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Old 11-25-2018, 11:42 AM   #41
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If the line is 1000# tensile strength and the weight is 1500#, Superman here will put 750# of force on the line to lift the weight. If the weight is 2001#, then the 1000# tensile strength line will break because Superman will have to exert 1000# 8oz of force to lift the weight. If you can demonstrate that this is incorrect, then a perpetual motion machine is within your reach.
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Old 11-25-2018, 11:45 AM   #42
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Originally Posted by Solly View Post
Guys when I went to school 5000 lbs. was 5000 lbs.
If you have a single line that will break at exactly 5000 lbs. and you lift 5001 lbs. it will break.

No mater how many pulley's and doo-dads you put the line around.

But with a 2 to 1 pulley system it would only need 2500.5 lbs. of force to lift it. But it would still break because the weight is over the lines capacity.

The weight of the block never changes.
What's amazing is that this question has created 3 pages of responses. To be charitable, we should assume that those still scratching their heads over this conundrum are, as XS said, confusing tensile strength of the line in a tackle with the mechanical advantage the tackle bestows.
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Old 11-25-2018, 12:10 PM   #43
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Unfortunately, the original question was asked in a confused way. Of course a tackle does not change the tensile strength of a line, but the tackle will change the amount of weight that the line can lift, provided the tackle hardware is up to it. Read carefully, the OP asked if the 4 parts of the tackle increased the tensile strength of the tackle, not the line:

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According to one strength chart I checked, the tensile strength of this line is 5,000 lbs. There are 5 pulleys that control one pole, thus 4 loops from the crane (?) to the near top and mid point of the pole. Does this translate to 20,000 lbs tensile strength?
The answer is "yes".

So yes, you can tie an oil tanker to the wharf with 1/4" clothesline, theoretically, though it would be practically inconvenient. On large very highly loaded sailing rigs, lashings are increasingly used in standing rigging. These make use of multiple turns of light line whose tensile strength is well below the working load of the rigging, yet multiple turns of this line well exceed the working load.
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Old 11-25-2018, 12:23 PM   #44
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Originally Posted by DDW View Post
Unfortunately, the original question was asked in a confused way. Of course a tackle does not change the tensile strength of a line, but the tackle will change the amount of weight that the line can lift, provided the tackle hardware is up to it. Read carefully, the OP asked if the 4 parts of the tackle increased the tensile strength of the tackle, not the line:



The answer is "yes".

So yes, you can tie an oil tanker to the wharf with 1/4" clothesline, theoretically, though it would be practically inconvenient. On large very highly loaded sailing rigs, lashings are increasingly used in standing rigging. These make use of multiple turns of light line whose tensile strength is well below the working load of the rigging, yet multiple turns of this line well exceed the working load.
Setting aside that block and tackles don't have tensile strength but mechanical advantage based on the number of wraps, the answer is still "no". Tensile strength is the amount of force that can be applied before the line breaks and that doesn't change regardless of how many pulleys there are in a block and tackle. In the OP's situation, the line will still break if he applies more than 5000# of force, even if he is able to lift 5000# with only 1250# of force. Attach that arrangement to a weight of 20,000# and the line will break.

And no, you can't tie up an oil tanker with 1/4" clothes lines by wrapping it around a bunch of pulleys, unless you eliminate the hauling part of the block and tackle. If you do, you no longer have a block and tackle as the OP describes, but a set of loops of line that will have a tensile strength equal to the number of loops times the tensile strength of one line. A totally different proposition from the arrangement for passive fins. There, a block and tackle is employed to reduce the amount of effort you need to pull the dolphin out of the water. That mechanical advantage has no impact on the tensile strength of the line.
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Old 11-25-2018, 12:32 PM   #45
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Last post and I'm done
Nobody here claimed anything... pulley, block or a magic hat can change the properties of any line utilized so we are in agreement there.
What several have tried to point out is that a line can support or lift many times its breaking strength by employing a block (multiple pulleys and wraps).
I'm not sure if you understand or are arguing that point... but I'm done.
I do hope the OP got his question answered.

Take the pulley Test
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Old 11-25-2018, 12:36 PM   #46
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Originally Posted by Delfin View Post
Setting aside that block and tackles don't have tensile strength but mechanical advantage based on the number of wraps, the answer is still "no".
This seems to be the sticking point in the argument. Why can't a block and tackle have a tensile strength?
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Old 11-25-2018, 02:39 PM   #47
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Originally Posted by Delfin View Post
Setting aside that block and tackles don't have tensile strength .....

And no, you can't tie up an oil tanker with 1/4" clothes lines by wrapping it around a bunch of pulleys, unless you eliminate the hauling part of the block and tackle. If you do, you no longer have a block and tackle as the OP describes, but a set of loops of line that will have a tensile strength equal to the number of loops times the tensile strength of one line.
Well we are just arguing semantics here but:

Everything has a tensile strength, including a tackle. Tensile strength is simply the load that can be applied to each end of an object without tensile failure.

A block and tackle with the running end tied off, is just a set of loops of line. And it can be used to tie up a tanker. It doesn't matter if the running end is tied off, or being held still by someone, it's still some loops of line and behaves exactly the same.

If you define a tackle narrowly as an arrangement of blocks and line in motion, that becomes a set of loops (and no longer a tackle) as soon as the running end is held still and motion stops, then what you say is true - but most would not agree with that definition. A tackle is still a tackle, even when tied off.
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Old 11-25-2018, 02:55 PM   #48
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Wow, anyone with a better link or real qualifications want to weigh in here?

I have my opinion, but there are some smart people here who have me doubting my many years of practical experience of dealing with blocks and tackle.
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Old 11-25-2018, 04:21 PM   #49
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Originally Posted by diver dave View Post
This seems to be the sticking point in the argument. Why can't a block and tackle have a tensile strength?
The line certainly has tensile strength. And if a block and tackle has a tensile strength it would only be because the line has one. The question was whether that strength of the line changes in a block and tackle and the answer to that is of course not, even if you can lift with less effort.

I think the bigger area of confusion is that some aren't considering that for any arrangement to be a "block and tackle", there is a hoist line, which is a single strand of whatever you're using. The tensile strength of that is the tensile strength of the whole system. For the OP, that is 5,000#, the same it would be without a block and tackle. Further, even if the load on the line in lifting a given weight decreases in a block and tackle, the tensile strength of the line certainly doesn't change. Now, if you do away with the live hoist end, and just double up the line, then there is an increase in tensile strength, but then you don't have a block and tackle, which is what the OP describes and what his question pertained to.
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Old 11-25-2018, 04:26 PM   #50
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Well we are just arguing semantics here but:

Everything has a tensile strength, including a tackle. Tensile strength is simply the load that can be applied to each end of an object without tensile failure.

A block and tackle with the running end tied off, is just a set of loops of line. And it can be used to tie up a tanker. It doesn't matter if the running end is tied off, or being held still by someone, it's still some loops of line and behaves exactly the same.

If you define a tackle narrowly as an arrangement of blocks and line in motion, that becomes a set of loops (and no longer a tackle) as soon as the running end is held still and motion stops, then what you say is true - but most would not agree with that definition. A tackle is still a tackle, even when tied off.
Just to make sure we aren't talking past each other, it sounds like what you are saying is that if I have a 2 wrap block and tackle with a 1,000# tensile strength line, that tackle now has a tensile strength of 2000# and can lift 2000# without breaking as long as the hoist is tied off, say, to a cleat, or held by someone?
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Old 11-25-2018, 04:37 PM   #51
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The fallacy in most of this thread is to suggest exceeding the breaking strength of the line used. For argument, give yourself a line that has enough strength so that even if used in a 4 part tackle, your fall has the strength needed to lift the load in a single pull. Then you only need to count the parts that give you mechanical advantage to know the total weight the tackle can lift. At the end of the fall, however, you are using a single part, so that one section needs to be strong enough for the full weight.
For example, if you want to lift 2000 lbs and use a 4 part tackle, you need to use a line with a breaking strength of more than 2000 lbs, but your winch can be turned with the same force as that needed to lift 500 lbs on a single part line, it just has to turn 4 times as many turns, thus lifting 2000 lbs.
The line will still register the full 2000 lb weight.
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Old 11-25-2018, 05:29 PM   #52
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For example, if you want to lift 2000 lbs and use a 4 part tackle, you need to use a line with a breaking strength of more than 2000 lbs, but your winch can be turned with the same force as that needed to lift 500 lbs on a single part line, it just has to turn 4 times as many turns, thus lifting 2000 lbs.
The line will still register the full 2000 lb weight.
True for a winch with a reduction gear, definitely not true for a block and tackle. If you have 4 lines headed for the bottom block, the load is shared by all. And, in every line segment, including the free end, the tension is the same and is 1/4 of the load supported.
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Old 11-25-2018, 05:38 PM   #53
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https://www.slideshare.net/superegof...e-body-diagram

this will save you from attending the first 3 weeks of statics class.
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Old 11-25-2018, 07:23 PM   #54
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Funny, I keep reading technical explanations that say that the line does NOT have to have the safe working load as the weight being lifted, but actually some reduced value involving other calculations other than mechanical advantage ones.

My suggestion is go to some "rigging" engineering websites if anyone is still undecided.
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Old 11-25-2018, 07:25 PM   #55
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Originally Posted by Delfin View Post
If the line is 1000# tensile strength and the weight is 1500#, Superman here will put 750# of force on the line to lift the weight. If the weight is 2001#, then the 1000# tensile strength line will break because Superman will have to exert 1000# 8oz of force to lift the weight. If you can demonstrate that this is incorrect, then a perpetual motion machine is within your reach.
Just for fun. The hook above the load is seeing 1500 pounds, the load lifted. The attachment point above the upper pulley is seeing 2250 pounds of force.
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Old 11-25-2018, 08:00 PM   #56
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Depends on the pull angle. At about 45 deg roof pull is less than 2200. Neglecting weight of pulleys and line, just under 2100 lbs. If the pull is made straight down, then it is 2250. Straight to the side, then the roof attach will see a shear load of 750 and a down force of 1500.
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Old 11-25-2018, 08:29 PM   #57
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Just to make sure we aren't talking past each other, it sounds like what you are saying is that if I have a 2 wrap block and tackle with a 1,000# tensile strength line, that tackle now has a tensile strength of 2000# and can lift 2000# without breaking as long as the hoist is tied off, say, to a cleat, or held by someone?
Not sure what you mean by a "2 wrap" tackle, but I'll assume a 2 part or 2:1:

The tackle has a tensile strength of 2000 lbs in your example, whether the running end is tied off or held or moving. Can it lift safely 2000 lbs? well, no, that is it's breaking point. It can lift 1999 lbs, and perhaps 500 lbs safely (with a 4:1 margin).

Let's be clear: a tackle has a running line rove through it, blocks at each end (and perhaps in the middle in a compound tackle), and additional attachments at each end fixing it to the anchoring point and load, these additional attachments might also be lines, but they are not the running lines of the tackle and not what we are talking about. Each of these has a tensile strength when considered individually. Let us assume that the blocks and additional attachments are strong enough that the running line will be the first thing to break.

In a simple 4:1 tackle, there are four sections of line that get shorter when I pull the running end. Any load applied to the tackle blocks will be shared by all 4 parts, and the tension in the line will be 1/4 of that load. A 100 lb pull applied to that line will put 100 lbs of tension into the line, in all parts of it, and 400 lbs pull on the each block. A line of 101 lbs breaking strength is sufficient to do this, with no margin for safety. You do not need a 400 lb strength line, the line only sees 100 lbs tension, anywhere in it.

If there is an additional, separate line involved attaching the block to the load or fixed end, then it must have a breaking strength of higher than 400 lbs. But that is a separate piece of line, not part of the running line of the tackle.

I am an engineer, I do these calculations all the time. I also sell commercially a piece of sail handling gear that depends on them, to the world's finest sailboats. This isn't rocket science, but it IS science and there is a right answer.
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Old 11-25-2018, 09:10 PM   #58
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Not sure what you mean by a "2 wrap" tackle, but I'll assume a 2 part or 2:1:

The tackle has a tensile strength of 2000 lbs in your example, whether the running end is tied off or held or moving. Can it lift safely 2000 lbs? well, no, that is it's breaking point. It can lift 1999 lbs, and perhaps 500 lbs safely (with a 4:1 margin).

Let's be clear: a tackle has a running line rove through it, blocks at each end (and perhaps in the middle in a compound tackle), and additional attachments at each end fixing it to the anchoring point and load, these additional attachments might also be lines, but they are not the running lines of the tackle and not what we are talking about. Each of these has a tensile strength when considered individually. Let us assume that the blocks and additional attachments are strong enough that the running line will be the first thing to break.

In a simple 4:1 tackle, there are four sections of line that get shorter when I pull the running end. Any load applied to the tackle blocks will be shared by all 4 parts, and the tension in the line will be 1/4 of that load. A 100 lb pull applied to that line will put 100 lbs of tension into the line, in all parts of it, and 400 lbs pull on the each block. A line of 101 lbs breaking strength is sufficient to do this, with no margin for safety. You do not need a 400 lb strength line, the line only sees 100 lbs tension, anywhere in it.

If there is an additional, separate line involved attaching the block to the load or fixed end, then it must have a breaking strength of higher than 400 lbs. But that is a separate piece of line, not part of the running line of the tackle.

I am an engineer, I do these calculations all the time. I also sell commercially a piece of sail handling gear that depends on them, to the world's finest sailboats. This isn't rocket science, but it IS science and there is a right answer.
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Old 11-25-2018, 09:28 PM   #59
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Whew! I'm glad that got cleared up. Physics 101 can get overcomplicated sometimes.
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Old 11-25-2018, 10:03 PM   #60
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I just can't quit this thread. I hope everyone ends up on the same (correct) page in the end! Reminds me of the Monty Hall Problem.
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