Wire knots

Press, 18 April 1980, Page 9

Wire knots

Since fencing wire is always knotted or tied when used in a fence, the type of knot used and the strength of the resulting joint determines the effective strength of the wire. In fact since the joint is the weakest part of a strained wire, the breaking strength of the knot is more important than the breaking strength of the wire. In order to clarify tins point, several tests were conducted on wire using three different knots. The knots used in this test included the following: A — Reef knot, B — Figure Eight, C — Double Loop. The knots were tested by The New Zealand Agriculture Engineering Institute. Two samples of each knot were tested by pulling to failure, while recording the maximum, load reached during the test. In all the tests 2.5 mm hi-Tensile galvanised wire was used. Results were as follows:— Reef knot Failed at 4448 N and 4403 N Average 4425 N Figure of Eight Failed at 4403 N and 4225 N Average 4314 N

Double Loop Failed at 2935 N and 2935 N Average 2935 N The ultimate strength of 2.5 mm hi-Tensile galvanised wire is 6650 N. The performance of both the Reef knot and the Figure of Eight is far superior to the Double Loop knot. However, the Reef knot and the Figure of Eight knots have a disadvantage for use when straining up a fence, because they cannot be tied without the wire losing 50mm or more of length before the knot takes the strain. This is a serious disadvantage and rules out its use for short strains. However, with long strains it is possible to pull in an extra 50mm of wire after reaching the required tension so that after the knot is tied the wire returns to this initial tension.

The use of some types of permanent wire strainers overcomes the necessity to join wire. The wire only needs to be tied off on the strainers and with the addition of a permanent wire strainer the wire can be tensioned up to the correct tension.