Method for controlling elongation characteristics of wire rope

Abstract

To provide a core for providing a wire ripe having an adequate elongation property.SOLUTION: A wire rope 1 is composed by a core 3 made by a solid resin, and six strands 2 spirally twisted around the core 3. Where the product of a contact resistivity between the core 3 and the strand 2 twisted round the core 3 and a compressive stress of the core is A, and a poisson's ratio of the wire rope 1 is B, the value of B represented by B=-0.1×A+6.0 is two or more and 5 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to core materials for wire ropes used particularly as running ropes in cranes, lifts, and other equipment or facilities, and to wire ropes. [Background technology]

[0002] Wire ropes with non-metallic cores, typically synthetic resins, are less susceptible to wear (especially fretting wear) than wire ropes with metallic cores (for example, those with an Independent Wire Rope Core (IWRC)). However, with continued use, the multiple strands that make up the wire rope tighten around the core, causing the rope diameter to gradually narrow and elongation in the longitudinal direction. Elongation of wire ropes is likely to occur when they are used as running ropes for cranes, lifts, etc., and those used in lifts in particular are used in a loop with both ends connected, so any elongation in the longitudinal direction must be cut off. Furthermore, as elongation causes the diameter of the wire rope to narrow, making the wire rope more susceptible to break than in its initial state, wire ropes are generally replaced with new ones when they elongate beyond the specifications of the lift (for example, 0.5% elongation).

[0003] Patent Document 1 describes a wire rope that uses a resin core. A resin core reduces its diameter little when tightened from the outside, and therefore a wire rope using a resin core stretches little in the longitudinal direction. Because it takes time to reach the specified stretch, it can be used for a relatively long period of time (so-called long rope life). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-229544 Summary of the Invention [Problem to be solved by the invention]

[0005] The wire rope is pulled (Step 1), the strands squeeze the resin core from all sides and crush it (Step 2), the crushing of the resin core causes the wire rope to stretch (Step 3), the stretching of the wire rope causes the wire rope to reduce in diameter (Step 4), and the reduction in diameter of the wire rope further crushes the resin core (Step 2), and this cycle is repeated.

[0006] To prevent wire rope from stretching, it is appropriate to use a resin core that is resistant to crushing even when tightened with a strong force from the periphery, in other words, that has a high compressive stress.

[0007] The higher the compressive stress, the less likely the resin core is to be crushed, which in turn contributes greatly to preventing the wire rope from stretching. However, if the compressive stress is too high, the resin core becomes brittle and cannot be bent (it will break (fracture) when bent). Breaking of the resin core directly leads to elongation of the wire rope, so this must be avoided at all costs. Conversely, if the compressive stress of the resin core is too low, the degree of diameter reduction when tightened from the surrounding area will naturally increase, making the wire rope more susceptible to stretching, so this must also be avoided.

[0008] In addition to the compressive stress of the resin core mentioned above, the size of the contact surface between the resin core and the strands twisted around it is also worth considering as a factor related to the degree of crushing of the resin core. If the resin core and strands are in narrow contact, the clamping force is concentrated in a small area, making the resin core more likely to crush. If the resin core and strands are in wide contact, the clamping force from the strands is dispersed, and as a result the resin core is less likely to crush.

[0009] This invention focuses on two parameters, "contact ratio between core and strand" and "compressive stress of core," and aims to derive the contact ratio and compressive stress that a wire rope should have in order to achieve appropriate elongation characteristics. [Means for solving the problem]

[0010] The core material for wire rope according to this invention is made of a non-metallic material, with multiple metal strands twisted around it in a spiral shape, and is characterized in that, where A is the product of the contact ratio between the core material and the strands twisted around it (hereinafter referred to as the "contact ratio") and the compressive stress of the core material (hereinafter referred to as the "compressive stress"), and B is the Poisson's ratio (lateral strain / longitudinal strain) of the wire rope, the value of B, expressed by B = -0.1 x A + 6.0 (hereinafter referred to as the "common characteristic formula"), is between 2 and 5 (hereinafter referred to as the "common characteristic range").

[0011] When stress is applied to an object within its elastic limit, the "Poisson's ratio," which is the ratio (lateral strain / longitudinal strain) of the strain that occurs perpendicular to the stress (lateral strain) to the strain that occurs along the stress direction (longitudinal strain), is determined by the material of the object. The Poisson's ratio of a wire rope is also considered to be specific to the wire rope. However, in a wire rope that has a non-metallic material, such as a synthetic resin core (resin core) at the center and is constructed with multiple strands twisted around it, when tension is applied, radial strain (lateral strain) occurs in the resin core that makes up the wire rope according to the resin core's Poisson's ratio. Superimposed on this is strain (diameter reduction) caused by the strands tightening from the periphery. For this reason, the Poisson's ratio of a wire rope with a resin core at the center cannot be determined solely by the material. In this sense, calling the ratio of the lateral strain to the longitudinal strain that occurs in a wire rope when it is pulled the "Poisson's ratio" is not accurate given the definition of the term, but since it is a value calculated using two strains that occur in directions perpendicular to each other when stress is applied, in this specification the ratio of the lateral strain to the longitudinal strain that occurs in a wire rope when it is pulled will be called the "Poisson's ratio."

[0012] Cores were made from various non-metallic materials, and several types of wire ropes were fabricated by twisting multiple steel strands around the cores. Various studies were conducted to determine the relationship between the Poisson's ratio of each wire rope and the contact ratio and compressive stress. The Poisson's ratio of a wire rope could be calculated by multiplying the contact ratio and compressive stress. This relationship between the Poisson's ratio and the contact ratio and compressive stress (these multiplied values) is the "common characteristic equation" mentioned above. By following the common characteristic equation, the Poisson's ratio of a wire rope with a non-metallic core can be known (predicted) by the contact ratio and compressive stress. Conversely, by determining the Poisson's ratio, the contact ratio and compressive stress that a wire rope with that Poisson's ratio should have can be determined.

[0013] Considering the elongation characteristics of a wire rope (its resistance to elongation when pulled) and the prevention of damage to the core material, the Poisson's ratio of the wire rope should be within a specified range. If the Poisson's ratio is too small, it means that the wire rope is hard (hard to deform), and this is caused by the core material being too hard. By setting the Poisson's ratio of the wire rope to 2 or more, it is possible to specify a wire rope that is flexible and whose core material is less likely to break. Conversely, if the Poisson's ratio is too large, it means that the wire rope is soft (large deformation) and prone to elongation. By setting the Poisson's ratio of the wire rope to 5 or less, it is possible to specify a wire rope with reduced elongation. The common characteristic range (Poisson's ratio between 2 and 5) is specified from this perspective.

[0014] According to this invention, the Poisson's ratio of a wire rope manufactured by twisting multiple metallic strands around a nonmetallic core can be controlled using two parameters: the contact ratio between the core and strands (i.e., the core structure) and the compressive stress of the core (the type of core material) using the above-mentioned common characteristic formula. By controlling at least one of the contact ratio and compressive stress so that the value calculated by the common characteristic formula is between 2 and 5, specifically by adjusting the shape of the core (controlling the contact ratio) and selecting the core material (controlling the compressive stress), a high-performance wire rope with good elongation characteristics and reduced core fracture resistance can be provided. Using the common characteristic formula and common characteristic range allows for a wider range of choices for the core material that makes up the wire rope. The appropriate core structure (contact ratio) can be determined depending on the compressive stress of the specific material used as the core.

[0015] The contact ratio can be maximized by heating a non-metallic core, such as a resin core, to soften it and then twisting the strands around the softened core. However, heating the core is laborious and time-consuming, and it also takes time for the core softened by heating to cool and harden. On the other hand, using a core that is not heat-treated results in a smaller contact ratio than a core that is heat-treated, which increases the wire rope's Poisson's ratio. However, by adjusting the other parameter, compressive stress, a wire rope with a core that is not heat-treated can be made to have a Poisson's ratio comparable to that of a wire rope with a heat-treated core.

[0016] Preferably, the non-metallic material is a solid resin. The compressive stress of the core material can be adjusted by selecting the resin, making it easier to manufacture a core material for wire rope that satisfies the above-mentioned common characteristic formula and common characteristic range.

[0017] More preferably, the outer peripheral surface of the core is formed with a plurality of spiral grooves for accommodating and twisting each of the plurality of strands. The strands can be twisted around the core along the spiral grooves, thereby increasing the contact ratio (i.e., reducing the Poisson's ratio of the wire rope). The spiral grooves may be formed in advance (before twisting the strands), or they may be formed when twisting the strands. For example, if a thermoplastic resin is used as the core material, the strands of the core can be softened by heating and twisted together in a spiral shape, and then the thermoplastic resin can be hardened (cooled) to form a plurality of spiral grooves on the outer peripheral surface of the core.

[0018] In one embodiment, the compressive stress of the core material is 10 MPa or more and 40 MPa or less. By making the compressive stress of the core material 10 MPa or more, it is possible to prevent the strands from digging into the core material and causing the wire rope to become thinner. By making the compressive stress of the core material 40 MPa or less, it is possible to prevent the core material from being unable to withstand the bending of the wire rope and breaking.

[0019] The present invention also provides a wire rope comprising the above-mentioned core material for a wire rope and a plurality of metal strands twisted helically around the core material for a wire rope. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a cross-sectional view of a wire rope having a resin core with a spiral groove. [Figure 2] FIG. 2 is a perspective view of a resin core that constitutes a wire rope. [Figure 3] FIG. 1 is a cross-sectional view of a wire rope having a circular cross-sectional resin core. [Figure 4] The graph shows the compressive stress of the resin core on the horizontal axis and the Poisson's ratio of the wire rope on the vertical axis, and plots the measured values ​​for each of the five types of wire rope. [Figure 5]The graph has the horizontal axis representing contact ratio x compressive stress and the vertical axis representing the Poisson's ratio of the wire rope, and plots the measured values ​​for each of the five types of wire rope, and also shows an approximate straight line for the plots. [Example]

[0021] Fig. 1 shows a schematic cross section of a wire rope. Fig. 2 shows an enlarged perspective view of a resin core provided at the center of the wire rope. Fig. 1 shows the cross section of one of the six strands in detail.

[0022] The wire rope 1 shown in Figure 1 comprises a resin core 3 and six metal (steel) strands 2 twisted around the resin core 3. When viewed from a cross section, the wire rope 1 appears to have six strands 2 with circular cross sections surrounding the resin core 3 with the resin core 3 at the center.

[0023] Here, the strand 2 is made up of a total of 19 steel wires twisted together in a sealed manner. The number of steel wires constituting the strand 2, the twisting structure, and the number of strands 2 constituting the wire rope 1 can be changed as appropriate.

[0024] The resin core 3 is made of high-density polyethylene and is made solid by extrusion or pultrusion. Six spiral grooves 4 are formed on the outer surface of the resin core 3, and a strand 2 is placed in each of the six spiral grooves 4, resulting in a wire rope 1 in which the six strands 2 are twisted around the resin core 3.

[0025] The spiral groove 4 formed in the resin core 3 is formed so that its cross section is an arc. Referring to Figure 1, if the arc of the spiral groove 4 has a length equivalent to α = 120° in the cross section of the strand 2 (one-third the perimeter of the strand 2), the groove 4 and the strand 2 can come into contact with each other over the maximum area.

[0026] For reference, Figure 3 shows a wire rope 1A equipped with a resin core 3A having a circular cross section. In the wire rope 1A, the strand 2 comes into line contact with the resin core 3A over a minimum area.

[0027] In the following explanation, the "contact area ratio" between the resin core 3 and the strand 2 is considered, and the contact area ratio when the resin core 3 and the strand 2 are in contact over the maximum area as shown in Figure 2 is normalized to "1," and the contact area ratio when the resin core 3A and the strand 2 are in contact over the minimum area (linear contact) as shown in Figure 3 is normalized to "0." As will be described later, the "contact area ratio" between the resin core 3A and the strand 2 is used as a parameter for defining a wire rope 1 that is less likely to elongate (has a small Poisson's ratio), and details of this will be described later.

[0028] The contact area ratio (hereinafter simply referred to as the “contact ratio”) can be adjusted by, for example, the depth of the spiral groove 4 formed in the resin core 3 .

[0029] As described above, the resin core 3 provided in the wire rope 1 of this embodiment is made of high-density polyethylene (HDPE), and its compressive stress is about 20 MPa. The higher the compressive stress of the resin core 3, the less likely the resin core 3 is to be crushed when tightly clamped by the strands 2, and as a result, the wire rope 1 is less likely to elongate. In addition to the "contact ratio" between the resin core 3 and the strands 2 described above, the "compressive stress" of the resin core 3 can also be used as a parameter for defining a wire rope that is less likely to elongate.

[0030] The compressive stress of the resin core 3 can be selected or adjusted by changing the material of the resin core 3. To give some examples of resins other than the high-density polyethylene mentioned above, the compressive stress of low-density polyethylene (LDPE) is about 15 MPa, the compressive stress of polypropylene (PP) is about 30 MPa to 50 MPa, the compressive stress of polyoxymethylene (POM) is about 110 MPa, and the compressive stress of polyamide (PA6) is about 47 MPa.

[0031] Table 1 shows the results of fatigue tests on five wire ropes, Samples 1 to 5. In the fatigue tests, a wire rope was attached to a 3,000 mm diameter sheave and moved repeatedly while applying a specified tension, with each pass of the wire rope through the sheave counted as one bend, resulting in 300,000 bends of the wire rope, after which the diameter and length of the wire rope were measured. Table 1 shows the transverse strain of the wire rope (= (diameter after stretching (after fatigue test) - original diameter) / original diameter) and the longitudinal strain of the wire rope (= (length after stretching (after fatigue test) - original length) / original length), using the diameter and length of the wire rope before the fatigue test and the diameter and length of the wire rope measured after the fatigue test.

[0032] [Table 1]

[0033] Samples 1 to 5 were all made by twisting six strands around a core material, with the strands being the same, but the core material being different for each sample.

[0034] Sample 1 is a wire rope (hereinafter referred to as "heated type") made by heat-treating a solid core with a circular cross section made of high-density polyethylene, a thermoplastic resin, and twisting strands around the core that has been softened by heating. The core deforms along the outer surface of the twisted strands, allowing for maximum contact between the strands and the core (contact ratio = 1.00).

[0035] Like Sample 1, Sample 2 is a heated wire rope made by heat-treating a solid core with a circular cross section made of high-density polyethylene and twisting strands around the core that has been softened by heat, but it was made using high-density polyethylene from a different manufacturer than Sample 1. Specifically, it used a high-density polyethylene core with a slightly lower compressive stress (20 MPa) than the compressive stress (22 MPa) of the core of Sample 1. The wire rope of Sample 2 also has maximum contact between the strands and the core (contact ratio = 1.00).

[0036] Sample 3 is a wire rope that has the same solid core material made of high-density polyethylene as Sample 1, but is not heat-treated; instead, a spiral groove is formed on the outer surface of the core material and strands are twisted along the spiral groove (hereinafter referred to as "non-heated groove type"). Compared to the heated wire ropes (Samples 1 and 2), the contact ratio between the strands and the core material is smaller (contact ratio = 0.60).

[0037] Sample 4 is a wire rope made by twisting strands around a core with a circular cross section (hereinafter referred to as "unheated round"), which has the same solid core material made of high-density polyethylene as Samples 1 and 3, but is not heat-treated and does not have a spiral groove formed on the outer surface of the core. The contact ratio between the strands and the core is significantly lower (contact ratio = 0.05).

[0038] Unlike Samples 1 to 4, Sample 5 is a wire rope made by twisting strands around a fiber core composed of a large number of thin-diameter filaments bundled together, rather than a solid core (hereinafter referred to as "fiber core type").

[0039] Figure 4 is a graph in which the horizontal axis represents the compressive stress of the core material and the vertical axis represents the Poisson's ratio (lateral strain / longitudinal strain) of the wire rope, and the values ​​of samples 1 to 5 in Table 1 above are plotted.

[0040] Poisson's ratio is the value obtained by dividing the wire rope's lateral strain ((stretched diameter - original diameter) / original diameter) by the wire rope's longitudinal strain ((stretched length - original length) / original length). Therefore, the larger the Poisson's ratio, the softer the wire rope is, and the greater the stretch that occurs when the wire rope is reduced in diameter. Conversely, if the Poisson's ratio is small, the wire rope is relatively stiff and does not stretch much.

[0041] Referring to Figure 4, by using a core material with a high compressive stress (Samples 1 to 4), the Poisson's ratio of the wire rope can be made smaller than when a core material with a low compressive stress is used (Sample 5). However, for Sample 4 (unheated round), although the compressive stress of the core is high (22 MPa), the Poisson's ratio of the wire rope equipped with that core is relatively high (5.79). Comparing Samples 1 to 4, although the compressive stress of the core is roughly the same (20 to 22 MPa), the Poisson's ratio of the wire rope differs (3.35 to 5.79).

[0042] Figure 5 is a graph with the horizontal axis representing the contact ratio between the strands and core material that make up the wire rope x the compressive stress of the core material, and the vertical axis representing the Poisson's ratio of the wire rope (lateral strain / longitudinal strain), on which the values ​​of the above-mentioned samples 1 to 5 are plotted, and an approximate straight line is drawn through the five plots.

[0043] As shown in Figure 5, the Poisson's ratio of a wire rope can be calculated by multiplying the contact ratio between the strands that make up the wire rope and the core material by the compressive stress of the core material (contact ratio x compressive stress).It is thought that the larger the contact ratio x compressive stress, the smaller the Poisson's ratio of the wire rope can be made proportionally.

[0044] The approximate line shown in FIG. 5 is a linear function with a slope of -0.1168 and an intercept of 5.9451, and can be expressed as follows:

[0045] Wire rope Poisson's ratio (lateral strain / longitudinal strain) = -0.1 (contact ratio x compressive stress) + 6.0

[0046] This relationship holds true for any wire rope that has a non-metallic core around which multiple metallic strands are twisted, regardless of whether the core is a heated type (Sample 1, Sample 2), an unheated grooved type (Sample 3), an unheated round type (Sample 4), or a fiber core type (Sample 5). The above relationship means that the Poisson's ratio of a wire rope with a non-metallic core can be controlled by considering the two parameters of contact ratio and compressive stress. In other words, a wire rope with a target Poisson's value can be manufactured by controlling at least one of the two parameters of contact ratio and compressive stress. The above relationship, which defines the relationship between the Poisson's ratio of a wire rope and the contact ratio and compressive stress, is hereinafter referred to as the "common characteristic formula."

[0047] For example, if the material to be used for the core is decided, then of the two parameters, contact ratio and compressive stress, compressive stress is fixed, so the only remaining controllable parameter is the contact ratio. By controlling the contact ratio, it is possible to manufacture a wire rope with the target Poisson's ratio according to the common characteristic equation. Conversely, if the contact ratio is decided, for example, by using a heating mold, and it is decided that the contact ratio will be 1.0, then of the two parameters, the contact ratio is fixed, so the only remaining controllable parameter is compressive stress. This allows for a wider range of choices for the type of core material.

[0048] As mentioned above, a wire rope with a large Poisson's ratio undergoes a large elongation when its diameter is reduced. For example, by adjusting the contact ratio and compressive stress so that the Poisson's ratio is 5.0 or less, a wire rope that is relatively resistant to elongation even with continued use can be provided. Of the above-mentioned samples 1 to 5, samples 1 to 3 have a Poisson's ratio of 5.0 or less.

[0049] The greater the compressive stress of the material used for the core, the smaller the Poisson's ratio. Since the maximum contact ratio is 1.0, the compressive stress of the core is the main parameter for reducing the Poisson's ratio of the wire rope. However, if the compressive stress is too high, the core will become brittle and will not be able to bend (it will break (fracture) when bent). Core breakage directly leads to a decrease in the strength of the wire rope and must be avoided at all costs.

[0050] To prevent the core from breaking, the Poisson's ratio of the wire rope should be 2.0 or more and the compressive stress of the core should be 40 MPa or less. To prevent the strands from biting into the core too much, the compressive stress of the core should be 10 MPa or more. [Explanation of symbols]

[0051] 1 wire rope 2 strands 3 Heartwood 4 grooves

Claims

1. A method for controlling the elongation characteristics of a wire rope comprising a wire rope core made of solid resin and having spiral grooves with arc-shaped cross sections formed on the outer surface for accommodating and twisting together portions of a plurality of metal strands, and a plurality of metal strands with circular cross sections twisted together in a spiral shape in the grooves of the wire rope core, The contact ratio is a value between the core material and the strand twisted around the core material, and is a normalized value where the value when the core material and the strand contact each other over the maximum area is "1" and the value when the core material and the strand contact each other over the minimum area is "0." When the product of this contact ratio and the compressive strength determined by the type of material of the core material is A, the value calculated by -0.1 x A + 6.0 is controlled so that at least one of the contact ratio and the compressive strength is 2 or more and 5 or less. A method for controlling the elongation characteristics of wire rope.

2. The compressive strength of the core material is 10 MPa or more and 40 MPa or less. The method for controlling the elongation characteristics of a wire rope according to claim 1.

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