4 sified by knot and rope. Effect Level Rope ArborPlex Double Esterlon Table 2. Least squares means (standard error) for breaking load (PMAX in kN) and specific strength [kN/(kg/100 m)] clas- n 20 27 Industrial Poly DB 12 Safety Pro-12 Stable Braid True-Blue XTC-12 17 22 20 20 Hitch z Clove hitch Cow hitch Running bowline Timber hitch 37 34 35 32 PMAX z 21.58 (0.49)a 38.16 (0.43)f 31.06 (0.64)d 25.64 (0.49)b 35.49 (0.55)e 27.89 (0.48)c 22.87 (0.49)a 28.85 (0.38)a 28.98 (0.39)a 29.72 (0.39)a 28.27 (0.40)a Specific strengthz 2.13 (0.04)a 3.16 (0.04)c 2.98 (0.05)bc 2.29 (0.04)a 2.90 (0.05)b 2.12 (0.04)a 2.29 (0.04)a 2.55 (0.03)ab 2.55 (0.03)ab 2.62 (0.03)a 2.49 (0.03)b Read down a column within each effect, least squares means followed by the same letter are not significantly different (P > 0.05) by Tukey’s Honestly Significant Difference test. Double Esterlon ropes 15.9 mm in diameter and 12.7 mm in diameter. Effect Hitch z Level Diameter 12.7 mm 15.9 mm Clove hitch n 15 9 13 Running bowline 11 PMAX z 38.67 (0.85)a 60.82 (1.06)b 48.75 (0.97)a 50.73 (0.96)a Specific strengthz 3.20 (0.07)a 3.03 (0.08)a 3.05 (0.08)a 3.18 (0.08)a Read down a column within each effect, least squares means followed by the same letter are not significantly different (P > 0.05) by Tukey’s Honestly Significant Difference test. clove and cow hitches did not affect the location of failure, nor did it adversely affect breaking load. Rotation of clove and cow hitches presumably was related to the way they were tied. Rota- tions appeared to begin to untie clove and cow hitches, which was consistent with the need to tie a half hitch and stopper knot to avoid the hitches untying before breaking the rope. Rotations did not consistently cause clove or cow hitches to slip off the pole. A more careful study of the rotation of clove and cow hitches may illuminate some performance deficiencies. In previous field tests, a cow hitch with a half hitch was exclusively used to attach a block to the trunk while removing the top and up to four ad- ditional pieces from each of 24 red pines (Pinus resinosa) (Kane et al. 2009; Kane, unpublished data). The hitch experienced im- pact loads up to approximately 18 kN, but it was never observed to rotate, even though—on a few tests—the impact load caused the sling to slide nearly 0.5 m down the trunk, de-barking it. The common location of failure was also consistent with differences in breaking load between ropes, which have inher- ently different breaking loads. The breaking load of an untied rope depends on many factors, including its material and con- struction, as well as the manufacturing process. The location of failure made mechanical sense because the rope would have experienced tensile stress due to the load plus stresses due to contact and friction (Milne and McLaren 2006) from the loop or bight through which the standing part of the rope passed as it cinched around the pole. The common location of failure sug- gested the importance of rope-on-rope abrasion at that location. ©2012 International Society of Arboriculture Table 3. Least squares means (standard error) for breaking load (PMAX in kN) and specific strength [kN/(kg/100 m)] of Kane: Breaking Load of Hitches and Ropes Used in Rigging Changes in the surface roughness of poles observed dur- ing testing did not appear to alter test results for two reasons. First, at the point of failure, video footage of several tests re- vealed that the part of the rope that failed was not in contact with the surface of the pole. Although video evidence was not available for every test, it was intuitive that contact between the pole and the part of the rope that failed was minimal consid- ering that for all hitches, another part of the rope was wedged between the pole and the part that failed. Second, the magni- tude of variability for breaking load and specific strength was small: the coefficient of variation of both variables for all ropes was less than 10% with the exception of True-Blue (10.4%). The analysis of specific strength highlighted differences between hitches as well as between the two rope construc- tions. Rope construction was clearly important in explaining differences in specific strength, since double braids had great- er values than single braids. This difference was likely due to the abrasion resistance provided by the outer jacket of fibers on double braids, which presumably protected the load-bear- ing inner fibers from rope-on-rope abrasion. The outer fibers on double braid ropes were also coated with urethane to re- duce abrasion, which may have enhanced abrasion resistance. On single braids, load-bearing fibers would have been im- mediately abraded at the point where ropes ultimately failed. It was unclear why the running bowline had a greater spe- cific strength than the timber hitch. Perhaps the loop (crossing rope parts) formed by the timber hitch around the standing part of the rope reduced rope strength more than the bight (parallel rope parts) that the running bowline formed around the standing part of the rope (Figure 2). Milne and McLaren (2006) also observed that knots with more crossing parts were weaker. The loop formed by the timber hitch around the standing part of the rope could have induced an additional torque, or it could have caused the scissor- ing effect that Leech (2003) described for rope fibers. While sta- tistically significant, the difference in specific strength between the running bowline and timber hitch has less practical relevance for two reasons: 1) the difference is much smaller than between different ropes, and 2) the hitches are typically used in different applications. The timber hitch is used to attach a sling to the tree, as an anchor point for a friction device or block; the running bow- line is used to tie off pieces of wood to be rigged (Lilly 2005). CONCLUSIONS Breaking load and specific strength were more closely tied to differences between ropes than between hitches. But consis- tent performance of hitches across many ropes is reassuring for practitioners. In the absence of data from dynamic loading tests, the results of this study provide a helpful baseline because tests on the utility pole better mimicked loading conditions in arboricultural rigging than previous tests. Practitioners can use results, with caution, to estimate the knotted strength of a rope that they typically use. However, arborists are strongly discour- aged from choosing a rope or sling based simply on its breaking load or specific strength. The choice of a rope or sling must also include consideration of its expected use (natural anchor point versus blocks and friction devices), range of loads, frequency of use, and cost. If rigging involves dynamic loading (which is usu- ally true), the rope or sling’s ability to absorb energy is a criti- cal point of consideration. Breaking loads measured in a static
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