SRT load question

bstewert

If in a basic SRT setup, a 100 load on a rope trunk tied over a single branch exerts 200 lbs on the branch, what happens when the rope now goes over two branches horizontal to each other?

Gunslinger

The load should be shared by the two branches. Load division would depend on the angle of the rope over each branch and the amount of friction at the live end over that branch.

bstewert

Assuming all matching angles, would you get your 100 lbs back, compared to tying directly to one branch?

Also, what kind of bird is that in your avatar?
Thanks!

Eric Frei

Oh, tricky question.

If we were to swap out those branches with real efficient pulleys what would the equation look like?

I'm more a visual person, I need to see things.

It would look like this .... and I wont give the answer yet however I can say you wont get rid of #100 that easy.

Attached Thumbnails

 

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bstewert

Your drawing is exactly my question. Sounds like you are saying a moving climber would exert more load on the branch than if he were just hanging there. But not factoring in that, just looking at the static load, would three branches get the 100# back?

Gunslinger

My avatar is a photo of an immature bald eagle, their heads turn white at about 5 years old. My wife snapped that photo one day when it was flying over our boat on a lake in New Jersey.

treejames64

It seems like the loads on each branch would still be 200 lbs however the angle of pull would be more on a 45 degree instead of straight down.

Eric Frei

Your answer lies in this thread, do read post 30

Make the calcs and you'll soon be the master rigger.

Doubling effect at pulley block

It is all about THE ANGLE OF THE DANGLE!

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Gunslinger

You really want us to attempt the math don't you?

moray

Ekka has gone about this the right way by putting in perfect pulleys. This means the tension on the rope is everywhere the same.

Analyzing the situation for pulley A, there is a rope leg pulling straight down with 100 lb. and a rope leg pulling horizontally to the right with 100 lb. You can't just add these up and get 200 lb.--you have to account for the 90-degree angle between the two ropes. This is done by vector addition, and the result is about 141 lbs. The pulley feels this 141-lb. load pulling downward to the right at a 45-degree angle.

These problems are easy to do with a diagram. Start at the pulley and draw an arrow pointing to the right that is 10 cm long. That represents the 100 lbs. pulling to the right. At the tip of the arrowhead, draw an identical arrow pointing straight down. This represents the downward 100 lbs. Now draw an arrow from the pulley to the second arrowhead. That arrow (vector) represents the force the pulley feels. Its direction represents the direction of the force the pulley feels, and its length, 14.1 cm, represents the magnitude of the force. Since 10 cm equals 100 lb., the pulley feels about 141 lbs.

Eric Frei

LOL, I was wondering when you'd show up, you cannot help yourself but to hang around these pulleys and vectors

I wonder what a Norwegians answer would be. Hahaha

Also I think you missed this one, go check it out and double check my math.

Results of tree-pulling (SIM) a possible fraud?

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moray

Ah, those damned Norwegians! Ekka, I did post a little something in the other thread, but didn't feel qualified to check anyone's math because I didn't think the question was well formed. Even a poorly formed question can lead to an interesting discussion...