Loop strength question

[Fumbler]

So...I'm no physicist, mathematician, or engineer but I'm curious.

Lets say:
You make a loop in a rope with a double fisherman's knot or in webbing by sewing the ends together. The original strength of the rope or webbing is 25kN.
If the double fisherman's knot or stitching do not cause any weak spots in the material, will the loops have a strength of 25kN or will it be more? (assuming the loops are not loaded by tying a girth hitch or anything like that)

 

[Ekka]

You double it.

Think about rope, loops, slings and any other similar things like this.

Count the number of parts (side/lengths/strip etc) of rope going to the tied item. That's roughly how many times stronger it is.

You get a piece of webbing and tie it to a limb. How many parts going to the limb? 1

Now you have made a miracle loop which is sewn with magical thread and has no loss in strength. You tie a limb up. How many parts going to the limb? 2

Now you got your miracle loop and actually twisted it and made a double loop. You tie a limb up. How many parts going to the limb? 4 so that means it's 4x stronger than the one part of webbing.

See, easy.

Now with webbing, and coincidently I bought 30' of the stuff today, you just make loops by tying beer knots. strength loss is only 20% because of the knot.

Now 1" webbing is 4000lb breaking.

You double it as it's now a loop ... 8000lb breaking minus the 20% strength loss = 6400lb breaking.

However, if you learn to use other ropes and splice your strength loss will only be 10%

 

[TheTreeSpyder]

Make sure that your knot/splice is on straight part of rig; not at bent part of choke, connecting krab etc. Part of the webbing's strategy is that it is flexible and lies flat on bend. Leveraging on bend is resistance to bend X bent dimension/axis on the bend. Webbing usually minimizes both multipliers; it flexes easy and lays flat/ has about Zer0 dimension/ axis on the bend. But, knotted/spliced that area becomes stiffer; and stands taller on the bend. So, this 'seam' takes leverage from flexible X ~Zer0 of web to stiff X increased height on bend.

Samson told me years ago that in their tests/ maintaining a conservative rating philosophy; they find/calculate a 100% strength 'seam'/knot/splice to make loop as 165% of original not 200%. We can argue this etc.; but in conservative terms; by their professional standard, this would be perhaps the route to take.

2" Seat belt webbing is ~6k; and the 1" loop webbing for 'loop runners' is 4k as Ekka says. 4k X 1.65 =6600 with fancy stitching stronger than webbing it self(?); and this is the number range i think i've seen quoted on the 1" loops in linear rating. i'd kinda give a knot in webbing less confidance than that personally.

Choke is 80% of linear; Basket is 200% of linear (which Samson claims is a convention and not exact science). Mathematically this makes a basket 2.5x stronger than a choke. That is a 'loose choke'; not one cinched down so tight as to not form teepee; like when you barely have enough sling and force it down to choke and krab is about to pop out. Then, with flat teepee we have highly leveraged the choke. That 80% is also for a fairly straight choke, where the Standing Part is bent only slightly by it's on restriction; not where the Standing Part is bent in a U; placing 2:1 on it's own restriction.


Orrrrrrrrrrrrr something like that!

 

[John Paul Sanborn]

Then there is the differeance between MBS, ABS and WLL.

The MBS and ABS are minimum and average tensil from lab tests.

WLL is your fudge factor for a work environment. IMO this should be taken from MBS, because ABS can be a great deal higher.

Though some say a factor (divisor) of 10 for WLL, this should slide depending on the shockload factor. Since i don't slam anything onto loop slings I use MBS:4=WLL.

Then there are so many other factors to look at, such as wear, damage, cyles to failure.

I've talked to several people who have done break tests of cordage, and it seems that the bend will allways be the weaker point compared cut yarn, boogers, picks and whatnot.

 

[Fumbler]

Thank you all for the responses.

Just to make sure I get this straight:
If I take webbing rated at 4000 lbs, magically sewed the ends together with no strength loss, clipped two carabiners through the loop and added a load, then the theoretical breaking strength is now 8000 lbs?

I just boggles my mind that doubling the parts doubles the strength when only one part actually goes around each biner. I understand that force is a vector, but I didn't know it would actually almost double the strength.

 

[TheTreeSpyder]

i believe we are talking about 1" loop runners on page 8 of CMI Catalog. Notice how this rating barely makes a choke in the sling legal for our climbing requiremeants.

You can find other ratings for same from 6-6.6k; but i personally think 8k is a lil'higher than i've seen reputedly quoted.

 

[ASEMASTER]

tying beer knots.?

What is and how do you tie a beer knot?

 

[Blinky]

What is and how do you tie a beer knot?

It's a waterknot but instead of backtracing the initial overhand knot with the other end, you slip one end of the webbing inside the tube of the other and backtrace. Webbing has to be the tubular nylon variety. Cute name, eh?

 

[ray benson]

Beer Knot
http://en.wikipedia.org/wiki/Beer_knot

 

[Kneejerk Bombas]

I just boggles my mind that doubling the parts doubles the strength when only one part actually goes around each biner. I understand that force is a vector, but I didn't know it would actually almost double the strength.

Think of it like this, you have a rope rated at 100 pounds. To pull 100 pounds, you need to have at least 100 pounds of resistance on the other end, or the rope would just move and you would not get the load.

Now, move the loads around a radius and change directions. The same rope is now able to hold 200 pounds Just like it always could.

It's so simple it boggles the mind.

 

[elmnut]

Knots reduce the strenght of rope. 100 lbs on each end of a rope puts 200 lbs of force on the object the rope is over. Am I missing something?

 

[TheTreeSpyder]

Place rope in overhead pulley support; anchor 1 end to ground with hitch; hang 100#load on other end. Anchor on ground pulls back 100# in response; this response pull plus the initiating pull of 100# places 200# on the bend of rope thru pulley. Without the anchor pull of 100#, the load falls; it must have it's equal/ opposite or travels (falls) until it finds it(in ground pushing back up against 100# with 100# force)! Everything with equal/opposite is satiated/in balance doesn't move. Motion is the imbalance of not enough equal /opposite; if not enough equal/opposite immediately; the amount of equal/opposite that is lacking is the amount that load moves/makes up to complete the necessary balance. Even E=MC squared; has the necessary equals sign at center; to show this balance must exist all ways.

The more friction at pulley, or line over branch instead of pulley; reduces the pull on ground anchor; that reduces the pull on bend through pulley friction or over branch instead to 100#load + the reduced pull(after friction) on ground anchor. This reduces load at bend/pulley/support for hold and lowering. But, untie anchor; and lift 100# on other end, now you fight friction and so you have 100#Load + 100#of your pull + amount of your pull to overcome friction all pulling on support/bend/pulley position.

We can contrive a situation where the 100#load pulls on one end, and the anchor tie off at other end of line pulls 100#; but the pulley/bend/support has less than 200#; by altering angles; by the anchor tie off position not being right under the pulley. The full 200# is not incurred unless Zer0 degrees of bend between lines/they are inline to pulley. This vector angle can also change the combined direction of the combined pulls of load and anchor if the directions are not balanced of load and anchor equally and opposite.

orrrrrrrrrrrr something like that:taped:

 

[Fumbler]

Think of it like this, you have a rope rated at 100 pounds. To pull 100 pounds, you need to have at least 100 pounds of resistance on the other end, or the rope would just move and you would not get the load.

Now, move the loads around a radius and change directions. The same rope is now able to hold 200 pounds Just like it always could.

It's so simple it boggles the mind.

That's a great explination

 

[Ekka]

Here's some more configurations.

Can you relate these to say a climber going up a tree? Notice how your weight will load the above branch between 100% to 200%.

 

[Fumbler]

Okay, if the single line's SWL is 50, then this system has a SWL of 100 lbs right?

What is the tension on the line right at the top of the radius?

 

[Ekka]

I found this too, might help.

 

[Ekka]

Okay, if the single line's SWL is 50, then this system has a SWL of 100 lbs right?

No! Because where the two ropes join is a single rope with only 50 SWL ... think of this as an eye sling.

What is the tension on the line right at the top of the radius?

50 lbs. But you are gonna say hey ... there's 50 pulling left and 50 pulling right so the total where they meet is 100. Sort of logical but wrong! Remember the tension in the rope is totally different to the combined force at the pulley.

I know how you are thinking, a 4000 lb piece of tape becomes 8000 lb but there's only one part over the biner.

I found this site that explains it well and verifies what I'm saying.

Another example of a state of equilibrium is the game of "tug-of-war." A rope is pulled taught between two teams; each pulls with a force that equals the force of the oppostie team. Assume in the figure that each team is pulling with a force of 220 pounds. As long as each team maintained a pull of 220 pounds the system is in equilibrium. If during this time a device would be inserted between the two teams to measure the magnitude of the tension force that the rope has anywhere along its length, it would read 220 pounds at each and every point. This would be true at ANY point along the rope.

Source:

http://www.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture6/461_lecture6.html

Hmmm, still not convinced hey. Gee, now I gotta try and explain in my words which is gonna be long and wierd.

Lets go back to the 4000 lb piece of tape. You tie a knot to the biner on the end of a single piece. What you have actually done in a strange and "twilight zone" sort of way is secured the tape with a 4000 lb opposite force. Imagine the knot is actually a pair of very strong fingers pulling the tape the other way to stop it coming off, some of those fingers also grab the biner.

Stay with me, wierd but will make sense. Now with the loop that goes over the biner (the magically sewn loop) instead of a knot or those magic fingers we have a 4000lb piece of tape on the left side pulling against the 4000 lb tape against the right side ... both can handle the 4000 lb. It's simply equal and opposite forces instead of a knot.

But hanging on the bottom of that loop is an 8000lb weight, however due to 2 parts that's 4000lb per side.

I hope that helped!

 

[Fumbler]

That helps
Weird stuff, but I understand now.

 

[Ekka]

Yeah, now you can see what happened to Spydie coz he loves this **** and then throw a few pulleys and redirects in and you got a serious headache! :hmm3grin2orange:

I just remember the rule not how it's derived. The ole 1 part, 2 part, 4 part rule etc.

Great thread, plenty of views, many learning but shy to ask or answer.:biggrinbounce2:

 

[TheTreeSpyder]

i think SWL should stand for Safe Working Load; a ratio of 4:1 or 10:1 etc. of the tensile strength to load. Like a 10k tensile strength line would have a 4k SWL rating. i'm not sure if i understand how that acronym is being used'ere.

Notice how the last drawing kinda resembles a DdRT climbing system most of us use; except the weight/ body would be not at the bottom, but where the 2 lines join (and bottom of line is slack). So, when 100# climber pulls 1 of the 50# lines to him/her; they have a 2:1 over themselves on their own line! The climber at that Y intersection will pull 1 50# line; and lift their 100# carcass.

Don't take this as weird stuff; but notice/ embrace it/ walk with it as all around you in all things; that you just now see differently/ more clearly.

A small point; the Upside down U's of 100# on each end, and 200# at top; should be straight down pulls; not deflected to the side. The slight side pulls to left and right fight each other; and take away slightly(at this angle) from the total 100# each force from purely pulling straight down to add up to the 200#. Every degree angle makes a difference, thus the scrutiny of alignmeant in mechanics.