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#37505 - 06/01/08 04:12 PM Z pulleys, mechanical advantage, and friction
Kent Offline
old hand

Registered: 01/21/00
Posts: 1038
Loc: The Bayards
 Quote:
you can't leave a z-pulley unattended, there are two friction knots that need to be repositioned every cycle. "Setting up a z-pulley" and then rapping away from it is totally pointless, the z-pulley can't be used by remote control.

If rapping down one can leave one end of a Z pulley unattended and that's all you need. Typically, in a low load system, the pulley/brake at one end of a Z pulley, the top, is an auto locking belay device. A Gigi (not Grigri) in a Z pulley, for example, does not need to be tended, adjusted, or moved. The other end of the Z pulley, a friction knot, is attached wherever you are, however far down, so it's easy to move if need be. One can easily set up the top of the Z pulley at the anchor, then rap down, and if need be set up the bottom of the Z pulley, and pull to boost someone through a move. Any reasonably experienced leader can learn to do this with just a little practice.

 Quote:
I suspect that in real situations it (friction) almost always overwhelms any mechanical advantage a z-pulley can provide.

As someone who has rigged mechanical advantages frequently, and for different applications, it's my opinion that the converse is true. In real situations, if you rig smart, the mechanical advantage almost always overwhelms the friction.

 Quote:
building a z-pulley up in the woods is never going to raise a climber hanging way below on High E.

Not true. When at the top of Erect Direction, belaying my second off a tree, she fell at the overhang and ended up 3-4 feet below the lip of the hang. She yelled "pull me up". Far be it from me to refuse a rigging challenge. I built a single Z pulley, enlisted the aid of a few passing climbers and, despite a great deal of rock friction, we pulled her straight up onto the face above the hang. Had I been alone I would have rigged a Z pulley on the Z pulley.

For anyone who might actually go out and practice any of this, extreme caution is warranted when using high mechanical advantages. They result in a lot of power which can be dangerous. For example, if you build a Z pulley on a Z pulley (9:1) and then put five guys on it, you can easily develop thousands of pounds of force, enough force to break gear and hurt or kill people. That kind of force requires inclusion of some kind of dynamometer, a backup belay, and people who do nothing but watch the system. Be careful. Take a course if you want to learn to do heavy lifts.

This stuff is a blast if you like gear.

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#37507 - 06/01/08 08:51 PM Re: Z pulleys, mechanical advantage, and friction [Re: Kent]
rg@ofmc Online   content
Pooh-Bah

Registered: 12/25/99
Posts: 2468
Loc: Poughkeepsie, NY
 Originally Posted By: Kent
If rapping down one can leave one end of a Z pulley unattended and that's all you need. Typically, in a low load system, the pulley/brake at one end of a Z pulley, the top, is an auto locking belay device. A Gigi (not Grigri) in a Z pulley, for example, does not need to be tended...


Well sure, and it also helps enormously to be carrying pulleys instead of carabiners. I wasn't referring to specially-equipped climbers. And, though you claim experience I don't I have, adding something like a Gi-gi plate reduces the efficiency of the top biner even more. My guess is your 3:1 system, which is only barely over 2:1 anyway, is under 2:1 with the additional friction.

For further reference, I'm speaking of climbers equipped with ordinary carabiners and whatever friction knots they can fashion from slings or dedicated prussik loops.

 Quote:
In real situations, if you rig smart, the mechanical advantage almost always overwhelms the friction.


Meaning, I suppose, that even though friction reduces mechanical advantage, there is enough left to complete the task. I guess this is the definition of rigging smart, so the conclusion is a tautology. I still have many doubts, but defer your experience.

 Quote:
...I built a single Z pulley, enlisted the aid of a few passing climbers and, despite a great deal of rock friction, we pulled her straight up onto the face above the hang.


The emphasis added is mine. Before I would count this as even a partial counterexample to my claim (which never envisioned a team of haulers), I'd have to know whether a few people couldn't have raised the hanging climber without the z-pulley. I mention this because I have on several occasions hauled someone up a few feet all by myself with just with leg power. Had I built a z-pulley and done the same, I might be inclined to credit the pulley for the success.

By the way, I don't think the z-pulley on the z-pulley, with its theoretical 9:1 mechanical advantage, actually makes to to 3:1 with biner friction. In reality, the second z-pulley only results in a 30% increase in mechanical advantage, and the 3:1 system has an actual mechanical advantage of (2.1):1. I think that such compound systems only make sense if one has pulleys.

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#37508 - 06/01/08 10:15 PM Re: Z pulleys, mechanical advantage, and friction [Re: rg@ofmc]
calamity Offline
stranger

Registered: 05/21/08
Posts: 18
I just used a C-pulley with just carabiner and prussik to pull down a smallish tree we were sawing.

It worked like a charm. A huge mechanical advantage, though I don't suppose quite enough for certain climbing situations.

The unpleasant surprise, was, I couldn't release the prussik under tension to slide it up for another pull. My mind sortta blanked out on how to do this.

Anyway we did the tree down, the C-pulley helped a lot, and didn't seem to damage the prussik.(!?!?!)...or the adjacent fence.

I hear using climbing gear interchangably with tree work keeps the stuff "broken in" and therefore much safer....Is this true?

--

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#37511 - 06/02/08 12:25 AM Re: Z pulleys, mechanical advantage, and friction [Re: rg@ofmc]
Kent Offline
old hand

Registered: 01/21/00
Posts: 1038
Loc: The Bayards
Hi Rich,

 Quote:
Well sure, and it also helps enormously to be carrying pulleys instead of carabiners. I wasn't referring to specially-equipped climbers. And, though you claim experience I don't I have, adding something like a Gi-gi plate reduces the efficiency of the top biner even more. My guess is your 3:1 system, which is only barely over 2:1 anyway, is under 2:1 with the additional friction.

An auto locking device isn't a specialized piece of equipment. Many of us routinely carry them. And sure they add a bit of friction but they have the advantage of having half a Z pulley built in whenever in use. So I'm talking about climbers usng ordinary gear too.

 Quote:
I guess this is the definition of rigging smart

My definition of rigging smart is picking your spots to reduce friction, using two carabiners instead of one to increase the radius, taking advantage of every opportunity to reduce friction with the gear you normally carry.

 Quote:
The emphasis added is mine. Before I would count this as even a partial counterexample to my claim (which never envisioned a team of haulers), I'd have to know whether a few people couldn't have raised the hanging climber without the z-pulley.

You might never have found out though because with your system, unless you go to the trouble of escaping the belay, you might not have had the opportunity to enlist anyone's help. You are stuck. With my system I'm already free of the belay. I'm free to move about and improvise.

 Quote:
I mention this because I have on several occasions hauled someone up a few feet all by myself with just with leg power. Had I built a z-pulley and done the same, I might be inclined to credit the pulley for the success.

No doubt you are a strong dude Rich, still. But your system is a one hit wonder in this lift. You're stuck where you are and can't take another bite without escaping the belay and rigging something like my system. With my system I can take as many bites as I like and I can rap 20' or 40' over the side and then take as many bites as I like.

 Quote:
By the way, I don't think the z-pulley on the z-pulley, with its theoretical 9:1 mechanical advantage, actually makes to to 3:1 with biner friction. In reality, the second z-pulley only results in a 30% increase in mechanical advantage, and the 3:1 system has an actual mechanical advantage of (2.1):1. I think that such compound systems only make sense if one has pulleys.

You could well be right. I've only rigged the double Z 9:1 with pulleys but it would be fun to try without to see what kind of real world benefit there would be. Sure it would be way less than 9:1 with all those biners. My guess is it would still have noticably more juice than just one Z. I say this because I've rigged a double rope come-a-long before and it seemingly had a lot more juice than a single. That's rope cutting against rope, twice, with no carabiners. You can't introduce too much more friction than that. For fun sometime I'll see if I can pull someone up the cliff by myself with a double Z using biners. If I do I'll let you know how it goes.

I only posted this stuff because your recent High E posts and other older posts, make it sound like no one in their right mind would consider using an auto lock belay device or the carabiner Z pulley it so easily provides.

Your serve.

Edited to add.....using the terms "your system" and "my system" makes it all seem so personal which I don't mean it to be. Besides, we didn't make' em up. So where it reads "your system" perhaps it should say "a traditional belay" and wherever it says "my system" perhaps it should say "an auto lock belay".

And also, I should know better than to volley with a mathematician about such things...:)







Edited by Kent (06/02/08 02:10 AM)

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#37512 - 06/02/08 02:09 AM Re: Z pulleys, mechanical advantage, and friction [Re: Kent]
fear Offline
enthusiast

Registered: 06/27/03
Posts: 221
Loc: New England
After many years of refining my system for 2-man glacier travel I have to add that the ability to raise a heavy person with a z-pulley setup is virtually impossible with the rope rubbing over rock during any training I've ever done. I'm pretty strong and it just ain't happening if there's dead weight on the other end.

Rig another C to the Z though and you've got a theoretical 6:1 which does work, albeit very, very slowly and with much effort. I've taken to bringing those pulley-biners (DMM revolver?) in lieu of real pulleys to save weight.

When people make statements like "Just rig a pulley system and haul her up" I always have to laugh. Go out and try to lift 200 pounds of dead weight over a rock edge.

Another problem is the immense strain on the anchors in such high-advantage rigs. I've uprooted fairly solid looking small trees during real-life practice sessions. You can easily not only blow your anchor but cut the rope if it binds over anything remotely sharp.

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#37513 - 06/02/08 05:38 AM Re: Z pulleys, mechanical advantage, and friction [Re: fear]
rg@ofmc Online   content
Pooh-Bah

Registered: 12/25/99
Posts: 2468
Loc: Poughkeepsie, NY
 Originally Posted By: Kent
I only posted this stuff because your recent High E posts and other older posts, make it sound like no one in their right mind would consider using an auto lock belay device or the carabiner Z pulley it so easily provides.

Whoa---I have an autolocking device and use it, though not very often. Perhaps it is true that I'm not in my right mind though.

As for z-pulley lifting, my experiences, which however are nowhere near as extensive as yours seem to be and have all been exercises, have convinced me that that its drawbacks make it the method of absolute last resort, and even then only if the rope makes few or no sharp bends as it runs over the rock. Add to this the fact that there is nothing in the elementary mathematical theory that gives cause for hope, because rope tension around a bend increases by a factor that is exponential in the contact angle. For example, two 45 degree bends over rock edges will probably entirely cancel a 3:1 mechanical advantage. Avoiding this is part of what you referred to as "smart rigging," of course, but such bends cannot always be avoided, and there are analogous effects from a larger number of bends, none of which is as big as 45 degrees.

But even if z-pulley hauling is possible, the question remains whether it is the optimal response. A free-hanging climber can prussik faster and easier and with far less strain on the anchors and far less tension in and abrasion on the rope, so I see no role for z-pulley raising in almost all ordinary rock climbing situations, "ordinary" meaning that the participants are conscious and minimally competent, and "minimally competent" meaning, among other things, the ability to prussik.

 Quote:
Your serve

Perhaps this is one of those situations in which our experiences have been different and we will have to agree to disagree. And meanwhile, Fear has already sent one into your court.

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#37518 - 06/02/08 04:00 PM Re: Z pulleys, mechanical advantage, and friction [Re: rg@ofmc]
wombat Offline
member

Registered: 05/27/08
Posts: 147
Loc: gardiner
i find that a body belay provides more than sufficient mechanical advantage to raise up a six pack (cans, we have to be safe) of beer.

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#37519 - 06/02/08 04:24 PM Re: Z pulleys, mechanical advantage, and friction [Re: wombat]
rg@ofmc Online   content
Pooh-Bah

Registered: 12/25/99
Posts: 2468
Loc: Poughkeepsie, NY
Some quotes from the "experts," with comments from this non-expert. Material in italics is from the specified texts. Material in brackets in roman type is mine.

From Tyson and Loomis, Climbing Self-Rescue, pp 137:

Building a wonderful 3:1 to pull up our 200-pound buddy is not such a rosy picture when we look at all the losses in the system...

[They go on to describe the reduction to (1.7):1 that occurs if climbing pulleys are used and the rope makes a 90 degree bend over an edge. They do not mention that if carabiners are used in this situation, all mechanical advantage is lost.]

From Nigel Shepard, The Complete Guide to Rope Techniques, pp 157--161:

There are so many factors that go against hoisting that it is important to establish that it is really necessary before you begin.

To be truthful the [Z-pulley] is very difficult to put into practice in a less than perfect situation, particularly when you have no other assistance.

From David J. Fasulo, Self-Rescue, pp 75--80:

...if you are alone and attempt to haul a victim using the Z-pulley, you'll find it extremely difficult...If you need to haul a victim by yourself, use the 5:1 system.

[If an autoblocking device is used, the 5:1 system can be rigged and operated remotely as Kent suggests, if an additional anchor for a double-length runner component can be built at the remote hauling location.]

The theoretical mechanical advantage of the z-pulley is 3 to 1...However, when improvising these systems the actual advantage is much lower.

[This refers to Z-pulley systems constructed without an autolocking device that will further reduce the mechanical advantage. The real-world mechanical advantage---without an autolocking element---of Fasulo's 5:1 system is about 3:1, which could be enough to raise a hanging climber if the rope running to the climber does not make too many bends over rock edges. An important observation is that the 9:1 system constructed by having a Z-pulley act on a Z-pulley does no better than the 5:1 system---actually a touch worse, in fact---after carabiner friction is accounted for, while still imposing a 9:1 stroke penalty that will nearly double the number of strokes needed for any given raising distance.]

None of these sources mention that hauling with a dynamic climbing rope involves not simply raising the hanging climber a certain distance, but also stretching the climbing rope enough to raise the tension in it to hauling levels. Consequently, the amount of rope hauled will have to exceed the distance raised. Sadly, stretch elimination may have to occur on every stroke, because the discrepancy between static and sliding friction allows the rope to recover at each stroke. This recovery, by the way, can be felt as a disconcerting upward bounce by the hanging climber.

An additional potential danger of hauling deserves mention. If anything jams and hauling continues, the mechanical advantage of the system can end up imposing gear and anchor-breaking loads in the case of jammed gear, and body-breaking loads in the case of the haulee getting stuck in a rock or vegetation feature. This means that hauling a person without excellent communication and a good view of the rope path is fraught with additional dangers.



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#37648 - 06/06/08 05:34 PM Re: Z pulleys, mechanical advantage, and friction [Re: rg@ofmc]
Kent Offline
old hand

Registered: 01/21/00
Posts: 1038
Loc: The Bayards
 Quote:
Building a wonderful 3:1 to pull up our 200-pound buddy is not such a rosy picture when we look at all the losses in the system...

[They go on to describe the reduction to (1.7):1 that occurs if climbing pulleys are used and the rope makes a 90 degree bend over an edge. They do not mention that if carabiners are used in this situation, all mechanical advantage is lost.]

No argument there. Even in my example from earlier in this thread, with a 110 pound woman a single Z, with 90 degree friction, I couldn't move her. You may well be right that the three of us that did pull her up may have been able to do so without the z pulley but the z with the auotlock belay device brake made it a very controlled process.

Z pulleys are, as previously discused commonly used to boost if they are having a hard time with a move. For the same boost I know your preference is to lock off your atc type device and stand up and that's fine but to me it's very limited in options.

The higher mechanical advantage of 5:1 for a single person raising a single climber is part of the AMGA guides test I believe. But in the AMGA test, or what it used to be, it's a staright hang, no rock friction to speak of.

 Quote:
An additional potential danger of hauling deserves mention. If anything jams and hauling continues, the mechanical advantage of the system can end up imposing gear and anchor-breaking loads in the case of jammed gear, and body-breaking loads in the case of the haulee getting stuck in a rock or vegetation feature. This means that hauling a person without excellent communication and a good view of the rope path is fraught with additional dangers.

With high mechanical advantage and multiple people yarding on the rope, sure. That's why in high advantage systems, a 9/16" static line, 9000lb test, is standard. As well, most employ a "poor man's dynamometer", which is a prussik. A 7 mm prussik on a 9/16" static line tends to start to core at about 1500 lbs and indicates that forces are getting too high, and that all pulling should stop immediately. Also in high advantage systems, anchors are reinforced, and a second 9/16 static line and anchor, are employed as a belay in case the raising system suffers a partial or catastrophic failure.

Like you Rich, I'm not an expert, but in training, with such a system, which if my memory serves me correctly was a double z pulley, using pulleys, we raised a "patient" and two attendants, one for the patient and one to tend the litter. That's 500-600 lbs straight up a rock face about 40' and the rope made a 90 degree turn over rock at the top.

So in different applications perhaps different solutions are useful. To me, learning about these possible solutions and being able to deploy them is an important part of ropecraft.

Regarding z pulleys, 5:1, or higher mechanical advantages, as only a "last resort" virtually assures one won't be able to employ them when necessary. Hence the agreement among several experienced climbers on the High E thread that a knife was an appropriate solution. In my opinion, it wasn't. The climber could have been easily raised by skilled climbers and her weight transferred to another line.

I bring this up to you Rich, because, if what is left of the gunks.com community has a patriarch, it's you, and I think you are potentially doing a disservice to your fans and mentees by discouraging them from learning about and practicing these important ropecraft skills.

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#37650 - 06/06/08 05:41 PM Re: Z pulleys, mechanical advantage, and friction [Re: Kent]
Smike Offline
Carpal Tunnel

Registered: 05/01/01
Posts: 3143
Loc: in your backyard
[Out of office reply: Too much babble to read. Gone climbing]

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