## 2 Materials and Methods  
The properties of the surfaces of materials change during their work due to both the work itself and the working conditions. The operation of a device is accompanied by a number of degradation processes, which frequently entail material losses, and these result in a reduced cross-section of the element and subsequently in the loss of its strength properties. During the transfer of bulk materials, wear is observed mostly as a result of impacts and abrasion in situations when two bodies have different relative speeds. Research into and analyses of the operation of transfer devices indicate that the speed of the material has the greatest impact on its wear processes [21]. Among numerous wear evaluation methods provided in the literature [22], the Burwell classification seems useful, distinguishing the following types of wear: adhesive, abrasive, corrosive, surface fatigue mechanisms, and minor wear types.  
The qualitative failure evaluation was performed on the basis of observations and photographic documentation. A visual inspection of the technical condition of individual elements and an inventory list of the observed surface changes allowed the identification of typical damage in the receiving bin and in the feeder carriage. The quantitative analysis of damage occurring in the operation of ore retention bunkers was performed on the basis of data collected and pre-processed for a 30-month period (between January 2018 and June 2020). The data concerned 41 receiving bins installed in 15 bunkers located in six units of a copper ore mine selected for the research. It provides information about the repairs and replacement of receiving bins' elements including the date, work shift, the name of mine unit, and the name of the retention bunker including number of receiving bins. The essential part of the research is that the analysis bases on the assumption that repair data reflects bunker failures. Furthermore, because the analysis is carried out for bunkers within the transportation system of the mine, Figure 4 schematically shows the locations of the bunkers (1-15). In the figure, blue color indicates smaller bunkers of a capacity up to 1000 Mg, and yellow color indicates larger bunkers capable of storing more than 1000 Mg of copper ore. Additionally, colors of belt conveyors define mining units (I-VI) and show the ore-flow routes.  
The qualitative evaluation of the bunker failure rate was performed with the use of the suggested **number-based failure indicator (failure indicator)** which describes the number of repairs per one receiving bin and with the **mass indicator**, which describes the frequency of a particular type of failure in relation to the mass quantity of the flowing ore. The above indicators allow for a different number of receiving bins installed under the analyzed bunkers.  
\[number-based\ failure\ indicator=\frac{number\ of\ repairs}{number\ of\ receiving\ bins} \tag{1}\]  
\[mass\ indicator=\frac{mass\ of\ the\ flowing\ ore}{number\ of\ receiving\ bins} \tag{2}\]  
To calculate mass indicator value, total masses of ore passing through individual bunkers over the analyzed time period were identified on the basis of measurement data provided by scales installed in the conveyor belts. Strain gauges installed in the scales allow measurements of the instantaneous mass acting on a particular idler set. The data include instantaneous linear speed of the belt. The product of the instantaneous mass and of the instantaneous linear speed is equal to the instantaneous capacity of the belt conveyor, and if this value is integrated over a defined time period, the result is the total transported mass over a period of time. Publication [23] indicates that the instantaneous capacity measurement error is \(\pm\)0.25%.