Neil Montgomery is an Australian caver who (it is claimed by the aussies) first coined the phrase Single Rope Technique (SRT) back in 1977 - sorry tree climbers, you did not invent SRT!
Montgomery's quote is useful as a reference for knot strengths versus splice strength. If we agree the following points: 1) knots are perfectly acceptable for use in a tree climbing system and 2) rope strength is compromised by the use of a knot by the approximate percentages noted above by Montgomery et al; then we can assume the break strength of a knot is the base level for acceptable splice strength.
In other words, if the splice breaks at a higher loading than the knot then it should be deemed acceptable in tree climbing competitions.
We appreciate purists will say individual knots produce different break strengths. Agreed, but the safety factors are so high in most of the test loads applied to ropes and splices it is more likely the climbers body would be broken well before the rope… so for the sake of this blog we will work with generalities. Download more on the testing of specific knot strengths here.
Another reference in Montgomery's quote is the severity of the rope bend and its effect on rope dynamics. Before Treetools conducted the series of tests on the DB Tree hand-spliced Armor Prus 8mm eye-to-eye friction cords we sought advice from Chris Cowell of the Treemagineers regarding the best procedure to adopt. Apart from cautioning us against the validity of the test results, Chris recommended we use a 10mm clevis pin on the test bed to better replicate the bend ratio of a carabiner. Consequently, all the DB Tree tests we conducted using the 10mm pin diameter. See previous splicing blog here.
8mm Armor Prus has a breaking strength of 27.46kN or 2800kgs according to Donahys specifications. If your friction hitch was tied with a knot (allowing for variances depending on the type of knot employed) it is likely to break at about 16kN (on a straight pull and if Montgomery's 40% figures is correct). That means, if the splice breaks at/or above 16kN it would be deemed to be as good as (or better than) a knot. Download more on high-strength cord tests here.
Treetools tested six DB Tree spliced 8mm Armor Prus friction cords hand-spliced by Drew Bristow. Three of the friction cords had a Type 1 splice eye-to-eye (on a Type 2 cord) and three were spliced using a locking brumell as recommended by rope manufacturers for this type of cord. Rob Telford from Chain and Rigging Supplies Limited in Onehunga conducted the tests on a Roberts Testing Machine which is fully certified and calibrated by Australasian Calibrating Services (last calibration 5 November 2009).
The eye-to-eye friction cords were attached to the hydraulic ram using 10mm clevis pins through each eye as noted above and broken on a straight pull. It should be noted the straight pull break-test method does not reflect the type of load encountered in the work application. Almost 99% of all eye-to-eye friction cords, when configured in a tree climbing system, are used in a closed hitch situation where the load is shared by both end splices, effectively doubling the break strength.
The straight-pull test results were as follows.
8mm Armor Prus friction cord with eye-to-eye Type 1 splice Test 1; 16kN Test 2: 16kN Test 3: 17kN 8mm Armor Prus friction cord with eye-to-eye locking brumell splice Test 4: 24kN Test 5: 22kN Test 6: 24kN
Remember, you can double these figures if the friction cord is used in a typical tree climbing configuration.
Both types of splice performed about the same as a knot (16kN) although we intend testing knots using 8mm Armor Prus on the same machine to see what the breaking strength actually is.
The higher rated locking brumell eye-to-eye friction cords all broke on the outside edge of the clevis pin. In other words, the unsheathed, exposed core of the Armor Prus broke not the splice - hence the high rating close the the actual rope strength.
The Type 1 splice broke where the taper of the 'bury' was at its weakest point and then the splice released. On a couple of the Type 1 test graphs (Tests 1 and 2) you will see fluctuations in the load line; the first peak is the whipping breaking and the second peak is the splice releasing.
If nothing else these tests show the quality consistency of the DB Tree hand-splices (as opposed to other hand-splices) - there is less than 1kN difference between each set of three splices.
Rob Telford preparing the test bed for the DB Tree splice tests. Click on the photograph to see more test photographs on Flickr.