Efficiency of Maintenance Activities in Aggregate Quarries: A Case Study of Wear Parts on Loaders and Excavators

Abstract

Technological equipment in quarries that extract and deliver aggregates for different uses operates in a predetermined flow depending on the type of rocks exploited and the dimensional characteristics imposed on the final products. In this context, the interruptions in operation required to replace high-wear parts (such as the teeth of excavators and bucket loaders) must be limited as much as possible through technological solutions to increase their service life. The evolution of the wear of the teeth of the quarry equipment that come into direct contact with rocks was concretely established in the production process, in parallel with the wear values obtained by simulating the wear phenomenon in laboratory conditions, in order to validate the data collected during the operation of the machines. Preventive–repetitive maintenance within the activities of reconditioning the worn surfaces of the teeth, through the charging process by manual electric welding with covered electrodes, was applied directly to the machine, which led to the shortening of the interruptions in operation necessary to replace these spare parts.

1. Introduction

Maintenance, defined as the sum of activities to maintain and restore a technical system to specific qualitative conditions [1,2], has a major influence on the profitability of production units, which has a significant impact on costs and operational safety. The importance of maintenance has increased with the achievement of high productivity through the development and implementation of strategies adapted to operating conditions, in particular those of technological equipment.

The final objective of maintenance is to maintain and return to the initial operating conditions of the equipment in order to fulfill the functions for which it was created [3,4]. At the same time, availability [5,6] represents the degree or probability that a product can be used and is in good working order whenever it is required in time and space. It depends on two factors, namely reliability (being a function of uptime) and maintainability (being a function of downtime or repair).

In view of the above, availability is considered to be the ability of equipment to perform its specified function at a given time or within a given time interval under the combined aspects of reliability, maintainability, and the organization of maintenance actions.

In the concrete case of quarry technological equipment [7], for example, those from Bata, Arad, the analysis of the production flow regarding the critical areas from the point of view of maintenance revealed frequent production bottlenecks; the design of the maintenance program must respect both the interventions in accordance with the technical specifications of the manufacturers, as well as the conclusions of the analyses regarding the nature and cause of the interruptions in the production activity. The disadvantages of the method consist of maintaining, with high costs, stocks of parts with a high frequency of replacement due to wear [8,9].

In the Bata quarry, Arad County, basaltic rocks are extracted, processed, and sold, mainly intended for the construction of highways and railways but also for other civil works. The physical–petrographic characteristics of the basalt are presented in

Table 1. The physical–petrographic characteristics of basalt.

No.	Characteristics
1	Color	Gray
2	Appearance	Homogeneous
3	Fracture	Angled, slightly irregular prismatic shapes
4	Texture	Compact, weakly vacuolated
5	Structure	Porphyric
6	Density	2.94 g/cm2
7	Compactness	93.5%
8	Appearance	1.08%
9	Appearance	6.5%
10	Compression strength	147 N/mm2
11	Friction wear resistance (at 440 rpm) in dry state	0.07 g/cm2

.

The production process of the Bata quarry is carried out with mobile, semimobile, and fixed machinery, corresponding to the three production flows, as follows:

• The primary rock crushing flow, composed of Sandvik CM1208i primary jaw crusher, Stockholm, Sweden (Figure 1), Liebherr 944 L excavator, Bulle, Switzerland (Figure 1), Volvo L220 E front loader, Gothenburg, Sweden (Figure 2);
• Secondary and tertiary rock crushing–sorting flow, consisting of Sandvik H4800 Secondary Crusher, Sandvik UH420 Tertiary Crusher, Chieftain 2100 Sorting Station, Volvo L220 E Front Loader;
• The rock loading–transport flow, consisting of excavator Caterpillar 345 CL and dumper Volvo A35D.

The activity on the primary flow allows the crushing of raw stone at a size of 0–300 mm; a hydraulic back cup excavator and wheel loader carry out the loading and unloading of the crushed material, which is then transported to the secondary–tertiary crushing–sorting flow for temporary storage or loading into trucks for delivery.

The machines that make up the primary crushing flow are mobile, a fact that requires the presence of a worker at each machine, both for their operation and for the execution of maintenance works, which are planned to be executed at the same time. The activity carried out on the secondary and tertiary crushing–sorting stream consists of crushing the broken stone of 0–200 mm to the size of 0–63 mm; a new crushing follows, as a result of which the sieves with dimensions of 0–25 mm are obtained, grouped into sizes of 0–4 mm, 4–8 mm, 8–16 mm, 16–25 mm.

The Volvo L220 E wheeled front loader from the primary crushing stream is used to load the 0–200 mm crushed stone, and another front loader of the same type is used to load the finished products into trucks. The proper functioning of the crusher, excavator, and loader in the production flow also depends on the operators’ ability to coordinate the operation of these machines; in parallel, it should be emphasized that the wear of parts in contact with the rock requires their constant monitoring to avoid unjustified operational interruptions and to prevent exceeding the maximum allowable wear limits of these parts.

The data collected from the quarry over a period of three years showed that the parts with the highest wear, which led to disruptions of planned maintenance activities [10,11], as well as interruptions in operation, respectively, had the highest costs with the spare parts for the bucket teeth of the Excavator Liebherr 944 L and the Volvo L220 E front loader.

The wear of parts in contact with rock is influenced simultaneously by four types of factors: the influence of the physical and mechanical properties of the rocks; the influence of the operating parameters of the part within a machine; the influence of the physical and mechanical characteristics of the part’s material; and the influence of the geometric and shape characteristics of the part. In the working conditions of quarry equipment, where there is a pronounced heterogeneity of rock properties, as is the case of the Bata quarry, the weight of each type of factor in the wear process of parts that come into direct contact with the rock is difficult to determine.

Finding ways to improve the service life of parts subjected to wear under these conditions can be approached by simultaneously analyzing these factors. Practical data collected directly from the Bata quarry have been complemented and supported by those obtained through laboratory tribological tests that allow the analysis of at least two groups of the aforementioned factors. In this paper, these are the factors related to the rock and those related to the part material that influence the evolution of its wear.

Theoretically, tribology studies, especially those on the wear of spare parts for quarry equipment, do not offer practical solutions for improving their maintenance activities, as they all depend on operating conditions, primarily on the properties of the rocks.

The solved problem is the reconditioning method proposed in this paper, namely on-site, in the quarry, by welding worn bucket teeth to reduce disassembly–assembly times in maintenance activities and associated costs, including downtime.

The purpose of this paper was to verify the concept of preventive–repetitive maintenance within the reconditioning activities so as to ensure the time interval between two successive interventions by using some additions called “wear”, whose size must be determined according to the specific wear process of that surface.

2. Materials and Methods

The methodology on which the analysis carried out in this paper was based involved the following stages: identification and analysis of the reliability of some components of the technological equipment in the Bata quarry; tribological analysis of the landmarks that show the greatest wear; and establishing solutions to make the career maintenance plan more efficient.

2.1. Reliability Distribution Law Types

The description of system reliability by means of a distribution law and related numerical characteristics represents a global approach to the problem in the spirit of classical statistics [12,13,14,15,16,17]. The analytical expression of the reliability function is a fundamental problem of reliability theory. In general, a distribution law expresses the dependence between the values of the investigated characteristic and the related probability. Distribution laws specific to mathematical statistics are adopted in reliability theory to the extent that they imply a reliability function of suitable form, to which a certain physical interpretation can be associated.

The times, as random variables or other variables such as the distance traveled or the number of request cycles, are distributed according to a statistical distribution law, highlighted by means of the distribution function F(t) or the distribution density function (frequency), f(t). As the random variable takes discrete or continuous values, the distribution is discrete or continuous. Several types of distributions are used for reliability:

• Discrete: binomial or Bernoulli, polynomial, Poisson, hypergeometric, etc.;
• Continuous: uniform, normal, lognormal, exponential, gamma, Rayleigh, Weibull, etc.;
• Specific: χ² (chi-square, Pearson), t (Student), F (Fisher-Snedecor), mixture, etc. [18,19,20,21,22,23].

To analyze the reliability of the components that presented the most defects, we used the Relyence Weibull 2020 software distributed by Relyence Corporation (Greensburg, PA, USA).

2.2. Laboratory Simulation Methodology for the Phenomenon of Wear at the Contact of Metal–Rock Surfaces

The phenomenon of dry wear of metal–rock surfaces in contact can be studied under laboratory conditions with the TRB3 tribometer, manufactured by Anton Parr (Graz, Austria). In the first phase, using the Sutronic S128 profilometer supplied by Taylor Hobson (Leicester, UK), the wear profile of the surface of the metal sample is determined [24,25,26], with all the profile roughness parameters being of the type shown in Figure 3.

The parameters of the test regime necessary for the tribological analysis are (Figure 4) the value of the static load exerted on the metal element by a basalt sample; the speed and number of rotation cycles of the metal element; and the radius of the wear surface of the metal element.

Completion of the test, after the predetermined number of rotation cycles of the metal sample, leads to the establishment of the values of the following parameters: wear, respectively, the profile of the worn surface of the metal sample (according to a model shown in Figure 3), and the coefficient of friction (Figure 5).

The wear of the metal sample at the end of the test serves to validate or deny the practical data obtained concretely in operation regarding the wear of metal surfaces produced by certain rocks with known properties.

2.3. 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_p), partial overhauls (R_p), and general overhauls (R_g), as well as the preventive–planned repair system, such as technical overhauls (R_t), current repairs (R_c), and capital repairs (R_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;
• 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. The planned preventive periodic functional maintenance system [29,30] pursues the following objectives:

• Avoiding equipment degradation and extending their life;
• Maintaining the functional parameters as close as possible to the initial ones;
• Elimination of accidental stops;
• Reduction in maintenance costs;
• Increasing safety in operation;
• Creation of important sectors of parts reconditioning.

The system applies to machines, equipment, and installations of a special character and to those that are part of continuous technological flows. The specific operations of the system are periodic check V_p; partial revision R_p; and general revision R_g.

• (B) The system of technical reviews and planned preventive repairs

In the production process, functional maintenance systems can avoid the occurrence of unanticipated wear and tear of machinery, equipment, and facilities, but not the normal wear and tear that generates repairs at well-defined time intervals. The objectives pursued by the system of technical reviews and planned preventive repairs [31,32] are as follows:

• Anticipated preparation of interventions to the prescribed quality. Thus, the drawings of the spare parts, their production, and their storage until the intervention; the purchase of specific spare parts (bearings, belts, sealing materials, etc.); and the replacement machines and equipment for the period of repairs are prepared;
• Carrying out the interventions on time, simultaneously with the assessment of the real technical condition of the equipment. It is necessary to take the decision on carrying out the interventions at the planned term, respectively, carrying out the interventions for some machines and postponing them for others, without affecting the production process. In parallel, the measuring and control devices are being prepared, namely the tools, devices, and verifiers necessary for the interventions;
• Modernization of equipment during capital repairs. The introduction of automatic drives, programming commands, operation limiters, etc., can be considered;
• Increasing the yield of machines and equipment. The mechanization of some technological operations, the automation of specific works, the provision of automatic work, and control and handling devices are considered.
• Maintaining or increasing the reliability of the equipment between two interventions by ensuring spare parts and carrying out interventions of good quality, while the operation is carried out according to the technological specifications;
• Reducing repair costs by reconditioning and reusing used parts, typing and standardizing parts, and reducing accidental stops;
• Optimum use of the labor force is possible through the nomination of concrete tasks, the control of their realization, and the material stimulation of employees;
• Reducing the consumption of energy, fuel, and lubricants by determining the consumption of cars and their judicious management, as well as removing any form of squandering.

Taking into account the structure, complexity, and regime of use of fixed assets, industrial practice outlines the following categories of revisions and repairs: technical review, R_t; current repair, R_C; capital repair, R_K.

3. Results and Discussion

3.1. Reliability Analysis of the Components That Have the Most Defects

In order to improve the efficiency of the maintenance activities of the technological equipment in aggregate mining quarries, the actual operating times (within a period of three years) between two failures are measured, namely the repair times (duration of the tooth replacement activity) for the bucket teeth of a Liebherr 944 L excavator (

Table 2. Values of operating time between failures and repair times of the bucket teeth of the Liebherr 944 L excavator.

Time	Value
Operating times between two failures (hours)	18	24	35	36	37	42	43	48	64	65
	66	67	72	73	78	96	108	120	126	138
	140	162	168	174	198	222
Repair times (minutes)	40	60	60	60	60	60	60	60	60	60
	60	60	60	65	65	65	70	70	75	75
	80	80	80	80	85	85

) and for the Volvo L220 E front loader (

Table 3. Values of operating time between failures and repair times of the bucket teeth of the Volvo L220 E front loader.

Time	Value
Operating times between two failures (hours)	7	7	7	14	14	14	14	14	20	21
	21	26	27	28	28	29	35	37	42	44
	48	55	56	63	65	69	70	72	76	77
	78	79	83	105	107	114	118	127	130	149
	160	178	211	227	347	533	538
Repair times (minutes)	120	180	180	180	180	180	180	190	200	205
	220	220	230	230	230	240	240	240	240	240
	240	250	260	260	260	260	260	260	270	270
	270	270	290	290	300	300	300	300	300	300
	310	310	320	330	360	450	840

). The results, totaling 26 for the Liebherr 944 L excavator (Table 2) and 47 for the Volvo L220 E front loader (Table 3), are presented in the table in ascending order.

The data thus obtained were analyzed with the Relyence Weibull software (produced by the Relyence Corporation) to obtain the reliability and maintainability variability of the bucket teeth from the mentioned machines.

The Relyence Weibull software initially ranks the goodness of fit of different distribution laws (Figure 6, Figure 7, Figure 8 and Figure 9) to the studied data (Table 2 and Table 3).

The assessment of actual operating times until bucket tooth replacement for both the excavator and the loader was obtained by analyzing the operating times between two failures, namely:

• By adopting the 3-Parameter Weibull distribution law (Figure 6) for the operating times between two failures of the Liebherr 944 L excavator’s bucket teeth, we were able to assess the evolution of failure times. Similarly, we used the Normal distribution law (Figure 7) to analyze the evolution of repair times for the teeth;
• By adopting the Lognormal distribution law (Figure 8) for the operating times between two failures of the Volvo L220 E front loader’s bucket teeth, we were able to assess the evolution of failure times. Similarly, we used the 3-Parameter Weibull distribution law (Figure 9) to analyze the evolution of repair times for the teeth.

In the next stage, the reliability variation curve was built for the bucket teeth of the Liebherr 944 L excavator (Figure 10), which shows that for a reliability of approximately 80%, a value imposed by the beneficiaries of the quarry equipment, the operating time without failures is only 29.71 h, for example, approximately 2 working days (the excavator works in two shifts of 7 h/day).

In the case of the bucket teeth of the front loader Volvo L220 E (Figure 11), reliability of 80% is obtained after about 21 h of operation, for example, after completing three working shifts (the loader works in two shifts of 7 h/day).

The maintainability data are not satisfactory, because to be able to obtain a bucket teeth change probability of 80%, it takes about 76 min for the Liebherr 944 L excavator (Figure 12) and 310 min for the Volvo L220 E front loader (Figure 13).

3.2. Tribological Test Results of the Benchmarks Showing the Greatest Wear

In order to be able to analyze, under laboratory conditions, the behavior of the bucket teeth of the two machines mentioned in contact with the mined rock, several cylindrical samples were made of the material from which the excavator tooth is made, both for the Liebherr 944 L excavator and for the Volvo L220 E front loader. Ten samples (Figure 14) from each of the two tooth materials were thus studied. The average values of the coefficient of friction (metal with basalt) were 0.0361 for the Liebherr 944 L excavator bucket tooth sample (Figure 15) and −0.0385 for the Volvo L220 E front loader bucket tooth sample (Figure 16).

The profile of the worn metal surface can be analyzed using the roughness parameter determined with the help of a tribometer and a profilometer, as described in Section 2.2. Following the friction test between the metal sample and the rock sample, the roughness parameter R_a is obtained, which represents the arithmetic average deviation of the worn metal surface profile from the ideal profile (represented by a straight line).

Regarding the amount of wear (Figure 17) on the surface of the metal sample of the Liebherr 944 L excavator tooth (Figure 18), a pronounced increase of 205.88% was found in correlation with its R_a roughness value and 31.03% for the bucket tooth sample of the front loader Volvo L220 E (Figure 19).

The obtained roughness value (R_a) can confirm or refute the actual wear value of the bucket teeth, as observed in the quarry. In the case of the bucket tooth of a Volvo L220 E front loader, the increase in wear of 31.03% was confirmed, corresponding to the Ra roughness of 0.76 microns, as observed on the tested metal samples (in contact with the same type of rock). The roughness value (R_a) established through tribometric testing can be used to determine the maximum allowable wear limit for the teeth, as agreed upon in the maintenance and reconditioning plan. The roughness of worn bucket teeth surfaces depends on both the rock properties and the material properties of the teeth. Therefore, tribological tests should be repeated whenever the rock structure in the massif changes, as the operating time until tooth replacement is required changes with the changing rock properties.

3.3. Establishing the Solutions to Make the Career Maintenance Plan More Efficient

The active surfaces of the equipment for crushing volcanic rocks from the igneous basalt group are subjected, in operation, to severe abrasive wear and mechanical shocks; it is necessary to apply layers of protective materials to the surfaces subject to wear, which ensure uninterrupted operation, respectively, to increase their life span. Abrasive wear of bucket teeth in production quarries of aggregates from basaltic rocks consists mainly of the change in dimensions and weight loss at the level of their active surfaces (Figure 20).

The experiments carried out made it possible to appreciate that the material of the tooth allows charging by welding with covered electrodes in order to increase its life and recondition it after wear [33,34,35,36,37]. The practical tests led to the choice of the charging procedure by manual electric welding with coated electrodes so that the existing equipment in the quarry could be used but also to the rapid reconditioning of the worn teeth without dismounting them from the machines. The choice of filler materials was made on the basis of data on wear resistance and welding behavior in difficult deposition positions.

The development of the self-protection system against tooth wear (Figure 21), which involves reconditioning areas through welding, was carried out differently depending on the wear surface position of the teeth, namely the worn edge of the tooth was protected with a compact layer of material, and the active surfaces were protected with rhomboidal systems.

In the case of the technical equipment of the quarry, the concept of preventive–repetitive maintenance involved the repetitive development, realization, and reconditioning, in conditions of economic efficiency, of the bucket teeth of the Liebherr 944 L excavator and the Volvo L220 E front loader; experimenting with reconditioned teeth demonstrated the usefulness of applying the proposed method by actually increasing their service life by 60%. The cost-effectiveness of the optimized welding process for bucket teeth is estimated at 1700 euros, based on a consumption of 20 teeth per quarry during 1.5 production cycles. The proposed method can also be used on other components that show pronounced wear, for example, the blades in the equipment of the buckets of front wheel loaders.

To streamline maintenance activities, it must be taken into account that replacement times depend on the training of maintenance workers, as well as the deformations of worn teeth which, in some cases, are blocked in the fastening devices.

4. Conclusions

The reliability analysis results of excavator and front loader buckets, obtained in this study, have allowed for the accurate determination of their behavior in the specific conditions of the basalt quarry. Efficient use of quarry equipment requires a reliability of approximately 80%. In practice, this was achieved through the faultless operation of the Liebherr 944 L excavator’s bucket teeth for an extremely short interval of 29.71 h. For the Volvo L220 E front loader’s bucket teeth, 80% reliability was achieved after about 21 h of operation, which is equivalent to three working shifts (the loader operates in two 7 h shifts per day).

During the three-year monitoring period of the equipment, significant differences were observed in the operating times of the worn parts, specifically the bucket teeth of the Liebherr 944 L excavator and the Volvo L220 E front loader, as well as the times required for their replacement. This led to the idea of studying the evolution of the wear phenomenon under the conditions of the rocks exploited in the Bata quarry through laboratory tests. In this way, it was possible to simulate the wear phenomenon at the metal–rock contact on a tribometer, leading to the knowledge of the wear evolution of a metal sample in a friction regime with the rock with known parameters.

The roughness value (R_a), obtained and measured with a profilometer, can confirm or refute the actual wear value observed on bucket teeth in the quarry. In the case of the Volvo L220 E front loader’s bucket tooth, the increase in wear of 31.03% was confirmed, corresponding to a roughness R_a of 0.76 microns, as found on the tested metal samples (in contact with the same type of rock).

The roughness value (R_a) established through tribometric testing can be used to determine the maximum allowable wear limit for the teeth, as agreed upon in the maintenance and reconditioning plan. The roughness of the worn surfaces of the bucket teeth depends on the properties of the rocks and the material of the teeth; therefore, tribological tests should be repeated whenever the structure of the rocks in the massif changes, as the operating times until the teeth need to be replaced change with the changing properties of the rocks.

The replacement times for the teeth depend on the skill of the maintenance workers, as well as the deformations of the worn teeth, which in some cases are blocked in the fastening devices. The operators’ ability to coordinate the proper operation of the equipment and the constant verification of the wear of parts that are in direct contact with the rock can help avoid unjustified interruptions in operation. In this context, and considering the uncertainties regarding sales volume, equipment availability, or other causes that may intervene in the production process, it is necessary to draw up a weekly maintenance schedule for each month, which specifies the scheduled interventions on the quarry equipment.

There is a clear need to increase the service life of replacement parts such as bucket teeth, especially through reconditioning, which is sometimes repeated. The economic efficiency resulting from the application of the optimized welding technology for bucket teeth is approximately 1700 euros for a consumption of 20 teeth per quarry, for 1.5 production cycles. The proposed and experimentally tested method of manual welding directly in the quarry for attaching bucket teeth has allowed the simultaneous achievement of two objectives: increasing the service life of bucket teeth and reducing downtime required for replacing worn ones.