Tools & Techniques:
Pulleys and Blocks

When Friction is Not a Friend

By Michael Tain

Pulleys and blocks play a major role in the modern tree climber’s bag of tricks, often performing vital functions in the most basic of tasks, such as hitch advancement, or the most complex, as in a multiple system rigging required tree removal. Regardless of application, the primary function of a pulley or block is to reduce friction as much as is possible within the given situation.

With the various types of pulleys and blocks available to tree crews today there’s one for almost every application, but along with the benefit of choice comes the burden of knowledge. The wrong block or pulley used in the wrong situation can be catastrophic for the pulley, the line, the tree and even the crew members in the path of destruction. A basic knowledge of some of the applications, suitability and safety concerns of these useful tools will go a long way towards ensuring that crews not only use them most efficiently, but also as safely as possible.

Basics

Pulleys and blocks perform the same basic function: providing a sheave or surface of some sort that reduces friction as much as possible on a running rope. In many industries the terms “block” or “pulley” mean the same piece of gear, this is not the case in tree care applications. While both perform the same basic function – and can even work interchangeably in the short term – the wrong choice can have disastrous results, especially when large woody debris at a height is involved.

Blocks, but not for the alphabet

The term arborist block is used to refer to a block/pulley designed to deal with the heavy loads and extreme forces of dynamic rigging situations. While there are a variety of designs and styles available, they will have several basic components in common: a bushing, a sheave and cheek plates.

Bushing, not bush league

The bushing in an arborist block is intended for sling attachment and will have some form of locking mechanism to ensure it does not release mid-load. Available mechanisms include spring locks and captured bushings with screw locks. Arborists in the market would be best served to purchase captured bushings regardless of locking mechanisms, as attempting to find a bushing dropped from 90 feet into a pile of pin oak brush can be a bit challenging. Whichever type of locking mechanism the bushing has, the user should be certain it’s correctly locked and secured after every load, or catastrophe may rear its ugly head.

As mentioned previously, the bushing – typically the smaller diameter of the two block ends – is intended for sling attachment, with the sling then attached to the tree with an appropriate hitch such as the cow hitch with a better half. The use of connecting links, even large heavy-duty rigging carabiners, from the sling around the bushing is a poor idea, as the possibility of side or cross loading is highly likely in dynamic rigging situations, and the metal-on-metal contact between connecting link and bushing quickly degrades the strength of both.