The Canadian Drop Loop system

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Postby fuzzy-hair-man » Oct 11, 2007 9:21 pm

NZcaver wrote:When I mentioned using the lower bollard as a pulley substitute, I meant just the lower bollard - no upper bollard, no progress capture function.


I guessed as much just though I'd say it can be used as a progress capture.

NZcaver wrote:Now I've also seen that Petzl Stop 3:1 diagram you showed, and I recall I tried it out one time. However I've never seen anybody rig it for practical use - anyone? It sure looks like it would introduce a lot of friction and inefficiency into the system, but I guess some people may find it useful in a pinch.

I can't remember where I saw it but I remember seeing just the lower bollard threaded but the tail comes up between the two bollards which means the Stop feature is operable, however this is not what Petzl show so it's probably no an approved use.

NZcaver wrote:Regarding your comment about the Stop not being hugely strong - the figures are obviously much lower than regular pulley ratings, but I'm not really seeing that as critical in this improvised/occasional use/single person load application. I also understand that more friction in the system means a potentially higher load on anchors and other components, but if you're only using the lower bollard it can't be any worse than using just a carabiner in place of a pulley can it?

Not worse than the carabiner as a pulley, just thought I'd say there are limitations to it's use as a pulley and / or progress capture which anyone using it as such should be aware of and that its use is possibly limited to self rescue single person loads when hauling.
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Postby fuzzy-hair-man » Oct 11, 2007 10:00 pm

Ok I found the example I was thinking about:

To explain a the 1.5 they multiply the 1kN force by this is the peak loads whilst hauling, so just like when prussiking you will generate more force than your body weight alone the hauling party will generate above the load force. LOAL measured this to be 1.3 times the load but for safety assumes it will be 1.5 times the load.

The 4.0 times multiplier is a measure of how much friction is adding to the hauling effort so it is the ratio of the forces seen either side of the pulley, in other words you need to pull 4 times the load to hold the load and up to 6 times the load to raise it. (at least that's how I understand it.....) see the table below

From Life On a Line 1: Part 1: Pages 82, 83

e.g.2 100kg mass lifted over 180° bend using a Petzl Stop in belay mode:
Again, the load side peak force is 1.5 x 1kN = 1.5kN, but the value of β is now 4.0 from
the table above, so the peak load on the hauling side is 6kN! As hauling and load forces are
approximately parallel again, they are additive:

The resulting anchor load is a massive 7.5kN (equivalent to a 750kg static mass)
These represent probably the best and worst cases, and as you can see even in the best solution
the anchor load is significantly higher than the lifted weight. At the other end of the spectrum
the humble Stop can create anchor loads of many times the lifted weight. Whilst 7.5kN is not
large compared to the strengths of anchors and karabiners we will be using, it happens to
exceed the 5kN rated test strength of the Stop itself. As we may on occasion try to lift a full
rescue load of 200kg only the foolhardy would rely on the ability of a device to work correctly
at over twice the test limit. This is a clear example that for rescue rigging we can easily push
devices beyond their designed limits, often without realising it.


The following table might be interesting seeing as we are discussing friction forces:
Image
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Postby delrio » Oct 16, 2007 7:00 am

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Postby Carl Amundson » Oct 16, 2007 12:07 pm



thank you sir, this is good stuff :kewl:

This is just what I was looking for.
It answers the question, "if you only had one pulley and need to make a 3:1 haul system, what is the best placement of that pulley?"

Image

Since the efficiency of the first pulley affects the tension in two parts of the tackle, the more efficient pulley should be placed nearer the puller.
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Postby delrio » Oct 16, 2007 5:08 pm

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Postby NZcaver » Oct 17, 2007 5:19 pm

delrio wrote:Another hoist-calculation by...

So if I'm reading it right, that diagram shows a compound 6:1 [correction, complex 5:1] calculated to lose about 10% efficiency per pulley? Honestly, when rigging a system I doubt I'll really feel the need to calculate pulling force down to 2 decimal places - especially with all the virtually incalculable variables induced by pulley drag and other edge friction etc in the system. Still, interesting info.

junkman wrote:(Quoting Bob Thrun) Since the efficiency of the first pulley affects the tension in two parts of the tackle, the more efficient pulley should be placed nearer the puller.

Ah-ha! Just as I thought. A really neat way to see this in action is by clicking on that Petzl pulley system calculator that I posted the link to previously. Select a system, and change around the pulley types and locations to get the resulting pull force (in kN). The Petzl Rescue pulley sheave is a 50mm bearing type (more efficient), and the Fixe, Oscillante, and Mini are all around 25mm. You can even swap a carabiner in and see the difference.

(And not to nitpick too ridiculously, but... the unit of measurement for force should be lbs force (not just lbs), or more ideally - kNs. Blah!) :tonguecheek:
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Postby dcfdrescue2 » Oct 19, 2007 10:10 am

Actually, the system DELRIO posted is a complex 5:1. I've heard it referred to as a "Crevasse 5:1"

The reason for this is (assuming you have more than one person hauling) that the person who is resetting the haul system can be attached to the end of the 2:1 portion of the system, right after it exits the pulley and is attached to the 3:1. Something like a butterfly would suffice, with a tail continuing on to be attached to the 3:1 pulley. This reset person is also walking back with the system and therefore becomes a hauler when they walk back.

Not only do they haul, they are using their body weight to haul, instead of their hands, because they are tied into th 2:1.

Because the 2:1 will only reset to a predetermined distance before the knot jams in the pulley, the person resetting the system will not fall into the crevasse (hence, the system name) or down the drop.

During the initial set-up of this system, the reset person can be belayed to where the edge by using a munter. When they are done rigging near the edge, the munter can be dogged or muled off (whatever you prefer to call it) thereby fixing it to the anchor.


The downside of this system as I've described it, is that you have a person tied into it. If an anchor catastrophically blows out, your load, along with a hauler will fall.

I realize this is kind of confusing and will try to draw it up pretty with my rescue rigger software.
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