In the last part of this series we began to look at the forces involved in using speedlines by looking at the calculations used for highlines; then also learnt that these did not really match our requirements where one of the anchor points was much lower and beneath the load being sent down the line.
In this part, we’ll look at the forces involved… or at least try to gain some understanding of the forces involved as trying to work out the exact loading is extremely complicated, not least due to having a moving load and not a static load as depicted in these images. The load is also being controlled by the rigging line which at this stage is being used as a control line, controlling the descent of the load down the speedline, so at this anchor point (A1) there is also a pulley where the loading at the attachment can be up to double the loading shown – as well as the speedline attachment point.
It’s a complex and confusing picture, but if we can gain some basic understanding of the loads involved then we can be more informed of the compatibility of system components.
Understanding The Forces In Speedlines
Factors In The Calculation
The following image shows the elements that we need to consider in our calculations – remember that we are only considering a static load here and we’re not factoring in any forces relating to the forward momentum of the timber section.
The tree that is being dismantled is on the right hand side of the picture, with our rigging pulley and speedline fixed at anchor point, A1. At the other end of the speedline is our anchor tree, shown on the left side of the picture.
Plugging In Some Numbers
First off, we’ll ‘cheat’ and rather than go through all the formulas, there are a number of websites that will allow you to calculated the forces and loading involved – although you do need to select carefully as many of them are for highlines, which are slightly different. I’ve used the Two Point Anchor Calculator at Ropelabs (http://www.ropelab.com.au/two-point-anchor-calculator/) for these calculations.
Anchor point A1 is 12m high from the ground, anchor point A2 is 1m above the ground level. At the point shown, the load has moved down the speedline until it is 7m away from the tree being dismantled and 13m away from the speedline anchor tree on the left. It’s also important to realise that anchor point A2 is beneath the load at this point, so measurement V1 is a negative number. Measurement V2 is 7m as the load is currently 5m above the ground, making the difference between it and anchor point A1, 7m (7m + 5m = 12m).
Whilst we could plug in the numbers as shown (and we should), it’s often better to calculate using a load = 1(kg). This means that we will end up with a multiplier that can be used for any weight, rather than one specific figure that only relates to one weight (25kg). Plugging the numbers into the Two Point Calculator gives us a multiplier of 2.04 at A1 (given as “Tension 2” in the screenshot) and 1.51 at A2 (“Tension 1” in the screenshot).
At the higher anchor point, our 25kg load exerts a ‘force’ of 25 x 2.04, or a fraction over 50kg. If that load had been 100kg, the loading would be equivalent to having a weight of 204kg at this point (100 x 2.04).
Interestingly, the loading at the lower anchor point has also increased to 25 x 1.51 = 37.75kg.
It’s important to note that these numbers are just vague approximations as there are many other factors to consider to arrive a proper figure – certainly far too many to be thinking about on an average working day when you need to get the job done!
Perhaps the best option when installing a speedline is to keep the timber sections being sent down the line to manageable levels, and to not drop the section directly into the speedline. If in any doubt, it would be worth considering the use of guy lines on the tree as well to provide support from the lateral loading that is also present at the anchor point at the highest part of the speedline.
Interested in achieving your aerial rigging qualification for arboriculture? Find out more about our course here.