Tools & Techniques:
Pulleys and Blocks
When Friction is Not a Friend
By Michael Tain
Sheave, not sleeve
The surface of an arborist block that the line is intended to run over is called a sheave, and it’s typically the larger diameter of the two block ends. This sheave should turn freely, minimizing the amount of friction the rope experiences when running over/around it.
Bend radius is a concept that must be understood when matching arborist blocks with rigging lines. In short, the sharper the bend in a rope passing over or around any object, the more strength the rope will lose. A bend radius of 8 to 1 is the most advantageous for minimizing strength loss, though a radius of 4 to 1 is acceptable in arborist rigging situations with lines of braided construction. As an example, a .5-inch rigging line should be used with a block with a 4-inch-diameter sheave for optimal strength retention, though a sheave diameter of 2 inches would be minimally acceptable.
Cheeks like a chipmunk
The cheek plates of a well-designed quality arborist block are thick and extend beyond the bushing/sheave with somewhat rounded edges. This thickness helps provide the strength required for enduring the loads and forces involved multiple times in tree removal rigging.
In addition, the extension of the cheek plates helps ensure that the sling remains in proper contact with the bushing, and the rigging line in proper contact with the sheave, while rounded edges help minimize rope damage should the line get trapped or run between the edge and the tree.
Pulleys aren’t bullies
Pulleys, as mentioned previously, are not the same piece of gear as a block in the tree industry. They should, perhaps, be thought of as the slender, more sensitive, but still quite useful, cousin of the big, burly arborist block.
While they do have the sheave of an arborist block, they lack the bushing for sling attachment, typically having an opening of some sort for the attachment of a connecting link. This particular lack is what prevents the safe use of pulleys in rigging applications involving dynamic loads and forces. The use of a connecting link through the opening provided can lead to cross or side loading in these situations. Given enough force or large enough load, the connecting link can tear entirely through the pulley’s thin, non-chipmunk like cheek plates, sending the load and pulley on a high-speed one-way trip to the unsuspecting branch manager.
(An arborist block-note the bushing-being used as the traveler in a balance slide line rigging system.)
An attempt to avoid the use of a connecting link by girth hitching the sling through the pulley’s provided opening is also problematic and unsafe, and can lead to bent and mangled cheek plates or slings severed by the sharp edges present in most pulleys.
There are a wide variety of pulleys available to modern arborists, and far too many configurations to all be discussed here. The few listed below give a good introduction to some of their appropriate applications.
Fixed, no not that way, sides – Pulleys with fixed sides or cheek plates typically, though not always, have a distance between the two sides equal to the thickness of the sheave. This allows their effective use in a number of applications, but common uses include as a fair lead for climbing hitches and as the traveler in a slide line system.
Multiple sheaves – Pulleys with more than one sheave are readily available and are particularly useful in the construction of mechanical advantage systems to provide lifting or pulling force greater than the input force.
Prusik minding – These pulleys are shaped in such a way that the cheek plates of the pulley, when properly used and configured, actually “mind,” or advance, a prusik or other hitch when it reaches the pulley. This feature is exceptionally useful when moving or advancing a load when it cannot be allowed to slip or travel backwards, or when attempting to apply gradual, constant force in a mechanical advantage system.
More attachment points – Pulleys, typically of a smaller size, are readily available with multiple attachment points that provide uses in both rigging and climbing situations that are only limited by the imagination of the user and the strength of the chosen pulley.
General rigging – While a pulley’s lack of bushings and more slender construction preclude their use in dynamic rigging situations, they are highly applicable and safe in non-dynamic applications, such as lifting, pulling or redirecting, as long as their safe working load is not exceeded.
A spider leg used to balance a load, one of the rigging applications that is much easier to control readily through the use of blocks and pulleys.
Loads, briefly
Terms such as breaking strength, safe working load and working load limit are a discussion all to themselves, but prospective block and pulley users should be aware of the strengths of their chosen devices and how they will function within the chosen application.
In short, a listed breaking strength is just that, the one-time load the pulley or block should take before breaking. Multiple uses will incrementally reduce that breaking strength, and multiple uses near the maximum strength of the block or pulley will reduce its strength even more severely, thus care and caution must be used when employing rigging systems and the blocks/pulleys within them.
(Multiple sheaves prusik minding pulleys being used in a mechanical advantage system.)
Pulleys and blocks can increase the safety and efficiency of both individual climbers and entire tree crews, but only if they are being used appropriately and correctly. The basic knowledge, descriptions and limitations discussed here are a good first step towards helping tree care professionals use blocks and pulleys to battle friction in the pursuit of safe, efficient tree care.
(Michael (House) Tain is a contract climber, splicer, educator and writer associated with North American Training Solutions/Arbor Canada Training and Education, currently located in Lancaster, Ky.)
(Article courtesy of Tree Services Magazine: http://www.treeservicesmagazine.com)
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