Canyoneers use mechanical advantage systems fashioned out of whatever gear they carry. In a frictionless world, this would typically result in a mechanical advantage (MA) of 3 to 1 or 4 to 1, etc. Friction with soft goods such as Spectra or nylon and carabiners rather than pulleys greatly diminish the MA. One concern with this is the load that can be put on an anchor. A weak anchor could blow out if too much force was applied to it.
The ACA teaches a number of techniques for a guided rappel. In particular one system utilizes a theoretical 5 to 1 MA. This system is set up at the bottom anchor. An inline figure eight satisfies two functions usually solved with a variety of gear. It is used in lieu of a prusik (rope grab) and pulley and in place of the progress capture device.
I tested such a system with Canyon Pro rope in a lab. Using a dynamometer, the real mechanical advantage was found to be approximately 1.6 to 1 rather than the theoretical 5 to 1.
I hadn't given much thought to how much mechanical advantage is lost to friction using this system, but 1.6:1 is still sufficient to accomplish the task and the benefits justify the reduction.
Majority of the friction is created where the two strands (from the bight that gets clipped into the anchor biner) pass through the inline figure eight loop. Rope-to-rope friction between the two strands and the loop and the two strands against each other.
Advantages include: Much faster to set up than a Z-rig with pulleys, grabs and a ratchet. Only requires one carabiner to set up. Self-locking without a ratchet.
Thanks for the great real world measurements that include friction.
I see Petzl makes a cheap, simple pulley that is basically a wheel, which can be clipped into a locking biner. Would this decrease the friction without making the set up any slower or significantly complicated? One could slip the wheel into the in line figure eight to set up with only one biner, but the use of a second biner seems obvious.
Also, is there any advantage to the in line figure eight over a butterfly knot in this design?
John
Last edited by johnswelchvi; 10-29-2005 at 11:58 AM.
One could slip the wheel into the in line figure eight to set up with only one biner, but the use of a second biner seems obvious.?
The rope-on-rope friction within the in-line figure 8 is what creates the ratchet in this rig, allowing no slippage when the rope is pulled to tighten it.
Quote:
Originally Posted by johnswelchvi
Also, is there any advantage to the in line figure eight over a butterfly knot in this design?
The in-line figure 8 lies parallel to the rope and the direction of pull, whereas a butterfly knot would be perpendicular to the rope.
For personal caynoneering (as opposed to SAR), I try to minimize how much gear I take. It is surprising how mechanically inefficient gear is. I used to carry small prusik minding pulleys in canyons. I had them rigged with 6 mm cord. Now I carry Spectra sling. If you need to raise something up a few inches or maybe a couple feet, make a 4:1 mechanical advantage out of 2 Spectra slings over carabiners instead of nylon over a small pulleys. Or consider carrying a Web-o-lette from MtnTools (or similar). I have tested 8 mm Spectra sling in the lab but not the Web-o-lette. White Spectra with very little colored material added is very slippery. The wider Spectra slings with a lot of colored material added might have more friction but not as much as nylon cord. If you are serious about raising something, say 30 feet, it will be worth it to bring bigger pulleys. I no longer carry the pulley sheave that fits over a carabiner. Keep in mind that depending on the system you create, you may greatly increase the force the anchor sees.
At the advanced class, I found it seemed to be more effective to first use a 3 to 1 mechancial advantage and pull the rope as much as possible then use the 4 to 1 with the rope locking.
Since the 4 to 1 had much greater friction, I wonder if you actually get more actual mechanical advantage with the 3 to 1 (which has less frictyion) then can use the 4 to 1 to essentially lock off.
Can you measure the mechancial advantage with the 3 to 1 set up, and see how close to 3 to 1 it actually comes, and whether it is in fact more or less than you get with the 4 to 1.
For personal caynoneering (as opposed to SAR), I try to minimize how much gear I take. It is surprising how mechanically inefficient gear is. I used to carry small prusik minding pulleys in canyons. I had them rigged with 6 mm cord. Now I carry Spectra sling. If you need to raise something up a few inches or maybe a couple feet, make a 4:1 mechanical advantage out of 2 Spectra slings over carabiners instead of nylon over a small pulleys. Or consider carrying a Web-o-lette from MtnTools (or similar). I have tested 8 mm Spectra sling in the lab but not the Web-o-lette. White Spectra with very little colored material added is very slippery. The wider Spectra slings with a lot of colored material added might have more friction but not as much as nylon cord. If you are serious about raising something, say 30 feet, it will be worth it to bring bigger pulleys. I no longer carry the pulley sheave that fits over a carabiner. Keep in mind that depending on the system you create, you may greatly increase the force the anchor sees.
The previous incarnations of the Spectra runners, including the Web-o-lette, are 50% Spectra and 50% Nylon, and I bet they have about the same friction as a nylon sling (at least it seems so from doing bachman knots with the two). The NEW super-cool skinny slings from Mammut on the other hand... very slippery.
Small Prussick-minding pulleys? like the Petzl Oscillante? 12$ retail, small and light - I was thinking of carrying some of these in my rescue kit.
I see Petzl makes a cheap, simple pulley that is basically a wheel, which can be clipped into a locking biner. Would this decrease the friction without making the set up any slower or significantly complicated?
Don't get too caught up in trying to decrease friction in this guided rappel tightening system. You can reduce friction, but at the cost of eliminating the self-locking mechanism. 1.6:1 works just fine.
Quote:
Originally Posted by johnswelchvi
Also, is there any advantage to the in line figure eight over a butterfly knot in this design?
Yes. The inline eight will pinch more efficiently for the self-locking mechanism.
At the advanced class, I found it seemed to be more effective to first use a 3 to 1 mechancial advantage and pull the rope as much as possible then use the 4 to 1 with the rope locking.
Paul, I mis-spoke during the course. The tensioning system is 5:1, not 4:1. The way you were pulling was a 3:1 with a change of direction. When the 5:1 is set up, the two strands of rope that run through the loop start locking even before the other strands of rope became taut. The way you were pulling eliminated the locking mechanism and the friction between the two strands, so it was easier to tension all of the strands, but it wouldn't lock until you pulled the other direction and returned to the 5:1. Worked great as a 2-stage process.
Even if the 3:1 results in less friction (because only one strand runs through the loop), I'd rather use the 5:1 for the locking mechanism.
I tested such a system with Canyon Pro rope in a lab. Using a dynamometer, the real mechanical advantage was found to be approximately 1.6 to 1 rather than the theoretical 5 to 1.[/QUOTE]
If I see the picture right Sonny? You are shown measuring the amount of force you are applying to a 3:1 MA. and because you are bearing into it rather than suspending just your mass (100kg??) you have arrived at 1.6:1 MA.????? Is the rope on the other end attached to a fixed object? If this is a 3:1 MA, then given the fact that you are using carabiners in lieu of pulleys, the carabiners are about .67% effeciency x 2 should be about a 2:1 actual MA. However because of the variable of friction this would be just an approximate.
Maybe I'm not looking at the picture right? Are you the load or is the load not shown in the picture?
Bo
Mechanical Advantage System You Show Sonny?
Linear Mode
