Doubling loads on an anchor point

[Will Hinchliffe]

Why when you tie a rope off to a stem then pass it through a crotch do you double the loading of the anchor point compared to loading both ends of a rope passed over the same crotch? Or tying the rope off to the crotch?

 

I find it a hard concept to get my head around.

 

Will

[Jamie]

I understand it, just i cant explain it.

 

how about you just accept it.....

 

not a good answer

 

Jamie

[Will Hinchliffe]

Im not rejecting the idea but would like to understand it better.

[Canal Navvy]

If you didn't tie off to the stem you would have to get a mate of

the same mass on the other end of the rope to stop it moving over

the crotch.Two of you is twice the load.

[Jamie]

when you are tied on to the limb or when you are hanging from it it's only your weight suspended. (on a climbing system each end of the rope takes half your weight).

 

when the rope is passed over the anchor and tied to the base your weight doubles due to reactionary force on the other end of the rope. there has to be an equal pull on both ends of the rope so the force at teh anchor point is double of the load on one end.

 

does that help???? a diagram would be good.

 

Jamie

[Peter]

Attached directly to an anchor, the load exerts force in one direction, down.

 

Attached elswhere, and then run through a crotch, the leg of rope with the load attached to it exerts the same force, and the other leg of rope exerts an equal force on the anchor. (assuming there is no friction in the anchor point.

 

If the force on either side becomes unequal the load moves.

 

Imagine you are lowering a piece with a pulley. To lift it by pulling your end of the rope down requires the same effort as it would to lift the piece directly. The pulley at the top is effectively doing both jobs simultaneously, therefore the loading is twice that of the actual weight of the load.

[Pete Mctree]

As has been said- if natrural crotch is used then the force is into the stem, as opposed to a block which is hanging from it, hence loading as opposed to disapaiting the force through it.

 

 

Spydie has the answers

 

http://www.mytreelessons.com/Pages/Tensionless_Anchors.htm

[bob]

if you were to lower a load of 100kg

the load would obviosly be 100kg

the other end of the rope you would have a 100kg force to hold

so there is a 200kg force on the pulley and anchor

[Will Hinchliffe]

Cheers I get it now thanks guys:2in1:

[Mikey P]

Just a quick question: does the angle of the rope into the pulley/over the crotch make a difference? The reason I ask is that, when you rig a rock/ice climbing anchor to, say, two anchor points and then equalise them, you want the angle between the ropes from the anchor points to be as small as possible.

 

If the ropes leave the equalisation point at shallow angles, they exert over twice the load on the anchors. Does this happen when using a pulley or running a rope over a crotch too?