Originally Posted by BAER
Putting the rope through a small pulley doesn't decrease the rope strength, it increases the drag, by up to 50%. What you said above would mean that every time you clip into a rope with a carabiner you are loosing 50% and that is just wrong. The bigger the pulley the less friction you will get.
Here is what the Rescue 3 Rope Rescue Technician book says (I used to be a Rope Rescue Tech.):
*Their purpose is to reduce friction on turns. Since turns in the rope weaken it, pulleys also increase strength of the rope at the turn.
*The strength of the rope is restored fully when it is put over a turn that is at least four times the diameter of the rope.
I won't bore you with the rest of the points, as it goes into variations in pulley construction, shape, axles, and bearing points.
This is why knots are categorized by different amounts of strength (related to how severe the bends/turns are in the knot).
Tensionless hitch (not exactly a knot obviously)= 100% rope strength
Barrel knots = 90% rope strength
Figure 8 series of knots = 80% rope strength
Bowline = 70% rope strength
Overhand - 50% rope strength
I was saying that I think a 50% strength lost is the most severe possible (though I'm not sure on that) from a rope being brought across a bend/turn, and I was saying/thinking if a rope is pulled 180 degrees across a biner is where the strength would be lost the most (which is a very likely shape in a z-drag set up). Apologies if I didn't word that clearly. I do believe it is ballpark up to about 50% rope strength lost in that scenario.
This also means that if you take a rope that will break at 2000lbs and tie an overhand knot in it, it will now break at approximately 1000lbs of load at the knot.
Thinking on it though you're probably right in that 50% is likely more of a guideline. If one takes the equation to infinity and makes the bend a knife edge- it will simply cut the rope.