch, and it immediately relaxed back. The center of the figure
shows the sheath immediately after ironing (yes, with a hand iron on
wool setting); it never got above 250 F (121 C). The bottom of the
figure shows the ironed sheath after stetching by hand; it immediately
relaxed back. The shrinkage is about 12 +/1%. Most polymers
shrink when heated above the glass point, as the increase in entropy
is taken up by increased disorder in the form of shortened, more
tightly wound molecular chains. Note this significant change
happens well below the melting point, and is probably not
noticed. This is a fairly good reason NOT to showboat and rap
fast on polyester ropes, especially if the cores are of a different
material. Even with all-polyester ropes, the core rarely gets as hot
as the sheath. Supposedly, Dyneema shrinks less than 1% at 100C, but
mariners report shrinkage of 3-4%, possibly from UV exposure.
10
Construction Stretch and Maximum Force
I always
test knots and other rope terminations by simulated falls.
The ceiling of my stairwell has been reinforced a LOT, so I
suspend about 10-12’ of rope down, clip my harness in, and take
“falls” of at least two steps (16”). This definitely hurts on a
high-modulus rope, but I do it about ten times to make sure I trust
the termination.
It is
relatively straight-forward to
calculate the maximum force in such a short fall, given the
moduli or the static elongation for a given rope, the amount of rope
played out, and the length of the fall. There are many derivations
on the web under “rope physics,” and one can do the energy balance
with high-school physics. There is about a 20% down correction for
the relative "softness" of the human body, compared to the inelastic weights assumed
in the simple equations [13].
I
repeated this test with 3/16” amsteel, and didn’t notice undue
discomfort. When I
calculated the supposed max force, it was around 2000 lbs.
I assure you I didn’t feel anything like that, compared to
other tests I have done.
The
answer to this conundrum is in two parts, First, if we look at the figure below, from [REF 10]. The
we see 11 tension cycles, with 9 to half-breaking-strength,
and the 11th to actual breaking. The "final" modulus is achieved only
after the rope has been tensioned a few times; even then it tends to
spring back and requires some light tensioning just to reach the
"linear" range. Perhaps more interesting, is that this test was done on just 10' of dyneema, and
gives a naive modulus just about 10% of the quoted modulus. The authors atribute that difference to construction stretch
in addition the the splices used to hold the test section.
(The hysteresis stretch is in addition to the quoted
elastic modulus, and takes several hours to relax back.)
There is a soft "construction stretch" in many braided dyneema
ropes, added when the separate fibers are woven into yarns, and when
the yarns are woven into a rope. Before ropes are tested to
determine elastic moduli and hysteresis stretch, they may be cycled
50 times, and can gain several % in length.
Why is
there so much construction stretch in Amsteel?
The main uses for Amsteel are in yachting, ocean sailing,
logging, and other heavy industries where ropes are tensioned to a
significant fraction of breaking as a matter of course. Slack is
taken up by capstans, knots, and electric motors.
In contrast, climbing
ropes rarely see more than body weight.
The
second part of the answer is a little more subtle. In sailing, much of
the construction stretch never comes back, because the ropes are nearly
always in tension. Loosely-braided ropes, with low lay angle, will gain
much of the construction stretch back, when they are simply bagged or
coiled. It's also rare that we use a rope without one or two knots in
the system; tightening the knots lowers the effective modulus of the
rope during a fall. How
much does this extra stretch moderate the "static" nature of dyneema?
Consider a practical example. I have 100' of Endura 6mm rope, with an
SK-78 Dyneema core, and a polyester sheath. I have taken many 800lb
falls on the rope to test terminations, and I have used it under my
body weight for raps; but I always bag (as a canyoneer might do) it
bewteen uses. New England Ropes claims this rope has 0.65% stretch at
900 lbs (20% breaking strength), for a linear modulus of ~1.38e5 lbs. I
put two knots as terminations on an 18' length, which was doubled back
through a 97% efficiency pulley, and brought the rope to 100 lbs for a
minute. Then I placed a Sharpie mark across both strands, and over
several minutes brought it to 300 lbs, using a come-along and
industrial scale. When the force plateaued I measured the stretch. The
corresponding linear modulus was 1.48e4 lbs, more than 9x lower than
the manufacturers specifications. Key
to this behavior is the 12-strand, low-lay angle braid of Amsteel and
many other dyneema ropes; they recover a softer effective modulus when
bagged or even coiled. In practice, the linear modulus of this 6mm rope
is very similar to that of 11.5 mm PIT/Maxwear nylon caving rope. The
knots also inherently reduce the effective modulus, unless they are
pretensioned by a hard pull. Note this soft braid is very different
from the tight, high-lay angle braid used in Dyneema slings.
11
Samthane
and Melting
Amsteel
is coated with a proprietary urethane coating, “Samthane,”
which
contains the coloring and supposedly increases abrasion resistance. The coating leaves the
individual fibers distinct; the coating thoroughly saturates the
thinner ropes, but is on the outside fibers for thicker ropes. Some
of the coating wears off after repeated friction, such as passage
through a Munter.
The most
obvious effect of the urethane treatment, is that it makes the
Amsteel almost impossible to melt with a flame. At best, the Amsteel
chars in a flame, leaving little burnt-rounded terminations on
groups of fibers. This
is quite unlike the other Dyneema and Spectra
cordage I have, and accounts for part of the odd results in
the first video linked in this document.
I melted the ends of two polyester-sheathed , Dyneema core
ropes. The first was
8mm Canyon Pro; the core was NOT urethane treated. The sheath and
core melted simultaneously with a flame, quite easily.
In contrast, below is a picture of Samson “WarpSpeed
II,” which has an Amsteel (urethane-treated) core; the polyester
melted away easily, leaving an unmelted
Dyneema core. To seal
the ends of Amsteel, I use Seam Grip urethane glue, and work it into
the fibers, then shape the ends as the urethane begins to cure.
What I
don’t like about this coating, is the uncertainty of what it does to
the CoF. Again,
to avoid surprises, put the entire rope through a weighted munter
several times. My experience is that the rope becomes
slicker as the coating wears off the very surface fibers.
The thin 6mm, polyester-covered dyneema (4900 lb test) actually works with Camp Solo2 ascenders, but the wear on the rope is noticeable. In addition, if one is going through the trouble of reducing weight with thin rope, all mechanical ascenders seem heavy. I have tested prusik loops (and pusik knots) made with 3.8 mm Spyderline (1900 lb BS, polyester-coated dyneema), and they work reasonably well. However, if one has mastered sewing dyneema (SEE CAUTIONS ABOVE), it is possible to make stronger, lighter "hybrid" prusiks that will work on the 6 mm polyester-covered cord, and even on 9mm double ropes. The video below shows the concept; the pair of hybrid prusik loops weighs 1.8 oz, for a BS > 2500 lbs. I recommend true prusik knots, NOT klemheist knots, on this thin, flexible 6mm rope. Klemheists have a very nasty tendency to pull the main 6mm "rope" into the friction knot, completely locking, unless one is very fussy with dressing the knots.
And for irony, the video below shows an “experimental” use of Amsteel. The setup uses prusik loops made from 1/8” Amsteel, tied with 5-turn klemheist knots, to 6mm polyester-sheathed Amsteel. I tried this test 5 times, and the knots were a bit tight by the time I shot the video. The analogous situation, with nylon prusiks on thin rope, is the one place I have melted nylon. On thin cord, klemheist knots can get very tight; despite being called a “friction” knot, much of the strength of klemheists comes from the deformation of the rope when the knot is loaded. With nylon, the relatively high CoF can actually melt the prusik knots when they are moved. For a time I had aramid prusik cords, because I was so afraid of melting the knots. I’ve learned how to loosen the knots, so the friction issue is less extreme, over the years. But in these tests, the prusik knots didn’t melt at all. I examined them in detail after the 5 tests; not a single melt spot.NOTE
these are NOT Dyneema WEBBING SLINGS. I have tried Dyneema slings,
and they are too stiff and slick. The key to the success of the
Amsteel is that it deforms, and is so thin that it is pretty easy to
tie a 5-turn (rather than 3-turn) klemheist.
The
Amsteel cord is about 3.2 mm (2500 lb
test); I used a knot to make a loop, then slid a 4000 lb
test piece of tubular webbing over the knot, and sewed the 4 pieces
of rope coming out of the knot to the webbing with 100 loops of 300
lb test spectra fishing line.
Then I soaked the tubular webbing in thinned Seam-Grip
urethane, so individual stitches were unlikely to pull through if
abraded, and the webbing was stiff enough to form a step.
These
results are somewhat consistent with [REF 11], though
they were testing the tightly woven Dyneema slings.
Does this
mean I recommend using thin Dyneema prusiks for ascending 6 mm
ropes? Of course not,
it’s a PITA. But it is
nowhere as unsafe as some predict. People with lower upper body
strength may have a really hard time with thin prusik loops on 6mm
cord, no matter what they are made of; the cord just deforms too
much.
13
Appearances Can Be Deceiving
Sometimes
the Amsteel rope deforms quite a bit, especially after untying a
knot; the deformation
may be disturbing to folks who are used to the kernmantle ropes that
seem almost indestructible. Usually
the deformation goes away simply by holding the rope on each side
and compressing it. Perhaps this will ease your mind about the
significance of the appearance ([REF 10], in a
discussion of logging):
“The
current project did not plan to test abrasion or degradation in
synthetic ropes, but we did obtain a sample of AMSTEEL 815 synthetic
rope used three years as a guyline for
tail trees and intermediate support trees. The 9/16 inch rope was
markedly increased in diameter (more than double) and had dirt and
debris among the fibers and strands. Yet the residual strength of
the test sections was more than 65% of the original specifications.”
I ask
again: are you big-boned? The
weight that can be handled by a rope roughly goes as the diameter
*squared*. So if an
8mm rope is “safe” for someone who weighs 150 lbs,
and you weigh 250 pounds, you need a 10.3 mm rope for a similar
safety margin. The
same rule applies for thin cords, except the strength rating may
drop off faster with diameter. In
kernmantle ropes, the sheath may be constant in
thickness, as the core size increases; and thin, purely braided
ropes have other issues with strength when the braids are loose.
But if you are just using Amsteel for a pull cord, 1/8 “ is
probably adequate. If
you want an emergency rap, go for the 9/64 with a 4000 lb
rating and supermunter.
* I wrote this guide in honor of Zadig (by Voltaire). Zadig, when asked about the laws against eating griffins (which were thought to be imaginary), tried to find a neutral course; but he still offended those who had never seen griffins, but harbored strong opinions.
**Why just 1100 lbs? I had an operation 2 days
ago, and the doc put me on lifting restrictions, so I didn't bother
setting up the mechanical advantage pulley system. Alas, the scale
tops out at 1100 lbs.
http://file.scirp.org/pdf/MSA20110500002_65580510.pdf
http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=15889&context=rtd
https://moodle2.units.it/pluginfile.php/96537/mod_folder/content/0/UHMWPE.pdf?forcedownload=1
03) Use of Dyneema in Europe. http://www.alpine-rescue.org/ikar-cisa/documents/2016/ikar20160404002117.pdf
04) Dyneema in French Canyoneering: http://cnc-ffcam.fr/wp-content/uploads/2015/01/cahiers-efc2.pdf
05) Temperature Effect on CoF.
Mens,
J.W.M. and de Gee, A.W.J. (1991) Friction
and wear behaviour of 18 polymers in
contact
with steel in environments of air and water .
Wear, 149, 255-268.
06)
Friction Coefficients of Synthetic Ropes. Available on web as
http://www.dtic.mil/get-tr-doc/pdf?AD=ADA036718 or
ADA036718 .pdf
Kane, B.C. (2007) Friction Coefficients for
Arborist Ropes Passing through Cambium Saver Rings. Arboriculture
& Urban Forestry, 33(1), 31-42.
07)
Viscous Friction in Brake Devices.
08) Rappel Devices and Rope Deformation. (http://www.bolt-products.com/Glue-inBoltDesign.htm ; https://link.springer.com/article/10.1007/s12283-013-0147-6
http://infohouse.p2ric.org/ref/08/07122.pdf
09) Strength of Dyneema Knots. Joel Hartter
thesis. https://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/9800/Hartter,%20Joel%20%20MS.pdf?sequence=1
http://depts.washington.edu/sky2001/proceedings/papers/Pilkerton.pdf
http://caves.org/section/vertical/nh/49/cthsc/cthsc.html
12) Temperature and tensile strength UHMWPE.
van der Werff,
H. and Pennings, A.J. (1991) Tensile deformation of high
strength and high modulus polyethylene fibers, Colloid & Polymer
Science Colloid Polym Sci 269:747-763
13 Holden, T; May, W.; and Farnham, R. (2009) On the Utility of Rescue Randy Mannequins in Rescue Systems Drop Testing. Available on the web as Rescue_Randy_Mannequins.pdf