_CORDAGE AND KNOTS_
The fibres used for the ropes and cords for scaffolding purposes are of
jute and hemp. The strongest rope is made of the latter variety, and
the best quality is termed the Manilla.
The cords are generally tarred for preservation, but this process has a
bad effect upon the strength of the fibre, more especially if the tar
is impure. The process of tarring is as follows:—
The fibres before they are formed into strands are passed through a
bath of hot tar. Immediately afterwards, and while still hot, the
material is squeezed through nippers, by which means any surplus tar is
removed.
The ropes, as used in scaffolding, are known as shroud laid or three
strand.
A shroud-laid rope consists of three strands wound round a core, each
containing a sufficient number of fibres to make an equal thickness. A
three-strand rope is similar, but has no core.
The following gives the breaking weights of scaffold cords and falls
according to circumference[2]:—
Size Cwts. qrs. lbs.
Tarred hemp scaffold } {2 inches in circumference 20 0 0
Cords } {2¼ inches in circumference 25 0 0
Tarred jute scaffold } {2 inches in circumference 14 0 0
Cords } {2¼ inches in circumference 18 0 0
White Manilla scaffold } {2 inches in circumference 32 0 0
Cord } {2¼ inches in circumference 39 0 0
{2½ inches in circumference 44 0 0
White Manilla rope for } {3 inches in circumference 70 0 0
pulleys and block ropes} {3½ inches in circumference 98 0 0
{4 inches in circumference 126 0 0
{4½ inches in circumference 158 0 0
It should be noted that all these weights are the actual breaking
strains of the ropes tested to destruction. In practice, one sixth
of the breaking weights only should be allowed, in order to leave a
sufficient margin for safety.
The strength of a rope is the combined strength of each separate yarn.
Therefore, if the fibres are not carefully twisted together, so that
each bears an equal strain, the rope is unsafe. Care should be taken
when choosing a rope that the strands are closely, evenly and smoothly
laid.
The rope is strongest when the fibres are at an angle of 45 degrees
to the run of the rope. When at a greater angle than this, the fibres
are apt to break, and when at a less angle, the friction between the
parts—upon which the strength of a rope greatly depends—is lessened.
The durability of tarred ropes is greater than when untarred, but not
to such an extent if kept dry.
A splice weakens a rope about one eighth.
Ropes are adulterated by the admixture of rubbish fibre termed 'batch.’
To test a rope as to condition, untwist it and notice whether any short
ends break upwards. If so, or if the tarring has decayed internally,
the rope must be viewed with suspicion.
Hemp ropes, after four to six months’ wear, are often one fourth weaker
than new ones.
Scaffold cords are from 15 to 18 feet in length.
Moisture will cause a shrinkage of 6 inches in an 18-foot cord.
A good hemp rope is more reliable than an iron chain, as the latter
sometimes snaps on surgeing.
The following diagrams show the various knots used in scaffolding,
both in the erection of the scaffold and for attaching materials to a
hoist. They are shown in a loose condition as being more useful for
study by a pupil. Several knots are, however, too intricate to explain
by diagram, and in these cases an attempt has been made, with but scant
success, to instruct as to their method of creation in the notes. It
will, however, be found by the student that half an hour with an expert
scaffolder, preferably an old sailor, will afford more instruction than
hours spent in studying diagrams.
The art of knot making is governed by three principles. First, a knot
must be made quickly. Secondly, it must not jam, as this prevents it
being undone easily. Lastly, under strain, it should break before
slipping.
[Illustration: PLATE III.]
=Plate III.=—No. 1. _Overhand or thumb knot._ Prevents the
end of a rope opening out or passing through the sheaves of a block.
No. 2. _Figure of eight knot._ Used as No. 1.
No. 3. _Square or reef knot._ Will sometimes jam with small ropes.
No. 4. The knot we all make until we learn better, known as the granny,
and will both slip and jam.
No. 5. _The bend or weaver’s knot_, used for joining ropes
together or securing a rope through an eye splice.
No. 6. _Wolding stick hitch_, is serviceable only in connection
with a pole used as a lever.
No. 7. _Bale sling_, for hanging on to hook of lifting tackle.
No. 8. _Magnus hitch, or rolling hitch_, for lifting material.
No. 9. _Two half hitches, or builder’s knot, or clove hitch._ Used
for tying ledgers to standards.
No. 10. _Loop knot_, used where ends of rope are not available.
No. 11. _Loop knot_, used where ends of pole are not available.
No. 12. _Boat knot._
No. 13. _Sheepshank, or dogshank_, a method of shortening a rope
without cutting it or reducing its strength.
[Illustration: PLATE IV.]
=Plate IV.=—No. 14. _Blackwall hitch_, very powerful, but
requires watching or may slip.
No. 15. _Midshipman’s hitch_, used as shown with a rounded hook.
No. 16. _Catspaw_ is an endless loop, and is used where great
power is required.
No. 17. _Capstan knot, or bowline._ After tightening it will not
slip.
No. 18. _Timber hitch_, for carrying scaffold poles. Take one turn
round the pole and standing part, and finish with jamming turns.
No. 19. _Artificer’s knot_, or half hitch and overhand.
No. 20. _Topsail halliard bend_, used as a timber hitch.
No. 21. _Bowline on a bight._ A board across the loops makes a
useful seat.
No. 22. _Racking or nippering_ is a method of temporarily joining
two ropes for lengthening purposes. The ends are laid side by side for
about 18 inches, and the marline or spunyarn is taken for about a dozen
turns round both, then by round turns over all and fastened with a reef
knot.
Nos. 23 and 24. _Round seizing._ With a slip knot at the end of
the spunyarn take a turn round the ropes to be nipped together. This
turn should be pulled tight, and continued for about a dozen turns
(No. 22); then take the end through the last turn, and take turns over
the first laid, finishing by carrying the spunyarn two or three times
between the rope and the seizing. Knot the end by jamming turns,
keeping the whole well taut (No. 24).
[Illustration: PLATE V.]
=Plate V.=—No. 25. _Butt or barrel sling_ when placed
horizontally.
No. 26. _Butt or barrel sling_ when placed vertically.
No. 27. _Double overhand knot._
No. 28. _Running bowline._
Nos. 29 and 30. _Band tie, marrying or splicing._ Commence as No.
29, and after continuing the turns until near the end of the rope, take
the rope twice between the poles and round the turns first laid, and
finish with jamming turns. Tighten with a wedge.
Nos. 31, 32 and 33. _Tying between standard and ledger._ Commence
with two half hitches as No. 31. Then twist ropes together as far as
they will go, and place ledger in position above the hitches (No. 32).
The twisted ropes are then drawn up in the front of the ledger to the
left of the standard, taken round the back of the standard, brought
again to the front and round ledger at the right of the standard, then
cross in front of the standard and round the ledger at the left of the
standard, and brought up and carried round the back of the standard.
This process is repeated until the end of the rope is nearly reached,
when it is given two or three turns round the ledger and fastened off
with jamming turns (No. 33). To tighten, drive a wedge at back of
standard.
[Illustration: PLATE VI.]
=Plate VI.=—No. 34. _Portuguese knot_ for shear legs, made by
several turns of the rope round the poles, and interlaced at the ends.
No. 35. _Running bow knot_—inferior to No. 28.
No. 36. _Bowline_—inferior to No. 17.
No. 37. _Double bend_—useful where a small rope is bent on to a
larger. The end of the rope is given one extra turn round the bight of
the other, with the consequence of a great increase of strength.
No. 38. _Fisherman’s knot._
No. 39. _Lark’s head_, fastened to a running knot.
No. 40. Where increased strength is required a small rope can be
attached to a larger one by means of a rolling hitch. The whole
arrangement comes apart as soon as the strain is removed.
No. 41. A method of lifting scaffold poles in a vertical position by
the use of the timber hitch and half hitch. If it is required to free
the upper end while the pole is being carried, the half hitch can be
replaced by a cord tied round the pole and the lifting rope.