184 Brian Kane: Compatibility of Toothed Ascenders with Arborist Climbing Ropes [e.g., Edelweiss Rescue and Beal Antipodes (Petzl 2010)], however, do not meet the Z.133 Standard (Anonymous 2006a) for appropriate climbing lines because they are made of nylon. It was unclear why the arrest distance for Tachyon, but none of the other ropes, was greater with 1000 mm of rope length com- pared to 2000 mm of rope length. Greater arrest distances with less rope in the system were physically intuitive since less rope in the system decreases the amount of kinetic energy of the falling mass that would be converted to strain energy stored in the rope (Smith 1998). By this reasoning, however, the impact load for tests of Tachyon with 1000 mm of rope length should have exceeded the impact load for tests of Tachyon with 2000 mm of rope length, which did not occur. This and other inconsistencies (e.g., greater and lesser impact loads with 1000 mm compared to 2000 mm of rope length for Safety Blue and Blue Streak, respectively) may be attributed to the complicating effect of the ascender cutting the rope. An initial speculation that variation in arrest distance among ropes and rope lengths caused stopping acceleration (and thus impact force) to vary did not fit with the observation that on every test maximum axial strain in the branch occurred immedi- ately after the mass was released. Assuming maximum impact load coincided with maximum strain, impact load seemed more likely to be related to the ease with which the rope was cut rather than the arrest distance. It was also possible that sampling for im- pact loads at 60 Hz introduced bias; the EN 12841-2006 Standard (Anonymous 2006b) requires sampling at 1000 Hz, so it is possi- ble that impact loads do not reflect the absolute maximum value. Impact loads at failure for arborist ropes were less than the 6.5 kN for ropes 13 mm in diameter noted in product litera- ture (Petzl 2010). This difference likely reflects the ease with which the ascender cut the rope, and the smaller mass used in the test. Impact loads were within the range of values presented by Bridge and Cowell (2009). Backing up the ascender with a friction hitch, as commonly shown in popular literature (Tres- selt 2006; Adams 2007; Clark 2009), did not meaningfully af- fect arrest distance, but it appeared to increase impact load, a negative outcome. Part of this disparity was presumably due to the fact that the VT was only mildly tightened prior to the drop test, which made the knot effective in only three of ten tests. The VT was intentionally not vigorously tightened prior to test- ing, to simulate what could happen to a friction hitch during a footlock, as the ascender pushed the hitch up the rope. Unless the climber repeatedly set the VT, it would likely be somewhat loose after ascending some distance. The backup friction hitch may still provide a measure of safety against other types of ac- cidents, for example, the ascender becoming dislodged from the rope by a branch. Impact loads for tests with Prusik loops were within the range of values presented by Bavaresco (2002). In addition to the limitation of the sampling rate of the dyna- mometer, the accuracy of impact loads may have been limited by two other aspects of the experiment. First, attaching the lan- yard to the drop mass with a carabiner allowed the possibility of cross-loading the carabiner. Second, branch deflection during testing would have stored strain energy converted from kinetic energy of the falling mass once the rope and branch begin to slow the descent of the mass. Neither of these limitations was considered serious because cross-loading was only observed on a few tests and branch deflection, which was conservatively esti- mated, was minimal. In practice, branches to which ropes would be attached would likely be smaller in diameter, but the rope would be placed closer to the point of attachment to the trunk. The variability of parameters that affect branch deflection (di- ameter, angle of orientation, elastic modulus, distance between rope and branch attachment) with respect to installing a climbing line precludes speculation about the effect of branch deflection on the conversion of kinetic to strain energy stored in the branch. It is important to remember that the drop test represents a Figure 3. Cut ropes at the point where the ascender was initially attached to the rope, showing remain inner strands of rope. From top to bottom: Tachyon, Safety Blue, Blue Streak, Velocity. “worst-case scenario,” and manufacturers (Kong 2010; Petzl 2010) and popular literature (Bridge and Cowell 2009) warn against dynamically loading toothed ascenders. In spite of the worst-case scenario nature of the drop test, it remains one of the tests that must be passed in order to meet the EN 12841-2006 Standard for Type B rope grabs (Anonymous 2006b). Tree climb- ers must be made aware that the use of some (and presumably most) arborist climbing ropes does not comply with the Ascen- sion and, presumably, most toothed ascenders. More generally, climbers should be made aware that adopting and adapting tech- niques and gear from related high angle disciplines comes with the critical caveat that such techniques and gear do not always safely translate into use in arboriculture. Such information should be widely disseminated and emphasized in popular literature, at climbing demonstrations and competitions, and in training venues. ©2011 International Society of Arboriculture
July 2011
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