By Michael “House” Tain
Rigging in tree care is a subject that can be either complex enough to fill an entire book, as it often has, or as simple as to be easily discussed and decided over the first cup of coffee in the morning right beneath the tree in question. However, in both simple and complex rigging, some basic knowledge and understanding is necessary to avoid situations involving insurance companies, friends who do roofing and gutters, or, worst case scenario, the emergency room of the local hospital. A good basic understanding of the terms, equipment, and forces involved in rigging in tree care will pay huge dividends in getting large, woody debris on the ground and into the chipper safely, easily and efficiently.
Terms and definitions
Breaking or tensile strength: The amount of force that will break or distort the particular rope or component. This should either be stamped on the particular component (block, pulley, etc.) or part of the literature that came with it. Putting a rope or component with an unknown breaking strength into a rigging system is a very bad idea.
Safety factor: The ratio used to determine the safe working load (SWL) or working load limit (WLL) of a particular component. For example, with a ratio of 10:1 the tensile strength would then be divided by 10 to arrive at a safe working load. This factor is often generated by the manufacturer of a piece of gear, but can also be used by tree crews to decide how far they wish to push their rigging system components. Typically, the lower the safety factor, the less use prior to breaking the component will provide as each use weakens the component slightly; and greater loads will weaken it more quickly, bringing it closer and closer to failure.
Safe working load, or working load limit: This amount of force, arrived at by using the safety factor and tensile strength of the component, gives the user a load limit that he or she can use the component at safely for an extended period of time. As mentioned previously, low safety factors will lessen the life of the component, while high safety factors should extend it, but also limit the size of the load put on the component.
Cycles to failure: The process by which repeated loading weakens a component in the rigging system. Ropes, connecting links, and other components are losing strength all the time through use. Loading them near their tensile strength will speed up this weakening process. This can be likened to the simple process or breaking a wire coat hanger, each bend weakens it more and more, until it separates. On a larger — and hopefully much stronger — scale, ropes, carabiners, and other components are very similar, being weakened slightly with each load.
Bend radius: The amount of bend formed in a rigging line when it passes over the sheave of a block or pulley. This bend weakens the strength of the rope, by compressing some of its fibers (as ropes are happiest, and strongest, when all their fibers are in a straight line). A bend radius for braided ropes of 4:1 or greater will retain more of the rope’s strength, and 8:1 would be preferred. Twisted rope construction, typically three strand, should have a bend radius of 10:1. For example a 1/2-inch double-braided rigging line should pass over at least a 2-inch sheave; and a 4-inch sheave would be even better for strength retention. This bend radius should also be kept in mind when not using blocks or pulleys, such as in natural crotch rigging; and care taken to use larger branches as the “sheave” of the bark-covered pulley.
Natural crotch rigging: Natural crotch rigging is a system where the natural features of the tree’s canopy are used as rigging points. This system’s advantages include a need for very little gear beyond a rigging line; and an easy set-up if the tree’s structure has all the rigging points in the right places that the crew needs. One major disadvantage can be all the additional friction and wear generated on the rigging line by running over the bark-covered “pulleys.”
Tools, ropes and equipment
Blocks: Tree care rigging — and rigging for removal in particular — generate extreme loads and abuse equipment; arborist blocks are designed to endure both these factors. They have a bushing for sling attachment and a sheave for less friction on the running rope.
Pulleys: Pulleys come in many configurations, including double sheave, Prusik minding, and those with fixed side plates. The primary difference between pulleys and blocks is that the pulley lacks a bushing for sling attachment, which also should restrict its use to static rigging systems. Use of a pulley in a dynamic rigging situation will lead to failure.
Double braid construction: A rope within a rope. Both core and cover work together to provide the overall strength of the rope. Low stretch and high strength for size. An excellent choice for rigging with blocks, pulleys, and lowering devices, but a bad one for natural crotch rigging as the cover tends to carry more of the load than the core due to increased friction.
Twelve-strand solid braid construction: Each of the individual 12 strands help bear the load. Good abrasion resistance and slightly more elongation than the double-braid constructions. This type of rope is a better choice for natural crotch rigging than the double braids.
Three-strand construction: Each of the individual three strands help bear the load. More elongation than the double braids and some 12 strands. Good choice for lightweight natural crotch rigging.
Twelve-strand hollow-braid construction: Primarily used to fabricate rope tools such as eye slings, spider legs, loopies, and whoopies. Its construction not only makes it fairly easy to splice, but also has shown in testing to retain more strength than the double-braid constructions in sling configurations. This construction also makes it a poor choice for a rigging or lowering line.
Endless loops: Loops of line, rope, or webbing formed through splicing, stitching, or the use of appropriate knots and hitches. Endless loops may be used in a variety of ways in both climbing and rigging operations.
Eye slings: May be manufactured from many different types of rope constructions, but 12-strand hollow braids and double braids are the most commonly used materials. Testing has shown that the 12-strand hollow-braid constructions retain more of their strength than do double braids in an eye sling configuration.
Whoopie slings: Primarily manufactured from 12-strand hollow braids through the use of splicing. A whoopie sling consists of a fixed eye on one end, and an adjustable eye on the other. It should be used with the fixed eye through the adjustable eye, and gives the climber or rigger more flexibility by allowing the use of one sling on various diameters of trees.
Loopie slings: Primarily manufactured from 12-strand hollow braids through the use of splicing. Loopie slings are adjustable endless loop slings. They should be used in a girth hitch around the branch or trunk, with the unspliced portion passing through the spliced portion. They give the climber or rigger more flexibility by allowing the use of one sling on various diameters of trees.
Spider legs: Primarily manufactured from 12-strand hollow braids through the use of splicing, spider legs are long eye slings with a large single eye used to balance limbs in rigging operations. Care should be taken to use a spider leg 1/8-inch smaller in diameter than the rigging line it is being attached to.
Connecting links: Tools such as carabiners, screw links, delta links, rigging rings, ladder snaps, and clevises. Their use in rigging systems should always be examined closely to ensure they are being loaded correctly, are strong enough for the application, and interact favorably with whatever other parts of the system they are attached to. They are perfectly acceptable in static rigging systems such as knotless rigging or balancing, but their use should be minimized as much as possible in dynamic rigging systems.
Lowering devices: Devices such as the GRCS, Port-a-wrap, or Hobbs allow for the management of friction in a consistent and controlled manner, increasing safety and productivity, but may not be necessary for very simple rigging jobs. However, rigging systems that involve the use of arborist blocks and pulleys will be much better served and controlled by the use of some sort of lowering device.
Rigging systems: All of these various components — rope, rope tools, connecting links, blocks and pulleys, lowering devices, and mechanical advantage systems — can then be used to construct rigging systems that allow jobs to be done more safely and quickly. However, the forces generated by these various systems must always be evaluated to prevent catastrophic failure of any of the various components. Tree crews should understand how much force is being put on the anchor points within the system by the various loads that are being suspended and moved around. This does not mean that the crew must have an exact idea of how much force exists at any given point, but rather that they should understand that the force generated may greatly exceed the weight of the piece itself. This may be better understood by examining the accompanying illustrations.
Tree care rigging is one of those topics that a professional arborist should always be gaining more knowledge and insight into, with each job providing more experience and information to store away in their mental toolbox. The basic introduction to terms, tools, concepts, and principles here should help climbers and riggers better understand how all the pieces of tree rigging go together, helping them build a system that not only works well, but also safely, in getting that large woody debris on the ground.
Michael “House” Tain is a contract climber, splicer, educator and writer associated with North American Training Solutions www.northamericantrainingsolutions.com and Arbor Canada Training and Education www.arborcanada.com He is currently located in Lancaster, Ky., and can be reached via e-mail at firstname.lastname@example.org