Rescue Pully Placement

[spotly]

Using a drop-loop 'Z' pully system, if you had one pully, where would you place it to get the best benefit? How about if you had two? I carry just one and have never had to use it but always assumed that lowering it to the victim was the way to go but area 1 in the pic below seems like it would see more friction. Do most here carry more than one? Any particular brand that you prefer? The $3.95 nylon Petzl seems like an "ok" 2nd pully option but I've read they tend to break - any experience with actual use of one of those?

 

Thanks

 

Edited February 5, 2009 by spotly

[mike_m]

For reducing friction in the system, I don't know if it matters if a single pulley is used as the fixed or floating pulley in a 3:1 system; the fixed one is weighted much more than the floater, but more rope travels through the floater. It does make sense, especially if the lone pulley is a prusik-tender, to use that pulley as the fixed one to make the best use of the ratcheting prusik, since if the system is set up right that prusik would not have to be tended at all, even during re-sets.

 

I usually only carry one pulley (a Petzl Mini PMP) unless I'm on a two-person rope team, but do carry a DMM Revolver biner (the one with the small pulley wheel built into the biner frame) to use as an improvised pulley if I need one. So if I was setting up a 3:1 with just my gear, the lone pulley (with a very short ratcheting prusik that needs no tending)would be the fixed one at the anchor and the Revolver would be the floating "pulley."

 

If the $4 petzl is that little nylon wheel to clip into a biner, I've never used one, but have heard the same stories of them breaking.

 

Hope that helps.

 

OK. With the drawing, I understand your "drop loop" better: a 3:1 piggybacked onto a 2:1. in this setup I still say the best place for the pulley would be at the anchor, so #1.

Edited February 5, 2009 by mike_m

[McGinnis]

I've used two of the nylon Petzl "emergency" pully and two tiblocs for doing short glacier crossings as a "just in case" set up. You can set these Z systems up with prussiks and ovals but it's not pretty. The nylon emergency pulley is the same thing used in the Sideswing Pulley but when you're using the nylon on an oval, if the pulley breaks you still have the oval holding your rope.

It depends on what your trying to accomplish and where you're trying to accomplish it as to what you might want to carry and have as well as how you want to set it up. What are you trying to accomplish?You're going to expirience alot of friction at your #1 and #3 points. If this is a "just in case" system I would say to go with the lighter nylon pulleys, you can carry 10 for the weight of a regular pulley. If you're going to be doing alot of heavy hauling get some nicer pulleys and get atleast two.

Edited February 5, 2009 by McGinnis

[spotly]

Good points. Especially regarding the prusik tending issue. Yes, I was referring to the little wheel. I've looked at the Revolver and I think that makes sense as a second pully, especially with the dual functionality. Thanks

[catbirdseat]

You always put your best pulley where the load is greatest. Just do the math. The coefficient of friction times the load is drag. More load, more drag. Lower friction, less drag. Use your blue pulley first, then the Revolver, then carabiners in that order.

[MarkMcJizzy]

 

 

OK. With the drawing, I understand your "drop loop" better: a 3:1 piggybacked onto a 2:1. in this setup I still say the best place for the pulley would be at the anchor, so #1.

 

Isn't the load at both (1) and (2) [ {2/3}X {1/2} load ], while the load at (3) 100% load. Therefore the best location for the most efficient pulley is at point (3) ?

[tomtom]

 

Isn't the load at both (1) and (2) [ {2/3}X {1/2} load ], while the load at (3) 100% load. Therefore the best location for the most efficient pulley is at point (3) ?

 

That sounds right. The second best pulley should probably go at point 2 because, although the loads are the same, it has three times the rope travel as point 1.

[DavidHiers]

3 shares the load with the victim's tie-in, so I'm pretty sure that it can't bear 100% of the load. Since the load is supported directly at two points and is free to move, each point must bear 50% of the load.

 

Ignoring the non-pulley losses such as edge loading, the frictional losses come from the rope/pulley surface, as well as the pulley/axle surface. Both load and speed will enter into the solution, which I suspect has some non-linear areas. I'm not sure which dominates.

 

There's going to be some losses internal to the rope as the fibers move over each other.

 

Connally [p322] claims that if you have only one pulley in the classic Z system, it goes next to the pulling hand. This suggests that speed dominates load in common climbing situations.

 

 

[ropegoat]

Connally's point makes sense in the traditional z pulley case, but not in the case of a drop pulley.

The tradeoff is the between the point that sees the most rope travel versus the point that sees the most stress.

Energy (aka work) = force * distance

In the case of the pulley, force is:

(tension across the pulley) * (friction coefficient)

so: energy = (tension)*(friction)*(distance of rope)

the tension across the pulley is 2 times the tension on each strand (the force in each strand running through the pulley is equal and must oppose the load on the opposite side of the pulley)

 

we get a 6:1 advantage in this system, meaning we have 1/6 the load on each strand, also, the tension across pulley 3 = the total load, the tension across pulley 1 and pulley 2 are both 1/3 the load.

for every 1 foot the victim moves up, you pull 2 feet through point 3, 4 feet through point 1 and 6 feet through point 2.

 

Mark hits it on the head:

so if you put the pulley at point 3, the energy lost is 2*load*coefficient

if you put the pulley at point 2, the energy lost is

6* (1/3) the load*coefficient

thus these two placements are equivalent. You want your worst pulley at point 1, because it only sees 4/3*load*coefficient

 

Catbird: your logic is right, but it's important to consider how much rope passes through each point, that's the difference between force exerted and energy expended. You can exert more force, but you have to balance that with your endurance. I prefer to look at system w/rt the total energy expended.

 

If this were a traditional Z pulley, the full load hangs from point 3, the tension across the pulley at point 1 is 2/3 the load, and the energy lost is 4/3, while the energy lost at point 2 is 2*load*coefficient.

 

David: unless your rope is sliding across the pulley surface without rotating the wheel (usually this wears a flat spot on your pulley), only the friction in the bearing of the pulley matters. I've definitely worn flat spots in two SMC pulleys- have you had this happen to you a lot? Also, the tension across pulley three MUST bear 100% of the load, otherwise the system would not be balanced.

[genepires]

put the pulley on #2. What matters is most is the amount of rope running through the pulley, not the weight held by the pulley. Especially with a 6 to 1, the point 3 will have very little rope motion per "cycle", therefore little friction even with the higher forces.

 

WHile I haven't used the revolver biner in a crevasse rescue, they have a rating where the pulley locks. It is not meant to rotate while catching a leader fall. Not sure what the force is, but the biner may not be a pulley in the #3 position. I suppose the lock off rating would have to be above a 1kn to not lock off holding a man.

[MarkMcJizzy]

This is truly a geek-fest. Ropegoats math seems the strongest, but I think I am going to revise my answer; put the best pulley at location (2).

 

If the pulley at (3) is 100% inefficient, then the system is still 3:1. If the pulley at (2) is 100% inefficient, then the system is only 2:1

[slogon]

If the pulley at (3) is 100% inefficient, then the system is still 3:1. If the pulley at (2) is 100% inefficient, then the system is only 2:1

 

This seems like a good way to think of it (of course when I started reading this thread I would have put the pulley at point #1). Also, I've tried the little plastic pulley (liked the weight savings), but had some trouble with the rope coming off the pulley, maybe that depends on the particular biner shape.

[ropegoat]

I forgot to include that my final opinion is also to put the pulley at point 2, even though point 2 and 3 might be the same, I think that with rope stretch and other effects (ropes rubbing against each other in your system, possibility of not having something set right, etc.) this makes the most sense.

 

Anyone in the PDX area and want to go play with this sometime on hood or somewhere?

[tomtom]

for every 1 foot the victim moves up, you pull 2 feet through point 3, 4 feet through point 1 and 6 feet through point 2.

The pulley at 1 acts as a redirect, so only 2 feet of rope goes through it when the rescuer pulls 6 feet.

 

One downside to this system is that there is no ratchet mechanism as shown. The rescuer would have to pull up on the tie in strand of the rope through the prusik at the anchor as well. Or one could add a prusik to the pulley at 1.

[G-spotter]

The best place for the rescue pully is the one having the most fun?

[spotly]

"One downside to this system is that there is no ratchet mechanism as shown." I was gonna draw that in at the #1 point but I figured...let the stick-man fall - he's not pulling his weight anyway

 

Good info from the thread. Thanks.

[mkporwit]

Genepires is right -- in a haul system, the most efficient pulleys should go where there is the most rope running through them. Typically that's closest to the hauling end of the rope. Here's a little Rescue Dynamics article about efficiency in a 3:1.

[Wastral]

To start with, this system only works when the person in the hole is not INJURED because they have to attack that pulley to their harness. Thus, is an idiotic way to "rescue" someone who is perfectly able to rescue themselves by simply using prussics!!! Which is far faster and you don't have to set up stupid pully systems to start with!!! DUH!!!

 

 

So, in my "humble" opinion, this type of Crevasse rescue is BS from the get go in case you didn't figure it out already... The "answer" to your pnderances is below...

 

 

 

Ok guys, think of it this way, where is the slowest part moving?

 

Pt. (3).

 

Thus, it has the highest mechanical advantage/gear ration.

Thus, it has the highest Force(load).

Thus, inefficiencies in the pulley will be multiplied at this pt.

 

So, if you were going to waste your time setting this system up with a perfectly able person in the hole put the pulley at pt. (3)... yea ok.

 

Do you really trust somoene to hook it up correctly who is on a GIGANTIC adrenaline HIGH?

 

Brian

 

 

 

 

BUT, we have friction.

1) Snow

2) Inefficiencies in the pullies

 

Since you only have 1 pulley

[Wastral]

oops forgot the last few lines were still there. Was going to get really "technical" then decided the heck with it, this system is BS from the get go, so why bother with all the math?

 

Brian

[Wastral]

Oh yea, if you actually do a correct z-pulley system, then once again attach your most efficient pulley at the slowest moving point where the force is the highest. Otherwise its far far faster to prussik yourself out of the hole if you can't simply be pulled out to start with.

 

Brian

[Wastral]

Uh, to mrporwit.

 

Your article showed a standard arrangement. It was most efficient because it was the slowest moving part of the system. NOT AS you postulated because the most rope goes through it.

 

In otherwords, you traded speed for force.

 

Force times speed = power, thus the efficiencies are multipled greatest where the Force is highest, and the Speed is lowest.

 

Brian

[genepires]

chill out. it is only a pulley question.

 

There are situations where the victim may be injured, unable to prussik out and still be able to help set up the system. How about a broken foot/ankle or one broken arm/hand? These people may find it difficult to prussik but sure can clip a biner to their harness.

 

The system is not BS. It is just a tool to use in certain situations. The use of wrong tools for a certain situation is BS.

 

As far as the victim being in a adrelaline high when it comes to clipping in a biner, by the time the biner actually makes it down to him or her (creating a anchor, getting safely to the hole, determine status, decide plan, excavate safe rescue hole / prepare lip) I would think the high has worn off enough to be able to clip in a biner to the harness. I would bet every climber is capable of clipping a biner. Even a newbie mountaineer.

[Wastral]

Well, Its far faster and safer to let the guys on top who aren't injured to set up a 2:1 or a 3:1. The only time you set up a system is when the person IS injured.

 

Not to mention that this system requires a TON of rope length to pull the guy out. I will bet hard money that you never have the rope length to do this set up unless the rear guy falls in the hole on a 3 or more person team, requiring all the other guys to get out of the rope to aquire enough rope length. = dangerous and stupid and more time consuming. :: You sure as heck won't have it if you are on a 2 person team.

 

In short, its a stupid way to do it because it takes more time, and on top of that it only works when the guy isn't injured.

 

Sorry, no, I would never trust the injured person to set up a 3:1. If I trusted them that much, let them prussic out. Its faster and simpler.

 

On top of those concerns; no sense learning 3 ways to do something when 1 way is simpler/faster and can be used in All situations. :tup:

 

Most people don't even know how to do the 1 normal way!

 

The (kiSS) principle notice which letters are capitalized keep-It-SIMPLE-STUPID Under stress people become stupid, thus keep it simple.

 

Brian

 

I am explaining because this is the NEWBIE forum and I have helped many newbies learn crevasse rescue and learning 3 ways is NOT the way to go about doing something. They can't even do the 1 normal way!

 

Edited February 7, 2009 by Wastral

[spotly]

Wasn't a question about the system. The best system to use is based on the situation. This happens to be one possible system and using it as an example for discussion regarding pulleys shouldn't lead you to think that it's the only solution that the questioner is aware of. If the diagram is uncomfortable for your viewing though, feel free to upload your own and we can use it for the discussion....about pulleys.

 

 

Edited February 7, 2009 by spotly

[Wastral]

??? The diagram is obvious. Read what I wrote.

 

Most efficient spot for a pulley is where the load is the greatest and coincidentally where the rope is moving the slowest. Simple physics, pt (3) for those who can't read what I wrote above.

 

Its a dumb scenario which should never be used and was only thought up because somoene forgot that if an uninjured clmber is in a hole they pull themselves out. You sure don't pull them out which is what these people are thinking because they are used to pulling "uninjured" people out of crevasse rescue practice scenarios and got bored and dumb in the process.

 

My whole point is that this "scenario" is pointless and should never be used because either the person in the hole is able to get themselves out via prussics or can't due to cut in, overhang, or injury. In which case this scenario is once again, useless and requires TONS of free rope which you don't have.

 

Why don't you have it? Because the most efficient way to haul someone out of a hole is to put the pulleys as far as possible away from eachother. Which means you won't have an extra 50 feet to drop down to the fallen climber.

 

Brian