Page 209 - J. C. Turner - History and Science of Knots

198 History and Science of Knots

Security section. At least in some conditions, knots in a rope may decrease the
shock force and increase the number of standard falls sustained, acting as if the
strength of the rope was increased rather than decreased. Some climbers use a
shock sling to limit impact force [18, p. 52]. This consists of some nylon tapes
sewn together with many sequential bar tacks that could each be ripped apart
by a fixed force, usually about 3 kN. If a force exceeding that level was imposed,
the bar tacks would rip out until the force was reduced. Cannon [12] tested
such a device, at a constant fall factor of 1.0, and found that an increase in the
distance fallen increased the number of bar tacks ripped: there was a longer
time during which the imposed force exceeded the rip point. Fewer bar tacks
were ripped if this sling was attached to the rope with a loose knot than if the
knot were tight. That is, the loose knot (identity not stated) acted as a shock
absorber, reducing the time of exposure to high forces while it tightened. A
similar shock-absorbing capacity for knots was noted by Marbach and Rocourt
[23], who tested two loop knots tied in the middle of static kernmantle rope
subjected to factor 1.0 falls with an 80 kg load. A new 9 mm rope without
knots sustained an impact force of 8.7 kN at the first fall and broke at the
second. If an Overhand Loop were tied in the middle of the rope, the rope
sustained three falls before breaking, with maximum impact forces of 3.7, 5.2
and 6.4 kN. Some used 10 mm rope was similarly tested; impact forces were
reduced and there was some increase in the number of falls held. More tests
of this kind seem desirable.

Other Tests of Shock Loads
Ashley [5, #68] described a method of testing the strength of knots by
subjecting them to a series of jerks `of increasing force', caused by a weight
falling from increasing heights. The tests are not described in sufficient detail
to know how the fall factor varied, but this seems unimportant since no results
are given. Barnes [7, p. 54] described a `dynamic shock tester' for monofila-
ment nylon line, in which a weight slid down a `practically frictionless' wire
on to a hinged rod suspended by the test line. The fall height of the weight
was progressively increased until the line broke (this is roughly equivalent to
a progressively increasing fall factor). The final height fallen is taken as a
measure of the dynamic breaking strength of the line. He found [7, pp. 55,
39] that knot efficiencies were more variable and lower in these dynamic tests
than in his more usual static tests using a hand pull against a spring balance
[7, p. 53].

The Security of Knots
When subjected to sufficiently high loads, knots fail before the main part of the
rope breaks. Up till now, we have only discussed failure through breaking, but