Page 211 - J. C. Turner "History and Science of Knots"

Studies on the Behaviour of Knots 201

knots in nylon [11, p. 90].
Mountaineers, cavers, rescuers and other users of life support ropes (see
Chapter 9) use friction hitches to provide a movable grip on a rope, and flowing
hitches to allow controlled movement of a rope past a point. Prohaska [31]
devised a simple test for friction hitches, to see if they would support at least
200 kg, in order to find out how many turns were needed in these knots under
various conditions.
Tests using shock loads have been used to study the braking effects of
various flowing knots and belay devices (some described in Chapter 9). Ex-
perimental details are lacking in some tests [18, p. 53] but fuller descriptions
are given by Chisnall [13, p. 378], who used dynamic kernmantle rope and
measured the impact force on the belaying devices while they held (with con-
trolled slippage) 81 kg loads, fall factor 1.5, bypass angle 90°, and by Dill [17]
who used static kernmantle and simulated conditions used in rescue work: a
load of 200 kg, fall factors 0-0.93 (mostly 0.33), maximum impact force al-
lowable 15 k. These results are of considerable interest to many users of life
support ropes, but they are specialised and dependent on the actual conditions
of use so that I do not discuss them further here.

Jamming of Knots

Many knotting books report that some knots are more prone to jamming than
others, that is, that after use they become particularly difficult to untie. This
tendency to jam is important in selecting knots that are expected to endure
heavy loads yet require frequent tying and untying. It seems that no one has
solved the problem of how to test that property reproducibly and objectively;
anyhow, no one seems to have reported such tests. This is a pity.

Conclusions

Most of the tests described above consider brand new rope, with or without
knots, subjected to loading in one of three ways: a constant load for a long time
(creep), simulating suspended loads; a steadily increasing load (the standard
method of measuring breaking strength) simulating overloading a winch; or a
shock force simulating a falling body. Most ropes that fail in practice are not
new, but have already been subjected to a variety of loads and, quite often, to
some amount of abrasion and weathering; the force that actually causes the
break is often a small jerk on top of a heavy pull. The laboratory tests can be
expected to give only a rough guide to what happens in the field.
There is still a lot to be learnt about the behaviour of rope and knots
under various circumstances. There are so many discrepancies between the