WHAT does a kink do to a wire rope?
In our discussion of the right and wrong way of handling
wire rope, we indicated one very common way to kink a rope.
While it is true that there are many ways by which
a rope in service may be subjected to conditions where kinking
is inevitable, unreeling and uncoiling a wire rope is a handling
that is common to all installations.
A kink, however, no matter how it is developed, has
the same damaging effect on the wire rope and should always be
carefully avoided.
Many of the reports of short rope-life that come
to wire rope manufacturers indicate that the rope has broken up
and worn out at only one point. An examination of a rope in this
condition frequently develops the worn part to be at the point
of a kink. Experience with kinked ropes indicates that the following
surface conditions are always evident:
(l) Where a kinked rope is subjected to severe bending,
the wires break up very rapidly, and many broken wires occur at
only one point in the rope. Examination of the remainder of the
rope shows it is in very good condition.
(2) Where a kinked rope is subjected to severe abrasion
it is always found that at one point the abrasion is more pronounced,
and the wires at this point break up rapidly, while the remainder
of the rope shows but very little abrasion and few broken wires.
In both of these cases the one point is always the
kinked point in the rope.
Frequently the rope user expresses his opinion that
in such cases the rope had either a bad spot in it, or the wire
was of an inferior grade.
A kink in a rope distorts the relation of all the
strands and all wires at this point from their original uniform
relative position as manufactured. This kinked condition causes
unequal tensions in the strands of the rope, frequently resulting
in an early failure at this point.
(3) There is another surface condition which frequently
develops that can be traced back to the kink as one of its causes.
This condition may be identified as "high strands" at
various points beyond the six-inch point as noted in Fig. 6.
With the continuous passing of the rope in service over sheaves
and drums, the looseness caused by the distorted condition at
the kink, frequently travels for considerable distance along the
rope, and shows up as high-strands which are subjected to additional
abrasion and consequent weakening of the rope as far as this strand
displacement travels.
While the visible damage done by the kink may be
concentrated at that point at the time the kink is made, it does
not follow that the resulting damage will always remain local.
A rope that has been kinked, does not therefore, always fail at
the point of greatest distortion.
It is possible for two different types of kink to
occur: one, a right-hand kink, and the other, a left-hand
kink.
In our Research Laboratories, we have made tests
of both of these two types of kinks, and we picture them here
for the information of wire rope users and to emphasize the great
need of care in handling wire rope, to prevent the development
of kinks.
In making these tests we used a 7/8-inch diameter
6xl9 Plow Steel Rope, right lay, regular lay, Seale construction,
with a hemp center.
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The start of a Right-Hand Kink Fig. 1 |
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| The start of a Left-Hand Kink Fig. 2 |
Fig. 1, shows the formation of a right-hand kink
in a right lay rope. Starting at point A and traveling in the
direction of the arrows it will be noted that the loop is formed
by the rope traveling in a right-hand direction, and end B passing
under A as the loop is finished. If several more such loops were
formed beyond this loop, and all were brought together one back
of the other, a right-hand spiral similar to a right-hand screw
thread, would be formed. A kink formed in this manner is termed
a right-hand kink.
Fig. 2 shows the formation of a left-hand kink in
a right lay rope. Starting at point A and traveling in the direction
of the arrows it will be noted that the loop is formed by the
rope traveling in a left-hand direction, and end B passing under
A as the loop is finished. Then, again, if several more such loops
were formed beyond this loop, and all were brought together one
back of the other, a left-hand spiral similar to a left-hand screw
thread would be formed. A kink formed in this manner is termed
a left-hand kink.
Two distinctions of these two kinks in a right-lay
rope are:-A right-hand kink will tighten up the rope lay and a
left-hand kink will loosen the rope lay, or tend to untwist the
rope. With a left lay rope, the results would be similar, only
arrived at in the opposite directions.
Fig. 3 shows a loop formed in the rope previous to
being pulled down into a kink.
Fi6. 4 indicates the loop pulled down, forming the
kink just before it straightens out. As the rope straightens out
an extra twist is thrown into the rope distorting the strand relation
of all six strands which is very detrimental to long rope life.
In Fig. 5, the kink is shown straightened out under
a tension of 5700 lbs. The rope has been damaged in the one spot
within the circle. The extra twist tending to tighten the rope
has been thrown into the rope and the distorted strands have been
given a set under strain.
Fig. 5b, is the same as Fig. 5 except that the rope
is under a 10,000-lb. tension. Note here. that the kink is still
very noticeable and this extra twist in the rope can never be
removed. The enlargement of the section within the circle, as
shown in Fig. 5a, gives a clear idea as to the distorted condition
of the strands.
Between taking the photographs for Figs. 5 and 6,
the kinked rope was subjected to a very high strain and held under
this strain for five minutes, then all strain was released.
In Fig. 6 the kinked rope is shown after all tension
was released, and it will be noted that the kink still remains
very prominent. The resulting strand distortion produced by the
kink will create high and drawn strands. This condition will cause
strand nicking and unequal tensions in the different strands,
and an early rope failure is the result.
A cross-section study of the rope at the kinked point
as indicated by A in Fig. 6 and another point, B, emphasize to
an even greater extent the damage done to a rope.
Cross-section of the rope at these kinked points-A-A
at a point B-B, 6-inches distant or approximately one rope lay
distant, are shown in Figs. 7 and 3.
This illustration (Fig. 7) shows a magnified cross-section
of the rope at the kinked point A-A of Fig. 6. With the tendency
of the right-hand kink to tighten up the rope, the hemp center
becomes compressed and it will be seen from the illustration that
some strands are drawn in and others forced out.
Hence, the high strands in the rope are subjected
to the severest abrasion and abuse, while the drawn strands are
subjected to a great strain as they carry a greater percentage
of the load. Whether this combination be concentrated at one point
in the rope, or as a result of this distorted condition high strands
develop at other points, it is very evident that the result will
be an early rope failure, while the rest of the rope at first
may appear to be in good condition.
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| Fig. 7 | Fig. 8 |
Fig. 8 is a magnified cross-section of the rope at
the point B-B six inches distant from the kinked point. Attention
is called to the very uniform strand relation of all six strands
at this point which is only six inches from the kink. However,
when the rope is put into service, the unbalanced strand relation
may develop at a more distant point.
Therefore, it is very evident from a comparison of
these two photographic reproductions that a kink in a rope is
a local condition at the time of kinkier and does permanent damage
to the rope at this point. While it is at this kinked point that
the rope is bound to fail, failure may also be evident at other
points.