The Weak Link

Try This Experiment

Tie a person between two large trucks using chain and then drive off. What happened? After that, try the same experiment using string not chain and see what happens.
If you had a choice, you would clearly choose to be tied by string and not chain for the obvious reasons. The string is the ‘weak link’ designed to fail before your body does.

Now consider your climbing rope is set high in a tree and you are limb walking a large low limb. You reach the target area to be pruned on the limb and clip in using a pole strap or flip line around the main stem. As you prepare to start cutting the limb snaps at a hidden defect back at the trunk. The total load of the limb is placed on your harness, climbing rope and body and something has to give and chances are it’s going to be your body. Don’t make your body the weak link.

The tree can be used as a weak link in some cases. The scenario above would have been much less severe if the climber had clipped the pole strap or flip line around a small limb approximately 25mm diameter. The climbers weight is supported by the climbing rope and the second point of attachment is only for stability so use a “weak link limb”. Thus if the large limb fails the “weak link limb” will break away from the pole strap or flip line. This technique may keep you alive or at least out of a wheelchair for the rest of your life.

In the last issue of Arbor Age I spoke about the weak link that should be incorporated into a chainsaw lanyard or strop. This weak link is designed to fail should the chainsaw become jammed in the fork of a falling limb or in the cut of a falling log. This breakaway link is vital especially when working on large trees and working with heavy loads. Since the last issue, I have been made aware of a purpose made chainsaw lanyard by Buckingham. This lanyard does have a built in weak link and also works as an energy absorber.

The friction knot or rope adjuster on your climbing rope should also be thought of as a weak link. That is under extreme loads it will slip down the climbing rope without failing or causing major rope damage. Would your friction knot pass the test or would it keep tightening until failure. Friction knots that are tied in the same size rope as the climbing rope slip best under extreme loads and are much less likely to fail.

The friction knot or rope adjuster can act like a weak link by slipping down the climbing rope, this prevents the climbing system being overloaded and failing.

Very few rope adjusters or ascenders offer this feature. Almost all cammed ascenders will rip the rope to shreds before failure. One device that does offer this feature is the Petzl Shunt and it will begin to slip when overloaded.

The weak link in the climbing rope system is designed to slip should the climber become over loaded in any way. The climber will slide down the rope until the load is released or the climber reaches the ground. This is a worst case scenario but it could happen.

Weak links should be kept to the climbing system and tool lanyards only. Never use a weak link in the rigging system. The rigging components should all be matched for rated strength, making all parts near equal. Keep in mind that all ropes and slings deteriorate in strength over time and wear.

Summary of Weak Links in the Climbing System

• The chainsaw stop or lanyard - Use a tear away lanyard or a weak link component that will break at a predetermined load.
• The pole strap or flip line - Connect to “weak link limb” when it's used as a secondary point of attachment.
• The climbing system - Use a knot or device that will slide down the rope when overloaded without causing major damage to the rope.

Conclusion

Climbing arborists often place their bodies between a large fixed object and a moving large object. Isolation or detachment from the moving object is the safest option but if this is not possible then exercise the weak link method. One reason why isolation is not possible may be due to a lack of stability when cutting.

An Example of a Fixed and Moving Object Situation

Imagine a large tree has fallen over and is hanging up in a second tree at about 45°. You ascend the standing tree and set your climbing rope before descending on to the fallen leaning tree. To connect your pole strap or flip line to this leaning tree would be crazy so you exercise the “isolation principle”. During one of the cutting sequences to dismantle this tree you require further stability and support to complete the cuts safely so you connect your pole strap or flip line to a “weak link limb”. This limb must be as small as possible but strong enough to offer the support required.

Warning

This type of tree work is highly dangerous and should only be undertaken by competent and experienced climbing arborists.

Clearly the weak link method is a last resort principle and may never be activated in a climbing arborists career. But it does offer peace of mind and does create a back up or an exit should things to horribly wrong or not as planned.

As usual these articles are meant to stimulate thought and conversation. Your views may be different to mine or you may have other ideas. I encourage you to comment in the form of a Letter to the Editor.

Remember that climbing is only a means of getting to work and it's what you do at work that counts.

Next: The Limiting Factor