Defects in the core of steel cord belts are identified by registering and analysing changes in the magnetic field of magnetized cables around their defects, e.g. cutting cables or their cuts, their crushing and corrosion. In the analysis of damage to hoisting ropes, special wire breaks can be identified. In the case of conveyor belts, cuts of several cables and their corrosion in the same cross-section of the belt are more critical. This type of damage weakens the strength of the belt, which may cause the belt loop to break. Corrosion of lines on a large surface can lead to holes in the belt, spillage of spoil or an emergency stop of the conveyor and as a result entail high costs of damage removal and significant production losses. The use of a magnetic bar which has a large number of sensors and offers a dense, two-dimensional image of core damage (as in the case of DiagBelt) allows tracking both the changes of defects in individual cables and damage development over time. Every significant defect in the core increases over time. Repetitions of the belt scanning process allow detecting these changes and building a model of the rate of damage development in both directions: along the axis of the belt (along the cables) and in the transverse direction. Damage development in this direction is particularly exciting, because it causes the development of cracks in the rubber, initiated by punching the covers into the core or out of it. During the bending of the belt on the drums and on the idler sets, the cracks increase, and the water begins to reach adjacent cables, causing corrosion. That is why it is so important to repair major punctures in order to delay the cracking process.
Development of cracks in rubber and adjacent steel cables corrosion requires further modelling similar to FEM analysis of the effect of radial expansion and crack width determination of corroded reinforcement of concrete [16, 43] and damage modelling for elastic and viscoelastic materials at large strain [29]. Application of DiagBelt for the first time allowed detection and measuring of similar phenomenon taking place in rubber belts.
The DiagBelt system has been described in a number of publications [e.g. 7, 12, 14]. Figure 1 shows the method of damage imaging with the use of the BeltGuard probe confronted with an image of a significant belt defect [13].
As the system is of high resolution and offers the possibility to analyze field changes on various sensitivity levels, it allows 3D damage visualizations (Fig. 2, on the right), and their aggregation to squares 10 cm x 10 cm provides a 2.5 D image, in which the background color represents mean signal intensity level and the central point represents the maximum for the area (Fig. 2, on the left).
The system has sufficient resolution to allow a precise inventory of defects and the representation of aggregated damage level measures, such as for example damage density and area per running meter and the damage histogram on the belt section for a particular belt fragment or for the complete belt section from splice to splice [14]. Aggregating numerous defects into a synthetic measure is necessary to evaluate the general condition of a belt section, which is coded with colors (Fig. 3) enabling the user to instantly evaluate the degree of wear and tear in each of the segments in the loop. Green color corresponds to no damage and red color indicates a significant number of defects which may pose a threat to the functioning of a transportation system (Fig. 3).
Visual representation of belt sections and their fragments allows both the scope of each defect and the damage density to be identified along the belt axis and in the cross-section (Fig. 4), and also provides information on the locations in need of repair (Fig. 1-4). Owing to the system’s resolution, the size of each defect can be estimated and tracked over time [11].
The possibility to analyze the distribution of belt core defects in the cross-section of the belt represents a significant improvement. Several research centers (e.g. Wroclaw University of Science and Technology, [4, 5] and in Košice, [18, 20]) conduct intensive studies into belt puncture resistance. The investigations include the influence of the energy and shape of material lumps on the type of damage to the belt and its core [1]. These are also classified [2, 33, 38] with the purpose to increase belt puncture resistance by modifying the belt support [21, 26], the belt design [4, 19, 39], the composition of the rubber mixture [1] or the introduction of breakers [ ]. The magnetic system [3] or other tools for evaluating the effects of punctures [17] allowed a non-invasive inspection of cores and belts subjected to punctures. Typically, puncture resistance tests are performed in laboratory conditions. Until present, no possibility existed to identify damage distribution in the cross-section of the belt. The DEM software [6, 15, 27,40, 41] enables evaluations of how the shape of the hopper or the transfer chute influences the energy of the lump and the drop locations [30, 31, 36], as well as wear processes in both the belt covers and the transfer devices [22, 42]. However, it was impossible to verify this data in practice – a procedure of great significance in the validation of simulation models. From the user's perspective, damage distribution should be importantly even and not concentrated in the central part (Fig. 4), as in such case the belt wears faster in the central part while its edges or side parts practically remain in good condition. A belt which is evenly worn in its central part cannot be refurbished and therefore its core cannot be reused (two or even three times). Belt refurbishment consists in milling its worn external covers and in re-vulcanizing new covers with the old core. As such, the procedure meets the conditions of circular economy [8, 9, 28]. Disposal of the belt due to its overly worn central part is considered wasteful, as a large part of the belt is still in operating condition. As can be seen in Fig. 4, more than 50% of the belt cross-section remains in good condition.
Results of measurements with the Diagbelt diagnostic system in an underground mine
The data on belt condition collected during a series of five consecutive belt scans [Table 1] and the identification of the number and size of cord defects allowed a statistical analysis of the rates of damage growth over time in both directions. The rate of new damage formation was also determined. Submission of these two processes allows forecasting future belt condition. For this purpose, specific measures of belt wear rate were introduced, such as the belt damage density (the number of defects per 1 meter of belt), the density of belt defect area (the area of defects per 1 meter of the belt) and the change in the average area of one defect over time. By determining the regression of these measures in time and the rate of damage development in both directions (along the axis of the belt and across the belt), it was possible to forecast future states of the belt, as well as to evaluate the costs of different belt replacement strategies and the economic rationalization of the decision to replace them.
The diagnostics of the loop of steel cord belt operated in a Polish underground mine on a conveyor 2200 m in length performed in order to evaluate the condition of its core provided results which served to demonstrate random belt degradation process and its change over time. The first inspection of belt St 3150, 1200 mm in width, was performed in March 2016, when the belt had been operated in the mine for 60 months (5 years). The tests were repeated after 6, 9 and 24 months. This allowed observations of changes in belt damage condition. The evaluation covered the technical condition of the core of steel-cord belt used in the transportation of copper ore. Fig. 5 shows the condition of a section of the inspected loop and its indicated damage. The diagnostic system was located in the vicinity of the conveyor’s head station, on the belt’s flat section. Two magnet heads and a measurement head were installed at a distance of 40 mm above belt cover. During the entire test period, the belt speed, as measured with a tachometer, was approx. 2.5 m/s. The measurement data were exported to separate *.CSV (Comma-Separated Values) files to facilitate their further processing and the statistical analysis of the defects.
The identified defects are indicated by a change of colour from yellow to blue (change of the magnetic field from negative to positive) (Fig.5). The total of the aggregated source signals from all sensors is shown above as a blue signal line in the grey background. Splices (7 and 8) are visible on both ends of the belt section. The required repairs are indicated above, along with their precise locations measured from the left splice in meters and with information on the number of defective cables.
The exported digital data regarding the condition of the belt section and the defects were aggregated by calculating the total number of defects per successive running meters of the inspected belt section between splices 7 and 8. The length of the belt section was 138 meters. Thus, aggregated belt condition measure was obtained – damage density (GUi). Damage density can be used as a basis to evaluate the technical condition of a complete belt section or of its individual parts. In this case the analysis covered successive 1-meter fragments (segments) of the inspected section. In the case of segments one meter in length, the density corresponds to the number of defects identified in the areas of successive segments (GUsi=Nsi). In this part, the analysis focused on changes in damage density distribution along belt axis during consecutive scans.
The numbers of defects (their densities) are natural numbers. Zero represents no defects and n represents n defects identified in a particular 1-meter segment. This is shown in the graph of Fig. 6. The graph in Fig. 7 shows the change of damage density over time in consecutive scans of the 7-8 section from March, September and December 2016, as well as from March 2018. As can be seen, even after many years in operation, some 1-meter belt sections still show no trace of damage. Some other fragments, on the other hand, reach 7 defects per 1 meter.
In order to investigate whether defects along the axis of the belt section are randomly distributed, changes of the number (density) of defects in consecutive 1-meter segments were analyzed for randomness in a series of tests.
The calculations of the number of defects in consecutive segments were performed by analyzing the defects identified in the digital image of magnetic field changes in the area of the 7-8 belt section, in its consecutive 1-meter segments (Fig. 8).
Table 1 Dates of the measurements
Scan number
|
Scan date
|
Belt age in months
|
Number of sections
|
Date of installation
|
Feb. 2011
|
0
|
32
|
1
|
Mar. 2016
|
60
|
39
|
2
|
Sept. 2016
|
66
|
39
|
3
|
Dec. 2016
|
69
|
40
|
4
|
Mar. 2018
|
84
|
44
|