2 METHODS AND MATERIALS Seven single and double braid ropes were selected from three rope manufacturers for testing (Table 1). Ropes were selected based on their frequency of use in rigging as judged by the au- thor and three ISA Certified Arborists who have extensive back- grounds in rigging and training. All ropes were nominally 12.7 mm in diameter, but had a range of breaking loads as rated by the manufacturer (Table 1). Ropes were hitched to a utility pole that was 20 cm in diameter and 2.5 m long. The pole was secured into the tensile testing machine at Yale Cordage (Saco, Maine, U.S.). Four different poles were used throughout the tests. Sur- face roughness of the poles was not quantified, but visually ap- peared similar when testing began. One end of the rope was se- cured to the hydraulic ram of the testing machine with four wraps around a bollard (10.2 cm in diameter) and a bowline around a steel bolt (2.54 cm in diameter). Ropes were precut to lengths of 4.6 m (when testing the running bowline and timber hitch) and 5.5 m (when testing the clove and cow hitches) to ensure the same length for the standing part of rope for each hitch. Approximately 2.1 m from the bollard, one of four common hitches was tied to the utility pole: clove hitch, cow hitch (Figure 1), running bowline, and timber hitch (Figure 2). Clove and cow hitches were finished with a half hitch around the standing part of the rope, and a stopper knot in the tag end of the rope to prevent it from pulling through the hitch. Finishing hitches in this way was not necessary when the running bowline and timber hitch were tied. Hitches were initially tied to be 30 cm from the end of the pole and parallel to its long axis (Figure 3). As testing contin- ued, ropes began to damage the end of the poles, and so subse- quent tests were moved farther away from the end (up to 50 cm) to maintain a reasonably comparable pole surface for all tests. Each hitch was consistently tied, dressed, and set by one of three ISA Certified Arborists who have extensive rigging experience. Testing was conducted in a randomized complete block design. Hitches were pre-loaded to 0.889 kN for 60 seconds to ensure similar alignment of the hitch and its orientation relative to the pole. The speed of the hydraulic ram on the test machine was 6.5 mm/s, a rate typical for testing synthetic ropes (pers. comm.: K. Buzzell, 07/23/2010). Hitches slipped off the end of the pole prior to failure three times; these tests were not included in the analysis. In addition to the breaking load, specific strength (or tenac- ity) of the rope was calculated as the ratio of breaking load to linear density of the rope (kg/100 m). This is a common mea- surement in fiber rope engineering (McKenna et al. 2004). An- other common measurement, efficiency, was not calculated because there was insufficient time and material to determine the unknotted breaking load of ropes considered in the study. Kane: Breaking Load of Hitches and Ropes Used in Rigging A two-way analysis of variance (ANOVA) was used to de- termine whether breaking load and specific strength differed among hitches, ropes, and their interaction. A small sample of Double Esterlon™ ropes measuring 15.9 mm in diameter was also tested with the clove hitch and running bowline. Greater breaking loads of such ropes began to damage poles, which limited the number of such tests. A separate two-way ANOVA was used to investigate whether breaking load and specific strength of Double Esterlon ropes differed among rope diame- ter, hitches, and their interaction. General linear models (PROC GLM) were used to analyze least squared means because of unequal sample sizes in each ANOVA. Tukey’s honestly sig- nificant difference test was used for multiple comparisons within significant (P < 0.05) effects. All analyses were conduct- ed in SAS (v. 9.2, SAS Institute, Cary, North Carolina, U.S.). RESULTS Hitches sometimes rotated circumferentially or slid axially on the pole depending on the type of knot that was tested. Neither of these motions was quantified, but rotation of clove and cow hitches was more common. On many tests involving those hitches, the hitch rotated clockwise (as viewed from the moving bollard on the test- ing machine) approximately 90 degrees from its original position. Rotations appeared to be more common on single braid ropes, but it was not possible to confirm this observation due to changes in the surface roughness of the poles over time. The location of failure was always at the first bend in the standing part of the rope where it entered the hitch and cinched around the pole (Figure 3). For running bowlines and timber hitches, the location of failure coincided with the location of the eye through which the standing part of the rope passed before cinching around the pole. Failure of clove and cow hitches occurred where the half hitch that fin- ished each hitch took a bight on the standing part of the rope. The breaking load of knotted ropes varied primarily among ropes (which explained 88% of the model’s variance). The range of values (16.6 kN) between ropes was much larger than the range of values between hitches (1.45 kN), for which val- ues were statistically similar (Table 2). Breaking load was greatest for Double Esterlon, which was greater than the other double braid ropes (Table 2). Breaking load was least for ArborPlex and XTC-12 (Table 2). Breaking load of True- Blue was greater than the other single braid ropes (Table 2). There were fewer differences between ropes for specific strength: all of the single braid ropes had similar values, which were less than values for the double braids (Table 2). The type of rope was again a more robust explanatory variable than the Table 1. Ropes used in testing, including their construction, manufacturer, material, nominal diameter (mm), rated breaking load (kN), and linear density (kg/100 m). The latter four values were obtained from manufacturers’ literature. Rope ArborPlex Double Esterlon Double Esterlon Industrial Poly DB Safety Pro-12 Stable Braid True-Blue XTC-12 Braid Single Double Double Double Single Double Single Single Manufacturer Samson Yale Yale New England New England Samson Samson Yale Material Polyester/polyolefin Polyester Polyester Polyester Polyester/polyolefin Polyester Polyester Polyester/polyolefin Diameter 12.7 12.7 15.9 12.7 12.7 12.7 12.7 12.7 Breaking load 26.69 48.04 75.57 44.04 29.36 46.26 32.47 26.69 Density 10.1 12.1 14.3 10.4 11.6 12.2 13.1 10.0 ©2012 International Society of Arboriculture
January 2012
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