Knot Break Strength vs Rope
Break Strength Dave Richards, Technical Director, Cordage Institute Abstract
All knots
reduce the breaking strength of rope. The question is, has been, and will
always be, how much? The general
rule has been a knot reduces strength approximately 50%. That is almost true. In early
2004 I asked and received permission from the Cordage Institute Technical
Committee to conduct a study on knot efficiency. The Institute solicited donors
of many types of ropes to be tested. In the climbing and rescue sector we chose 3
types of ropes, 7mm Acc. Cord, 10.5mm Dynamic, and 12.5mm Static ropes, with
conventional fibers i.e. nylon and polyester. With help from knowledgeable
people in the industry 8 knots were chosen; bowline, fig. 8 end, fig. 8 bight,
butterfly, fisherman's knot, double fisherman's knot, sheet bend, and double
sheet bend. To remove
as many variables as possible, it was decided to break 5 specimens of each size
to establish a benchmark. Then we conducted 5 breaks for each knot. All the
knots were tied by the same person and the test were conducted in a
relatively controlled environment. The data was collected on an Excel Spread sheet for the various graphs and charts. This paper
discusses the methods, procedures, mistakes, successes, and will display the
findings. There are still many more tests to conduct. This is a work in
progress and may not ever be complete. Introduction
Testing
of fiber rope is always a challenge. There are many variables in any test, but
fiber ropes with knots have even more. The key to good tests is the
repeatability. First priority was to select the types and sizes of rope to be
tested. This was done with the advice of the industry. Three types and sizes
were selected, Static Life Safety 12.5mm, Dynamic Climbing 10.5 mm, and
Accessory Cord 7mm. The next was to select the knots to be tested. Again we
went to those with the knowledge of what is being used. Eight knots were
selected; Bowline, Fig. 8 end, Fig. 8 bight, Butterfly, Fisherman's knot,
Double Fisherman's knot, Sheet Bend, and Double Sheet Bend. The next decision
was quantity of tests. There are two factors involved, amount of rope, and time
available to do the tests. Since this was strictly an allvolunteer project, we
decided to keep the quantity of rope to 300 ft. of each size and type. This
allowed 5 breaks of each type and size to establish a benchmark, and 5 breaks
for each knot. This will give an average strength and a standard deviation to work
with. PMI provided all the rope used in this segment of testing. The next
segment includes 1/4" and 1/2" diameter ropes of various constructions and fibers. Equipment
The test
equipment is a standard hydraulic ram pull against a fixed load cell. Ram length
is 36" with a pulling speed of approximately 39"/minute. The load is recorded
and peak load retained with a digital readout. The clevis has a 1" diameter
pin. This can be used to hold the rope or the 4" diameter bollards. The overall
length of the test bed is six feet with ram extended providing nine feet of
usable bed length. The fixed end is moveable thus allowing shorter lengths to
be tested. The load cell is calibrated to ASTM E4 Standard annually. This is
an open test frame with protection for the operator. It is possible to observe
the reaction of the knots as tension is applied.
15K TEST
FRAME WITH 1" DIA. PIN & BOLLARD
Method
This type
of testing is to determine the percentage of the knot strength vs. rope
strength. The first step is to determine the average of each type rope with
five breaks in accordance with CI1801. The ropes were tested and the mean and
standard deviation was determined. Each of the knots was tested and the break
strength recorded. The Bowline, Fig. 8 end and bight, and the Butterfly knot
had one end on the bollard. Bollards on both ends held the Fisherman's, and
Sheet Bend knots. The mean and standard deviation were determined. This was
then compared to break strength of the rope and percent of break strength determined.
In all testing there is some variation, to eliminate as much of this as
possible all the knots were tied by the same person using the same procedures.
The data was then recorded on Excel and the various graphs and charts were
compiled. Data
Normally there are three categories referring to types of configurations in the end or joining of rope. Knots, Bends, and Hitches. However for simplicity and clarity in this paper we will divide this data into two categories; End line knots and joining knots. The Bowline, Figure 8 end and bight, and Butterfly are end line knots that are to hold on to something. Sheet Bend and Fisherman's Knot are to joining two ropes of the same size and type together. To determine the consistency we used the Standard Deviation divided into the mean break strength. Standard Deviation is the square root of (the sum of the squared deviations from the mean, divided by the sample size minus one) In formulae it is often represented by the letters SD or the symbol (Greek letter) sigma. The lower the percentage the more consistent the knot. It was necessary to add backup knots, as noted, to get the rope to break.
Fig. 1
Fig. 2
Fig. 3 Comparison Charts
Fig.
4 Fig.
5 Fig.
6 Fig. 7
Fig. 8 Comments
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