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Discussion Starter · #1 ·
I dont want to start WWIII, but can someone explain to me the practice of running a winch cable from your truck, through a snatch block (anchored to a tree or whatever) and back to the truck and proceeding to winch the truck out of trouble. As I see it there is no Mechanical Advantage (MA) to this practice, in actual fact you are increasing the load on the winch because of the friction created by the snatch block pulley on the shaft. Admittedly there is less friction in the snatch block than running the wire around a tree trunk, the principle is the same though. To halve the load on the winch you would need to have a MA of 2, to achieve this you require 2 snatch blocks. MA is defined, in this scenario, by the number of parts of rope (wire, etc.) supporting the MOVING BLOCK. In the first case the Block (snatch) is not moving, but anchored and you are pulling to disadvantage, in other words the wire is being wound back onto the winch, therefore it is moving in the opposite direction to the load (truck).
If you ran your wire from the winch to the anchored snatch block and back to your truck and then through a second snatch block attached to the truck and back up to the anchor point, you will have 2 parts of wire supporting the moving block and you would be pulling to advantage thereby halving the load on the winch (+ the friction in the 2 blocks). would be interested in hearing from anyone on this subject. Frank.
 

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You do have 2x the mechanical advantange using a single block anchored to a tree or rock, provided that the other end of the rope/cable is attached to the winching vehicle. If it is not, then, there is no mechanical advantage.
 

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Discussion Starter · #3 ·
Disco said:
You do have 2x the mechanical advantange using a single block anchored to a tree or rock, provided that the other end of the rope/cable is attached to the winching vehicle. If it is not, then, there is no mechanical advantage.
If you run your cable to an anchored, not moving snatch block and back to the truck you do not get a Mechanical advantage. Try this set up as above, winch in 1 metre of wire and the truck will move one metre, the only difference between a straight pull and a anchored snatch block is that it takes twice as long to rewind the cable, because there is twice as much cable out and the friction in the snatch block. Do away with the snatch block and disregard any friction and put the cable around the anchor tree, same principle, does the tree halve the load on the winch? Regards Frank.
 

· MG
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I have to go with Disco on this one.

Frank, you make the argument yourself: the force of the winch remains the same, the distance travelled by the rope is doubled, thus, the work is halved - simple mechanics. The winch "works" (expends the same amount of energy) in either case, but with the snatch block setup you describe, it does so over twice the "distance" - mechanical advantage of 2.

It's a "point of reference" issue, where the snatch block should actually be considered the "moving pulley" because the winch and the vehicle are the same (the force and the anchor point, respectively, are rigidly connected to each other).

Here's an example from winkipedia (http://en.wikipedia.org/wiki/Mechanical_advantage):

"A man sits on seat that hangs from a rope that is looped through a pulley attached to a roof rafter above. The man pulls down on the rope to lift himself and the seat. The pulley is considered a movable pulley and the man and the seat are considered as fixed points; MA = 2"
 

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Not quite.

The formula is F=1/n x W, Where F is the force (on the winch, in this case), n is the numer of cables, and W is the weight of the load.

If your vehicle weighs 4000 lbs, and you use a single pull (number of cables is 1), the force on the winch is 4000lbs (4000=1/1x4000)

If you use a pulley, attached to a fixed point, and terminate the free end back to the vehicle, you now have 2 cables, so the force now equals 2000, 2000=1/2x4000.

Taking the formula one step further, if you run the cable out to a pulley attached to a fixed point, back to a pulley attached to the vehicle, then terminated it at the fixed point where the pulley is attached, or any fixed point, the formula is now F=1/3 x W (1333.33= 1/3 x 4000)
 

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Discussion Starter · #6 ·
MG, Disco and TerryS, I am a fully qualified Class 1 ticketed rigger with over 20 years experience and a 12 month tech. course under my belt. You want the formula for determining Mechanical Advantage (MA) for pulley blocks, this is it. To realise a MA using ropes, wire cables, pulleys or blocks (Block and Takel (correct spelling) etc. a few basic unchangeable principles apply.
1. the MA is determined by THE NUMBER OF PARTS OF ROPE SUPPORTING THE "MOVING" BLOCK.
2. you must PULL TO ADVANTAGE.
Take the example put forward by MG
"A man sits on seat that hangs from a rope that is looped through a pulley attached to a roof rafter above. The man pulls down on the rope to lift himself and the seat. The pulley is considered a movable pulley and the man and the seat are considered as fixed points; MA = 2"
If the man sitting in the seat (Bosuns Chair) weighed 100kilograms (220lbs) MG says he has a MA of 2, that means that he would only require 50kilos (110lbs) + up tp 20% friction to lift himself off the ground, amazing, that little pulley has just defied the laws of physics. In the above situation, if you suspended 100-kilos on each rope the weights would balance each other out, do you agree with that, if you wished to raise one side you would have to apply enough pressure or weight to overcome the friction caused by the pulley. so you would require at least 100kilos + up to 20% or 20 kilos to move the opposite side. Look at it this way with your thinking, if the above system, as you say gives a MA of 2, then you would only require 50 kilos to lift 100 kilos, it don't add up. Grab your snatch block, hang it off the beams in your garage or tree, fill a bucket with sand or water and attach it to one side of the rope thru the pulley. Half fill the other bucket and attach it to the other side and come back and tell me which bucket drops to the ground. Still don't believe, put a loop in one side rope, stick your foot in the loop and just see how high you can lift your self, if you weigh a 100 kilos (like me) then you should be able to lift yourself up with only 50 kilos of downward effort, if your theory is correct it would mean that a person weighing 100 kilos could lift 200kilos on the other side of the pulley, Magic Pulley or what. In MG's proposal above the true weight to raise 100 kilos when pulling to DISADVANTAGE as described is another 100kilos + 20% friction which means that the rope or sling holding the pulley or snatch block will be supporting 100 kilos each side + 20 kilos friction = 100+100 +20 +220kilos. MG yousaid that "Frank, you make the argument yourself: the force of the winch remains the same, the distance travelled by the rope is doubled, thus, the work is halved - simple mechanics. The winch "works" (expends the same amount of energy) in either case, but with the snatch block setup you describe, it does so over twice the "distance" - mechanical advantage of 2". MG the rope is doubled over a straight pull and it takes twice as long to retrieve because it is twice as long as a straight pull. If you put a marker on your winch rope at 1 metre from the winch and wound it in through the pulley the hook end of the rope would move 1 metre, if there was a MA of 2 then the hook end would only move 1/2 a metre, which is physically impossible in this situation.
Regards Frank.
 

· MG
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I still think that you're approaching it from the wrong vantage point.

Your buckets of sand analysis is correct, but then, the "fixed block" is at the roof, thus it isn't an analogous situation.

As you say, tha MA is determined by the number of ropes pulling the "moving block" - in the situation where both the motive force and the attachment point are rigidly connected, the "stationary" block (the one that does not go anywhere in space-time) becomes the "moving block". If you like, think of the vehicle/winch/recovery point as stationary, and the tree (and the rest of the world) as the object to be moved - there are clearly two ropes going to the snatch block.

Again, all your analyses are correct (and I certainly can claim NO expertise in winching or block and takel operations), but you're not taking into consideration the "special condition" that arises when the weight to be moved and the motive force are co-located. It's the co-location that makes the snatch block the "moving pulley" - draw a free body diagram and you'll see.

Of course, neither Disco, TerryS nor I have really taken into account friction like you have. But let's say that it is in fact 20% - a 100 kg person pulling himself up 1 meter will expend only 70 Newtons (kg m/s2), while if someone else were pulling on the same system, they would expend 100 N.

The other thing we could do is setup a seat on a pulley and try it out. I'll bet you a beer that I can pull myself up more easily (but with twice the rope) than you can...
 

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Nice video, ok, so this all got me thinking.....
And damn I've got a civil engineering degree. Just never thought about applying that knowledge to getting a truck out of the mud, lol....

Anyways in actual fact I agree with FrankS. There is no mechanical advantage using a snatch block attached to a "fixed" point such as a tree.
Only if the pully was attached to the "movable" object would you get an advantage. Using the snatch block on the fixed point, you actually lose advantage because you add in the friction of the pulley usually assumed as 20%. The only advantage a fixed snatch block gives you is to allow you to change your winching direction such as shown in the 3-point recovery on that Ramsey winch video.

It's hard to describe, so I searched for a bit and came up with this website it has most of these scenerios, just picture the weight(W) as your truck and the wooden beam being the tree (as shown in the "Basic Pulley Physics section)
http://www.the-office.com/summerlift/pulleybasics.htm

That's my $0.02 and I also re-learned some physics that I haven't used in quite a while....
 

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Discussion Starter · #10 ·
MG, I don't understand how you can say the staitonary block should be viewed as the moving block, it doesn't compute. I will say it again and I am quoting from "The Riggers Guide" the Text book for Riggers doing the 12 month Rigging course to obtain a Class1 Rigger's ticket, recognised as the benchmark worldwide.
A MA is ascertained by "THE NUMBER OF PARTS OF ROPE SUPPORTING THE MOVING BLOCK" end quote. Emphasis on "MOVING" if it ain't moving it's not working.
If a block is Stationary, in the situation we are discussing then you are pulling to DISADVANTAGE, in other words if you are pulling a rope towards you and the load on the other end is moving away from you then you are pulling to disadvantage. MA can be gained in a number of ways, by gears for instance, a small gear turning a larger gear is a MA the ratio of that advantage is determined by the number of teeth on the gears, i.e. 10 teeth turning 40 teeth gives you a ratio of 4:1. in this case the small gear turns 4 revs and the big gear turns once, a MA. Now if both gears are the same size then one rotation of one gear results in one turn of the other, no MA. Try this simple test, put a piece of rope thru the snatch block, hold the ends of the rope in each hand, pull the rope back and forward, the rope will move evenly both sides, i.e. if you pull the rope 6" with your left hand, your right hand will move forward 6", 1 to 1, no MA. In your scenario one side of the rope (right hand) would move 3", PHYSICALLY IMPOSSIBLE, WHERE DOES THE MYSTICAL POWER IN THE SNATCH BLOCK COME FROM.
1st rover, I just looked at that Ramsay video, if you want to believe that crap, go ahead, that 3 Point recovery was a Doozy. In that stuation with the cable thru the snatch block and at over 90 degrees angle at the snatch block, those clowns doubled the load on each side of that snatch block. Here's how it works, if you have a 45 degree angle at the snatch block you impart the actual load to each side of the snatch block, at 90 degrees you double it. e.g. 2 Ton load @ 45 degrees becomes 2 ton TENSION on each side of the block, for a total of 4 Tons TENSION. @ 90 degrees the Tension increases to 4 tons each side of block. Seems the good 'ol boys from Ramsay forgot to tell you that you need an 8 ton cable to do what they did. Alright they were only probably only pulling 1/2 ton, but what happens to the poor [email protected]$tard that is well and truly stuck and is pulling 2 tons deadweight, your geniuses at Ramsey need there ar$es kicked for showing that garbage. You noticed I used the word TENSION to describe what was happening. When you use a setup like them 2 dopes did and make the angle between the winch and the fnial anchor point you create incredible tension in the cable, why? because the winch is trying to straighten the rope out, it is trying to pull the final anchor point into a straight line. Try this, put a housebrick in each hand and hold them by your side, quite easy, yes, now lift your arms out to the side to form a 45 degree angle between your arms with your head at the theoretical centre. Fairly hard to keep them out there, now move your arms out to 90 degrees and see how long you can keep them up. That strain on your arms at 90 degrees was hell of a lot harder than when they were at your sides. That sort of stupidity needs to be stopped and NO I dont believe anything your mates at Ramsey says, listen to someone who has been there and done that, Regards Frank.
 

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Discussion Starter · #11 ·
02RhinoRange, thank god I thought I must be the only person in the World that could see what was happening, check out my reply to 1st rover about Ramsey's dangerous situation in their 3 Point Recovery, thank you very much for that pulley web-site, much obliged, Regards Frank. PS 20% friction is a worst case scenario, e.g. ungreased pulley shaft with no bush or bearing, agood roller bearing, greased could be as low as 5%. I'm going to bed now, it's almost 5:AM here in OZ
 

· MG
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At the risk of boring the spectators and sounding a bit pigheaded (which I am, btw), I'm going to try to convince one more time - after that, I'll just agree to disagree until we can work it out in a boatswain's chair and a couple of beers.

Let's stick to the boatswain's chair, as it's the simplest analogy of the underlying discussion. I like your use of the word tension, it comes in handy.

Step 1: When we're talking about a 100 kg boatswain's mate (me) suspended from the ceiling, the rope is under 100 kg of tension. No friction, for now.

Step 2: No pulleys, yet. Let's suspend me by two ropes from the ceiling, like a swing - each rope is under tension at 50 kg.

Step 3: When we add a pulley and pass a single rope through (no motion, yet), as in the simplest boatswain's chair setup, each side of the rope is still under 50 kg of tension - just adding a pulley can't change the basic mechanics of a swing, right? Thus, if you agree with me on this one, the bitter end of the rope only requires 50 kg to hold me up.

Step 4: If I were to pass that same bitter end to someone on the ground, the tension would go up to 100 kg, because now I would be suspended by only one rope (as in Step 1). That's statics - still no motion. So far, I can't see where there would be disagreement. Everyone, drink beer if you agree with MG (I know, not fair).

Step 5: I think you smell what I'm stepping in by now. So I'm sitting in my boatswain's chair at a full 100 kg, excerting only 50 kg of tension on the bitter end of the rope. When I add 1 kg of tension to the loose end, in a friction free world, the pulley allows that tension to be transmitted throughout the rope, which means that the system can now carry 102 kg and I should begin to rise. MA = 2.

Try this: take a halloween basket and suspended from anywhere you want, looping a rope around a pipe, for instance (it'll help if the rope and suspensión point are pretty smooth). Put two oranges of similar weight into the basket. At this point, the basket should just hang there.

If you're right, duck taping a third orange to one side of the rope should do nothing - I would need a fourth orange (plus something for the friction) to make the basket rise.

The bootstrapping you try to compare to the chair isn't quite accurate because you don't have a pivot point (the pulley) to leverage force against (or through/around).

Oh, and I agree with the quote from your reference book - I insist that you need to see the snatch block as the "moving block". And my wikipedia cite supports my assertions - without any interpretation.
 

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And for that equation that popped up , either under this thread or the same thread in the "Off Road" catagory, you know the

F=1/n x W where F is the force, n is the number of cables/rope and W is the weight or load. Well although the n is the number of cables, you subtract the first cable, you only count cables that are past the first pulley.

So in the example with only one pulley attached to a fixed object n=2-1=1
so the equation would be F=1/1xw so F=W

Two pulleys give 3 cable spacings so n=3-1 so F=1/2 x W so F=1/2W that's where the mechanical advantage comes in.

Damn this is an indepth subject eh? I've been through 5 years of engineering and it still puzzles me a bit. But I studied more about buildings, concrete, steel, etc... not so much rigging...
But I'm pretty sure I've got it now, just took some review.
 

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Discussion Starter · #15 ·
02RhinoRange, before I went to Technical College to do my Riggers course, I had been working as a Rigger in the Construction Industry, mostly large Minesites, Whyalla SA, Gove NT, Mt.Isa and Mary Kathleen Qld and then in Shipbuilding and repairs. and I Thought I knew all there was to know about Rigging. I was shocked to know that for years as a Rigger there was a lot more to this game. Over 200 applicants applied to do the course at the TAFE College in Sydney University, Sydney Australia. The first day was a Maths exam which left less than a 100 applicants, about 70 appl. graduated after 12 months. There is so much to rigging that doesn't meet the eye, so many dangerous practices that cause maiming and death. Our Teacher was (is) a Dept. of Labour and Industrial Relations (DLIR) Inspector, his job was to investigate workplace accidents. One of the accidents he told us about was man who was killed by a falling load. Seems he was asked to believe that this poor man had been using a single pulley and rope to lift a load to the second floor when the rope securing the pulley to an overhead beam broke and the load fell and killed this man. When he arrived at the scene he was told that this man was alone pulling the load up to the second floor, trouble was the load weighed 500kg and the man was only 80kg, so the scenario he was given was false. It came out at the Coroners Inquest that there were 7 men pulling on the rope when the Snotter rope holding the pulley broke and fell on this unfortunate person, so it pays to KNOW what you are doing, remember that the rope or tree trunk protector that is connected to the snatch block needs to be equal to the Task. If you use a single pulley and anchor it to a tree, (why anyone would want to is beyond me), it needs to be strong enough to support the Tension on both sides of the pulley, especially if you use that ridiculous 3 Point Recovery shown in that Ramsey Video, Regards Frank.
 

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Discussion Starter · #16 ·
me.guevara said:
I have to go with Disco on this one.

Frank, you make the argument yourself: the force of the winch remains the same, the distance travelled by the rope is doubled, thus, the work is halved - simple mechanics. The winch "works" (expends the same amount of energy) in either case, but with the snatch block setup you describe, it does so over twice the "distance" - mechanical advantage of 2.

It's a "point of reference" issue, where the snatch block should actually be considered the "moving pulley" because the winch and the vehicle are the same (the force and the anchor point, respectively, are rigidly connected to each other).

Here's an example from winkipedia (http://en.wikipedia.org/wiki/Mechanical_advantage):

"A man sits on seat that hangs from a rope that is looped through a pulley attached to a roof rafter above. The man pulls down on the rope to lift himself and the seat. The pulley is considered a movable pulley and the man and the seat are considered as fixed points; MA = 2"
MG, this is the 1st 2 Paragraphs from your quoted web-site "wikipedia",
Mechanical advantage

Consider lifting a weight with rope and pulleys. A rope looped through a pulley attached to a fixed spot, e.g. a barn roof rafter, and attached to the weight is called a single fixed pulley. It has a MA = 1 (assuming frictionless bearings in the pulley), meaning no mechanical advantage (or disadvantage) however advantageous the change in direction may be.

A single movable pulley has a MA = 2 (assuming frictionless bearings in the pulley). Consider a pulley attached to a weight being lifted. A rope passes around it, with one end attached to a fixed point above, e.g. a barn roof rafter, and a pulling force is applied upward to the other end with the two lengths parallel. In this situation the distance the lifter must pull the rope becomes twice the distance the weight travels, allowing the force applied to be halved. Note: if an additional pulley is used to change the direction of the rope, e.g. the person doing the work wants to stand on the ground instead of on a rafter, the mechanical advantage is not increased.
MG, need I say more, Regards Frank.
 

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I cannot believe the depth of discussion that this has received.



http://www.technologystudent.com/gears1/pulley8.htm

Run winch line from vehicle to pulley attached to a tree, and back to vehicle. Afix the end of the winch line to the vehicle. Vehicle winch is now able to gain a 2x mechanical advantage (minus friction, etc) over the tree.

In other words, force required to pull 4000 pounds from the winch exerts a force equivalent to 8000 pounds on the tree, which places a corresponding pull force of 8000 pounds on the vehicle.

The point of reference is the anchor point (on the vehicle). The pulley, though affixed to a tree, moves with respect to the reference point.
 

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Discussion Starter · #18 ·
Pugsly said:
I cannot believe the depth of discussion that this has received.



http://www.technologystudent.com/gears1/pulley8.htm

Run winch line from vehicle to pulley attached to a tree, and back to vehicle. Afix the end of the winch line to the vehicle. Vehicle winch is now able to gain a 2x mechanical advantage (minus friction, etc) over the tree.

In other words, force required to pull 4000 pounds from the winch exerts a force equivalent to 8000 pounds on the tree, which places a corresponding pull force of 8000 pounds on the vehicle.

The point of reference is the anchor point (on the vehicle). The pulley, though affixed to a tree, moves with respect to the reference point.
Pugsly, in all of the graphics it shows the block moving, I will say it one last time IF THE BLOCK IS STATIONARY i.e. NOT MOVING, THEN THERE IS NO MECHANICAL ADVANTAGE, your web-site backs up what I am saying, Regards Frank.
 

· MG
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It would appear that we're destined to disagree on this point until we gather around a keg of beer and a boatswain's chair.

It's all good, though - that's what beer is for, right? :drink1:
 

· MG
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02RhinoRange said:
And for that equation that popped up , either under this thread or the same thread in the "Off Road" catagory, you know the

F=1/n x W where F is the force, n is the number of cables/rope and W is the weight or load. Well although the n is the number of cables, you subtract the first cable, you only count cables that are past the first pulley.

So in the example with only one pulley attached to a fixed object n=2-1=1
so the equation would be F=1/1xw so F=W

Two pulleys give 3 cable spacings so n=3-1 so F=1/2 x W so F=1/2W that's where the mechanical advantage comes in.

Damn this is an indepth subject eh? I've been through 5 years of engineering and it still puzzles me a bit. But I studied more about buildings, concrete, steel, etc... not so much rigging...
But I'm pretty sure I've got it now, just took some review.
So if there's only one rope, n=1-1=0, thus F=0xW=0 => effortless pull.
 
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