July 16, 2018

Isuzu

A Guide to assisted felling: Part one – Using an offset system – David Vickers

Vickers

In our brand new series, David Vickers looks at how to set up an assisted felling system to give greater control of the felling direction and to remain safe

Using ropes and winches to assist with felling trees is fairly common, especially with boundary trees such as those on woodland edges. Such trees will have healthy canopy growth on one side due to phototropism and limited growth on the other, where other trees have been competing for the light. The result is an unbalanced tree weighted towards the neighbouring land or property. Setting up an assisted felling system gives us greater control of the felling direction, minimising the risk of the tree landing in the wrong place. With a boundary tree, it is likely that we will be felling against the weight of the canopy but there is only so much that can be done with a felling lever.

Vickers

Whatever the actual situation, unless we have rope or winch cable that will allow us to stay out of the danger zone (two tree lengths) for a straight pull, we will need to set up a configuration that allows us to remain safe. That means using an offset pull.

Fully loaded
Once using an offset system, we need to understand the loading applied to the offset anchor and related equipment such as slings, shackles and the pulley itself. This loading at the offset pulley can be calculated using the following formula: pull = 2t(cosine x) where t is the force (in tons) exerted by the winch and x is half of the angle
the wire/rope passes through the pulley (the direction that the offset pulley would move if its anchor point were to fail).

Vickers
Let’s assume the cable passes through 90° in the offset pulley. In this case, x = 45°.

We do not gain any mechanical advantage on the tree to be felled, so a 1.6t winch exerts a pull of up to 1.6t on the tree. But it’s a different story for the offset anchor, pulley and associated equipment. According to the above formula, the pull exerted on the offset anchor can be calculated as follows: 2 x 1.6t x cos(45) = 3.2t x 0.707 so pull = 2.26t

If you are using a webbing sling rated at 3t chokered around the anchor, that reduces the Safe Working Load of 3t by 20% to 2.4t. That’s very close to the calculated pulling force on the offset anchor. If you used a carabiner rated at 23kN to attach the offset pulley, the pulling force on the offset anchor is so close to the above figure of 2.26t that you must consider upgrading these components or downgrading the winch.

Next month we’ll look at creating a ready reckoner guide to help with just this sort of situation and delve a bit deeper into assisted felling.

Vickers

Drivelink Training provides City & Guilds NPTC forestry and arboriculture short courses run by David Vickers, a City & Guilds NPTC approved trainer and assessor, qualified teacher with a BA (Hons) Ed. and QTLS. Drivelink Training provides training from basic maintenance to dealing with windblown trees, from basic tree climbing to rigging / dismantling. Visit www.drivelinktraining.co.uk to find out more.

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