Webbing strength in anchors

by Stefan Junghannß
February 24, 2014
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This article will discuss the various anchoring options of slackline webbing. The investigation uses the very widespread Landcruising White Magic webbing. White Magic is a webbing made of polyester fibers in a 2 layer design with moderate strength of min. 30 kN and relative low stretch of 5% at 33% of its break strength (e.g. 10 kN).

Explanation of the testing setup

Based on the tensile strength in the free length, which is approved by the manufactuer with every batch production, there were various webbing anchors analyzed on lab testing machines since 2009. Special thanks is contributed to Institute of Solid Mechanics of the Technical University of Dresden for providing the testing fascilities. New webbing was used for every break test. Partly a red sibbling of White Magic (Verve) with the same strength was used. The value of the break load efficiency of every anchor refers to 32 kN (the absolute strength of White Magic). All tests were run at low speed. Old webbing was tested in a separate investigation for developing conclusions about the fatigue behaviour and failure load reduction.

Detailed informations on the tests

Factory test of the free length strength

The webbing manufacturer tests with every batch the absolute breaking load and the stretch parameters of the webbing. An amount of 3 test samples is used to give informations on the scatter. The results are written in the inspection report.

Zwick webbing testing machine
Zwick webbing testing machine
The free length strength is tested on tensile testing machines of the company Zwick. The anchoring is carried out over big webbing sample holder. This ensures that the webbing does not break inside the anchor, but in its free length. Thus the absolute breaking strength of the webbing can be achieved.

The strength of the webbing is part of the product definition and must always be greater than 30 kN for the White Magic webbing. Since webbings are subject to certain fluctuations due to there manufacturing process, the strength of the webbing is made somewhat higher from the outset. White Magic has for all batches a strength higher than 32 kN (100%) and an average strength of 33 kN.



Knots

Knots provide the easiest way to attach a slackline to a carabiner or shackle. Knots are used to achive a backup behind the webbing main anchor. Therefore a knowledge of the breaking load reduction is also very important. We examined all slackline relevant knots. All knots have the weak point of loading the webbing in a local point and asymmetrical, so their breaking load efficiency is in general fairly low. In following the presentation of results:

The overhand bend is a very dangerous knot when used with webbing. This knot should be avoided (in literature the knot is in entitled as death knot).

Knots are in general not suitable as webbing main anchors, due to their low break strength efficiency and the pretty difficult untying after high loads.

The doubled figure of eight knot is a good solution for backup the line additionally after the main webbing anchor.

A small graph shows the results of knot strength investigation:

knoten-bruchlast-diagramm

The reinforcement of knots by threading the webbing inside outer protective tubular webbing is a special technique. The reinforcement can be achieved also by putting additional webbing of same width in between. This knot method is known as "Frost Knot". Many american highline pioneers used such knots for their highline setups (for example Dean Potter). Landcruising used in the years 2007 and 2008 a double tube reinforced overhand knot in 4-ply style for highlining.

Linelocker (chain links)

Linelockers are webbing anchors realized with the help of single chain links. Those chain link lockers offering a very simple and fast attachment to shackles, carabiners and quicklinks. They are specially suitable for beginners. However, the tests showed that these anchors haven´t any advantage over knots regarding break strength efficiency. Additionaly most of the chain links provide sharp edges at the welds. These edges do not affect the general break strength of the webbing, but they can damage individual fiber bundles.

Normal unreinforced linelockers are not suitable for slacklines with high load potential (longlines, highlines, tricklines), due to their low break strength efficiency.

Ringlockers (rings + shackles)

Webbing anchors with the help of rings are widely used in America and representing an alternative with a high break strength efficiency. The clamping mechanism is similar to the bolt lockers and the bananas. However, there must be paid particular attention for a clean centering of the rings to the shackle.

Boltlockers

Boltlockers can be realized with the help of smooth steel bolts sitting directly on the shackle. They are established as very efficient and affordable anchors. The locking principle is friction and deviation based. The failure mechanism of the webbing uniform across the whole width. The breaking load efficiency is directly related to the diameter of the bolt. However, large diameter are unhandy and heavy. Accordingly this principle is mostly used with smaller bolt diameters as lightweight application.

Sewn end loop

A sewn end loop is probably the easiest way to attach a slackline to a fixing point. The break load depends largely on the seam and the used yarn. The webbing is very high loaded at the inlet of the seam. Special seam patterns can enable a smooth force transition and therefore higher break load efficiency. Modern banana webbing anchors are superior to sewn loops regarding handling flexibility and break load.

Banana anchors

Webbing anchors with 3 pins between 2 sideplates are called bananas. A friction based method locks the webbing securely with the help of the two front bolts (Attention: dyneema or rubber printed webbings need special methods). The particular advantage of the banana is that the webbing can be pulled hand tight prior to tensioning. The insertion and the removal of the webbing is very comfortable and fast too.
banana-lcoking-principle

The first webbing anchor of this class was the Slackdog, invented by Scott Balcom. On this basis the "Ur-Banana" was built by Junghannß/Zak in 2007. It was a real heavy but sturdy anchor. Michi Aschaber picked up this design in 2008, and published it to a wider audience and made it commercially available under the neologism Banana. Landcruising released in early 2010 new optimized bananas with pioneered inventions such as quick release pins, large middle rollers and direct shackle attachment. The following break tests with the White Magic webbing were made with different generations of the Landcruising Lynx and Zilla series. The size of the middle roller defines mainly the break strength efficiency. The webbing is loaded very smooth and symmetrically. This results in a high breaking load of the webbing.

Banana style webbing anchors are proned to webbing slippage under very high loads (generally > 20 kN). All Landcruising Lynx and Zilla anchors were tested with White Magic without slippage up to the break of the webbing. Slippage at lower loads can occur with Dyneema, Dyneema hybrid and rubber printed webbings. Additionally some bananas from other manufacturers can be more "slipping friendly" due to there geometry. Please ask the specific manufacturer for more infos.

Webbings with lower friction properties need 1.5 or double wrap inside the locker. The pretightening option does not work with this wrapping methods unfortunately.

Special high strength anchors

Landcruising realized in spring 2010 a new locking principle for high tensioned slacklines. The so-called Revolve anchor was especially designed for extreme longlines with Dyneema webbings and very thick polyester webbings.

Investigations on old webbing

The breaking load reduction of webbing by repeated tigtening and loosening cycles, mechanical abrasion, UV radiation, or the influence of moisture and sand is a sure fact. The question is how big this reduction is. We tested a 2 years old piece of White Magic, which was used very often as longline and waterline. The tests were done with the Lockman and a linelocker. The webbing itself showed no signs of wear on the outside or fiber abrasion.

  • On 3 tests with the Lockman, the average break strength of the used webbing was 20 kN (63%), the scatter of the 3 samples was fairly low. This represents a reduction of 4 kN (relatively based 17%) compared to new webbing (24 kN).
  • On 3 test with the favorable threaded linelocker, the average break strength was tested to 16 kN. This gives a reduction of about 1 kN (relatively based 6%). This low reduction is maybe based on the different failure mechanism of shear.

This leads to the conclusion, that even very carefully treated webbing with no visible harms shows a reduction of about 20%, specially with highly efficient anchors. The fatigue behaviour of polyester webbings needs to be examined more detailed i an a separate article.

Summary and discussion

The following diagram shows the relevant webbing anchors and there break loads in combination with White Magic:
diagramm-einspannungen-alle

Following statements can be formulated evaluative:

  • simple knots have due to local stress concentrations and asymmetric loading a very low break strength efficiency, especially the simple overhand knot should not be used
  • doubled or reinforced knots show a substantial increase of the breaking load efficiency, the doubled figure of eight knot is very suitable as webbing backup knot
  • linelockers with chain links provide only low break strengths and should not be used in slacklines with higher tension and high safety requirements (jumplines, longlines, highlines) (exception are reinforced linelockers)
  • bolt- and ringlockers are very cost efficient webbing anchors with high strength capabilities
  • modern webbing anchors based on the banana / slackdog principle are the solution for efficient, comfortable and safe fixture of jumplines, longlines and highlines
  • sewn end loops generally do not reach the breaking load efficiency of bananas with large roller radius (>20 mm diameter)

How behave other slackline webbings made of different material or with different strength?

  • the more stretchy the slackline is (higher elongation parameters), the greater the breaking load efficiency, asymmetric stress distribution can be mitigated with the greater elongation, thus the break load increases (polyamide webbings have in general higher efficiencies with the same anchors compared to polyester)
  • conversely, a lower elongation of the webbing increases the asymmetry, the breaking load falls (low stretch webbings require bigger middle rollers)
  • the webbing thickness and the weave design are crucial for the breaking load efficiency too, thick webbings need bigger middle rollers to reach similar efficiency values in respect to thin webbings

The breaking load efficiencies of slackline webbings vs. webbing anchors are displayed in the base tables.

Finally, some other reference articles are given in the links below:

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