Pulley block rope question

Physics is not my strong point.

When you use a pulley block, say hang it in a tree that you are pulling, does it increase the load on the rope as effecting the working stress that is being put on it? It seems like at the point where the rope goes around the sheave, the tension is being doubled, as opposed to a a direct pull with no block. With a Maasdam puller and the half inch 3 strand polyester line, though the tensile strength of the rope is high, it seems like you would be exceeding the recommended safe working load of the line at 3/4ton pull using a pulley, being somewhere between five and ten percent of the tensile strength. Yes/no?

Thanks for the help, I'd rather know earlier than finding out definitively at a job coming up.

The load on the rope remains constant on either side of the pulley. If you are standing under a branch with a pulley on it and run a rope through said pulley to a 100kg weight next to you it will require ~100kg of pull on your side to lift the weight. The branch and inherently the sling connecting the the block to the branch will have 200kg of load on it. The rope retains 100kg at all points.

With a Z-Rig you can change the force on certain legs in the rope system though..

"At all points". That's what I wanted to know. Great, thanks Nick.

That is if you are both directly under the block. When you change the angles either entering the block or exiting the block, the loads will then also change.

I guess the locations of the line being attached and pulled from being 180 degrees to the block gives max rope strength? I'm specifically thinking about pulling with a Maasdam puller, or some other device using wire rope.

Jogged my memory, Bud, I recall seeing how sling or wire rope strength changes when spread out differently over loads being crane lifted. More horizontal spread out to the lift becomes weaker.

Essentially Nick is right. The total load will not change. I will often set up my MA system first then use my Maasdam on that. Even one extra block is amazing.

Husabud said:

That is if you are both directly under the block. When you change the angles either entering the block or exiting the block, the loads will then also change.

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Yes the loads on the block will change but the loads on the rope stay the same. If you have a pull line attached to a tree and run it through a block on the ground and then back to another block attached to the line with a prussick you will have 3:1 (right or is it 2x?) set up behind the single strand. That single strand will then have 3x (or 2x if I got it wrong) the force as the piece that you are holding and pulling with.

I assume that with different angles to the block, the loads on the rope will remain the same, but the ability to withstand those loads decreases as the angles spread out from the block. Safe working strength denotes only the calculated percentage of tensile strength in direct line with the load.

3x

woodworkingboy said:

I assume that with different angles to the block, the loads on the rope will remain the same, but the ability to withstand those loads decreases as the angles spread out from the block. Safe working strength denotes only the calculated percentage of tensile strength in direct line with the load.

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In theory as soon as you bend a rope, whether on a biner, a pulley, or around a hot water tank it loses some strength.

bonner1040 said:

In theory as soon as you bend a rope, whether on a biner, a pulley, or around a hot water tank it loses some strength.

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Yes, but that's two completely different points here.
Take a web sling on a flat grooved block and the strength loss tends to zero, but the "different angles to the block" thing remains the same.

For a given load, the more you have to pull side way ( in relation to direction of intended move) the more you have to pull on the rope. Its purely geometrical and its the same application as to pull a high tree with a too short rope, or straightening a heavy back leaner.

Straight in line with the move, the better, the both sides of the rope put 2x force on the block, so on the tree being felled. If the both sides spread at 120°, each one puts only half its load on the block, and you loss completely the MA. If the angle comes close to 180°, the block sees nearly nothing and you can break easily the rope before the tree began to move.
Its the reason why of the decreasing ratings on the slings for crane work. For a heavy load (in relation to the WL), choke the slings as close as possible. If the stability is a concern and needs a wide spread of the tie-in points, take some stronger slings or they are likely to break.