Talk:Z-drag

Self tending

I removed the bit about needing two people to operate a z-drag. When rigged as shown in the diagram with a progress capture prusik the system is self tending; the operator(s) can let go completely and nothing bad will happen. So one person can haul until the two pulleys are together then let go of the hauling part and reset the movable pulley while the progress capture prusik hold the load. --john.james (talk) 12:44, 19 April 2010 (UTC)

Mechanical advantage of illustrated configuration

Hello there, I appreciate your drawing and explanation of a Z-drag but I believe that this arrangement offers a 2 to 1 mechanical advantage, not a 3 to 1 as stated. There is only one dynamic pulley (#1)in this system. The pulley on the fixed object offers no mechanical advantage; it simply facilitates a direction change for the pull. Prussik (#1)allows one to attach a dynamic pulley to an object that is at some distance away. In effect, pulley #1 acts as though it is on the object. Every length of rope pulled will result in the object's moving one half of the length of the rope pulled. That is a 2 to 1 advantage. I set up this system in my front yard just now and verified this relationship. — Preceding unsigned comment added by Outof lifelines (talk • contribs) 16:24, 6 September 2011 (UTC)

by the way, if I haven't heard a credible response within a day or so to explain the 3-to-1 information, I plan to edit your original posting with what I believe to be the correct information. thanks — Preceding unsigned comment added by Outof lifelines (talk • contribs) 19:52, 6 September 2011 (UTC)

3:1 is the correct advantage, when used in rescue situation, where the puller is next to pulley #2. Looking at the picture of Z-rig, it is easy to see that there are three ropes parallel between pulleys 1 and 2 and when pulled together, the system moves the load the distance that the pulleys have initially and all rope exits, thus giving the 3:1 advantage. The point is that you should measure the advantage at a fixed point next to pulley #2 (where the rescuer stands), not at pulley #1. Also, If you had only pulley #1, you would get 2:1 advantage (known as C-drag or C-rig in literature), pulley #2 is part of the dynamic system and changes the advantage to 3:1. It is correct in that the Prusik #1 is not relevant to the analysis and you might as well assume that the pulley #1 is attached to the load. For further confirmation I recommend googling "z rig rescue advantage" or seeing http://en.wikipedia.org/wiki/Pulley#How_it_works and Diagram #3 there. Kiravuo (talk) 13:38, 18 September 2011 (UTC)

sorry, but you are incorrect. pulley #2 is indeed not part of a dynamic system and does nothing other than allow the rope to change direction. basic physics tells you that with one moving pulley, the advantage cannot exceed 2:1. the other pulley (and line) that you refer to only allow for a change in direction of the pulling force. it is common for people to count the lines and assume this is also the mechanical advantage. but, it is incorrect in this case. it works where you have a double pulley attached to the object. in that case there are 4 lines and the advantage is 4x. there is simply no construction you can do that gives a 3:1 advantage. the first pulley gives 2:1 and a second pulley doubles that, i.e. 4:1. if you set one up at home and measure the amount of line that is pulled versus the distance the object moves, you can prove it to yourself. please leave my change to "two" until/unless you are willing to take the time to demonstrate the concept in real world. thank you. — Preceding unsigned comment added by 174.28.28.138 (talk) 14:52, 27 October 2011 (UTC)

Added reference to Ashley. 3:1 should be clear now. Kiravuo (talk) 18:51, 11 November 2011 (UTC)

And if Ashley is not enough, let's try to explain this once more. It is just not true that a stationary pulley could not provide an advantage. To illustrate: Imagine a rope tied to ceiling, when climbing the advantage is 1:!, i.e. no advantage. Switch a pulley to the ceiling and have somebody pull you up, it is still 1:1. But pull yourself up the rope which is going around a pulley, and you get 2:1 advantage. The pulley is fixed, but the point at which the force is applied is moving, that's the difference. And that is the difference between C- and Z-rigs. If the pulley #2 would be useless, lots of rescue people would have been fools over the years to bother with it.

Or to go back to high school physics. Work is force multiplied by distance. The string counting method is just a simple implementation of this rule. In a Z-rig when you lift a load, you pull three times as much rope as load moves. Since the work done is same (discarding friction), but you have applied a force to a distance three times as long, it means that the force was one third of the force applied to the load. Or in my example above, when somebody is pulled up to ceiling with a rope, the work is applied at ground level and the length of rope being pulled is equal to distance moved. But when somebody pulls themselves up with a rope going over a pulley, the force is applied at same place as the load the lengt of rope being pulled is twice the length of distance being moved. Kiravuo (talk) 13:19, 12 November 2011 (UTC)

your statement "In a Z-rig when you lift a load, you pull three times as much rope as load moves." is verifiably incorrect. are you too lazy to set up the Z-drag you draw and measure the values in question?? it is easy to do and it shows conclusively that the amount of rope you pull is exactly twice the amount the object moves. that, my friend, cannot happen with any advantage other than 2:1. the rest of your arguments and citations are specious but are not worth the trouble to argue since you won't take 15 minutes on a sunny afternoon and do the test. it's much easier than going back and forth like this. when you get back and post here that you measured the rope distance and found it to be 3X the object distance, then you will have established some new physics and i will be very interested. by the way, the reason people use the second pulley has nothing to do with mechanical advantage, it is solely to allow an attachment to the object that can be implemented from shore via a prussik or an ascender and you need to change the direction of pull if you do that - thus the need for pulley#2. if you have a pulley already installed on the boat, a z-drag offers no additional advantage. CHECK IT OUT!!! — Preceding unsigned comment added by 174.28.138.132 (talk) 00:20, 24 November 2011 (UTC)

The Z-drag doesn't exactly match traditional block and tackle configurations one usually finds documented in non-climbing/rescue books. I hope the following thought experiment will be helpful.

For clarity I will use the labels as shown in the configuration of ropes, pulleys, etc. shown to the right:

Pretend for a moment that "Pulley 2" is jammed: the rope is completely stuck at that point and it cannot move either forwards or backwards. In this case, if the user hauls on the rope at "Pull here" two units, the "Object" will move one unit, giving a ratio of 2:1. I will assume we all agree on this.

Imagine what has happened to the rope between "Prusik 1" and "Pulley 2" after we've completed the pull described above with the jammed pulley... It will have gone slack, right? And how much slack will have accumulated in that section between "Prusik 1" and "Pulley 2"? Since the "object" has moved one unit, there will also be one unit of slack in that section of rope.

Now imagine the object has been stabilized in place and then "Pulley 2" is unjammed. If the hauler takes that one foot of slack out of the system, that will mean the hauling end has moved three units total: 2 original units and 1 unit to take up the slack from between "Prusik 1" and "Pulley 2". So that means with "Pulley 2" operational, three units of rope need to be pulled by the hauler for every unit the "Object" is moved, thus the theoretical ratio is indeed 3:1.

There are other ways to analyze this apart from the ratio of movement... You can read about "rove to advantage" vs. "rove to disadvantage" in the block and tackle article, and in particular the section on rigging methods. This is the way one can arrive at odd ratios of advantage; roving to advantage adds one unit to a given configuration. Advantages of 3:1, 5:1, 7:1, etc. are quite possible. --Dfred (talk) 02:07, 24 November 2011 (UTC)

and yet, all you need to do is set this up and test for yourself that you will pull exactly twice the amount of rope as the object moves. why won't you do that? do you agree that if you pull twice the amount of rope then the mechanical advantage is 2, not 3? will we keep going back and forth forever or will you just run the test and see for yourself???? — Preceding unsigned comment added by 174.28.138.132 (talk) 14:59, 24 November 2011 (UTC)

I have set this up. I have measured the rope. I have seen myself. It is 3:1. See http://www.kiravuo.net/z-drag/ Kiravuo (talk) 16:20, 24 November 2011 (UTC)

You know very well that you have not set this up and measured 3:1. Anybody who is willing to lie about it is not going to be honorable in their descriptions or analysis. This is the problem with Wiki, there is no protection against pseudo experts who are willing to lie in order to make a point. I won't belabor this anymore since it is clear that you are determined to have it your way, right or wrong. It truly is a shame and shameful that you claim to measure a system that you obviously never have. I only hope that independent readers will go out and verify for themselves that this is 2:1 not a 3:1 mechanical system. — Preceding unsigned comment added by 174.28.138.132 (talk) 19:03, 26 November 2011 (UTC)

You are both funny — Preceding unsigned comment added by 184.59.69.80 (talk) 06:11, 5 January 2014 (UTC)