Friction is something I’ve known a bit about for a long time however I have only come to understand its application to rope based activities recently.
In rock-climbing rope friction can be a friend and a foe. All of our belays function because of friction. We can hold falls easily because of friction over edges and through runners.
It is only when we want to establish a hauling system to raise someone that we realise how much friction can give us problems. We has used and taught simple Z-drag 3:1 systems for decades but we’ve been teaching it all wrong. We’ve only taught half the story: namely the Ideal Mechanical Advantage (IMA) of the system. What about the Actual Mechanical Advantage (AMA)?
The IMA of a Z-drag is 3:1. When we set these systems up in the real world we have friction over rock and through carabiners and belay devices. Two simple rules-of-thumb for evaluating the AMA of these rope based systems are:
– 90 degree bends over rock or concrete create an effective disadvantage of 1:2
– 180 degree bends over metal objects an effective disadvantage of 1:2
So, unless we specifically choose low-friction devices to manage all of the rope bends in our systems, the 3:1 Z-drag often has an AMA of less than 1:1.
RRG Products Used in this Solution:
[products skus=”D14,202392 P,293008 P,P59A”]