Why special wire ropes?
Standard ropes often do not meet the high requirements of many applications of wire ropes. Higher demands for rope lifetime, breaking strengths, rotational stability, flexibility, structural stability and spooling behavior can only be fulfilled by special wire ropes. It is for these reasons that many engineers and end users resort to verope special wire ropes.
Many verope products have a plastic layer between the steel core and the outer strands. This intermediate layer stabilizes the form stability of the rope like a flexible corset and increases the lifetime of a rope especially under difficult working conditions. The intermediate plastic layer avoids the infiltration of water and dirt, which helps avoiding corrosion in the steel core. This cushion avoids internal steel-to-steel cross over contacts and limits as such the damage caused by this phenomenon (figure 29).
The main advantages are:
• Prevents internal wire breaks
• Seals in rope lubricant
• Keeps out infiltration of water, dust, etc…
• Reduces the internal stress
• Improves the form stability of the rope
• Absorbs dynamical energy
• Reduces the noise level
verope Special Wire Ropes are designed to achieve high breaking loads and better strength to weight ratios. High ductility wires drawn to controlled tolerances are stranded and closed into a rope constructed with optimized gap spacing between the individual rope elements. verope products achieve an increased fill factor by using compacted strands as well as rotary swaging in their method of rope construction. Parallel lay elements in the rope composition increase the metallic cross sectional area. Crane designers use the technical advantages provided by the rope manufacturers to reduce the drum and sheave dimensions in line with maintaining the recommended D/d ratios. The material cost and weight saving effect on the static design of the crane elements is substantial.
Breaking Strengths and Swivel
The minimum breaking strengths that is given in catalogs is valid for wire ropes whose ends are protected against twisting. The breaking strengths of non-rotation resistant ropes is reduced significantly by the usage of a swivel. Even if the rope would not immediately break under the nominal load, several now overloaded elements of the rope will disproportionally become fatigued. Also structural changes, like basket deformation could appear very fast. Therefore non-rotation resistant ropes should not be used with a swivel.
Bending fatigue and rope lifetime
verope operates the first two bending fatigue machines world-wide built according to a revolutionary concept. The steel wire rope is installed in the test machine, put under tension, and then the rope travels back and forth over five test sheaves until it finally breaks in the middle. Only then the rope analysis starts: To the left and to the right of the broken section, which during the test has travelled back and forth over five sheaves, the machine has rope sections which only travel over four sheaves and don’t make it to the fifth. Regardless of what the number of cycles to failure will be, these sections will always have made 80% of this number. These sections, and the further sections which will have travelled over 3, 2, 1 and 0 sheaves only and which will represent the condition of the rope after 60%, 40, 20% and 0% of the rope life are cut out for analysis (figure 35).
One of the two sections of each condition is used to determine the number of external wire breaks and the changes in rope diameter and lay length. Then the section is taken apart in order to also determine the number of internal wire breaks on the underside of the outer strands, on the outside and inside of the IWRC and on individual strands as well as changes in the IWRC and strand diameters and lay lengths. This way the sections will tell you how the external wire breaks develop over the lifetime of the rope, how the internal wire breaks develop over time, how the plastic infill looks at different stages of the rope life and which elements start to deteriorate first. These results can help verope. to improve the product design of a new ope after only a single test. The comparable 80%, 60%, 40%, 20% and 0% sections on the other side of the break are subjected to pull tests to destruction. This way verope. can determine how the strength of this rope design, its modulus of elasticity and its elongation at break develops over the lifetime of the rope. A steel wire rope should have a breaking strength as high or higher than new until it reaches the discard number of wire breaks (figure 34).
figure 34: Rope breaking strength in % of the breaking strengths of a new rope dependent on the lifetime until break. A steel wire rope should have a breaking strength as high or higher than new until it reaches the discard number of wire breaks.
Due to the detailed analysis of the several working sections, the development of external wire breaks over the lifetime can be evaluated very precise (figure 42).
figure 42: Number of visible (pulled trough line) and invisible wire breaks (sketched line) dependent on the rope lifetime. After the ending of the bending fatigue test the analysis of the rope sections with the different fatigue numbers shows the marked numbers of wire breaks.
On disassembling the rope pieces the internal wire breaks depending on the lifetime can be evaluated (figure 43). At the veropro 8 construction the number of visible wire breaks is higher than the number of internal (invisible) wire breaks.
figure 43: Development of the visible wire breaks on the wire rope surface and the wire breaks inside the rope that are visible from outside
figure 44: Diameter modification of the rope in the bending fatigue test
figure 45: Diameter modification of the steel rope core in the bending fatigue test
Bending fatigue tests are taken normally until the break of the rope or a strand. The exact point of discard can be determined by evaluating the single rope sections.
This results in the so called “rest-lifetime” (lifetime between discard and break) (figure 46).
figure 46: Number of bending cycles until discard and until break
figure 47: Number of bending cycles until discard and until break (non-rotation resistant ropes, equal load)
figure 48: Number of bending cycles until discard and until break (rotation-resistant ropes, equal load)
Bending fatigue when using steel or plastic sheaves
The lifetime of a rope is significantly influenced by the sheave material. By the use of plastic sheaves the bending fatigue rises clearly in comparison to the use of steel sheaves. The remaining “rest-lifetime” of the rope after the achievement of the discard criteria until the break of the rope, is with regard of the bending cycles more or less the same, nevertheless it drops significantly in percentage. Therefore the rope inspection must be carried out especially carefully when using plastic sheaves. verope recommends plastic sheaves, hence, only in applications where the ropes are checked magnet-inductively or where the rope gets damaged primarily outside like in multi-layer spooling (figure 49).
Bending fatigue of ungalvanized and galvanized ropes
A comparison of the bending fatigue of ungalvanized and galvanized ropes until the achievement of the discard criteria according to ISO 4309 and until break of the rope shows, that galvanized ropes usually reach more bending cycles. The zinc coating offers better “emergency operating features” when the rope is not lubricated any more and protects the rope from friction corrosion (figure 50).
Bending fatigue in dependence on the line pull
The appealing line pull has a considerable impact on the bending fatigue. While for example with a line pull of 2t, still 950.000 bending cycles are reached, with a line pull of 4t only 290.000 bending cycles are reached (figure 52).
Bending fatigue in dependence on the groove diameter
According to ISO 4309 the groove of a sheave should have a diameter, that is 5% to 10% bigger than the rope diameter. During the operating time, the rope diameter will decrease. With this decreased diameter the rope will dig itself in the sheave groove and will reduce the groove diameter. Therefore, with the installation of the rope it should be considered, that the groove diameter of the sheave is at least 1% bigger than the measured rope diameter. If the groove diameter is too big, the support of the rope is not very good anymore and the surface pressure increases. Consequently the lifetime of the rope decreases steadily with an increasing groove diameter. If the groove diameter is too small, the rope will be squeezed and the lifetime d Steel sheaves rops extremely.
In many applications the exact knowledge of the deformation behavior of wire ropes is of great importance. verope. has investigated in many work-intensive tests the modulus of elasticity (lengthwise and transverse), the elastic and plastic deformation as well as the diameter reductions of its products. Many technical parameters of the rope can be determined by the creation of a load-elongation diagram (figure 54). verope. loads and relieves the ropes in steps and determines out of this the elongation under load as well as the remaining elongation after discharge. The elasticity modulus is determined from the gradient of the linear area of the load curves. At the same time the diameter reduction in dependence of the load is measured. In order to be able to determine also the breaking strength and the elongation at break, the ropes are loaded up to the break.
Modulus of elasticity
Within a rope construction the modulus of elasticity varies slightly in dependence of the rope diameter, the lay type (lang’s lay or regular lay) and of the tensile strength of the wire (figure 55). As a rule, the modulus of elasticity of wire ropes increases over the lifetime of the rope.
In particular with suspension ropes, but also with running ropes an exact knowledge of the elongation of the rope under load and the remaining rope lengthening after load is important. verope has measured these relevant values for all its products with high precision on long test lengths. You will find here measured values of typical verope rope constructions. We are pleased to provide you with the results of other verope rope construction for your interpretations.
A rope becomes longer and thinner under load. The diameter reduction can influence the rope behavior in multi-layer spooling strongly. verope has measured the diameter reduction of all its products and will be pleased to provide you with the measured values if required.
Lateral stability with and without load
In multi-layer spooling wire ropes are additionally to tensile and bending loads exposed to enormous transverse loads. In order to be able to withstand these loads and to avoid spooling problems, a high degree of radial stability is necessary. The radial stability of the rope also influences the deforming behavior of the drum. That’s why it is important for the designer of the drum to know the radial stability in the form of the transverse modulus of elasticity of the ropes. Radial stability is defined as the resistance of a wire rope against transverse (radial) deformation (Ovalization). verope measures the radial stability of its products with (figure 62) and without load (figure 60 & 61).
Measurement under load
At the determination of the transverse modulus of elasticity under load the deformation behavior of the rope is measured under various tensile loads and different transverse loads (figure 62). verope has determined the transverse modulus of elasticity for all its products and will be pleased to provide them to designers when required.
To evaluate the rotational behavior of a wire rope the rope torque and the rotation angle are measured. For the measurement of the rotation angle a smooth swivel is fastened at the end of the rope. During the test the twist of the rope is measured in dependence of the load. The twist usually is given in degree per 1000 x rope diameter. To measure the rope torque both rope ends are protected against twisting. At one end of the rope the rope torque in dependence of the load is measured, with which the rope wants to twist the end-fitting.
The flexibility of a rope is a measure of how easily a rope allows itself to bend around a given diameter. The flexibility of a rope is among other things dependent on the line pull. The flexibility of an unloaded rope can be measured by the sag of a rope under its own weight. Figure 67 shows the maximum sag of the rope for different free rope lengths (expressed as a multiple of the rope diameter). The flexibility of ropes under load is measured as the efficiency factor of the rope while running over a sheave.
Figure 68 shows a typical diagram of a rope efficiency factor under line pull. In many specified standards one finds the reference, that for dimensioning a reeving system using roller bearings it should be calculated with an efficiency factor of 0.98, this value is marked in figure 68. However, the designer of a reeving system needs the efficiency factor under high line pulls (area B in the diagram, here the efficiency factor is higher than 0.98) for the calculation of the required drive power. In order to calculate the minimum weight of the unloaded bottom hook block the designer needs the efficiency factor under relatively low line pulls (area A in the diagram, here the efficiency factor is clearly lower than 0.98). To help the designer in his interpretation, verope measures the efficiency factor of its products in the low-load range and in the range of high loads with high accuracy (figure 69 & 70).
As the very first special wire rope manufacturer, verope has measured the efficiency factor of its products over the lifetime of the ropes. Typically the efficiency factor of the rope improves first over the lifetime and drops later to reach the initial value at discard. Figure 71 shows a typical example. Under higher loads the efficiency factor of verope special wire ropes with a D/d ratio of 20 or higher lies demonstrably above 0.99. Therefore shall for example cranes that are certified by Germanischer Lloyd using various verope special wire ropes be interpreted with an efficiency factor of 0.99. Please contact us for further details.