## 2 Materials and Methods
### Maintenance Types  
In order to obtain a given level of reliability, respectively, a level of designed availability or safety of a product or system, planned or unplanned actions of testing, maintenance, and repairs are required. Maintenance systems [27; 28] are based on the logical organization of work according to resources, the use of automatic data processing means, the reduction in maintenance, repair, and storage costs, the elimination of accidental stops by preventing malfunctions, permanent wear, etc. The adopted systems must engage the workers who service these machines to carry out maintenance and repair operations, in this way appreciating the maintenance work and the knowledge from a technical and functional point of view of the equipment they work on, in order to exploit it to the allowed parameters and grant greater attention to daily maintenance work and monitoring of operational behavior. Maintenance works are carried out based on the annual maintenance schedule for all equipment in the quarry, which is drawn up taking into account the estimated sales volume for the year in question. Each piece of equipment is budgeted for the number of hours necessary to achieve production. The annual maintenance schedule for equipment comprising the production processes is drawn up at the end of the year for the following year and includes specific tasks for both the preventive-planned periodic maintenance system, more precisely, periodic inspections (V\({}_{\text{p}}\)), partial overhauls (R\({}_{\text{p}}\)), and general overhauls (R\({}_{\text{g}}\)), as well as the preventive-planned repair system, such as technical overhauls (R\({}_{\text{t}}\)), current repairs (R\({}_{\text{c}}\)), and capital repairs (R\({}_{\text{k}}\)). Due to uncertainties regarding sales volume, equipment availability, or other factors that may intervene in the production process, a weekly maintenance schedule is drawn up at the end of each month for the following month. This schedule, which is adjusted to the current situation, outlines the planned interventions on quarry equipment. Daily routine checks, partial overhauls at 50 operating hours, general overhauls at 100, 500, 1000, and 2000 operating hours, technical inspections, and routine repairs are carried out by the quarry equipment operators and maintenance personnel and verified by the maintenance department manager. General overhauls at 2000 operating hours, as well as capital repairs, are carried out by service technicians from a service company approved by the equipment manufacturer. The system-level strategies to reduce corrective maintenance are as follows:  
* Fault detection, identification, and isolation. Fault indicators and symptoms should be clearly defined through maintainability design to attract the operator's attention for isolation. Testing equipment, usually incorporated into \"hot spots\", and well-trained personnel are necessary;  
Figure 5: The variation in the coefficient of friction at the surface of the metal sample. * Accessibility. System elements with a high failure rate should be easily accessible, a condition that must be considered and provided for from the design phase;
* Functional and physical interchangeability, which facilitates the disassembly and replacement of components, reduces downtime, and has a positive impact on the need for spare parts and inventory;
* Redundant elements. If technical and economic considerations allow, the design should provide for standby components to avoid system downtime and enable maintenance operations to be performed with adequate quality. In the quarry studied, two maintenance systems are applied, as presented below: (A) The system of periodic functional maintenance of the planned preventive type  
The system emphasizes the prevention (rather than the elimination) of failures. The prevention of failures, with a certain probability, allows the knowledge of the parts that need to be replaced (before the failure occurs), which leads to a correct dimensioning of the spare parts requirement.