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Old 11-20-2018, 04:59 PM   #1
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Line Strength

I just replaced the old lines that control the paravanes. New line is 3/8” double braided polyester, roughly 75’ in length per pole. 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?
If you look at the top right of the attached photo you can see the pulleys.
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Old 11-20-2018, 05:22 PM   #2
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Photos of stabilizer.
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Old 11-20-2018, 05:25 PM   #3
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Pretty sure the mechanical advantage imparted by using pulleys doesn't change the tensile strength of the line. That is determined by the type, braid, etc of the actual line itself.
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Old 11-20-2018, 06:25 PM   #4
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Pretty sure the mechanical advantage imparted by using pulleys doesn't change the tensile strength of the line. That is determined by the type, braid, etc of the actual line itself.
I agree with that. Four lines working together can lift a heavier weight than one line, yes? So no tensile strength for one line but four together increases tensile strength for the bundle?
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Old 11-20-2018, 06:31 PM   #5
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Not quite. Strengh remains the same, but the force in the line is reduced by the pulleys.
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Old 11-20-2018, 07:00 PM   #6
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The strength of the line does not change, just with the mechanical advantage you don’t have to lift as hard to raise a given weight.
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Old 11-20-2018, 07:50 PM   #7
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That looks to me like a 3-1 system, and the bend radius will reduce the load a bit. I would guess the breaking strength at 10-12k lbs. However, I would guess the weakest link in that system is the pulleys, not the rope. I bet if you tested it to failure one of the pulleys would fail.
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Old 11-20-2018, 07:52 PM   #8
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Thank y’all for humoring my curiosity. Let me go at this a different way. Leave out the pulleys and mechanical advantage. Tie four of the 3/8ths line to a block of granite, with a lifting eye, weighing 7000 lbs. Each line has a 5,000 lb tensile strength. Will they lift the 7K block or snap? This is a straight lift, no mechanical help. One line alone would snap, but four?
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Old 11-20-2018, 07:54 PM   #9
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That looks to me like a 3-1 system, and the bend radius will reduce the load a bit. I would guess the breaking strength at 10-12k lbs. However, I would guess the weakest link in that system is the pulleys, not the rope. I bet if you tested it to failure one of the pulleys would fail.
Yeah, I would agree with that point. The pulleys are top notch equipment but also 25 years old.
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Old 11-20-2018, 08:01 PM   #10
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Quote:
Originally Posted by Gulf Comanche View Post
Thank y’all for humoring my curiosity. Let me go at this a different way. Leave out the pulleys and mechanical advantage. Tie four of the 3/8ths line to a block of granite, with a lifting eye, weighing 7000 lbs. Each line has a 5,000 lb tensile strength. Will they lift the 7K block or snap? This is a straight lift, no mechanical help. One line alone would snap, but four?

They will lift the block. Even 2 lines would probably lift 7k.
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Old 11-20-2018, 08:04 PM   #11
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Quote:
Originally Posted by Gulf Comanche View Post
Thank y’all for humoring my curiosity. Let me go at this a different way. Leave out the pulleys and mechanical advantage. Tie four of the 3/8ths line to a block of granite, with a lifting eye, weighing 7000 lbs. Each line has a 5,000 lb tensile strength. Will they lift the 7K block or snap? This is a straight lift, no mechanical help. One line alone would snap, but four?

You still have a single part (line) at the final attachment to the load. Mechanical advantage of the block and tackle does not matter here. The final part has a tensile strength of 5,000 lbs. The load is 7,000 lbs.
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Old 11-20-2018, 08:10 PM   #12
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You still have a single part (line) at the final attachment to the load. Mechanical advantage of the block and tackle does not matter here. The final part has a tensile strength of 5,000 lbs. The load is 7,000 lbs.
I think his proposal is to tie all 5 knots to a padeye of infinite strength. In this case, there is no single part line, but all lines in the system are attached independently to the load. The knot could weaken things, but in the spirit of the question, I think we can assume the knot does not weaken the line.

In this case, 2 lines, each with a 5000 lb breaking strength, could lift a 9999 load.
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Old 11-20-2018, 08:13 PM   #13
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Quote:
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I think his proposal is to tie all 5 knots to a padeye of infinite strength. In this case, there is no single part line, but all lines in the system are attached independently to the load. The knot could weaken things, but in the spirit of the question, I think we can assume the knot does not weaken the line.

In this case, 2 lines, each with a 5000 lb breaking strength, could lift a 9999 load.

Post #2 clearly slows blocks and the lines forming a block and tackle arrangement.
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Old 11-20-2018, 08:14 PM   #14
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The bigger issues are the forces on the fore and mast stays that connect to the tops of the poles and also those on the chain/cable to the paravanes. The forces on the rigging that raises and lowers the poles is minor in comparison.

Jim
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Old 11-20-2018, 08:22 PM   #15
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Go to an engineering web site.

I think block and tackle do allow a line to lift more than it's normal working load, but what percentage...???? I would have to look it up.
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Old 11-20-2018, 08:32 PM   #16
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Quote:
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Post #2 clearly slows blocks and the lines forming a block and tackle arrangement.
I think you need to be looking at Post #8, not post #2 if you want to answer OPs question.
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Old 11-20-2018, 08:35 PM   #17
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If I understand the setup correctly, in a perfect frictionless system, you could pick up 20,000 pounds without exceeding 5000 pounds load on the rope.

In the real world you want to design your system using “Safe Working Loads” not Tensile Strength or Breaking Load. Don’t forget the pulleys and attachments need to be strong enough as well.
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Old 11-20-2018, 08:43 PM   #18
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If I understand the setup correctly, in a perfect frictionless system, you could pick up 20,000 pounds without exceeding 5000 pounds load on the rope.
I still think the system is 3-1, so in a perfect system you could pick up 15k.
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Old 11-20-2018, 09:09 PM   #19
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There are four lines going to the moving part (the boom). All four get shorter when you pull on the tail. Therefore it is a 4 part system, if 5000 lbs pull is put on the tail the boom will pull 20,000 in an ideal world. In this world, figure 20% loss for friction, plus those blocks will let go at probably something less than 5000 lbs and certainly less than 20,000.
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Old 11-20-2018, 09:32 PM   #20
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Also keep in mind that rope has a working maximum that if exceeded will damage the rope. I don't know the numbers for polyester but for nylon it's about 20% of breaking strength.

Ken
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