Page 25 - The Ashley Book of Knots

THE ASHLEY BOOK OF KNOTS

Many experiments have been made with the object of determining

the relative strength of knots. But so far as I know the quality of

security has not been considered, or else has been regarded as one of

the properties inherent in, or covered by, the term "strength." Yet

the two cannot be measured at the same time, and both are not pres-

ent in any two knots in the same degree. A secure knot often breaks;

a strong knot often slips.

The standard laboratory test of a knot has been to subject it to a

gradually increasing load. Eventually either the knot has slipped or

the material has tired and broken, which was usually at a point just

outside the knot.

In everyday service, however, rope is seldom subjected to just that

sort of wear.

In common use knots and rope generally break under a sudden

jerk, or a series of jerks, or else under a sudden access of load.

63. Several years ago the Collins and Aikman Corporation, manu-

facturers of piled fabrics, asked me to find a knot for them that would

not slip in the particularly coarse variety of mohair yarn required

in automobile upholstery coverings. If this yarn once broke it was

so "springy" that, with the knots then used, it untied over and over

again before it was finally woven into the cloth. The ~reat number

of knots that had to be retied slowed down productIOn seriou~y.

Eventually the problem was solved and the knot evolved was put

to use.

I started my experiments with the following premises for a point

of departure.

A. The security of a knot is determined by the stress it will endure

before it slips. To determine security a material i!;i required that will

slip before it breaks.

B. The strength of a knot is determined by the stress it will endure

before it breaks. To determine strength a material is required that

will break before it slips.

Some especially made mohair yarn, of large size and even quality,

was provided, and I made myself some testing apparatus of material

64 secured from a local junk yard.

My test of security consisted of a series of uniform jerks applied

at an even rate of speed, using the drip of a faucet for a metronome.

A bag of sand provided a weight, and the jerks were continued until

they either amounted to one hundred in number or else the knot

spilled.

Only bends were tested. Ten bends of each kind were tied with t~e

ends trimmed to an even length. Only six knots failed to slip and only

one of these was a well-known knot. One knot slipped at the first jerk

each time it was tried, and other well-known knots gave unexpected

results. These will be found tabulated on page 273.

67 64, 65. During the course of these experiments another question
66

suggested itself, which was: what effect, if any, has the direction

of the lay or twist of rope on the security of a knot?

Right-laid rope and left-laid rope have opposite torsion. It was

found that the regular RIGHT-HAND SHEET BEND (1166), tied in the

two different lays, slipped at about the same average rate, but the

variation of the number of jerks required was about twice as great

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