In the last article of this series, we introduced the use of speedlines in arboricultural; in this one we’ll take a quick look at the components that make up the system. There are three areas to consider when installing speedlines.
- The Rigging System
- The Speedline
- The Control and Safety System
The rigging system remains pretty standard, if you’re sectioning down the stem of a tree then you’d install your friction device [typically] at the base of the tree and the pulley just underneath where you intend to place the felling cuts. However, you may need to leave a little more space between the felling cuts and the pulley as you’ll need to fit the speedline between the two.
The rigging line must also be long enough to reach the end of the speedline, so it will need to be substantially longer than twice the height of the pulley.
This line will be fixed between the upper pulley and the ground and will be used to send the section of timber down to the ground, but it should be able to be tensioned and de-tensioned so requires a reasonable understanding of how it should be installed by the ground crew… it’s not as simple as just tying it off around an anchor point.
Calculating The Forces
Unfortunately trying to understand the actual potential loading on various components on the system is extremely difficult and although there are formulas that can be used from creating zip-lines (typically recreational lines), there are a number of factors that these do not take into account, such as tree species and lateral loading, loads taken by the control line, forward thrust or friction within the system. Indeed, even going to the widely cited (and excellent) Rigging Research document RR668 says that “no guidance is available, only the experience of arborists in finding the right balance, between the slack required to minimise forces and the tension needed to carry the weight of the log over the length of the speedline” (2008, p.253)
There are basic [ideal] formulas for high-lines, where the anchor points are at the same height and the load is hanging down between them; these are a good place to start for drift-lines but not for speed-lines where one anchor point is not only much lower than the other, but the load is above the lower anchor point and beneath the upper one.
What we need is to change the configuration to bring the anchor point A2 down to (almost) ground level, where it would be sited by the ground crew… in the next article we’ll look at how we can get a feel for the forces involved with a load on a speedline, as we lower the load from the tree on the right (A1) of our picture to the ground anchor (A2) as per the picture below.