Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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"MULTI-STRAND STEEL WIRE ROPE"
BACKGROUND TO THE INVENTION
THIS invention relates to a multi-strand steel wire rope and to individual
strands of a wire rope.
A multi-strand steel wire rope has steel wires spun into strands, and the
strands are then laid up helically, typically about a core, in one or more
layers. Figure 1 shows a typical example of a conventional single layer
multi-strand steel wire rope, indicated generally by the numeral 1. The rope
1 has a core 2 about which a single layer of strands 3 is laid up. The core 2
is typically made of a fibre such as sisal, a synthetic polymeric material
such as polypropylene or another steel wire strand, or the core area may
be vacant. In this example of a conventional construction, the numeral 4
designates the core of each strand, the numeral 5 an inner wire of the
strand and the numeral 6 an outer wire of the strand. If the outer wires 6
are laid up in a helical direction opposite to the helical direction of the
strand as a whole, the rope is referred to as being of ordinary or regular lay
construction. If the wires 6 are laid up in the same helical direction as the
strand itself, the rope is referred to as being of Lang's lay construction.
Ropes with a single layer of strands, for convenience referred to in this
specification as "single layer ropes", generally generate a torque when
subjected to tensile load. The result of this is that if one end of the rope
is
free to rotate the rope as a whole will tend to untwist in order to alleviate
the
torque which is generated. This untwisting may be highly undesirable in
certain applications, for example where the rope is used to raise a load
which is not restrained from spinning. In such cases, non-spin ropes are
used. Such ropes generally have more than one layer of strands, with an
outer layer of strands laid up on an inner layer of strands but in the
opposite
helical direction to the strands of the inner layer. In this way an attempt is
made to balance the torque generated by the respective layers when the
rope is under tensile load.
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For convenience in this specification, ropes with more than one layer of
strands are referred to as "multi-layer" ropes.
Although multi-layer ropes can counter the tendency of the rope to untwist
they are generally less stable and robust than single layer ropes. Also,
magnetic non-destructive testing of single layer ropes tends to be more
accurate and reliable than is the case with multi-layer ropes.
The strands of known steel wire ropes may take different forms. An
example of a round strand is shown in Figure 2 in which the numerals 7
and 8 respectively indicate the "height" and "width" of the strand. The
"height" of the strand is the cross-sectional dimension of the strand
measured, in the laid up rope, in a radial direction corresponding to the
indicated Y-Y axis.
The "width" of the strand is the cross-sectional measured, in the laid up
rope, in a circumferential or tangential direction corresponding to the
indicated X-X axis. In the case of a round strand such as that of Figure 2,
the ratio height:width is substantially equal to unity and the bending
stiffness of the strand about the X-X axis is substantially equal to the
bending stiffness about the Y-Y axis.
Figure 3 shows an example of a known triangular strand in which the core 4
has the cross-sectional shape of an equilateral triangle. In this case, the
ratio height:width will typically be of the order of 0.98, i.e close to unity.
As a
result the bending stiffness about the X-X axis is again substantially equal
to the bending stiffness about the Y-Y axis.
Figure 4 shows an example of another known strand form known as an "8
over 2 wire" strand composed of wires 9. In this case, the ratio height:width
is substantially less than unity and may for instance be about 0.69. The
bending stiffness of such a strand about the X-X axis is substantially less
than its bending stiffness about the Y-Y axis.
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SUMMARY OF THE INVENTION
According to the present invention there is provided a multi-strand steel
wire rope comprising multiple strands laid up helically on a core, at least
some of the strands being deep strands, i.e. strands with a height:width
ratio greater than unity, preferably 1.04 or greater.
In the preferred embodiments, the rope is of a single layer construction.
There may for instance be a single layer of deep strands, and no other
strands, laid up helically on the core. Alternatively there may be a single
layer of strands, including both deep strands and other strands, laid up on
the core.
Each deep strand may includes a core having the cross-sectional shape of
a non-equilateral triangle. Alternatively each deep strand may comprise, in
cross-section, parallel rows of wires arranged generally radially.
Further according to the invention there is provided a strand for a multi-
strand steel wire rope, the strand being a deep strand having a ratio
height:width of 1.04 or greater. As indicated above, the strand may have a
core with the cross-sectional shape of a non-equilateral triangle and wires
laid up on the core, or it may comprise, in cross-section, parallel rows of
wires arranged generally radially.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in more detail, by way of example only,
with reference to the accompanying drawings in which:
Figure 1 illustrates a conventional multi-strand steel wire rope;
Figures 2 to 4 illustrate different, conventional strand configurations
used in multi-strand steel wire ropes;
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Figure 5 illustrates a deep strand of a multi-strand steel wire
rope according to the present invention; and
Figures 6 to 10 illustrate different multi-strand steel wire ropes
according to the invention.
SPECIFIC DESCRIPTION
The multi-strand steel wire rope seen in Figure 1 has been described
above, as have the different strand configurations seen in Figures 2 to 4.
Figure 5 illustrates a deep strand 10 according to this invention. The strand
has a core 12 having the cross-sectional shape of a non-equilateral
isosceles triangle, with two sides 14 and 16 of equal length and a shorter
third side 18. An inner layer of steel wires 20 is laid up helically on the
core
12 and an outer layer of steel wires 22 is laid up helically on the inner
wires.
In Figure 5 the numeral 24 indicates the cross-sectional height of the strand
10. As in the above description of conventional ropes and strands, this is
the radial dimension of the strand, i.e. the cross-sectional dimension of the
strand, when laid up in a multi-strand steel wire rope, measured in a radial
direction with respect to the central axis of the rope.
The numeral 26 indicates the cross-sectional width of the strand 10, i.e. the
circumferential or tangential cross-sectional dimension of the strand,
measured perpendicularly to the radial direction, when laid up in the rope.
In Figure 5 the parameters are such that the ratio height:width is of the
order of 1.12.
For convenience the strand 10 of Figure 5 is referred to as a deep
triangular strand. It will be understood that a deep strand according to the
invention may comprise a shaped core, as in Figure 5, with only a single
layer of wires instead of multiple layers of wires.
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Figure 6 shows a cross-sectional view of a multi-strand steel wire rope 28
which has a core 30 and five closely adjacent and equally spaced deep
triangular strands 10 of Figure 5 type laid up helically in a single layer on
the core.
Figure 7 shows a cross-sectional view of a single layer multi-strand steel
wire rope 32 which has a core 34 and nine closely adjacent and equally
spaced deep triangular strands 10 of Figure 5 type laid up helically in a
single layer on the core.
Figure 8 shows a cross-sectional view of a single layer multi-strand steel
wire rope 36 which includes deep strands having a form different to the
deep triangular strand 10 of Figure 5. In this case each deep strand 38 has
ten steel wires 40 arranged in generally radially extending, parallel rows 41,
such that the height 42 of the strand is greater than the width 44 thereof,
i.e. the ratio height:width is greater than unity. The ratio height:width may,
for instance be of the order of 1.46:1.
In Figure 8, three deep strands 38 are spaced apart from one another and
alternate circumferentially with three conventional, round strands similar to
the strand 3 described previously with reference to Figure 2.
Figure 9 shows a cross-sectional view of a single layer multi-strand steel
wire rope 46 which includes deep strands 38. Once again, the ratio
height:width of each deep strand 38 is greater than unity and may, as in
Figure 8, be of the order of 1.46:1.
The embodiment of Figure 9 differs from that of Figure 8 in that there are
four spaced apart deep strands 38 alternating with four round strands 3 of
Figure 2 type.
Figure 10 shows a cross-sectional view of a single layer multi-strand steel
wire rope 50 which includes four closely adjacent deep strands 38 and four
round strands 3.
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The round strands 3 are laid up helically as fillers between the deep strands
38 but do not cover them so that the construction does, in effect, remain a
single layer construction.
As indicated above, it is recognised that tensile force applied to a single
layer multi-strand rope will generate a torque, i.e. a force tending to
untwist
the rope, when the rope is subjected to tensile load. The present invention
is based upon the recognition by the inventor that the tensile force in a rope
can be resolved into components of torque-generating shear force and
longitudinal force. The inventor has furthermore recognised that in order to
reduce the tendency of a single layer multi-strand rope to untwist under
tensile load, the bending stiffness of the strands of the rope about the
appropriate axes should be increased relative to the torsional stiffness of
the strands, i.e. the resistance of the strands to twisting under the shear-
generated, applied torque forces acting about the axes of the strands.
As a result of the fact that its ratio height:width exceeds unity, in most
cases by a substantial amount, the deep strands 10 and 38 described
above will exhibit increased bending stiffness about the axis A-A in Figure
6, perpendicular to the radial direction and corresponding to the axis X-X in
Figures 2 to 4, compared to conventional strand configurations where the
corresponding ratio is unity or less.
It is accordingly perceived that the ropes illustrated in Figures 6 to 10 will
have a reduced tendency to untwist under tensile load, or that such ropes
may have no such tendency at aIl to untwist or even a tendency to twist up
slightly when loaded.
It is furthermore considered most beneficial, in order for the rope as a
whole rope to enjoy an appropriately reduced tendency to untwist under
tensile load, that there should be three or more deep strands having the
desired, increased bending stiffness about the axis A-A, although it will be
understood that some beneficial anti-twist effect will be experienced even if
there are less than three deep strands.
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It is noted that the principles of the invention are applicable to various
different types of multi-strand steel wire ropes, including ropes with right
hand or left hand lay, ropes with Lang's lay or ordinary lay, ropes in which
the strands are simple strands with a singfe layer of wires over the core,
ropes in which the strands are compound strands with two or more layers
of wires laid up on the core, irrespective of whether the wires are laid up in
the same or different helical directions in the different layers, ropes in
which
the strands have metallic or non-metallic cores, irrespective of whether the
strand cores are of plaited or other construction, ropes in which the rope
core is metallic or non-metallic or in the form of a strand or otherwise, and
ropes in which the strands and/or ropes themselves are encapsulated.
The major benefits of the invention will be realised in single layer ropes,
but
it is envisaged that a reduced tendency of a rope to untwist can also be
achieved in the case of multi layer ropes.
