Rabu, 11 November 2009

Dynamic ropes

Technical reference = norme EN 892
Number of falls:
To meet the standards, dynamic rope must withstand at least 5 successive falls of factor 1.77. The number of falls before break decreases with wear, and the number indicated on the technical notice must be no greater than the least number found after 3 laboratory tests.
A rope with a greater number of falls rating will thus give security for longer.

Impact force:
For a single rope, the Standard EN 892 requires a maximum value of 12kN during the first factor 1.77 fall arrest, with a mass of 80kg, the same value as for twin rope (12kN for 80kg) but on two strands.
For a double rope, the impact force has to be less than 8kN for factor 1.77 with a mass of 55kg.
The impact force given in the technical notice must be no less than the least good (highest) result found by the laboratory in tests on 3 samples of the rope.
Note that the impact force rises as the number of falls sustained increases.

Dynamic extension:
This is the measure of the stretch of the rope (≤=40%) during the arrest of the first fall (with 80kg for a single rope, 55kg for a double rope, and 80kg on two strands of twin rope).
Diameter and weight:
Ropes of bigger diameter generally have a longer lifetime. At the same time they are heavier and not so nice to use. On routes, where weight and ease of movement are important, it is preferable to choose a thinner rope. The overall performance of a rope may be considered as a balance between its weight and its dynamic performance.
Sheath slippage:
The core and the sheath are two independent components that have the tendency, if the construction is not carefully matched, to act separately and slide one on the other. The sheath may deform little by little under the effect of belaying and create a slack zone around the core and sag outwards “the nora batty stocking effect”. This phenomenon results in more rapid wear, particularly in intensive or top-roping use, and brings the risk of jamming in descenders or belay devices. Note that the risk of sheath slippage is increased in wet conditions.

Summary of standard EN 892

Impact force
Number of UIAA falls
≥ 5
≥ 12
Dynamic elongation
≤ 40%

Sheath slippage
UIAA ≤ 20 mm
Extension at 80kg load
≤ 10%
≤ 12%
≤ 10%

Unit of measure of the impact force

The KiloNewton: kN

1kN ~ 100kg force