Forums: Climbing Information: The Lab: Re: [JimTitt] Spanish Burton conversion to 7:1 & other musings: Edit Log

Partner rgold

Nov 21, 2011, 3:44 PM

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Registered: Dec 3, 2002
Posts: 1804

Re: [JimTitt] Spanish Burton conversion to 7:1 & other musings
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JimTitt wrote:
I think there is a slight error in your thinking, in all the systems you show with a downward pull you can never get a load on the anchor of more than the combined weight of the two people, either you go up or he goes up! What you do in the middle with purchases doesn´t make you any heavier.

True, but this does not mean there is an error in the "anchor loads," other than the fact that the term might be a misnomer. If L is one of the calculated anchor loads, that means the load on the anchor is L times the pulling force (and not, as Jim implies, L times the weight of the raised object). If you are raising a mass M with a A:1 system whose anchor load (as used in the diagrams) is L, in the absence of friction your pulling force is M/A and so the actual load on the anchor is (L/A)M. So, for example, the anchor load for the Spanish Burton 3:1 system is (4/3)M, which is less than the 2M upper limit Jim proposes.

The problem occurs when there is a lot of friction outside the pulley system, which is often the case; at the very least the rope will probably run over some edges. in this case the pulling force has to be raised and the anchor load goes up because of that. If there is only one rescuer, they presumably cannot pull with more than their own body weight, which would result in actual anchor loads the numbers in the diagram times the rescuers weight.

Even more problematic is high-friction situations in which multiple rescuers or perhaps even motorized winches are used to pull on the system. The pulling-force multiplying factors shown can then lead to very high anchor loads.

(This post was edited by rgold on Nov 21, 2011, 3:49 PM)

Edit Log:
Post edited by rgold () on Nov 21, 2011, 3:49 PM

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