Archives for November 2015

Is DdRT really 2:1 mechanical advantage?

First and foremost let me say that I have never been, nor am I, a wealth of knowledge when it comes to mechanical advantage with ropes and pulleys especially when systems start getting complicated. So I welcome all feedback and other points of view.

The reason I decided to write this article is because over the last few years each time somebody mentions Ddrt being a 2:1 MA (mechanical advantage) or SRT being 1:1 MA it automatically presses the ‘On’ button in the mechanical advantage section of my brain, because I feel it is not quite right. So I decided to do a bit of research.

doubled rope techniqueBefore I start explaining my thoughts on this subject, I’d like to say that it doesn’t change how hard or easy climbing is in the real world, it is merely food for thought, and making sure we are well informed and use the correct terminology.

I will begin with this image of a standard DdRT (doubled rope technique) setup, using one rope passing through a pulley doubled over, then connected back together with a friction hitch and Karabiner with the addition of a micro pulley under the hitch for ease of tending slack. Now, in theory and real life if the tail of the rope is pulled upwards against the micro pulley then this appears to me to be a 3:1 MA system. As tree climbers we don't climb like this because trying to pull the tail upwards uses weaker muscles and can't generate as much force, but I’m sure those climbers that have used this method to lift a casualty in an aerial rescue scenario can attest to this seeming much easier.

a rope passing through a pulleyHere is a picture of a rope passing through a pulley, which in Mechanical advantage terms is a 1:1 because there is no advantage gained when lifting a weight, the only purpose of that pulley is to change the direction in which the weight is being lifted/pulled from.

If we now look at the image below of the Ddrt climbing system being used by the tree climber to ascend, the way in which we use the system reverts it back to a basic change of direction. As tree climbers we use techniques that make ascending a bit easier than physically hauling ourselves up on a 1:1 system, for example the hip/body thrust.

 the SRT configurationLets move then to the SRT configuration, as climbers we physically climb up the fixed rope using devices designed for this application, meaning we make greater progress much quicker utilizing our legs unlike DdRT where we primarily use our arms. If you think about the rope itself it isn’t moving, if the rope is fixed then there can be no mechanical advantage theory applied, in the same way we don’t apply this theory to climbing a set of ladders. The only ’mechanical advantage’ we have as climbers are the mechanical tools we utilize e.g. rope ascenders.

prusik/rope grab/ascenderWhen making a difficult limb walk maybe on a sloping limb a prusik/rope grab/ascender can be placed above the climbers friction hitch the the tail of the rope passed up and through a carabiner or pulley and back to the climber. When returning from the limb walk if the climber pulls on the tail coming from above him this creates a 2:1 MA which is why it feels easier than climbing on a DdRT system. This also appears easier than PULLING UP on a regular DrRT system (3:1), the reason is because pulling down on a system that is attached to you as you also move is easier than pulling upwards on a dead weight.

Definition of Mechanical advantage:
Ratio of effort/force applied (input), to the force being applied to the load (output).