INTRODUCTION

With in the technological process of rice production, the use of combine harvesters is an issue of undoubted interest. The deterioration and years of operation of the means used during the technical assistance of these equipment, has caused the decrease of the technical availability of them, which affects the harvest productivity . Hence the importance of the proper execution of technical maintenance operations and repairs to guarantee the work of the machinery in operation, until they can be completely renewed, according to the economic possibilities of the country.

In Cuba, rice harvest is an operation carried out with the help of agricultural machinery, which represents strong investments; for this reason the dynamics of the production process requires that harvesting machines are in perfect technical condition during this period. Consequently, it is necessary to properly organize and plan the maintenance and repair system to ensure the increase of productivity efficiently (Miranda et al., 2003; Mora, 2005; Izmailov, 2007; Amu, 2010; García de la Figal, 2011).

Studies carried out indicate that, when the daily technical maintenance is carried out in the established time and all the operations foreseen are performed, the flow of machine failures decreases by 50%, under average operating conditions, which allows reducing the expenses for concepts of repairs and time labor (Sotskov, 1972; Miranda-Caballero et al., 2004; Shkiliova et al., 2007; Paneque-Rondón et al., 2009; Matos-Ramírez et al., 2014; González-Cueto et al., 2017).

In the Agroindustrial Grain Company (EAIG) “Los Palacios”, as in the whole country, the deterioration of machines requires greater demand for technical assistance and with it the need to organize the maintenance and repair process, laying out strategies that ensure an optimal decision at each moment, based on maintainability criteria.

Mesa-Grajales et al. (2006), point out that reliability, availability and maintainability are powerful tools that help maintenance personnel to make decisions. These tools provide the criteria to develop strategic maintenance management and, in order to increase availability, priority must be given to maintainability. Miranda-Caballero et al. ( 2004), state that the little attention to the planned technical maintenance influences the increase in the amount of breaks in harvesters because they are responsible for the control and prevention of failures. Thus, measures can be taken that help to increase harvesters’ productivity, once they know which the pieces that fail are and the effect they cause. Another negative impact during the maintenance of the machines, is not having the organization, resources and trained personnel that responds to the current requirements of improving the operating conditions and the improvement of the reliability indexes Miranda-Caballero et al. ( 2004). Each one of the investigations carried out previously reflects the need to comply with quality maintenance in order to increase the operational reliability and al though it is characterized by the time and resource expenses for the maintenance of its working capacity, it is limited to consider only the expenses of times, by concept of live work of people, spare parts and other materials for repair. All the characteristics and previous events that occur before reaching the state of normality such as design, assembly, operations, skills of the operators, modifications made, previous repairs, operation capacity, reliability, maintenance executed th rough out the useful life of the equipment, environment, legislation or indications, quality of the spare parts, cleaning and environmental impact generated, evaluating the management and operation of maintenance, are other issues to be taken into consideration.

According to the existing problems, for the first time in the conditions of the EAIG “Los Palacios” and in Cuba, scientific basis support the parameters that characterize the maintainability during the technical maintenance, that is, the ease and economy in the execution of the maintenance of an element, device or equipment that can be restored, and specifically rice harvesters, which, in turn, provides a methodological basis for the solution of such important scientific and practical problem in the organization and streamlining of maintenance and, therefore, in the working capacity of the harvesters.

Taking into account the influence of the technical maintenance in the assurance of the working capacity of the harvester and the ignorance of the level of compliance of the technical maintenance and repairs in the programmed time, it is necessary to estimate the maintainability of the rice harvesters during the daily technical maintenance, every 30 hours during the harvest period in the conditions of the EAIG “Los Palacios”, since it is the variable that directly influences the ease and economy in the execution of maintenance.

METHODS

The experimental research was carried out in EAIG “Los Palacios”, in Pinar del Río Province. The field work and the characterization of the experimental site, was carried out under the repair and harvest conditions of the UEB A “Sierra Maestra. The evaluation of the factors that influence maintainability were carried out in three models of New Holl and L -521, L -624, L -626, during the daily technical maintenance (10 h) and the M T-1 (every 30). h ), during the rice harvest periods June-July and September-November 2012.

Methodology for Collecting Information on Maintenance Times

For the collection of information, different Cuban norms were used, such as PNO PG-CA-043 (2013) and PNO PG-CA-043 (2013). A series of measuring instruments (digital chronometers with an accuracy of 1/10 s (1%), control models of the harvest peloton work (worked area, quantity of harvested grain, fuel consumed and duration of the working day), documentation of the UEBA (prepared by the head of batch) (field , area, field performance, variety) and a set of tables prepared in advance for the collection of the primary data. Table 1 reflects the types of technical maintenance performed. In column 1, the model and chassis number of the machine are set; in column 2 the type of maintenance is indicated and in column 3 the tools used are shown. Average time used in maintenance (main and auxiliary time) is set on columns 4 and 5. Column 6 provides information on the number of people involved. This table is filled taking into account Table 2, where the results of the photo-chronometry of the maintenance carried out are established, taking the time at which the maintenance starts and the final time, by difference between the initial and final time the maintenance duration time is calculated. The time of each operation is timed accordingto the type of maintenance (daily maintenance, periodic MT1 every 30 h).

Photo-timing begins with the first task of the day that is the daily technical maintenance, which in some cases can coincide with the periodic technical maintenance M T-1 (every 30 h). With the help of the clock the start and end time are taken and with the chronometer the time of duration of each maintenance operation is taken into account.

Before starting the observation, it is necessary for each machine to take the data related to its characteristics (name and brand, date of manufacture or repair, name of the production or repair plant, place where the observation is made, start dates and completion of observation); working conditions and useful work of the machine; cases and causes of machine shutdowns due to technical problems (technical maintenance, fault elimination and repairs). Table 3 is the control model of daily work in operation, it is used to record work done daily by machine. Information on the field is obtained from UEBA documents (technological letter of the area). The amount of initial and final fuel is measured with the graduated rod that each machine owns. The work volume is taken from the report or harvest part issued by the platoon leader and then corroborated with the document issued by the reception center (conducts), the data from clean work time, shift hours and other harvest materials.

Based on the manual of employment and care of the harvesters (New Holl and L series) in Table 4, in columns 1 and 2, the data taken from the timing of the operations of daily maintenance, periodic every 30 h are settle, coming from tables 1 and 2. Column 3 takes into account the location of the point to be served (On the side of the machine, P1; Top of the machine, P2; Below the machine, P3; Front of the machine, P4; Rear of the machine, P5). In column 4, information is provided on the w ork positions of the personnel working in the maintenance, according to Kopchikov (1980), quoted by Shkiliova & Fernández-Sanchez (2011), standing working at the height of the shoulders; A, standing leaning forward, B; standing working above the head, C; bent, D; bent on knees with straight spine, E; on knees inclined forward, F; lying on his back, G; sitting leaning back, H; seated leaning forward, Iand lying face down, J.

In columns from 5 to 8 the time that is used in the work, element to work, tools used and the people involved in the operation are established. These data are taken from Tables 1 and 2. Once ordered these data are averaged by operation and machines which will allow the analysis based on the theoretical basis.

Methodology for Processing the Data That Influence Maintainability Levels

For the processing of the indicators that influence maintainability, the UNE-EN 61703: 2002, UNE-EN 13306: 2002, UNE-EN 61703: 2003 2002a, 2002b, 2003 , standards were used and are grouped into tables that allow calculating the data of technical maintenance by type (Table 2), which will allow the elaboration of graphics necessary to show the results of the investigation. Using the professional Mathcad 2000 calculation software, the average daily technical maintenance times are calculated, every 30 hours, and the maintainability estimate is grouped in Microsoft Excel database. That will allow the statistical processing of arith metic means, standard deviation, coefficients of variation, Kolmogorov test for goodness of fit and probability charts in the statistical program STATGRAPHICS PLUS, Versión 5.1 (Statistical Graphics Crop, 2000).

Evaluation and Determination of the Economic Effect

The economic effect was based on the data obtained during harvest period 2009, with the evaluation of the machines in operation. The amount of grain left to harvest was determined by the equation:

Where:

Adcpm-Amount of grains left to harvest;

TMdmd- Average daily maintenance extra time;

TMdm30 h- Average maintenance time every 30 extra hours;

Wexp- Productivity; N- Number of maintenance.

RESULT AND DISCUSSION

Technical Maintenance Time

Supported by the data obtained from the timing of the technical maintenance carried out during the harvest period, average values were determined (Figure 1). The average time of technical maintenance of the harvesters is composed of the main and auxiliary average time, the latter prolonged, which makes it difficult to optimize the technical maintenance time. It shows that in daily maintenance between 99.5 and 108.6 min (59 and 64% ) is the main time and between 35.4 and 42.2 min (36 and 41%) is auxiliary time, which indicates the excessive loss of time due to lack of tools, and devices adequate. In the maintenance every 30 h, time between 101.6 and 121.9 min (50 and 55%) is used in th e main operations and time between 68.3 and 74.2 min (45 and 50%) is used in the waiting and preparation of tools, devices, transfer between points to be attended and positioning to execute operations, among others.

Estimation of the Maintainability of Technical Maintenance

From the results obtained in the estimation calculation, in Figure 2 and Table 5 it is shown that in the L-521 harvester, there is a 97% probability that the daily technical maintenance is performed in 2 hours; in the L-624 harvester there is the 96% probability that the daily technical maintenance is carried out in 2 hours, and in the L-626 combine there is a 95.8 % probability that the daily technical maintenance is carried out in 2 hours. It is demonstrated that the two regulated hours of daily maintenance is not enough time for that and it provokes that they use more time (2.40 h), causing loss of operating time of the harvester.

For the periodic maintenance every 30 h the L-521 harvesters, there is a 78 % probability that the technical maintenance every 30 h is done in 2 hours, in the L-624 harvester there is a 75% probability that the technical maintenance every 30 h is done in 2 hours and in the combine L-626 there is a 74% probability that the technical maintenance every 30h is done in 2 hours. That proves that the two hours regulated to perform periodic technical maintenance every 30 hours is not enough; so it is necessary to increase the time up to 3 hours which causes loss of operating time of the harvester. Therefore, the probability of performing maintenance at a zero time is zero, as the execution time increases, the maintainabil ity curve increases to become maximum in a longer or infinite time; this reveals that to the extent that a larger and maximum time is assigned to perform a maintenance, the successful probability of realizing it grows.

As it can be seen in Figure 3, the greatest loss occurs in L -624 harvesters that stopped harvesting 112 7.86 twhich represents 225 571.08 Cuban pesos (CUP) or 9 022.84 convertible Cuban pesos (CUC) and of them 129 601.08 CUP or 518 4.04 CUC belong to the extra daily maintenance time. While L -626 harvester stopped harvesting 779 .46 t representing 155 892.8 CUP (6 235.71 CUC) and of them 90 518.40 CUP (3 620.74 CUC) belong to the extra time of daily technical maintenance. L-521 harvester has more discrete values, the loss of rice left to harvest is 621.99 twhich represents 124 396.65 CUP (49 75.87 CUC) of them 3 353 3.01 CUP (1341.32 CUC). So the situation could improve if the daily technical maintenance extra times due to the deficiency of the logistics system (spare parts, tools and devices to carry out maintenance in the field) are reduced, and personnel training is also implemented in order to diminish losses because of production stops.

CONCLUSIONS

• In the harvesters evaluated, the maintainability curve shows that there is between 95.8 and 97 % probability that the daily technical maintenance is carried out in two hours; while in technical maintenance every 30 hours the probability is between 74 and 78 % in th same period of time.

• The level of maintainability of the rice harvesters during the daily technical maintenance and every 30 hours in the harvesting process in the EAIG “Los Palacios” is less than the possible to be reached, mainly due to the logistics system factor.

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