Designing, developing and employing rigging systems is, by necessity, part of most tree care professionals’ daily life. Along with knowledge of what forces a rigging system might generate and the strength and weight of various tree species, climbing arborists must be informed and educated as to what rope, gear and equipment choices to make. The different uses, applications and scenario suitability of the wide variety of arborist blocks and pulleys available is a subject that can not only be confusing, but extremely dangerous if inappropriate or misinformed choices are made. Both blocks and pulleys perform similar functions, providing a sheave of some sort that will reduce friction for a running rigging line; and in many other industrial or recreational applications, the term block or pulley is considered interchangeable. This is not the case in the arborist industry, as these two types of devices have very different applications. Basic information and a little knowledge can help tree crews make informed, and safe, choices.
Arborist blocks are designed to take the extreme loads and abuse of dynamic rigging situations. They have a bushing, for sling attachment, with a variety of locking mechanisms available, ranging from spring locks to captured bushing screw locks. Regardless of which locking mechanism is present, users should make sure it is locked correctly and securely prior to load. Slings should be attached directly to the bushing without the use of any connecting link, such as a shackle or carabiner. This soft-to-hard connection of sling to bushing is the manner in which these blocks are designed to be used. A hard-to-hard connection through the use of a connecting link not only exposes the link to possibly dangerous cross loading in dynamic rigging situations, but also quickly degrades the strength of the bushing and block itself through metal-on-metal wear and stress. The sheave of the arborist block is larger than the bushing, and is intended to allow the rigging line to run over it freely with a minimum amount of friction. The bend radius of the rope passing over the sheave must be considered when choosing the appropriate block and rigging line. A bend radius of eight to one is preferable, though four to one is acceptable, when using rigging lines of braided construction. For example, when using a .5-inch rigging line, the block’s sheave diameter must be at least 2 inches—4 inches would be even more advantageous. Blocks typically have thick, extended cheek plates with somewhat rounded edges. This increases strength and protects the rigging line from inadvertent friction against the bark of the tree, while also lessening the likelihood that a cheek plate’s sharp edge might sever the line should contact be made during a dynamic rigging scenario.
Pulleys come in many configurations, including double sheave, Prusik minding and ones with fixed side plates. They range in size from the small diameter, 1.25-inch sheave of a Petzl Fixe, to the much larger sheave diameter, 4 inches, of a CMI rescue pulley, and some are even larger. The obvious and most important difference between pulleys and blocks for arborist rigging applications is that a pulley lacks a bushing for sling attachment. The thin metal cheek plates of a pulley can only be attached to a sling through the use of a connecting link, and this requirement should restrict its use to static rigging systems only. The use of a pulley in a dynamic rigging situation can lead to failure in many spectacular and dangerous ways. The connecting link used to attach the sling to the pulley may become cross or side-loaded, leading to its failure at the moment of greatest load, the thin metal cheek plates of the pulley itself may fail at the point of contact with the connecting link under the severe shock load of dynamic rigging situations, or, if a sling has been attached directly to the pulley in an attempt to avoid the use of a connecting link, the sharp edges of the pulley’s side plates will, in all likelihood, sever the soft fibers of the sling or the rigging line. These disadvantages do not in any way preclude the use of pulleys in tree care rigging systems, they simply steer pulley use to non-dynamic rigging situations. Applications such as redirects for rigging lines, generating mechanical advantage to lift or pull in a rigging system, static loads with no shock loading or as travelers/carriages in a slide line system are among the many appropriate uses for pulleys.
The use of blocks and pulleys in arborist rigging systems will often lead to safer jobs. The basic information discussed here will hopefully help assist climbing arborists to make safe decisions about which particular block or pulley to use in a given scenario.
Michael (House) Tain is a contract climber, splicer, educator and writer currently located in Lancaster, Ky. He can be reached at firstname.lastname@example.org.