Life on a line
12: Rope testing
The numbers seem a bit small, since 200kg is more than twice 80kg and yet the forces are less than twice the values above. However, our equation takes in the fact that a larger mass makes the rope stretch further and so the energy is dissipated over a longer time, making the peak value less than you may expect. Having said that, forces over 8kN are not to be sneezed at! The calculations above make two critical assumptions that mean we can’t expect those forces to be present in a real drop-test: 1. Some energy is dissipated in tightening knots and in non-elastic effects inside the rope, making the peak force smaller, especially on the first drop. 2. Repeated drop tests reduce the elasticity of the rope (decrease s) and so the peak force rises after each drop. The result is that the rope experiences different forces on every test, and for semi-static ropes where s is small, the non-elastic effects are significant and can even dominate. This change in peak force also helps to explain why a rope will break after a long repeated set of ‘identical’ drops – as the forces are far from identical! The only reliable way to obtain the peak force during a drop test is to measure it. Many people have tried to improve on the simple equation above, but to put it simply; the rope is too complex to let itself be written down in an equation! A little note on the construction of drop test rigs
The majority of home-built drop test rigs are based on a solid mass raised by some winching system and released, with the rope tied between this mass and a fixed ring or peg mounted above it. This works fine for fall factors less than 1.0, but to achieve FFs greater than that, the mass must be released from a point above the fixed anchor. There is a problem with this of course – you need the mass to fall vertically and for the rope to be vertical also, hence it seems the mass needs to pass through the tope anchor as it falls! A lot of test rigs (including the NCA device) offset the anchor just enough to let the mass fall past it, but therefore impart a horizontal component to the forces. There is however no reason why the mass needs to be connected in any way to the rope until the point at which the force is applied – i.e. when the mass reaches the bottom of the fall. Petzl and a few other manufacturers have designed their test rigs to use this principle in a design called a ‘catch plate rig’. Here, the rope is hung vertically between a framework of vertical guides, and on the bottom of the rope is a light but strong plate or bar, called the catcher. The mass is unconnected to this bar, but instead moves freely within the confines of these guides (usually two u-channels, or two round bars). The catcher is designed so that as the mass falls past it, it is hit and dragged down, thus transferring the force to the rope. The big advantage of this catch plate design is that you can apply fall factors of any value – including values greater than 2. There is no horizontal force on the rope, and the mass is safely contained by the guides. The disadvantage is engineering – the mass must move without friction as it falls, so bearings and careful shaping of the rig and mass are needed – plus a bit of thought into the catch plate. The falling mass must of course clear the rope itself as it falls, and so a common design has a round or square mass with a large central hole, inside which the rope is hung. The catch plate in that case is just a bar or plate slightly bigger than this hole, so the mass hits it and drags it down.
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