Monday, 22 October 2012

Anchors, part 1

I have watched students tasked with attaching a line to a fixed anchor point struggle and retie knots several times to achieve a slack fee connection. Try this method and you will achieve results every time.

How much do I allow for the knot. Trial and error?

create a bight in the line (11mm) and pull the line and anchorage together tightly, adjusting the bight so that the two just meet. Note: the tighter these two are pulled together, the tighter will be the attachment.

Extend the bight an additional 8" or 200mm, as a guide, measure and use the span of your fingers as a guide. Then tie a figure of eight knot, dress and tension the knot in the normal manner.

Finally make the connection, you will find the knot to be in the perfect position. The system works with all lines, the length of the addition will need to be adjusted to accommodate different line diameters.

The major consideration in establishing any anchorage system is the prevention or at least the management of shock loads. These unwelcome forces can be caused by a number of reasons, whatever their cause they can seriously overload a system, causing the failure of knots and anchorages alike.

Prevention has always been better than cure. When a rope is under tension and then shock loaded we have relied in the past on the elasticity built into the line and the fact that knots will tighten slightly.

Energy dissipater

The energy dissipater has been around for a while, but like most items was designed for a different use. Petzl use a similar method of reducing shock loads in their Via ferrata lanyards, yet it has never been actively applied to rope terminations.  

The rope in use, either an 11mm or even 9mm is passed through the holes in the device, it is very important to ensure that the correct sequence of holes is followed as they differ in size for the varying diameters of line. A diagram engraved on the body of the dissipater helps with this. Personally, for our lines that are packed and ready for use, we leave them in place. Most important is to leave a tail of about a metre and finish with a stop knot.

Under normal use, this termination is as secure as any knot. However should the rope be shock loaded the dissipater will allow the rope to pass through, absorbing energy in the process. The stop knot is a preventative measure to stop the rope from unravelling completely. Should you wish to prevent any movement in the rope pass the knotted tail beneath the last turn on the dissipater.

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