Arboriculture & Urban Forestry 38(1): January 2012 Arboriculture & Urban Forestry 2012. 38(1): 1–5 1 Breaking Load of Hitches and Ropes Used in Rigging Brian Kane Abstract. The incorporation of hardware like blocks into the rigging system has prompted a change in the types of ropes used as slings to attach blocks to trees. Since large forces can be generated while rigging trees, it is important to determine the breaking load of hitches used to attach a sling to a tree or the rigging rope to a piece of wood. Breaking load and specific strength (the ratio of breaking strength to linear density of the rope) were measured for four common hitches and seven ropes often used in arboricultural rigging. Hitches were tied around a utility pole to sim- ulate field conditions, and tested with a gradually increasing load. Breaking load was similar between all hitches, but varied widely among ropes, while specific strength differed between ropes and hitches. Tying hitches around the utility pole mimicked the arboricultural application of hitches and ropes, but the static application of the load, which did not reflect dynamic loads often generated during rigging, was an important limitation. Key Words. Breaking Load; Hitch; Rigging; Rope; Specific Strength. Rigging is one of the most dangerous aspects of arboriculture because large, swinging pieces of wood can generate dynamic loads of great magnitude. Pieces of wood in motion have sub- stantial momentum, and, if uncontrolled, can injure the climber or ground workers and damage property. Introduction of me- chanical devices like blocks and lowering devices (e.g., Good Rigging Control System, Hobbs Lowering Device, and the Port- A-Wrap) have led to the incorporation of different types of rig- ging ropes (e.g., double braids) better suited for use with such devices. In rigging, laid three-strand and single braid (12- and 16-strand construction) polyester ropes have, to some extent, been replaced with double braid ropes. However, double braid ropes are inappropriate for use in natural anchor point rigging (Donzelli and Lilly 2001) because of their relatively loose con- struction, which is susceptible to snagging and allowing abra- sive particles into the core fibers (Smith and Padgett 1996). In arboricultural rigging, ropes and slings are often tied in hitches, and while spliced eyes are generally the most ef- ficient permanent rope terminations (McKenna et al. 2004), they require special expertise to fashion and are peculiar to a particular rope material and construction (Milne and McLaren 2006). A permanent termination-like a splice is not always useful in arboricultural rigging. Hitches also provide greater flexibility because of the ease with which they can be tied around trunks or branches of varying diam- eter and the ability to remotely tie off a limb. Some hitches also offer the advantage of being easy to untie after loading. Many hitches are used in rigging, and technical publica- tions (Donzelli and Lilly 2001; Lilly 2005) describe the ap- propriate arboricultural uses of each. Cow and timber hitches are commonly used to attach a block or friction device to the tree, and the clove hitch and running bowline are used to at- tach the lowering rope to the branch or piece of wood being removed. The timber hitch and running bowline offer the ad- vantage of being easy to untie; clove and cow hitches pro- vide a secure attachment to the tree even when not loaded. Smith and Padgett (1996) note the wealth of information on the efficiency of knots; they also highlight the inherent variability in test results depending on numerous confounding factors (e.g., rope diameter, construction, and material; test method and condi- tions; person who ties the knot). Simon (2002) noted that there is no theory to quantitatively predict the breaking load of knots, pre- sumably because of the wide array of complicating factors. Aside from one previous study at Samson Rope in 2004 (cited in Detter et al. 2008), testing knots has mostly originated in other disci- plines, such as caving (Richards 2005), rock climbing (Brown 2008), and sailing (Milne and McLaren 2006). Such work is informative, but the ropes tested and the testing conditions of- ten differ from arboriculturally relevant ropes and conditions. Ropes commonly used in rigging include single braid and double braid ropes. Single braid ropes are used for natural anchor point rigging and typically have twelve strands and a hollow core. The strands of single braid ropes provide abrasion resistance and bear loads. In contrast, double braid ropes, which are used when rigging with blocks and lowering devices, consist of a braided sheath of fibers (of varying thickness) that resist abrasion and car- ry some of the load, around a core of primarily load-bearing fibers (McKenna et al. 2004). In arboricultural applications, single braid and double braid ropes consist primarily of polyester, although small amounts of other fibers are sometimes incorporated into the rope to achieve a specific purpose, such as reduced weight. In light of the scarcity of robust data to describe arboricul- turally-relevant ropes and hitches, the objectives of this study were to determine a) the breaking load of hitches commonly used in arboricultural practice, and b) whether the break- ing load differed among ropes commonly used for rigging. ©2012 International Society of Arboriculture
January 2012
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