Assessment of rope condition either by visual examination or drawing a specimen rope length and subjecting it to destructive evaluation seldom speaks about integrity of the entire rope length in the installation[sup1]. Further, ropes that are used on friction winders and aerial ropeway passenger cable car installations cannot be assessed at any cost due to non-availability of rope length for destructive investigation. Under these situations, non-destructive investigation is the only means for its evaluation to study the behavior.
Magnetic flaw detectors are used to determine rope wearing[sup2]. The detectors can measure loss of metallic area (LMA) and detect local faults (LF) such as broken wires and other faults of the rope. Deterioration of a rope during its lifetime leads to a reduction of the rope safety and to its possible destruction. Electro-magnetic nondestructive testing allows an increase in the safe use of steel ropes due to objective, reliable and documented evaluation of the real rope condition and by ensuring timely rope replacement.
Objectives
Following are the main objectives for nondestructive evaluation of steel wire ropes used in mine winders and in aerial ropeways:
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study of the condition of ropes over a period of time at regular interval of three months/six months/one year depending on age, condition of ropes etc. in the installation;
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assessment of the suitability of ropes by non-destructive evaluation: and
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achievement of the optimum safety, economy and reliability during operation of the ropes in their current installation.
Available methods and scopes
The most conventional inspection method for wire ropes is visual inspection where experts observe the surface and assess the rope condition empirically. They cannot evaluate inner failure such as corrosion. The visual method can also be an inadequate inspection due to its subjectivity. On a practical basis, it is hard and nearly impossible to review thoroughly a rope covered by lubricant. Only surface faults of the rope can be detected and this is insufficient to define its condition correctly.
Visual inspection alone is inadequate to provide a real definition of the rope degradation level, even if the inspection is fulfilled conscientiously. Using the electromagnetic method, a rope expert is better able to estimate rope condition. Electromagnetic nondestructive evaluation of wire ropes has been in regular use in a number of countries for inspection of hoisting ropes in deep mines and inspection of aerial ropes. The principle of operation for electromagnetic non-destructive evaluation of wire rope systems employ: measurements of fringe fields near the surface of the rope to detect local defects such as broken wires, corrosion pitting, local wear, and measurements of changes in magnetic flux passing through a short length of rope to quantify changes in metallic cross-section.
Instrumentation recently used for nondestructive testing of steel wire ropes generally uses the "DC Magnetic Method" for magnetization of the rope with permanent magnets and detection of the changes of magnetic field around the rope and total magnetic flux. Discontinuity in the rope, such as broken wire or corrosion pit, creates radial magnetic flux leakage and the sensor detects it as the rope passes through the sensing head. Other sensor measures total axial magnetic flux in the rope. It provides information about loss of steel due to missing wire, continuous corrosion or abrasion.
Detectability of rope defects depends mainly on the sensing head employed but readability of its signals and ease of operation depends mainly on the recording/processing instrument. The sensing head brings the running sector of wire rope to a condition close to magnetic saturation and provides signals from its sensors.
Rope rejection criteria can be divided into qualitative and quantitative ones. The qualitative criteria are: various types of deformation; damage as a result of high temperature or of a flash influence; and strand or metallic core break. The qualitative criteria are: diameter change, surface or inner abrasive wear and (or) corrosion of wires which lead to loss of metallic cross-sectional area (LMA), quantity of breaks of outer and inner wires per definite length (usually per 6d, 30d or 40d, where d in mm is nominal rope diameter). The last criterion belongs to the rope local faults (LF) whereas the first one (LMA) belongs to the dispersed faults. The LF and LMA signals of the non-destructive evaluation instruments represent the electronic equivalent of the mechanical anomalies present in the wire rope. The saturating magnetic field of the instrument makes the anomalies visible to the magnetic sensors placed around the rope. This process is somewhat similar to making non-destructive examination of a human body with X-rays, where density variations of the patient are made visible by greater or lesser absorption of these rays.
Critical Rope Assessment and Evaluation
A few case studies in mine conveyor system, mine winders and aerial ropeways have been cited below:
Two steel wire ropes each of 51 mm dia., 6×25 FW construction, fiber main core, in a material handling cable belt conveyor system have been considered investigated in-situ in four phases for evaluation of the condition over nearly three years. A concentration of flaws (broken wires) in different places on the ropes has been noticed in the last phase. Within a few splicing zones, broken wires have also been observed.. The last phase investigation has revealed more flaws compared to the results found during the previous one. An increase in fatigue resulting significant raise in broken wires over stipulated length of 2.04 meters (40d) has been observed. Immediate replacement of those two ropes has been recommended.
Two cage ropes and two skip ropes (each of 28 mm dia., full locked coil construction, right hand Lang's lay, galvanized) used for a multirope friction winder in uranium mines in India have been considered over nearly 48 months at regular interval for nondestructive evaluation. The cage ropes have been replaced after 36 months and the skip ropes the first time after 26 months and then after 24 months from the date of installation. Cage rope 2 has shown a higher number of flaws/100 m lengths. Skip rope 1 has registered comparatively more flaws/100m lengths during the first study after 25 months whereas skip rope 2 (new) has shown more flaws/100 m lengths during the second study after 17 months in the installation. Relative losses in the metallic cross-sectional area compared to the healthy portion of cage and skip ropes have been observed as nearly equal in each investigation. Lay lengths of winder ropes range from 6.3 to 6.7 times the nominal diameter. Locked coil ropes have constant cross sectional area. Comparatively less effect of corrosion has been noticed during the study. Steel cross-sectional area for locked coil rope has been assumed about 85% of the full (nominal) cross-sectional area.
Two ropes (each 60 mm nominal dia., construction 6X49, preformed, right hand Lang's lay, galvanized, Independent Wire Rope Core [IWRC]) used in two Koepe cage winders for coal mines have been considered for non-destructive evaluation. The relative loss in metallic cross-sectional area compared to a healthy portion of the rope is negligible in all six observations in both the cage winder ropes. The steel cross-sectional area for the IWRC rope has been taken about 60% of the full (nominal) cross-sectional area. The trend of occurrence of flaws as observed per 100 meters length of ropes is nearly the same in the ropes in the two cage winders. The lay length values have been found within the range of 6d to 8d. The ratio of observed dia. and nominal dia. has decreased with time. Presence of corrosion has also been noticed. The wire role length of about five meters each from both cappel ends are not subjected to non-destructive evaluation due to infrastructural disadvantage. It has been experienced through a destructive study with the 3 m sample of ropes from cappel ends that generally broken wire criteria is not observed throughout this length from cappel ends. Localized flaws in the form of broken wires generally concentrate near the point, which crosses and halts in the pulley whenever the cages/skips stop at boarding-deboarding/loading-unloading stations. Typically, they are caused by bending-over-sheave fatigue cycling.
A haulage rope of approx. length of 4.5 kms in continuous monocable with automatic grips of passenger cable car has been considered for non-destructive evaluation. For calculation of relative loss in cross-sectional area, it has been taken into account that steel cross-sectional area for stranded rope is about 55% of the full (nominal) cross-sectional area. This non-destructive evaluation on the rope has been carried out nearly seven years after its installation. The maximum length of the rope has been found as heavily corroded. A significant number of flaws, 52, have been observed and maximum relative loss in metallic cross-sectional area is more than 6% compared to a healthy sector except splicing. There exists five numbers of splices in the total length of rope.
The haulage ropes at six different mono-cable aerial ropeway installations have been studied using non-destructive evaluation method and the findings have been detailed below:
First investigation has been carried out on the rope (nominal dia. 40 mm) for the first time after five months in the installation. The lay-length is 6.7d and the spliced length is 1212.5d where d is the diameter in mm. No flaw has been observed and the % loss in relative cross-sectional area has been negligible.
Second investigation has been carried out on the rope (nominal dia. 40 umm) for the first time after eight months in the installation. The lay-length is 6.8d and spliced length is 1075d where d is the diameter in mm. Eight numbers of flaws have been observed and the relative loss in cross-sectional area has been found as 1.6% in corroded rope.
A third investigation has been carried out on the rope (nominal dia. 36 mm) for the first time after nine months in the installation. The lay-length is 6.9d and spliced length is 1167d where d is the diameter in mm. One flaw has been observed and maximum % loss in relative cross-sectional area has been found as 0.87%.
A fourth investigation has been carried out on the rope (nominal dia. 40 mm) for the second time after 72 months in the installation. There are two splicing zones in the total length of approx. 2150 meters. The number of flaws has been observed as 56 (48 found after 51 months) and the maximum % loss in relative cross-sectional area has been found as 4.07%. The lay-length and spliced lengths are 7.3d and 1325d respectively where d is the diameter in mm.
A fifth investigation has been carried out on the rope (nominal dia. 33 mm) after 24 months in the installation. The splicing portion is not good enough. The number of flaws and maximum % loss in relative cross-sectional area compared to a healthy portion of rope has been observed as 7 and 5.4%. The spliced length is 1242d.
A sixth investigation has been carried out on the rope (nominal dia. 36 mm) after 24 months in the installation. The number of flaws and maximum % loss in relative cross-sectional area compared to a healthy portion of rope has been observed as 8 and 3.5%. The spliced length is 1138d.
Wirerope Defectograph MD 120B, with its magnetic head 2-sh suitable for wire ropes varying dia. from 20 to 60 mm, of Polish origin has been used for the present investigation[sup7,8]. All the ropes have been calibrated first with metallic rod of known cross-sectional area of 80 sq mm/20 sq mm before actual scanning of ropes, The rope speed has varied from 0.6 to 1.2 meter/second. The lay-lengths observed are within the range of 6d to 8d. The spliced lengths are from 1000d to 1300d where d is the nominal diameter in mm.
Conclusions
A complete documentation of gradual rope deterioration throughout a rope's entire service life by periodic inspection from rope installation would enable the operator to arrive at a decision for preventing premature rope failure under adverse conditions or extensions of rope life in deserving cases.
The study depicts the present condition of ropes only. The measurements are intended to identify rope wear and other deterioration so that a wire is removed from service before it becomes hazardous to use.
Application of non-destructive evaluation procedures makes it possible to improve the reliability of detecting broken wires over the available rope length for evaluation. This non-destructive investigation on winding ropes does not include the aspect of fatigue that may develop in rope in course of time.
The reliability of electromagnetic inspection has made it a universally accepted method for the inspection of wire ropes in mining, for ski lifts, chair lifts, cable cars and many other applications.
With this in view, the Electrical Laboratory of the Central Mining Research Institute (CMRI), Dhanbad, India has been engaged over a considerable period of time in performing nondestructive evaluation on steel winding ropes used in mine winders and in aerial ropeways in different parts of the country and abroad.
ADDED MATERIAL
Debasish Basak received B.E. (Electrical Eng.) in 1986 from National Institute of Technology (formerly Regional Engineering College), Durgapur, India and M.E. (Electrical Engg.) with specialization in System Eng. & Operations Research in 1988 from I.I.T. Roorkee (formerly University of Roorkee), India, and an MBA degree in operations management in 2002 from IGNOU, New Delhi, India. He has worked nearly one year at IIT Kharagpur, India as "Junior Research Engineer" in a project sponsored by ISRO, Bangalore, India. In 1989, he joined Central Mining Research Institute, Dhanbad, India where he is currently Scientist "EII" and head of the Electrical Laboratory. He has published about 20 papers in national/international conferences/journals. His research interest includes support vector machines, optimization, reliability, nondestructive study of steel wire ropes etc. This paper was presented at WAI's International Technical Conference, New Delhi, India, October 2006.
Acknowledgements
The author is grateful to the Director, CMRI, Dhanbad, India, for his kind permission to publish the paper. The views expressed in the paper are of the author's and not of the organization he serves.
D. Basak
