32 Kane: Friction Coefficients for Arborist Ropes determined by a randomly generated number corresponding to the number of lengths to pull through the rings before testing. The random numbers were large enough to ensure that samples were not concentrated in one particular section of the rope. Because each rope was tested on three cambium saver rings, there was incipient wear on the tested sections. Although incipient wear never amounted to more than a few broken fibers, when it coincided with a test, the rope was pulled further through the cambium saver rings to avoid test- ing that section again. Static and kinetic friction coefficients and the difference Figure 1. Diagram illustrating equation (1): a rope (dashed line) traveling over a tree branch in cross- section; T2 motion. T1 is the tension in the rope in the direction of is the tension in the rope on the opposite side of the branch from motion. is the angle of contact, in radians, between the rope and the branch, and R is branch radius. outer fibers of all arborist ropes are made of polyester, sur- face coatings on individual rope fibers and the ropes vary widely and may affect friction coefficients. Some rope manu- facturers present friction coefficients (Anonymous 2004), but these usually refer to fiber on fiber testing and do not account for the effects of coatings and braiding. METHODS AND MATERIALS Static and kinetic friction coefficients (s, k, respectively) were determined using equation (1) for 12 types of climbing rope on cambium saver rings made of raw aluminum, pol- ished aluminum, and steel. The difference between s and k was also calculated for each rope and ring combination. For one randomly selected rope of each rope type, the following attributes were also measured: braid angle, the angle a strand diverges from the long axis of the rope; and braid length, the axial distance between the points where the same strand re- appears above itself after having made one rotation. The av- erage of three measurements of braid angle and braid length were taken for each rope while the rope was under the same tension applied during testing. Surface roughness of cambium saver rings was measured with a Starrett Model 3800 pro- filometer (Starrett Machine Co., Athol, MA). Table 1 pro- vides the names, abbreviations, and pertinent product details for the ropes and cambium saver rings. The length of an individual rope varied from 7.62 m (25.15 ft) to 45.72 m (150.87 ft). For each type of rope, at least two and as many as five individual ropes were tested and each individual rope was sampled five times. All ropes and cambium saver rings were tested before any field use. Ropes were sampled by pulling a length of rope through the cambium saver rings before each test. The length of rope that was pulled was ©2007 International Society of Arboriculture between them were also determined for two types of climbing rope after they had been used in the field. The used ropes had been in service for at least 2 years and were visibly and tangibly “fuzzy” (Figure 2). The two types of used rope were tested on each ring material. At least three individual ropes were tested and each individual rope was sampled five times. To quantify rope wear, the ropes were photographed and ImageJ software (Wayne Rasband; NIH, Bethesda, MD) was used to determine the surface area for a 12.5 cm (5 in) length of rope. Four high-resolution (approximately 2000 dpi) digi- tal images were taken of a randomly selected rope of each type of new rope tested. Images were taken at intervals along the length of the rope; intervals were determined randomly as described above. Each rope was suspended vertically and loaded with 22 N (5 lb). Because there was greater variation in fuzziness of used ropes, six images were taken of each individual rope. The surface area of four types of used rope was compared with their new counterparts. However, only two types of used rope had enough replicates to be used in the analysis of friction coefficients. A Buckingham cambium saver (Model 57; Buckingham Manufacturing, Binghamton, NY) was installed over a beam and a spread of ≈10 cm (4 in) was maintained between the rings to ensure that the rope legs remained parallel to one another throughout the test. Keeping the rope legs parallel maintains radians (180°) (equation [1]). During use in the field, the rings of the cambium saver will press against one another, but as long as both legs of rope are parallel, radians. Cambium saver rings also have a tendency to move vertically relative to one another depending on which direction the rope is moving through the rings. As long as the legs of rope remain parallel and the rings remain in contact, however, radians. To avoid overly abrading one area of the rings, they were rotated counterclockwise after testing each rope and two dif- ferent cambium savers for steel and polished aluminum rings were used. Because only one cambium saver with raw alu- minum rings was used, the rings were smoothed more quickly and only five rope types were tested on them. For analysis of the effect of ring material on friction coefficient, only the five rope types tested on all three ring materials were included.
January 2007
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