Help with Pulley System

Actually, it is not always true that "the mechanical advantage is equal to the number of ropes supporting the movable pulley."

For more than you ever ever ever wanted to read about this, see TCC link about RADS and Mechanical Advantage .

There is a lot of 'stuff' there. Some of the clutter could be eliminated by using a ring/ring FC and a delta link.

The illustration might not be quite right. I would need to get out the gear to rig it. But...I know that you can eliminate the tube FC and a lot of knots by using a sling or ring/ring FC.

If you continue to use the setup like you've illustrated you should replace the upper biner that the pulley hangs from with a delta/pear screwlink. Carabiners are not meant to have cross loading. I could see some configurations where you could crossload a biner.

Jim, good point in that TCC thread! I didn't consider that pulling on the rope distributes your weight to the third line. The advantage is 3:1.

Your example of pulling yourself up with DRT vs. pulling somebody else up was quite convincing. I was wondering why I had such trouble pulling my mother up a tree (120 pounds), when I could pull myself up easily (140 pounds divided by 2 equals 70), and that explains it!

You were right, moss!

-Link

Geof, I may well be missing something here, but I don’t see what the purpose of this arrangement is. Well, that’s not quite accurate: It just seems that there are simpler systems to do the job.

1 --- What is the purpose of the figure-8 knots adjacent to the cambium saver (CS)? If the rope is supposed to pass through the CS, the knots will prevent/inhibit it, or will pull the CS off the limb. If the rope is not to move across the limb, then why use the CS?

2 --- Looking at the drawing, what is the purpose of the three figure-8s on the left-hand-side (lhs) rope? Pulling on them does nothing (except waste energy). If you want something to pull to assist in ascending, then the knots would have to be on the rhs, but then the ascending knot couldn’t pass them. I read what you said about getting into a standing position, but it doesn’t make sense to me (of course, I haven’t tried it, either).

It just seems that grabbing the “down” rope is better: At least in that case, the force is going toward ascending.

3 --- Although popular in some circles, I see no value in the figure-8 in the bridge.

4 --- Tom beat me to it about the carabiner. A screw link should be used.

All that having been said, your artistic skills more than make up for the design!! I do stand in awe of those skills!

Why not use a Z-pulley (as it is known to mountaineers) configuration in the arrangement known as RADS (sometimes called yo-yo)? RADS is pure simplicity **if** you’re willing to use mechanical devices (an ascender and a GRIGRI). I’m pretty much a “knot kinda guy” myself, so don’t use the system.

Well, in answer to my own question, there is the consideration of switching from ascending mode to descending mode . . . not to mention the cost of the mechanical devices.

I suggest that you take a look at the RADS anyway, just for educational purposes. Click the link in my previous post, go to page 2, and look at the picture Joe posted on 2/14/2007 at 2:00:46 PM. Joe showed me this system a few years ago and it’s really slick.

Although I have not taken the time to try to work it out, I really think that a RADS method using knots only could be a workable system. Geof, you sound like a really creative person, so maybe that’s a contribution you could make. Whatcha say?

Link, I suggest that you be macho about it and not let on, even a little bit, that it’s more difficult to lift your mom. If she sees you lifting yourself relatively easy, and then sees that you are huffing and puffing while lifting her, **while using the very same pulley arrangement,** it might cause her to be concerned about her weight.

Furthermore, it’s one thing to upset one’s mom that way, yet a whole order of magnitude greater to do the same to a wife/girlfriend. Be careful; don’t say I didn’t warn you!

The rope isn't running over the branch loaded, so the leather sleeve isn't needed.

Honestly, with all those knots, I would aim to clean things up. It really looks like a mess. I would splice all the ends, and switch to a split tail.

Overall, it looks like you're taking the scenic route when there are much easier ways to get to where you're going.

love
nick

It just seems that there are simpler systems to do the job.

That is one of the reasons for my post - to learn from the community what are some additional systems.

What is the purpose of the figure-8 knots adjacent to the cambium saver (CS)? If the rope is supposed to pass through the CS, the knots will prevent/inhibit it, or will pull the CS off the limb. If the rope is not to move across the limb, then why use the CS?

The 8s on either side of the CS help in repositioning of the system when (if) it creeps off the limb.

Looking at the drawing, what is the purpose of the three figure-8s on the left-hand-side (lhs) rope?.....(of course, I haven’t tried it, either).

I would invite you to try it. Keep in mind that this system is for a first time climber (client) that needs assistance in climbing. If you try it out, don't thrust it - climb as a newbie would. You'll see quickly how easier it is vs. not pulling on the knots.

Although popular in some circles, I see no value in the figure-8 in the bridge.

You are probably right - Just the way I was taught. Old habbits...

Tom beat me to it about the carabiner. A screw link should be used.

Because of cross loading? The drawing did show cross loading, but in reality there is none.

All that having been said, your artistic skills more than make up for the design!! I do stand in awe of those skills!

...More than make up for the design? Ouch! Thanks for the compliment. This was my first attempt. I hope to draw more again.

I need to read more on the Z-pulley, RADS, Yo Yo, GRIGRI, & YuGiO.

PS - I wouldn't even try to lift up my mother-in-law, ouch!

Thanks!
Geof

Geof, I am concerned about the system you described. I woke up this morning with the strong urge to do a force analysis on it. I had some time at work and did that (don't tell the boss).

The system does have a force imbalance: The top pulley has more force pulling it down than it has pulling it up, so the rope through the cambium saver is being pulled in a “clockwise” direction through the CS.

I think if it weren't for (a) the figure-8 knot not allowing the rope to pass through the CS, and (b) the friction between the CS and the tree bark not allowing the CS to slip, the system would collapse. I think the only thing that has kept it from doing so in practice is that the rope has been prevented from passing through the CS.

If there is not enough force holding the rope in place over the limb, what will happen is that the top pulley will move down, thus allowing the lower pulley to move down, and this will continue until “the system reaches equilibrium,” which will occur (a) when the two pulleys meet and jam against each other, or (b) when the climber becomes more intimate with the earth than s/he might care to be.

I strongly recommend that you not use this system.

Next, I guess what I meant as a compliment about your excellent artistic skills did turn out to be a “left-handed compliment.” My apologies for that! I really am envious of your talent.

Originally posted by Jim W

The system does have a force imbalance: The top pulley has more force pulling it down than it has pulling it up, so the rope through the cambium saver is being pulled in a “clockwise” direction through the CS.[/qoute]

this is what i was thinking this morning too....that force pulling down couldbe very detrimental to to the limbs structural integrity, possibly overloading the limb and snapping it.

Easy does it guys I've seen this system (minus all the extra F8's) rigged with 1/2" Safety Blue easily support a 300+ lb. climber. The upper pulley does not move down, there is equal and evenly distributed force on the each side of the cambium saver (even assuming there is not an F8 at the top on one side of the saver to prevent the top pulley from moving down).

As far as causing structural degradation of the limb (as SRT-Tech suggested)... there is no more chance of that then there is with standard DdRT tie-in with the same cambium saver. Assuming that the tree and branch have been assessed to be capable of supporting the weight of the climber it is very safe. There is no rotational torque or multiplication of forces on the branch beyond typical dynamic forces (caused by climbing motion) added to the weight of the climber.

Try rigging it (without the extra F8's for the sake of clarity), it's a very effective and well thought out mechanical advantage DdRT system.
-moss

Sorry, moss, but it is not balanced at the CS; there is a "rotational" force.

I do not disagree that it can/will support a climber without incident, but that will be only because the "frictional force" between the rope and the CS is high enough to prevent the rope from moving. When the coefficient of static friction times the normal force of the rope is exceeded by the imbalance, the climber will unexpectedly descend (apologies for the technical talk).

I also have to disagree that it is a "well thought out" system. It is an accident waiting to happen.

I know I'm going out on a limb here (gee--I wonder where that saying came from) by asserting these things, but I wouldn't do it without certainty.

That certainty comes not only from a force analysis, but also because I just now did model it with pulleys and it did collapse. Try it.

As for SRT's concerns, I agree with moss.

Jim,

Can you post a diagram of your force analysis? Intuitively I agree with moss's experience.

Did you use a pulley to represent the cambium saver in your model? I don't think that would be sufficiently accurate, since a rope slipping through a cambium saver has a much higher coefficient of both static and *kinetic* friction.

I think the coefficient of kinetic friction will overcome any tendency for the system to spiral out of control like you describe.

I don't really buy the fact that there is an imbalance to begin with, which is why I would like to see the force diagram.

In your model, if the friction was way too low, a slight imbalance, might cause the results you describe. I think this is equivalant to saying a blake's hitch won't work by modeling it with fishing line.

-Link

Thanks for the interest, Link. I have no way to post a diagram (actually, I guess it’s more that I am totally ignorant of how to create and post a diagram). Let’s see if words will work.

First, yes, I did use a pulley to replace the CS for the physical model. I do agree, as I said before, that friction at the CS can keep the system in equilibrium. The question, of course, is, do we want to use and/or recommend something that depends on that unknown amount of friction to keep it from failing. Do we want to bet our life on an unknown?

For example, we all know that we can simply wrap a rope around a branch a few times and the resulting friction will be enough that the rope will support us without slipping; we would not “need” a knot. But ask yourself if you would be willing to use that arrangement to allow your child/wife/mother/self/other-loved-one to climb to 100 feet on it. Would they be safe? Yes, almost absolutely. But why take the chance?

As for whether using pulleys to physically model the system would be “sufficiently accurate,” as I said, I agree that it does not provide the friction of the design in question--Geof's drawing. But my point is that the system *is* indeed unbalanced, so why take the chance with it. My purpose in using all pulleys is to demonstrate that the system is unbalanced. As I suggested before, try it if you don’t believe it.

BTW, the coefficient of kinetic (or dynamic) friction (mu-sub-k) is not the important parameter here. Because it is *always* lower than the coefficient of static friction (mu-sub-s), the latter is the one that is important. The point is that the system in question here will remain static (won’t collapse) until the resultant linear force on the rope exceeds the “mu-sub-s times N” and then the lower-valued mu (mu-sub-k) will be in effect and the system will continue to slip. Said in plain English, once it starts slipping, it will continue to slip.

I submit that the imbalance between the force pulling on the lhs and that pulling on the rhs (see Geof’s drawing) is one-third of the weight of the climber. Link, put your 120-pound mom on this system and the rhs is pulling 40 pounds more than the lhs. 40 pounds. Do you want your mother in this situation?

This already is a long post, so I’ll close and put some analysis in the next one.

Without doing a complete analysis now, let’s just start it off.

First, a fundamental fact: When there is tension in a rope, that same amount of tension must exist everywhere in the rope. The only way a rope can have different amounts of tension in different portions of it is if there is some additional (external) force applied to the rope. Here, we’re talking about a rope with no additional external forces applied.

Example: If there is 10 pounds of tension in a rope at point A, there must be 10 pounds at point B. If that were not so, the rope would move (accelerate) because of the force imbalance. This is common sense (at least to Mr. Newton).

Yes, I grant that a vertically hanging rope will have more tension at the top than at the bottom (because of the finite weight of the rope). We’re ignoring that here because it is inconsequential and irrelevant.

Definitions: “CS” is the cambium saver. “TP” is the top pulley. “BP” is the bottom pulley.

Let’s put Link’s 120-pound mother on this system (don’t worry, Link: low and slow). Look at Geof’s diagram and consider what is supporting her: three ropes. Her weight is being supported by the two ropes through BP, and the one rope that has the foot loop on it.

Note that all three of these “ropes” are really one continuous rope, and they are (it is) passing through pulleys (so there are no external longitudinal forces being applied to the rope). By the “fundamental fact” about rope tension, the tension in all three ropes must be the same. So the 120 pounds must be divided equally among them. Therefore the tension in the rope is 40 pounds.

That means that the tension in the rope on the lhs of CS is 40 pounds.

I’ll stop now and leave it as an exercise for the student to give us an analysis of the forces in the rhs of the CS. This should include (a) the tension in the rope at the rhs of CS (moss and Link claim that it is the same as the tension on the lhs); and (b) a force analysis at TP (including the one rope above it and the two ropes below it).

I anxiously, and with great interest, await your responses!

Jim, are you missing that the redirected force (though the pulleys) when the climber ascends (pulls on the down rope) is loading the left side of the rope going over the branch and countering potential downward movement of the upper pulley? Did you account for this force value in your analysis and physical model?

Something not shown in the drawing is that normally the screwlink "noose" should choke the limb. This may provide additional stability.
-moss

Analysed as a static system (when the climber isn't ascending) it is imbalanced. The Blake's Hitch however locks off and prevents movement of the rope so there is no pulley movement. When the climber pulls on the down rope there is still imbalanced force in a static analysis of the pulley/climber/rope configuration but... the redirected force of the downward pull through the pulleys ends up on the left side of the system and successfully counters the static imbalance on the right side of the system. I don' think friction between the rope and the inside of the pipe is a factor.
-moss