Arboriculture & Urban Forestry 36(3): May 2010 129 trast’ statement was used to separate means within each signifi- cant level of the dominant effect. Contrasts were developed for linear through the appropriate polynomial, depending on the degrees of freedom of the effect. The contrast with the great- est F-value was used to describe the relationship. For example, since there were three degrees of freedom for the effect of swing, the authors developed linear, quadratic, and cubic polynomials to separate means. Simple regression was used to quantify the relationship between percent cut and impact force of the blade. RESULTS Zubat blades cut similarly (p = 0.0690) through Poison Ivy (least squares mean = 69%) and XTC (least squares mean = 71%), so pooled results are presented for the remainder of the paper. Swing was the dominant effect determining percent cut; in the ANOVA, its F-value (705) was nearly six times greater than the F-value for rope tension (120). Percent cut increased quadratically with increasing swing height (Figure 2). The F-value for the quadratic contrast of swing heights (1,982) was 27 times greater than the F-value for the linear contrast (72.5), and 48 times greater than the F-value for the cubic contrast (41.6) even though all three were significant (p < 0.0001). Full swings cut completely through 53 of 54 ropes tested; one rope was cut through 89%, at the smallest rope tension (143 N). In contrast, only two ropes were completely severed during quar- ter swings of the pendulum, both occurred when ropes were under maximum tension (400 N). Excepting full swings [within which the effect of rope tension was not significant (p = 1.000)], within each swing height, percent cut increased quadratically with increased rope tension (Figure 2). The effect of rope ten- sion was most pronounced at quarter swings, within which the quadratic relationship among rope tensions was positive. Within half and three-quarters swings, the quadratic relationship among rope tensions was negative, which reflected the complete sever- ing of all ropes when rope tension exceeded 311 N and 156 N for one-half and three-quarters swings, respectively (Figure 2). DISCUSSION In light of the many ways a saw blade and rope may come into contact, the most important finding is that across a realistic range of impact forces and rope tensions, it is easy to cut through one’s climbing rope with a handsaw, supporting previous research (Kane et al. 2009). As previously speculated (Kane et al. 2009), impact force and rope tension both affected the likelihood of a climber cutting through his or her rope, although impact force was clearly the dominant effect. Part of its dominance may be due to the wider range of impact forces measured, which was 1.4 times greater than the range of tested rope tensions. At full swings equivalent to the impact force a climber can Figure 1. Experimental setup: (top) pendulum attached to beam, the arrow points to the jig that held blades; and (bottom) a close- up view of the jig, where the “X” marks the tooth that first made contact with the rope. levels of the subordinate main effect (as determined by F-value) within each level of the dominant main effect. Then the ‘con- exert with two hands on the handsaw, a climber can easily cut his or her rope, even if it is under little tension. Standing on a branch, but otherwise fully supported by his lanyard, a 75 kg climber exerted 222 N of tension in the lanyard (pers. obs.). It is unclear whether the maximum impact force would mimic the force a climber exerted as he or she actually cut through a branch, but many climbers have prematurely severed a branch while still exerting considerable force on the saw. Perhaps the greater risk would occur when a climber is making a cut but unaware the rope is in the path of the saw blade. Another dangerous scenario ©2010 International Society of Arboriculture
May 2010
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