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INTRODUCTION
The man, from the beginning of his history, has tried to facilitate his work with the aid of machines or tools. The growing agricultural borders and volume of production, make to increase the speed of work (Paneque-Rondón et al., 2018). With the appearance of the agricultural machines, an infinite field of development of machines for each function opened: to work the ground, sowing, harvesting and product collection, load and transport (Rivas, 2004).
Nowadays, agriculture demands optimal operation of the processes mechanized, concentration and specialization of the production and increase of work productivity based on agricultural yields and diminution of the production costs (Igarza, 2012).
One of the tasks to maintain the levels of functionality and readiness of the corn harvester Massey Ferguson model MF 7252 is the regulation of its systems and components, through periodic works of maintenance. On the other hand, during the operation of this equipment, different failures may happen, which, even being insignificant and not opportunely treated, could become a serious problem, that risk the availability of the equipment.
During the corn harvest, in the agricultural land of the Polytechnic Territorial University of the Estado de Portuguesa, the Massey Ferguson model 7252 corn harvester, presented faults in the transmission system by belts and it caused delays in the grain harvest. This situation led to organize the maintenance service in order to control failures, breakdowns and to improve the availability of it.
Nowadays, Six Sigma is considered like a tool of generally accepted management in countries like United Kingdom, France and Spain, as it has demonstrated its validity and potential in financial sectors like automotion, services, discharge technology, manufactures, chemistry, aeronautical, information technologies, software, public bank, administrations and hospitals, independently of the business size and volume (Zuluaga, 2016).
According to Felizzola and Luna (2014), Six Sigma is supported in a methodology composed of five phases: To define, To measure, To analyze, To improve and To control, commonly call DMAIC, and it has as objective to increase the capacity of the processes. It has engaged many organizations in implementing Six Sigma like strategy of businesses to increase their yields, to improve the quality of their products and services, as well as to improve their productivity and competitiveness.
The application of it by several investigators in different processes and machines is related next:
Sexto (2004), in its article published in 4to Peruvian Congress - Engineering of the Maintenance raises "the maintenance to critical systems, with inferior levels sigma to six, can carry inadmissible consequences and losses for the security, the environment, the production and injures to the credibility and organizational sustainability”.
Leal (2005) applies methodology MAMC of Six Sigma to assets of the heavy machinery sales department in Guatemala.
Díaz (2005), Jiménez (2005) and Leal (2005), use the Six Sigma method in heavy machinery, respectively, to control the processes in the sales department of a trading company and in the housing construction industry. In addition, Mendoza et al. (2014) point out that there are several applications of the Six Sigma methodology in different areas and in a considerable number of important companies from different productive sectors. The steps of the methodology guarantee an in-depth analysis of the problem to study.
Mendoza et al. (2014) in their research only develop three phases of the Six Sigma methodology (definition, measurement and analysis). In this work, several techniques were applied, such as studies of repeatability and reproducibility, linearity, control graphs, process capacity and experimental design in order to determine the significant variables in the calculation of the uncertainty of the Diesel emissions mapping test.
Zegarra (2014), expounds the administrative method Six Sigma, will help the results analysis of the maintenance management and will allow the continuous improvement. Hence, it will improve the mechanical availability and will secure the machines useful life and therefore, the efficiency in the factory managing.
Jasso et al. (2014) outline that "Six Sigma methodology helps to identify, to reduce and to eliminate defects of any product, process and transition. Six Sigma is a dynamic strategy, flexible and is an initiative of processes that helps an organization to discover solutions. Using Six Sigma to identify and to correct the greater problems will create real data that will previously discover unknown solutions, solutions that would remain hidden without the implementation of the methodology ".
Santiago et al. (2014), indicate that Six Sigma is a methodology of quality management, centered in the process control whose objective is to manage diminishing the number of defects in the delivery of a product or service to the client. The goal of Six Sigma is to produce pieces with a maximum of 3.4 defects by million opportunities, considering defect, any product/service that does not manage to fulfill the requirements of the client.
Gurrola et al. (2014), offer the following definitions with the purpose of facilitating the understanding:
Six Sigma is a methodology for solving problems that helps the improvement and good organizational performance of the company, which adopts it.
Performance of Six Sigma is a statistical term for a process or procedure that looks for generating 3.4 defects or errors per million of opportunities.
After analyzing the use of the Six Sigma method by several authors in different processes, it is concluded that, it has not been used as a maintenance strategy to improve the availability of the belt transmission system of the Massey Ferguson model 7252 corn harvester, belonging to the Production Unit of Mijaguito of the UPT. "J.J.Montilla" Estado Portuguesa, Venezuela, which is the main objective of the investigation.
In this way, the problem of the Six Sigma project is that the little organization, planning and control of the tasks related to the transmission system, influence the emergence of failures and the low availability of that system.
METHODS
In order to develop the investigation whose results are exposed in this article, theoretical methods of investigation were used, such as analysis and historical synthesis and logical for the study of the object through the time and to arrive at a rational analysis.
The research was developed in the production unit of Mijaguito in 2015. It is located in Mijaguito Town, Páez Municipality and currently has 214 hectares, where academic activities are carried out as well as research and production projects of milk, sheep, laying hens, worm humus, sugarcane, corn, citrus and fodder.
For the application of the Six-Sigma method, the authors developed the following stages: define the problem, measure, analyze the root cause, improve and control.
The techniques used to collect the data were participant observation and consulting of official documents of the production unit (technical files and registries of maintenances).
As there was no historical data of excellent quality, the criticality analysis was carried out through interviews with maintenance personnel and the operator to know the necessary information on the frequency of failures and their consequences. The interview allowed defining the value given to the priority of the project.
Implementation of the Methodology of the Six Sigma. Calculation of the Six Sigma
To calculate the value of the Six Sigma, the following data were collected during the work of this machine: number of observed defects (d) four belts fail, number of units (U) 14 and number of opportunities (O) two.
The defect per unit (DPU) was calculated by the following formula:
This metric reveals that there are 0.286 defects per unit (per belt).
Defects per opportunity (DPO)
This metric measures the non-quality of the process; in this case, it is only 14.3% of quality.
Defects per million opportunities (DPMO)
This index determines the predictable defects per million of failure opportunities, in this case, 143 000 failures are the defects expected in a million of opportunities.
According to Sexto (2004), the sigma level can be known depending on the defects per million opportunities (DPMO), in this case, it is 143 000. Therefore, sigma value is greater than two value, but smaller than three value.
From this sigma value, it is concluded, that the system can cause inadmissible consequences and losses during the corn harvest, therefore, it is necessary to implement the Six Sigma methodology in the belt transmission system.
First Stage, to Define the Problem
The combined of maize Massey Ferguson model MF 7252 of Mijaguito Production Unit (see Figure 1), presents different systems, 18 altogether, that allow the correct operation of this equipment. They have presented different failures during the operation.
The presence of faults in the machine systems, led to carry out interviews to the maintenance staff and the machine operator to know the most critical system. It allowed concluding that the faults that occur in the belts of the transmission system are responsible for paralyzing the machine and the corn harvesting. They occur with the following frequency: between 250 and 300 hours of work or between 350 to 400 hectares of work. This frequency is lower than that recommended by the manufacturer for changing the belts, which is between 500 and 650 hectares of work.
The main failures that happen in the system of transmission by belts (see Figure 2), are the following ones: the tensions become loose, the belts stretch, vibrations of the system and the belts are broken.
Critic Variables for the Work Quality (VCC) of the Combine
Critic variables that act in the failures and affect the quality of transmission towards the different subsystems that form the machine are proposed. They are quality of the belt material, tension mechanism of the belts, vibrations in the machine and the existing process of maintenance for the machine.
From the analysis of Table 1, it is concluded that the critic variables and of greater influence on the failure occurrence in the transmission system are the following ones: material of the belts, belt mechanism of tension and maintenance. Its priority is located between 4 and 5 and maintenance is the one of greater priority because it allows controlling the adjustments of the tension mechanism and, simultaneously, maintaining the system and the machine ready.
TABLE 1 Critic variables and their priority
| Variable | Importance | Present situation | Priority in the project |
|---|---|---|---|
| Material of the belts | For the belt resistance. | It is not controlled | 4 |
| Mechanism of tension of the belts | It is in charge of the tension and gets out of order with regularity. | It is controlled when the failure happens | 4 |
| Vibrations | They act on the belt mechanisms of tension and other mechanisms of the combined. | It is not controlled | 3 |
| Maintenance | To maintain the function of the transmission system. | It is not controlled | 5 |
The quality of the belt material is out of the machine operators’ control and the vibrations can be controlled with an effective process of maintenance.
From this study, it is concluded that the variable maintenance process is the one with the highest priority of the Six Sigma Project and it is the critical quality variable (VCC).
The previous analysis allows to list (Table 2), the critic quality variables of the maintenance process that influence in the availability of the system studied.
TABLE 2 Maintenance variables and their priority
| Variable | Importance | Present situation | Priority in the project |
|---|---|---|---|
| Organization, planning and control | They allows planning the maintenance correctly and increasing the availability of the subsystem. | Bad | 5 |
| Human | Its qualification is important for the fulfillment of the technical services tasks of the maintenance cycle. | Regular | 4 |
| Technological | To know the characteristics of the machine in order to measure the belts tension and alignment. | Bad | 4 |
| Direction | It coordinates and controls all the activities of the maintenance process. | Good | 3 |
From Table 2, it is concluded that the variable of greater priority is the organization, planning and control of the process of maintenance related to the system of transmission of the combined. This way the problem of the project Six Sigma is as follows: the little organization, planning and control of the tasks related to the transmission system influence in the appearing of failures and the low availability of this system.
Problem Definition (D)
Problem: the appearing of failures in the belt transmission system of the combined of maize Massey Ferguson model MF 7252 causes the low availability of the machine.
In order to define the problem the technique of the four which is applied:
-
Which is the problem with the availability of the belt transmission system?
The appearing of failures in the belts after 250 and 300 working hours or 350 to 400 hectares of work.
-
Which is the magnitude of the problem?
The failures arise before the 500 hectares recommended by the manufacturer to replace the belts.
-
Which is the index of availability of the system compared with another combined of the same mark and model?
This datum is unknown.
-
Which is the impact of the low availability of the belt transmission system in the productivity of the machine?
The low availability of the system causes that the main mechanisms of combined do not work. This situation causes that the machine cannot collect the harvest until the system is available again.
Final Definition of the Problem: The appearing of failures in the belt transmission system causes the substitution of the belts before the hectares recommended by the manufacturer. That situation provokes the low availability of the system and, simultaneously, it avoids the machine collects the harvest until the transmission mechanism is ready again.
Second Stage, to Measure (M)
This stage was not developed because there was not any element related to the critic variables for the work quality of the combined.
Third Stage, to Analyze the Root Cause (A)
The objective of this stage is to identify the root cause(s) of the problem (to identify the vital X(s)), to understand how it (or they) generate the problem and to confirm the cause(s) with data. In order to reach this objective the techniques of the five why and the diagram of Ishikawa were used.
The use of the technique of the five why gave the following answers:
Why? The low availability of the machine
Why? Failures arise in the belt transmission system at random.
Why? The failures are not detected in advance in the transmission system
Why? The ignorance of the period of intervention and the tasks to execute in the transmission system.
Why? The little organization, planning and control of the process of maintenance of the combined
The root cause of the problem, according to this technique, is the little organization, planning and control of the maintenance process of the combined.
The diagram of Ishikawa allows analyzing systematically the effects and the causes that produce the main problem. This analysis is developed through a brainstorm (Figure 3).
From Ishikawa diagram, the following is concluded:
Fourteen causes were determined that affect the availability of the belt transmission subsystem.
Of them, eleven were related to the process of maintenance of the subsystem, representing 78.5 % of all the causes.
Therefore, the previous conclusion is related to the maintenance and to the organization, planning and control of the process of maintenance of the combined, therefore, this the root cause of the defined problem.
Next, the causes analyzed in Ishikawa diagram are related to the critic variables of quality of the maintenance process referred in Table 2.
Variable Direction
There are not causes related to this variable.
The rest three different causes (suppliers, humidity of the land and type of maize to be harvested) do not have relation with the variables analyzed in Table 2.
Fourth Stage, to Improve the VCC (M)
The objective of this stage is to propose solutions that allow improving the root cause of the problem defined in the stage first. The possible solution that sets out for this problem is the elaboration and organization of the technical service tasks to be developed by the maintenance personnel of the belt transmission system of the combined.
The authors consider that a routine maintenance program, will keep the belts working without problems for a long time. Inspecting the belts before they fail allows avoiding production stoppages and costly delays.
Procedure to Fulfill for the Planned Preventive Maintenance of the Belt Transmission System of the Combined (Gates.com, 2014)
To maintain the security in the means of work, machine turned off;
To follow a program of routine inspection of the transmission;
To follow suitable procedures for belt installation;
To know the characteristics of the belts;
To perform yield evaluations of the transmission;
To know how to locate failures and to correct them.
The first step to elaborate the tasks of the technical services to carry out in planned preventive maintenance will be to fix the frequency of the inspections, according to the criticality of the transmissions.
After knowing the frequency of inspection, the technical services necessary to develop in the belt transmission system of the combined are determined. They will be two: simple inspection (IP simple) and complete inspection (IP complete).
Cycle of Repairing of Three Months (own elaboration)
After knowing the frequency of inspection, through an analysis of failure modes and effects (FMEA) developed together with this investigation, the necessary technical services that make up the repair cycle to be developed in the belt transmission system of the harvester are determined. They will be two: simple inspection (simple Ip) and complete inspection (complete Ip).
Tasks to Develop in the IP Simple (Weekly)
-
Observe and listen.
Be attentive to observe and to listen to any vibration or abnormal sound while the operation of the transmission reviewed. A designed and maintained affluent transmission will work of smooth and quiet form.
-
Inspect the protection and the temperature of the belts.
Inspect the protection to see if it is loose or damaged. Maintain it free of residues or accumulation of dust and dirt. Temperature constitutes an important factor of the belt yield and duration. For example, if the temperature of the elements that surround the belts increases over 60 °C, the internal temperature of the belts increases in 10 °C.
-
To review if there are any oil and grease leaks.
The leaks of oil and grease can indicate too much lubricated bearings. If the oil or the grease touches the rubber components, these ones could be swollen and twist, causing an early failure of the belts.
Retire the protection and review if there are damages. Inspect if there are indications of wearing or friction between the components of the transmission. Clean the protection if it were necessary.
-
Inspect the belts to check if there were wearing or damage. Replace them if it is necessary.
The analysis of signals of wearing or abnormal damages allows locating and correcting possible problems in the transmission.
Review the cracks of the belts, areas with breakage, cuts or indications of abnormal wearing. Verify the temperature of the belts in case there is excessive heat.
Although the belts in fact warmed up during their operation, their temperature never must exceed certain limits. If the belts are too hot, maybe some maintenance change could be necessary. Human hands can tolerate temperatures until around 45 °C. That could provide some reference on the belts temperature: if the operator cannot maintain the belts on his hands, he should check the cause of the excess of heat. The belts have to be replaced if obvious indications of cracks, breakage or abnormal wears exist.
Inspect the other components of the transmission such as bearings, axes, assembly of the motor and adjustment track guides.
-
Review the tension of the belts and tighten them if it were necessary.
If little tension is applied, the trapezoidal belts can slide or the teeth of the synchronous belts can jump.
The correct tension is the lowest tension to which the belts can transmit power when the transmission works to full capacity. The general procedure to verify the tension of the belts is according to Gates.com (2014) as follows:
Measure, in the center of the branch (t), the deflection force necessary to obtain an arrow of 2 mm every 100 mm of length of the branch (synchronous belts) or 1 mm every 100 mm length of the branch (trapezoidal belts) from the normal position of the belt.
If the force measured is less than the recommended minimum deflection force, the belts must be tensioned again.
New belts have to be tensioned until the deflection force is as close as possible to the maximum recommended deflection force.
To facilitate the measurement of the voltage, use a sonic tension meter
Review the alignment of the pulleys.
Reinstall the protection of the transmission.
Make the transmission system work. Observe and listen to any indication outside the normal functioning.
Tasks to Develop in the Complete Inspection IP. (Every three months)
To make all the tasks of the simple inspection.
To inspect the pulleys in case wearing or damage exists, to change if they wore away.
The wearing down is not always noticed. Use gauges to inspect the trapezia grooves.
To review the alignment of the pulleys.
It is always advisable to review the alignment and the suitable assembly of the pulleys.
In order to inspect the alignment, a rule is the only device required, and for systems of transmission with long centers, a rigid cord.
Fifth Stage, to Control to Maintain the Improvement (C)
Aspects to control in the maintenance process of the combined:
The emission of the orders of works corresponding to the planned maintenances and the unexpected failures.
The tension of the belts and the alignment of the pulleys according to the frequency fixed to the described technical services in the fourth stage.
The fulfillment of the tasks of the two technical services presented in stage four.
CONCLUSIONS
The application of the Six Sigma method in the Massey Ferguson corn harvester model 7252 in the Production Unit of Mijaguito of the UPT. "J. J. Montilla "Portuguesa State, Venezuela allowed knowing that the variable maintenance process is the highest priority of the project of the Six Sigma, which is the critic variable for the quality of the work of the harvester and the belt drive system.
The application of the Six Sigma method in the Massey Ferguson model 7252 corn harvester allowed for the belt transmission system determining its repair cycle and the tasks of the two technical services that make it up. The application of the Six Sigma method contributes to the improvement of the maintenance process of the Massey Ferguson corn harvester model 7252, in the Production Unit of Mijaguito of the UPT, "J. J. Montilla " Estado de Portuguesa, Venezuela. It also contributes to the continuous improvement of the management of the combined readiness.
