This invention relates to cores for wire ropes, particularly steel wire ropes.
GB-A-2 219 014 describes a core for wire rope comprising a fluted member of polymeric or elastomeric material having helical grooves symmetrically spaced around its outer surface, each groove in cross-section (in a plane normal to the longitudinal axis of the core) having the form of an arc of a ellipse. The properties and control of manufacture of the core may be substantially improved by incorporating within the extrusion a reinforcing member or cord constituted by a fibre core or independent wire rope.
However, the use of conventional types of reinforcing members such as are illustrated in GB-A-2 219 014, has certain drawbacks. Both fibre cores and independent wire rope cores, by the very nature of their helical construction, entrap a quantity of air which has a detrimental effect on the solidity of support that the core is able to provide in the rope. This is especially important in a wire rope, where the core is typically subjected to very high radial pressures; any compressibility of the core results in a reduction in rope diameter with tension and, hence, an increased rope stretch characteristic, which is often undesirable in service. Additionally, any pronounced reduction in rope diameter will result in the outer strands of the rope coming into contact with one another at the wire crowns, with very high stresses at the contact points. In a rope which is working backwards and forwards over sheaves or is otherwise repetitively flexed, these stresses will cause fretting damage to the outer wires, with a consequent loss of fatigue performance.
GB-A-2 219 014 discloses a core according to the preamble of claim 1, in which the fluted member is tubular with a cylindrical internal region for containing a reinforcing member. Practical experience with the conventional types of reinforcing member disclosed in GB-A-2 219 014 has revealed that it is possible for the reinforcing member to fail prematurely, before there has been any significant deterioration of the outer strands of the rope (which are readily inspectable). This mode of failure is not only undesirable from an inspection viewpoint, but can also lead to a loss of load carrying capacity and structural integrity of the wire rope.
What is desired is a core which avoids or mitigates these disadvantages.
The present invention provides a core for wire rope, the core being made of plastics material and comprising a tubular member of extruded plastics material having a cylindrical inner surface and having a fluted outer surface, formed by extrusion, with symmetrically-arranged spaced-apart helical grooves for receiving respective wire strands of the rope, characterised by a stiffer solid cylindrical central member of extruded plastics material, the inner surface of the tubular member being in intimate contact with the central member, the diameter of the central member being at least 70% of the root diameter of the fluted outer surface, and the cross-sectional area of the central member being at least 40% of that of the core.
Preferred and optional features are set forth in claims 2 et seq.
The invention will be described further, by way of example only, with reference to the accompanying drawings, in which :-
The core illustrated comprises a tubular outer member 1 of plastics material and a solid cylindrical central member 3 of plastics material. The outer member 1 has a cylindrical inner surface in intimate contact with the central member 3. Optionally there may be a film of lubricant or release agent at the interface between the members 1,3. The outer member 1 has a fluted outer surface with symmetrically spaced helical grooves 2 for receiving respective steel wire strands 4.
In the illustrated example of a wire rope, shown in Figure 2, there are six strands 4. However, it will be appreciated that a different number of strands may be used (e.g. 5, 7, or 8). Each strand 4 is a round strand whose circumscribing envelope E - in a cross-section perpendicular to the strand axis - is a circle of diameter d1. Since each strand extends helically, the envelope E appears slightly elliptical in a cross-section in the plane of the drawings, which plane is perpendicular to the axis of the core. Each groove 2 - in a cross-section perpendicular to the groove - is an arc of a circle of diameter d1.
To maximise the area of contact between the wires of one strand 4 and the core, the strand is a Dyform (Trade Mark - Bridon plc) strand which - in cross-section perpendicular to its axis - has a compacted structure in which the outer wires 6 have eternal edges 6a which are arcs of a circle of diameter d1. However, conventional strands made up of round wires may be used instead. Also, non-round strands may be used, with grooves of suitable cross-section.
The core, having a solid cross-section, provides good support to the strands 4. By providing the core with a sufficiently large cross-section with grooves 2 of sufficient depth, a working clearance between the adjacent strands 4 can be ensured, even up to the breaking load of the rope. The use of two distinct (outer and central) members 1,3 enables one to select the properties of the members 1,3 to satisfy various requirements for the physical (and chemical) properties of the core as a whole.
In order to define the preferred size of core, reference is made to Figure 1, wherein dimension d1 represents tne diameter of the outer strands 4 of the rope and diameter d2 denotes the root diameter of the fluted memoer 1, i.e. the minimum distance between the bottoms of any two diametrically opposing grooves. For any given rope type and construction the minimum root diameter d2 may be expressed as a function of the outer strand diameter d1. The exact relationship will depend primarily upon the number of outer strands in the rope and the lay angle (or pitch) at which they are closed together, although additional account may be taken of the strand construction and service duty. For example, different factors will be required for ropes in which the strands are of quasi-triangular shape. For the more common round-strand rope constructions and lays, the following relationships are recommended :
The two members 1,3 may be made of the same type of plastics material or different types. In each case the plastics material may, for example, be selected from the following types:
Specific grades of material, and their characteristics physical properties, are shown in the following Table 2 :
The above properties are cited for the purpose of illustration only. Additional property enhancement may be obtained by going to harder grades of material or incorporating reinforcing fibres (e.g. glass-fibre) into the polymer, albeit with some loss of rope flexibility.
The optimum blend of core properties may be obtained by using two dissimilar materials to produce the core. For example, the outer member 1 may comprise a material with good elongation properties to accept the high bending strains and also be capable of deforming locally to provide close conformance with the surface geometry of the outer strands of the rope, whereas the central member 3 may be beneficially composed of a harder and more rigid material, perhaps incorporating reinforcing fibres.
Whatever the choice of materials, the core is preferably prepared in two separate extrusion operations, the first of which produces the solid cylindrical member 3. Optionally, a lubricant or a release agent is applied to the solid cylindrical member 3. This is then fed into an extruder for a further operation which produces the fluted member 1, the central member 3 greatly assisting in the control of the second extrusion operation.
The size of the central member 3 in relation to the size of the core may vary, so that one size of central member can be used for a range of core sizes, thereby achieving an economy in manufacturing costs. However, the cross-sectional area of the central member 3 is at least 40% (more preferably at least 45%) and preferably at most 60% (more preferably at most 55%) of the cross-sectional area of the core, a convenient ratio being about 50% (enabling extrusion speeds to be equalised). Accordingly, the central member 3 has a circular cross-section whose diameter d1 is at least 70% (more preferably at least 75%) of the root diameter d2; preferably d3 is at most 90% (more preferably 85%) of d2, a convenient ratio being about 80%.