Knot failed, lightbulb clicked on ! attaching ropes to trees........

[preventec47]

I want to verify my thinking. I tied a pull rope 30 feet up in the air around a tree using a standard bowline. While winching hard, the rope broke right in the bowline knot.
After thinking about it for a long while, I realised that if I was using a running bowline with the knot being inside of the loop around the tree,
that the stress on the knot is reduced by 50 percent. I say this because I think that when the rope reaches the tree and forks right and left to go around the tree in
two directrions, I figure the strain on the loop is reduced by half. The takeaway being I should always use a running loop of some kind with the loop knot
in the part wrapped around the tree. Am I analyzing this correctly ?

 

[buzz sawyer]

Sure seems like it would spread out the force. I've always used a running bowline, mostly because you can tie it from the ground.

 

[Del_]

You are running your ropes to hard.

Don't exceed 10% of minimum breaking strength for bull ropes.

A bowline reduces rope strength 50%.

And you must have been at about 50%.

Cycles to failure = 1.

 

[TheJollyLogger]

Something is not computing. From the trees you have described, it shouldn't take nearly that kind of force to pull them over. Was it notched and backcut when the rope broke, or were you pretensioning them.

 

[ATH]

I haven't been following along...what rope were you using? Sounds like more of a rope problem than a knot problem... Yes, knots weaken rope, but like @Del_ said, you need to know the WLL of the rope and stay within that.

 

[TheJollyLogger]

I haven't been following along...what rope were you using? Sounds like more of a rope problem than a knot problem... Yes, knots weaken rope, but like @Del_ said, you need to know the WLL of the rope and stay within that.

It's 5/8 poly 3 strand, I think. Should be around 8k break strength, 800 wll. But the trees he is pulling over are just 10-15" poplars, as I recall. I worry he is pretensioning them too much, could end up with a barber chair situation.

 

[benjo75]

An 800lb working load on a rope could be easily achieved by a 3 to 1 and one good strong man.

 

[benjo75]

Then if you factor in 50 percent decrease in strength due to the knot you're down to 400lbs. Figure in a little age, wear or uv damage you might just break it by hand.

 

[lone wolf]

I want to verify my thinking. I tied a pull rope 30 feet up in the air around a tree using a standard bowline. While winching hard, the rope broke right in the bowline knot.
After thinking about it for a long while, I realised that if I was using a running bowline with the knot being inside of the loop around the tree,
that the stress on the knot is reduced by 50 percent. I say this because I think that when the rope reaches the tree and forks right and left to go around the tree in
two directrions, I figure the strain on the loop is reduced by half. The takeaway being I should always use a running loop of some kind with the loop knot
in the part wrapped around the tree. Am I analyzing this correctly ?

Was this when chunking down sections or pulling a whole tree with a truck or Skid Steer? What size rope 1/2?

 

[EchoRomeoCharlie]

The stress on the knot in a running bow are reduced significantly due to friction of the tree on the rope as well.

A running bow around a tree can handle significantly greater pulling force (as measured on the line going from the tree to the pulling device) than a bowline tied with the tree in the middle of the bowline loop. Yes.

Also yes, bowline is not a great knot as far as rope strength is concerned.

 

[windblown]

Yes, a running bowline should distribute some of the forces involved off of the knot. The exact percentage would be tough to calculate, lots of variables involved.

Breaking ropes unexpectedly is cause for a serious stop and re-think on a lot of levels. I've only broken a heavy working rope once. I had a very heavy pull I wanted to try but didn't want to risk overloading a good rope. I had an aging static bull rope that I had retired to general "farm" duty so used it since it was a situation were a failure carried no risk. It failed, my curiosity was satisfied.

Either your rope was already toast or you seriously miscalculated the load you were putting on it. If the later you might want to consider picking up a load cell and playing with one if your pulls will be such that a rigging failure could be dangerous.

 

[Del_]

Then if you factor in 50 percent decrease in strength due to the knot you're down to 400lbs. Figure in a little age, wear or uv damage you might just break it by hand.

I believe the 10% safe working load takes into consideration rope strength lost to knots.

So I'd say we are still at 800 lbs safe working load.

It is surely a point to be debated and it's good to work those numbers around in one's head.

 

[rabbit box]

Remember the diameter of the cable or rope and the length of the cable or rope when the belly of the line is just tightened can put 40 to 600 lbs. pull on the line without pulling (talk about a run on sentence and a play on words, please forgive).