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Topic Date User

Sliding X revisited 28-Jan-2015 At 11:07:02 AM Dave_S

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On 28/01/2015 One Day Hero wrote:
>
>Do you also make sure to double up your belay device, carabiner, harness,
>and rope?

Nope. Ropes and harnesses are much, much more reliable than slings due to their having protection against wear built-in. I've never even heard of a harness failure ever occurring. (Opening scene of Cliffhanger does not count.) Ditto for belay device and belay carabiner, which additionally are not susceptible to damage from chemical exposure, UV exposure, or being weakened by small cuts.

Additionally, the fact that redundancy is not practical in every single piece of equipment you use is not a reason to forego redundancy when it would have been practicalto have it.

On 28/01/2015 Wendy wrote:
>
>Seriously though, whilst there are a few single pieces in the system,
>why choose to add more to the system, especially in a configuration that
>is going to provide nylon on nylon friction? We all agree you don't run
>a rope through slings because of that. In the course of self equalising,
>a sliding x has constant small amounts of movement and if one fails, a
>slide along half the length of the sling rubbing on itself.

Definitely agree with the point on redundancy, but fabric-on-fabric friction is only a problem when one piece of fabric is moving and the other isn't, at when this happens the heat all concentrates in one spot on the non-moving strand and melts it. When both strands are moving against each other, as in a sliding-X, the heat generated is distributed across the length of the sling, and doesn't cause melting. This is the same reason why a Munter hitch won't melt your rope.

On 28/01/2015 kieranl wrote:
>
>No the sliding X does not ensure that both pieces are weighted equally
>- it ensures that both pieces are kept in tension. The force on each piece
>will be equal only when the ttie-in point is centred between the two anchors.
>If that point shifts more force will be applied to the leg which has just
>been shortened somewhat. It's basic rigging physics, change the angle,
>change the forces (a diagram would explain this better).
>
>Also, if there is a shock load off the anticipated line of force, the
>transition to the new orientation isn't instantaneous, momentarily one
>side of the anchor will take a hit until the krab moves to the new alignment.

Incorrect. The only degree to which magnitude of load on the anchors can differ with a sliding-X is due to friction between the carabiner and sling, which is negligible in comparison with the load force. The carabiner simply cannot exert force on one end of the sling more than the other, which would be required in order for different magnitudes of force to be placed on the anchors. The only way in which forces on the anchors can differ is in direction.

Even in shock loading or sudden change of direction, forces on the anchors are equal, but the system is not in equilibrium because the is a net sidewards force applied to the carabiner, which causes it to shift until the system is balanced.

(And if you cite the DMM article which incorrectly makes this claim that anchor loads are not balanced in a sliding-X I will be forced to put my face through my desk and then start drawing free body diagrams!)