The Scoop on
Sling-on-Sling Connections
Note:
The information contained in this article is gleaned from the web
column of Black Diamond Equipment engineer Kolin Powick, whose
blog:
Kolin's Corner or QC with KP, is
an ongoing "off the record" project involving tensile pulls and
drop-testing of climbing gear and systems to ascertain more objective
information regarding the strengths and weaknesses of the tools
that help keep climbers alive.
Kolin's testing
of used equipment and systems has helped to inform
climbers of the limitations, considerations and most importantl, shed
light on the primary safety question for those who cling to the
vertical with only a rope and equipment as
their life-line: What are
the weakest links in my rope system and the more
likely causes of
system failure?
|
Sling-on-sling
connections are a common necessity in order to reduce friction
through
rope-drag on a pitch which can be a
limiting factor and source of
frustration when trying to "send the project".
We
commonly build anchors with slings and may choose to
connect slings
together without carabiners to minimize links or due to
a lack of equipment.
The
harness and belay loop are also made of nylon and we usually
girth
hitch slings to either of these points as an extension for
rappel and
belay anchoring in the form of a "daisy chain" or longer sling
(24"-48").
|
A 48"
sewn sling girth-hitched to the belay loop for an "extended rappel".
|
|
Any
connection in a climbing system could potentially be a weak point in
that system, so knowing which components are your more likely sources
of
concern in regard to system failure is key in the
decision-making
process.
|
A
close-up of a girth-hitch being used to join together two
pieces of
equal diameter nylon webbing. With materials of equal
diameter, the
loss in strength is approx.
30% in this connection using a girth-hitch.
|
The
girth hitch is one of the most commonly used hitches in climbing for
joining sling-to-sling. Examples of these connections would
include:
Belay loop to sling (or daisy chain) and sling to sling (including cam
slings).
With
newer materials being introduced into climbing each year, and
with
thinner and smaller being the focus of many manufacturers and the
desire of many climbers, what happens
when we girth-hitch materials of smaller diameters together?
Here is
where the problems begin:
|
When two
10mm dynex slings are girth-hitched together, the loss of strength is approx. 47%;
enough of a strength loss to be of concern.
|
|
Due to
this significant difference in strength loss, it is recommended to
girth-hitch only similar diameter slings
made of nylon.
|
And when
an 11/16" sling is girth-hitched to an 10mm dynex sling, the loss in
strength is approx. 54%
which could be a big enough reduction in
strength to become the weakest link in a climbing system.
|
|
With any
other choice of materials, using carabiners
would be the strongest method for sling-on-sling attachment.
If any
of the carabiners are being levered over or easily opened by the rock,
they can be doubled-up for a fool-proof connection.
|
 |
What
about using other hitches for joining slings
together?
The
strop
hitch is very similar to the girth-hitch, but for joining nylon to
nylon, this is a stronger method due to the symmetry of the
curves of the hitch.
|
The
strop-hitch used to join two nylon slings together reduces the
strength of this connection by only 20%
which is 10% stronger than a girth hitch.
|
Despite
the strength gain of using the strop-hitch with two nylon slings of the
same diameter, the same advantage is not realized as much with smaller
materials or materials of different diameters.
There is
one exception
gleaned from Kolin's tests:
The
strop-hitch being used to join a 10mm dynex sling to a nylon
sling is a significantly stronger hitch.
|
The
total loss of strength in this connection is approx. 46%
compared to the 54% loss of strength that occurs when connecting these
two materials with a girth-hitch. Neither loss of strength is
ideal,
but the strop-hitch might bring the connection strength up to a
"strong-enough" level for some climbing applications.
|
A
summary of this sling-on-sling connection testing is:
1.
Joining two slings together using a hitch reduces the ultimate strength
of the material up to and over 50%. Only a
nylon-on-nylon connection
with a hitch seems to result in a reasonable
amount of strength loss
compared to other sling materials.
2.
In general, the narrower the material used, the greater the reduction
in strength when joined together with any hitch.
3.
In general, materials of different width joined together with
a hitch results in a significant reduction of strength.
4.
If you must link two runners together, a carabiner is
stronger. If you
must use a girth hitch then put the girth hitch on the web that has
higher strength or use a strop-hitch.
5.
Better yet - use a longer runner altogether or a thick nylon
daisy-chain for harness connections.
Many
thanks to Kolin Powick and Black Diamond Equipment
for their work
and support in the testing of commonly used techniques and
equipment
in various climbing applications.
|
