Technical Field
[0001] The invention relates to a steel wire rope comprising a fiber core element and an
outer strand layer.
Background Art
[0002] In rope manufacture and use of steel wire ropes, core plays the role of supporting
the outer strand layer of steel wire rope, keeping rope structural integrity, improving
the flexibility of the wire rope. In addition, the core can store lubricant, thus
keep the outer strand layer lubricated to reduce friction, abrasion and corrosion.
[0003] Fiber rope made from natural plant fibers, including hard fibers (e.g. Manila hemp,
etc.) and soft fibers (sesame, sisal, cotton yarn, etc.) are widely used as the cores
in the manufacture of steel wire ropes. One of the common disadvantage of natural
fiber cores is its poor resistance corrosion due in wet environments. Organic cellulose
a common building block of natural fiber core has strong water absorption ability
and thus easily deteriorate and lose the original performance properties such as radial
stiffness. If the rope core has poor resistance to crushing, the outer strand layer
becomes unstable and the contact stress between adjacent strands will increase causing
the steel wires to wear and eventually break. The hygroscopicity of fiber core will
further increase the moisture content within the rope causing corrosion of the steel
wires in the steel wire rope. Such corrosion will intensify the brittleness of the
steel wire and lower the tensile strength of the steel wire rope. This would shorten
the service life of the wire rope.
[0004] For applications where steel wire ropes are subjected to wet and acidic environments,
the rope core is commonly made from synthetic fiber instead of natural plant fiber.
However, sisal rope is widely used as core for steel wire ropes in some industrial
and agricultural production. It has characteristics that are not possessed by synthetic
fiber rope core and is irreplaceable in some industries.
Disclosure of Invention
[0005] It is a main object of the present invention to develop a steel wire rope having
considerably increased resistance to corrosion of the fiber core and in particular
suitable for critical applications, such as used in wet acidic mine shafts.
[0006] It is another object of the present invention to devise a steel wire rope having
improved radial resistance and capacity for dynamic transverse impact pressures.
[0007] According to a first aspect of the present invention, there is provided a steel wire
rope comprising a core element surrounded by at least one outer layer, said core element
containing natural plant fibers, said at least one outer layer comprising a plurality
of steel wire strands, wherein said core element is sheathed with a polymer having
a thickness in a range from 0.2 to 2 mm.
[0008] Herewith, the steel wire rope can have a diameter in a range from 10 to 100 mm, e.g.
from 30 mm to 60 mm, e.g. 30 mm, 40 mm, 50mm or 60 mm. The diameter of the core element
is preferably from 30% to 60% of the diameter of the steel wire rope, and more preferably
from 40% to 50% of the diameter of the steel wire rope. The sheath of the core element
has a thickness in a range from 0.2 to 2 mm, preferably in a range from 0.3 to 1.5
mm and more preferably In a range from 0.5 to 1 mm. The thickness of the sheath on
the core element can be selected depending on the diameter of the core and the steel
wire rope.
[0009] The natural plant fiber referred in the present application can comprise, but not
limited to hemp (e.g. Manila hemp, ramie or jute etc.) and soft fibers (sesame, sisal,
cotton yam, etc.).
[0010] The core element can have any constructions known for fiber ropes. For instance,
a rope construction of 3 strands, 4 strands, 1x4 or 1x6 strands. In such rope constructions,
the ropes are made up of strands. The strands are made up of rope yarns, which contain
natural plant fibers. Methods of forming yarns from fiber, strands from yarn and ropes
from strands are known in the art.
[0011] In addition, the natural plant fiber rope can be preconditioned before further processing
through e.g. pre-stretching, annealing, heat setting or compacting. The constructional
elongation can also be removed during the rope production by sufficiently pre-tensioning
the core before applying a sheath or during closing the outer wire strands onto the
core.
[0012] The core element of the present invention can be lubricated with lubricant. The lubricant
can have a weight in a range of 10 to 25 % of the total weight of said steel wire
rope, which is less than normally used for fiber core element without sheath. For
example, the lubricant can have a weight in a range of 10 to 12 % of the total weight
of said steel wire rope.
[0013] The steel wire rope according to the present invention has at least one outer layer
consists of steel wire strands, e.g. 6, 8, 12 or 16 steel wire strands. The steel
wire strands can have any construction which is known in the prior art. The steel
strand can have a triangular or round shape.
[0014] As another example, the core element can be surrounded by two outer layers and each
outer layers consists of a plurality of steel wire strands. Preferably, the twist
direction of the two outer layers are different.
[0015] The steel wire strands can be made from high-carbon steel. A high-carbon steel has
a steel composition as follows: a carbon content ranging from 0.5 % to 2.15 %, a manganese
content ranging from 0.10 % to 1.10 %, a silicon content ranging from 0.10 % to 1.30
%, sulfur and phosphor contents being limited to 0.15 %. preferably to 0.10 % or even
lower; additional micro-alloying elements such as chromium (up to 0.20 % - 0.40 %),
copper (up to 0.20 %) and vanadium (up to 0.30 %) may be added. All percentages are
percentages by weight.
[0016] Alternatively, the steel wire strands can be made from low carbon steel (a carbon
content ranging from 0.05 to 0.30 wt %), medium carbon steel or stainless steel.
[0017] According to the invention, the steel wire strands can be coated with zinc and/or
zinc alloy. Preferably, the steel wires of the steel wire strands of the outer layer
are coated individually with zinc and/or zinc alloy. More preferably, the coating
is formed on the surface of the steel wires by galvanizing process. As an example,
zinc aluminum alloy can be applied. In contrast to zinc, a zinc aluminum coating has
a better overall corrosion resistance and is more temperature resistant, Still in
contrast to zinc, there Is no flaking with the zinc aluminum alloy when exposed to
high temperatures. A zinc aluminum coating may have an aluminum content ranging from
2 wt % to 12 wt %, e.g. ranging from 5 % to 10 %. A preferable composition lies around
the eutectoid position: aluminum about 5 wt %. The zinc alloy coating may further
have a wetting agent such as lanthanum or cerium in an amount less than 0.1 wt % of
the zinc alloy. The remainder of the coating is zinc and unavoidable impurities. Another
preferable composition contains about 10 % aluminum. This increased amount of aluminum
provides a better corrosion protection than the eutectoid composition with about 5
wt % of aluminum. Other elements such as silicon and magnesium may be added to the
zinc aluminum coating. More preferably, with a view to optimizing the corrosion resistance,
a particular good alloy comprises 2 % to 10 % aluminum and 0.2 % to 3.0 % magnesium,
the remainder being zinc.
[0018] The sheathed polymer on the core element can be any thermoplastic polymers. For instance,
it can be selected from Arnitel®, Hytrel®, Polyethylene (PE), Polypropylene, (PP),
Polyurethane (PU) or Polyvinyl chloride (PVC). As a preferred example, the core element
can be coated with a polymer having copolyester elastomer containing soft blocks in
the range of 10 to 70 wt %. Preferably, the hardness Shore D of the copolyester elastomer
as measured according to ISO 868 is larger than 50. In a preferred embodiment, the
copolyester elastomer contains soft blocks in the range of 10 to 40 wt %. In a more
preferred embodiment, the copolyester elastomer contains soft blocks in the range
of 20 to 30 wt %. In a most preferred embodiment, the copolyester elastomer contains
25 wt % soft blocks. The modulus and the hardness of the copolyester elastomer depend
on the type and concentration of soft blocks in the copolyester elastomer. The advantage
of using the copolyester elastomer containing soft and hard blocks in the manufacture
of steel wire rope is that a hard transition layer established in-between the core
and the outer layer. Less concentration of soft blocks in the copolyester elastomer
can make the elastomer harder. Thus, the application of copolyester elastomer layer
between the core and outer steel layer improves the fatigue resistance of the steel
wire. Furthermore, the copolyester elastomer containing soft blocks is compatible
with the inner fiber core element and the outer layer. Also, the material has outstanding
resistance to flexural and bending fatigue both at high temperatures and sub-zero
temperatures. This makes it particular suitable for applications such as crane ropes,
which are subjected to a wide range of temperatures and also encounter very high levels
of flexural fatigue and compression.
[0019] Suitably, the copolyester elastomer is a copolyesterester elastomer, a copolycarbonateester
elastomer, and /or a copolyetherester elastomer; i.e. a copolyester block copolymer
with soft blocks consisting of segments of polyester, polycarbonate or, respectively,
polyether. Suitable copolyesterester elastomers are described, for example, in
EP-0102115-B1. Suitable copolycarbonateester elastomers are described, for example, in
EP-0846712-B1. Copolyester elastomers are available, for example, under the trade name Arnitel®,
from DSM Engineering Plastics B.V. The Netherlands. Preferably copolyester elastomer
is a copolyetherester elastomer.
[0020] Due to the compression force applied on the core element during rope closing, the
sheathed core element of the invention steel wire rope can be compressed by the outer
layer. The sheath also protects the core fibers from damage during rope closing compression.
The sheathed core element may go in between the steel wire strands of the outer layer
and have a natural star shape in cross-section or helical flute shape in three dimensions.
The natural star shape in cross-section of the core element can be kept during the
lifetime of the steel wire rope.
[0021] According to a second aspect of the invention, it is provided a method of manufacturing
a steel wire rope, comprising the steps of:
- (a) providing a core element, wherein said core element containing natural plant fibers;
- (b) extruding said core element with a polymer having a thickness in a range from
0.2 to 2 mm; and
- (c) twisting a plurality of steel wire strands around said core element to form an
outer layer such that said steel wire rope is closed.
[0022] According to the invention, the extruding in step (b) is preferably a - jacket extrusion.
The extruded polymer can be selected from Arnitel®, Hytrel®, Polyethylene (PE), Polypropylene
(PP), Polyurethane (PU) or Polyvinyl chloride (PVC).
[0023] To the knowledge of the inventors, specialist equipment and skill is required to
extrude polymers over natural plant fiber, in particular sisal, since the natural
plant fiber is easy to burn. On the other hand, in order to sufficiently avoid penetration
of liquid, e.g. water, the sheath should have good impermeable property. Moreover,
the sheath on the core is desirable to survive during the lifetime of the rope. Due
to the abrasion of the core element with the outer steel wire strands, the thickness
of the sheath is preferably in a range from 0.2 to 2 mm and more preferably in a range
from 0.5 to 1 mm. The extrusion process used to form the polymer sheath on the natural
plant fiber core element is preferably a well-designed and concentric jacket extrusion.
The advantage of applying jacket extrusion is to ensure a uniform and consistent thickness
of the polymer layer around the fiber core element and along the length of the core,
as the shape and diameter of a fiber core is less stable under tension than steel
wire ropes. On the other hand, the applied temperature of molten polymer and residence
time of fiber core to elevated temperature during the jacket extrusion process should
be affordable to the natural plant fiber, thus avoiding the risk of burning of the
natural plant fiber core element. The technique involves passing the fiber core through
an extrusion head whilst overlaying the polymer over the fiber core at a predetermined
draw down ratio and optimized line speed to result in a tight and concentric jacket
over the fiber core.
[0024] According the invention, in step (c) said core element can undergo compression during
rope closing such that said core element takes a natural star shape in cross-section
or helical flute shape in three dimensions after rope closing.
[0025] The invention illustratively described herein may suitably be practiced in the absence
of any element or elements, limitation or limitations, not specifically disclosed
herein. Thus, for example, the terms "comprising", "including", "containing", etc.
shall be read expansively and without limitation. Additionally, the terms and expressions
employed herein have been used as terms of description and not of limitation, and
there is no intention in the use of such terms and expressions of excluding any equivalents
of the features shown and described or portions thereof, but it is recognized that
various modifications are possible within the scope of the invention claimed.
Brief Description of Figures in the Drawings
[0026] The invention will be better understood with reference to the detailed description
when considered in conjunction with the non-limiting examples and the accompanying
drawings, in which:
Fig. 1 is a cross-section of a steel wire rope according to a first embodiment of
invention.
Fig. 2 is a cross-section of a steel wire rope according to a second embodiment of
invention.
Mode(s) for Carrying Out the Invention
Steel wire rope 1
[0027] Fig. 1 is a cross-section of an invention steel wire rope according to a first embodiment.
The invention steel wire rope 10 comprises a natural plant fiber core 12, a coated
polymer layer 13, and an outer layer containing steel wire strands 16. The steel wire
rope 10 as illustrated in Fig. 1 has a 6XV39(K3/9-12-15)-FC rope construction (the
designation of the rope is according to BS EN 12385-2:2002 +A1:2008 Steel wire ropes
- Safety - Part 2: Definitions, designation and classification, V stands for triangular
and K for compaction). This refers to a rope design with an outer layer having six
single triangular wire strands 16 and a fiber core (abbreviated as FC) 12. The triangular
wire strand 16 has a K3/9-12-15 structure. The four layers of the outer steel wire
strand 16 are preferably-twisted in the same direction, i.e. if it is a right hand
rope, the 9 over 3, 12 over (3-9) and the 15 over 12 are all right hand (Z), so the
rope is a Lang's lay rope.
[0028] As an example, a twisted 3 strands sisal rope 12 is used as the core of the steel
wire rope. In a next step the sheathed polymer layer 13, such as Arnitel®, is extruded
on the core 12 using a jacket extruder with the designed processing conditions. The
technique involves passing the fiber core through an extrusion head whilst overlaying
the polymer over the fiber core at a predetermined draw down ratio and optimized line
speed to result in a tight and concentric jacket over the fiber core. The sheathed
polymer has a thickness in a range from 0.2 to 2 mm, e.g. a thickness of 0.2 mm, 1
mm or 2 mm and preferably in a range of 0.5 to 1.5 mm.
[0029] The steel wire rope is obtained by twisting six steel wire strands 16 around the
sheathed sisal rope core 12 to form an outer steel wire strand layer such that the
steel wire rope is closed.
[0030] It should be noted that the sheathed core element undergoes compression during rope
closing such that said core element takes a natural star shape in cross-section after
rope closing, as shown in Fig.1.
Steel wire rope 2
[0031] Fig. 2 is a cross-section of an invention steel wire rope according to a second embodiment
of the invention. The invention steel wire rope 20 comprises a sisal fiber core 22,
an extruded thermoplastic jacket 23, and an outer layer containing steel wire strands
26. In this embodiment, the wire rope 20 has a rope construction of 6xK19(1-9-9)-FC.
In contrast to the above first steel wire rope, the outer steel wire strand 26 in
this second steel wire rope is a compacted independent wire rope and has a round shape.
As shown in Fig. 2, the strand 26 has a 1-9-9 structure.
1. A steel wire rope comprising a core element surrounded by at least one outer layer,
said core element containing natural plant fibers, said at least one outer layer comprising
a plurality of steel wire strands, wherein said core element is sheathed with a polymer
having a thickness in a range from 0.2 to 2 mm.
2. A steel wire rope according to claim 1, wherein said natural plant fibers are sisal.
3. A steel wire rope according to claim 1 or 2, wherein said core element is an independent
sisal wire rope having a rope construction of 3 strands, 4 strands, 1x4 or 1x6 strands.
4. A steel wire rope according to claim 3, wherein the sheathed polymer is an extruded
layer,
5. A steel wire rope according to any one of the preceding claims, wherein the sheathed
polymer has a thickness in a range from 0.5 to 1 mm.
6. A steel wire rope according to any one of the preceding claims, wherein said core
element is lubricated with lubricant having a weight in a range of 10 to 25 % of the
total weight of said steel wire rope.
7. A steel wire rope according to any one of the preceding claims, wherein said at least
one outer layer consists of 6, 8, 12 or 16 steel wire strands having a triangular
or round shape.
8. A steel wire rope according to any one of the preceding claims, wherein said steel
wire strands is made from low, medium and high carbon steel or stainless steel.
9. A steel wire rope according to any one of the preceding claims, wherein said core
element is surrounded by two outer layers and each outer layers consists of a plurality
of steel wire strands,
10. A steel wire rope according to any one of the preceding claims, wherein said steel
wire strands are coated with zinc and/or zinc alloy.
11. A steel wire rope according to any one of the preceding claims, wherein said polymer
is selected from Arnitel®, Hytrel®, Polyethylene (PE), Polypropylene (PP), Polyurethane
(PU) or Polyvinyl chloride (PVC).
12. A steel wire rope according to any one of the preceding claims, wherein said sheathed
core element is compressed by the outer layer and has a natural star shape in cross-section.
13. A method of manufacturing a steel wire rope, comprising the steps of:
(a) providing a core element, wherein said core element containing natural plant fibers;
(b) extruding said core element with a polymer having a thickness in a range from
0.2 to 2 mm; and
(c) twisting a plurality of steel wire strands around said core element to form an
outer layer such that said steel wire rope is closed.
14. A method of manufacturing a steel wire rope according to claim 13, wherein said extruding
in step (b) is a jacket extrusion.
15. A method of manufacturing a steel wire rope according to claim 13 or 14, wherein in
step (c) said core element undergoes compression during rope closing such that said
core element takes a natural star shape in cross-section after rope closing.