Mechanical Advantage Pulley Quiz

Eric Frei

It is 3:1 not 2:1.

Look again, at 8 you have 2:1, then the red line has another guy pulling at the rate of 100kg as well, so a total of 3:1

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Fairfield

Assuming that the rope tied to the tree has a rope grab of some sort attached to it makes that rope just an extention of the tree it's self. When you attached the second line for the 2:1 That is where you have had the MA come into play. You could just attach the pulley directly to the tree and have the same thing.

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moray

Yes, it is the same thing. Either way the MA is 3:1.

Fairfield

Ok, in no way am I ever going to say that I know everything so I guess I jus am not following at all.

In the last pic it shows a line tied off to the tree that is to be pulled. That line goes to (lets say a prussick) and then nothing ( just slacks). Then that prussick is attached to a singal pulley. That pulley has a diffrent rope that starts at an anchor runs through the pulley and to the person pulling.

That being what I think is going on, the line on the tree has nothing pulling it other then the pulley ( that we all agree that portion is a 2:1).

Am I right thus far?

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Fairfield

Ya know what I think I am losing it a little sorry boys. I will go back on what I sais about the very first pic and stay with my first thought of it being a 6:1. I would still keep my thought on the last one being a 2:1 MA. Would like someone to show me with a weight measurement that I am wrong though. Heck if I am wrong then I really want to know.

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Eric Frei

Step by step.

Here we have the basic 2:1



Here we get 3:1 because we have another 100kg input force.



Here we have the original set up, 3:1

Attached Thumbnails

     

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Fairfield

Ok like I said I think I had lost my head for a little bit, I agree with the first and third one but the second I still am not seeing how you got it. You still have to pull the line 2 feet to get the object to move 1 foot. Making it a 2:1.

Hahaha I'm going to drive someone up a wall I just know it.

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Fairfield

EEFFFFF ME!!!!!!! I just seen what you are saying..... sorry I was looking at the red part as if it was just going to nothing an not tied off...... sorry, think I will have another

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Eric Frei

Pic 2 and pic 3 are the same thing.

Forget the amount of rope you pull through, irrelevent for this. Imagine you have a 2:1 set up on a tree and you agree at 8 there is 200kg of force. Now a mate comes along and attaches a prussik on the same rope coming from the tree, he also pulls like a frieght train with another 100kg of muscle. You have your 200kg of force plus his added 100kg = total of 300kg.

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Fairfield

Ya sorry Eric, just one of those few days for me I guess. I compleatly se what you are saying now. For what ever reason I just was not opening my eyes.

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moray

The Norsemen were pikers. Ekka, you could have saved us all a lot of trouble had you found this link by--can you believe it--a bunch of Aussies!

Attached Files

Friction%20Testing%20and%20Pulley%20Systems%20in%20Vertical%20Rescue.pdf (112.6 KB, 67 views)

Jeff Darby

What and made too easy for you all, Have you not benifited from this exercise ?..Ekka could spoon feed us all but what would we really learn.

On the other hand you seemed to stepped up with your research, another positive outcome. Every one has to remember no one pays for this mans time.

Ekka releases good information to the public for free when a lot of blokes would let you stay in the dark or ask for a fee for their time.

Thanks for your time and effort Ekka.

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Eric Frei

He was confused and unable to understand from the thread and existing diagrams, where answers were already provided, then how do you expect he would have coped being dumped on with that PDF?

He needed to be explained to, step by step, broken down into components, in the context of his own working environment.

This is why many struggle to learn because they "don't get it" when reading a volume of stuff, and unable to interact. Sure he can read the answer is 3:1 or 6:1 in the thread or the PDF but why .... is what he doesn't get.

Thanks for finding the PDF though, very good, but Moray I know you have the skills to do more ..... hopefully you will.

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Fairfield

Hahaha, I was really just being blind to what was plainly in front of me like a terd!! but thanks for putting it in nice terms, dont worry I hit myself in the back of the head for that one

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Russian

IMHO
1-600
2-200
3-200
4-200
5-100
6-100
7-500
8-400

serialtreekiller

I have one of these pulleys i don't know how to use it

Garry Brockley

If you buy 1 more you can copy the diagram and practice this system useing some prussics and karribinas, just set it up as the diagrams show and you will be pulling trees over correctly in no time

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Rescueman

Sorry to revive this old thread, but I stumbled on it through a Google search and just had to register so I could correct a whole slew of misunderstandings about mechanical advantage and friction.

You could say that you achieve a total force of 300kg on the tree with this crazy hybrid system, but it's NOT a 3:1 MA system. What you depict is no different than a simple 2:1 with another man pushing on the tree with 100 kg force.

I'm afraid that Fairfield shouldn't have hit himself over the head - since he was right. For this to be a 3:1 MA system (which your third image is) it has to multiply a single input force 3x to the output end, and the input force has to move 3x as far as the load.

Rescueman

The Norwegians are wrong on almost every conclusion, but not for the reasons you offer.

As far as how much a typical man can pull on a rope MA system, what they measured was basically the friction of weighted sandals on a grassy field. They show the hauler with the rope wrapped belay-style around his body and digging in as hard as he can.

Testing performed by Rigging for Rescue in British Columbia CAN a number of years ago demonstrated that a typical, physically-fit rescue team member with leather gloves can exert an average of 50 lbs (22.7 kg) force on a 7/16" (11mm) rope. You can do better with a larger-diameter rope, but not by much. In most real scenarios, we pull hand-over-hand and don't use our full bodies.

The problem with their alleged MA output numbers is that they assumed that, if a man can exert 40 kg on a 1:1 system, then each time he pulls on the input of another system, he's exerting the same force. The only way they could determine the MA ratio for each system would be to measure both the input force and the output force.

The issue was not the difference between static and dynamic friction, since the purpose of pulleys is to virtually eliminate that difference. There is a significant difference in static and dynamic friction with rope over a fixed capstan (whether carabiner or tree).

And the rated efficiency of rescue pulleys includes the friction in the pulley bearing and the internal friction in the bending rope. The primary determinant of the efficiency of a pulley - all other things being equal - is the diameter of the sheave (wheel), as that determines the internal friction of the rope.

Eric Frei

I think this is the system you are referring to.



It is not a crazy system, it is a common system used to pull trees, often. We do this because we have lots of pulleys and do not want a pulley up in the tree that might get smashed when the tree comes over. The pulley we put on the rope coming from the tree is usually out of harms way and at worst falls to the grass or field beneath.

This system does offer 3 x input force, you agree with that:-

Now from your diagram you posted (thankyou) the most similar set up I can see is the one I have put in the red box below. And that has a number 3 on it. Can you see that or do you think because the pulley on our rope coming from tree is not hard up on the tree it is different? To me I see 3x.

Attached Thumbnails

 

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Rescueman

No, I was referring only to pic 2, which you described as:
They are not the same thing. Pic 2 is a pig rig with a 2:1 ganged onto a haul line. Pic 3 is a 3:1 Z-rig using a single rope.

Not crazy at all. In fact, it's the most common MA system used in rope rescue, and the one I just taught to a trail-building crew for hauling 400 lb rocks up along a cable highline for building stairs. It offers the most bang for the buck - the optimum MA with the least amount of equipment and the least friction loss.

If the travelling pulley is secured to the tree (load), then it's a simple 3:1 MA. If the travelling pulley is secured to the haul line with a rope grab (prusik), then it's a Z-rig 3:1. If there is also a progress capture device (another rope grab/prusik on the haul line below the fixed pulley at the anchor, then it's a resettable Z-rig 3:1 which can also hold tension on the load.

Eric Frei

So some keys points.

1. I was correct, the output load is 3X the input force. Whether you want to call that 3xMA or 3X is irrelevant.

2. Pic 2 and Pic 3 both have the same output force except for the sake of explaining how that happens the components were broken down and separated in pic2, it helped Fairfield understand how that occurred, which he "got" in the end. Pic 2 is the "working out" in the mathematical equation just like pic 1 is. You are failing to look at the working out of how all the forces come into play to deliver the 3X in pic 3, you have taken each component out of context and used it standing alone.

3. Your version of MA is inconsistent with the scientific version which is:- MA is the number of times a machine multiplies your effort of force. In this instance it is 3X which you agree on. Where is "law" or "nomenclature" that 3x MA with rope and pulley systems must have 3x more rope pulled through the system than the load physically moves?

So where exactly is my misunderstanding, seems to me I have been spot on and consistent the entire way through and regardless of all your nonsense the numbers stack up accurately which you agree to anyway.

Your diagram of the Z rig with a back up prussik is old news, what we do often, but you again say it is 3X which is what I have said all along.... and have the audacity to say Fairfield was right but agree with my numbers.

Then you go further to say:-

When in fact they measured the forces so they are not wrong nor lying.

You wrote:-
They did measure the output force in each case and applied the input force consistently. They also had a similar Z rig set up and managed to get around the 3xMA we are discussing.

One could reasonably assume that the input force would not vary greatly if the same method was being applied. The fact that Cannucks aren't as strong or getting the same input force is not grounds to throw out their findings, maybe grounds for the Cannucks to man it up a bit more.

To suggest that when we use a Z rig and put all our force on the rope, not hand over hand but body weight and really heave we can only apply 22.7kg is a joke, like most of your argument has been thus far.

On forums I call your type of conduct trolling ..... a lot of finger pointing and noise but nothing really delivered and just more confusion that frankly has led to the same result that already existed, a total waste of my time explaining more of the same to an argumentative troll.

Squawk on, lets see some more of the genius you possess.

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Rescueman

I dropped in to this site to clear up some confusion and misinformation that was posted here - mostly by you - about mechanical advantage systems and friction in pulleys.

Obviously, anyone who advertises that they are "Australia's most prominent Arborist" has a lot more ego than common sense - and far too much arrogance to be able to learn from someone who knows a related subject far better than you.

You may have something to teach me about trees (though I've been felling timber for 30 years), but a true master like Tom Dunlap has the knowledge and experience to share, the wisdom to know what he doesn't know and the humility to learn from those who know more. I learned a great deal from him and he relied on my knowledge of rigging.

You are clearly in a different category and will label anyone a "troll" who dares correct your mistakes.

So, even if you're not capable of admitting error and learning from a true rigging master, perhaps you'll at least have the decency to let others here benefit from a more knowledgeable perspective.

Not "irrelevant" at all. You can get 3x the output by having three men haul on a single directly-tied rope, but that's not a 3:1 mechanical advantage system.

It actually confused the heck out of Fairfield, because your explanation was wrong. But he finally figured out what you were trying to get at.

I took nothing "out of context". You made some incorrect statements specifically about pic 2 and I corrected them.

There is no free lunch. Which is saying the same thing as the first law of thermodynamics: energy cannot be created or destroyed. But you don't seem to understand the fundamentals.

The physics of mechanical advantage is simple and consistent, whether you're talking about a lever, a pulley, a screw or a ramp. The only way to get more force at the output than is supplied at the input is to have the input force move a greater distance than the output load, proportionate to the force multiplication. That way, since work is force multiplied by distance, the amount of work is the same at both ends of the system and the first law is satisfied.

So, if you don't have a dynamometer to compare input and output forces, you can determine the MA of a pulley system by comparing distance in vs distance out. Every 3:1 MA system requires the input distance to be 3x as much as the output distance. That's basic physics.

Pic 2 is NOT a 3:1 MA system - it's a 2:1 and Fairfield was correct.

Of course, I never suggested that they were "lying", merely wrong on almost every count. You also stated that they were wrong, but mischaracterized their mistake. Now you claim they were right.

That they knew little about mechanical advantage is also evident in their statement "In addition the participants are better able to load the system since they are more stable whilst travelling 1/4 distance under pulling due to 4:1 system." If they were pulling on a 4:1 system, they would have to pull 4x the distance compared to the direct pull 1:1. Thy have it backwards.

They also wrongly concluded that "the additional pulley has negligable effect (friction) on the system", without having measured both input and output forces at each pull. Since a good pulley will reduce the applied force by only about 10%, and the (unmeasured) input force can easily vary by more than that, it's impossible to make any valid conclusion about the friction loss in the pulley. To suggest that they can get better than 100% of the input force through a pulley system is further evidence of their nearly total ignorance of the subject.

I suggested nothing of the kind. I said that we use mechanical advantage systems so that we don't have to dig in our heals and use full body weight. And those "Cannucks" who did the hand grip strength testing (Arnor Larson of the British Columbia Council of Technical Rescue) are the foremost rope rescue instructors in the world.

It appears that you're the only one who needs to "man it up". But, perhaps "Australia's most prominent Arborist" is unable to admit when he's wrong and incapable of learning since he believes he already knows it all.

Eric Frei

I will now deal with this squawking genius.

Big tickets on yourself their buddy and sucking up Dumblap's butt is nothing to pat yourself on the back about.

Like I said, I have not made an error. You walk into the middle of a thread, do not read it all, take pics that are a dissection of the whole system to explain to Fairfield how we get to the answer (which he got in the end) and apply your bullshit.



Here again is that squawking genius. Hey genius, you have 3 men pulling the rope so input force is 3X and the output force is 3X, where is the MA? There is none, wow, your real smart eh.

Fairfield was confused to start with, the breaking down of the components lead him to the answer. To squawk all over that is stupid. Pic two resembles the line from the tree in a different perspective, showing the red line with force1 and the blue line with force2. A sensible and methodical way of approaching the final outcome of pic3.

Pic 2 beneath


Pic 3 beneath


I have a very good understanding, seems much better than yours.

I think you have not seen that pic 2 has force1 and force2 applied. Yes force2 is a 2:1 and then there is an additional force. Again the pic was to show how 300kg of force is applied. It is you who is misconstruing this, not I.

I actually wrote this but it appears your comprehension skills are lacking.


Well, what a barrage of rhetoric.. They were measuring output forces and using the same method for input force. Hello, is there an echo or are you not reading what I previously wrote. Seems you not only like to hear the sound of your own voice but also running out of space putting those tickets on yourself.

Lordy help people with a squawking fool like you around.

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Eric Frei

The Z rig is a 3 x MA system, 3 X as much is pulled through to get the load to move 1.

Pull 3m of rope to move load 1m

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Rob Chace

why is this still going on, is it not blatantly obvious 3:1 reduction?

Eric Frei

Because a half cocked out of context comprehension challenged fool wandered in then squawked and crapped everywhere . So I sorted it.

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