Modelling of Maintenance Management on the Basis of Given Operative Availability

Hasan Avdić University of Tuzla, Faculty of Mechanical Engineering

Univerzitetska 4, Tuzla, Bosnia and Herzegovinaavdic.ha@bih.net.ba

Mehmed HasanovićCoal Mines “Banovići”

Armije BiH 52, Banovići, Bosnia and Herzegovinahasanovic.mehmed@gmail.com

ABSTRACT

The maintenance management of technical systems should, first and foremost, provide operational or short-term (daily) planning, but also the planning for a longer period of time, as well as the innovations of the maintenance system and technical systems (design requirements of the maintenance system, the preparation of technical systems for exploitation and, finally, suggestions for withdrawing technical systems from the usage). On the other hand, it is known that operational availability is the likelihood of the technical system, when used under specified conditions, to function satisfactorily at any point of time; the time considered here includes the use of the time (effective time) and time delays. This paper will present one maintenance management system model in the Coal Mines “Banovići”.

KEYWORDS: modelling, management, maintenance, operational availability

1 INTRODUCTION

The competitiveness and the survival of companies on the global market depend largely on the maintenance system. Enhanced requirements requested from the equipment and decreased maintenance costs, either direct or indirect; present an important factor for the enhancement of competitiveness. The fact that the best maintenance departments in the world strive to improve the maintenance system by applying new materials and technological achievements presents an important reason why it is essential to direct company developments in our area. The importance of maintenance is constantly rising, especially due to the fact that there is an increase in availability and accessibility of equipment and its safe functioning.

In broad observation, the maintenance system presents part of the business system through which design and redesign the following maintenance parts are integrated: an optimal organisation, relevant technology and adequate strategy, and make up one maintenance system. Special importance is put on the implementation of informational systems in the framework of the maintenance system as the basis for the unification of available resources and engineering economies.

The Coal Mines “Banovići” joint-stock company, possess a large number of different kinds of excavators, some of which were bought in the fifties in the past century and some up-to-date excavators, bought only few years ago. The fact that the excavators are produced by different manufacturers (P&H, EKG, Marion, Terex, Liebherr), that they range from old ones to the most recent ones, that they are of different kind (hydraulic and conventional), makes it difficult to find an

EM12044-1

optimal solution for maintenance applicable to all excavators. Irrespective of the part of the maintenance system of the excavator: strategy, organisation or technology, it is impossible to find a common solution to all excavators, which would be a solution to the minimum and maximum of an excavator function.

2 THEORETICAL RESEARCH

2.1 Maintenance system

The maintenance system of technical systems (Figure 1) can be realized in different ways, in many different variants. Some variants or some solutions of the maintenance system are different in a range of details and in some basic, for the system important characteristics. This regards the concept of the maintenance system and also, the applied technology and organisation. The concept of the maintenance system presents the principle of decision-making about the time when to apply the maintenance process (a series of activities). There are some maintenance opportunities: preventive, corrective and combined maintenance. Preventive maintenance implies necessary actions beforehand, and corrective maintenance implies actions when the failure happens. The technological aspect regards the kind and presentation way of the maintenance process. The organisation of the system depends on the relation between single levels where the maintenance process is held (by the way, the maintenance process presents those activities carried out in the technical system in order to prevent failure – overhauling, the replacing of some parts, etc).

The concept of the maintenance system presents its basic characteristic which influences the general quality of the maintenance system. This term implies the system characteristic which depends on the principle according to which decisions in the maintenance proceedings are reached. There are three basic conceptual possibilities: preventive, corrective and combined maintenance. In the first case, the maintenance proceedings are taken before down time occurs. In other words, the task of maintenance is to prevent or postpone the appearance of down time. Corrective maintenance implies taking actions after it comes to down times. In this case, their task is to put an “IN FAILURE” system into a “WORKING” system. Both concepts can be connected – it is a combined maintenance. Combined maintenance can be realised in many ways. Usually it is realised in the way that specific technical system parts (specific elements) are maintained preventive, whereas other system parts are maintained after down time occurs. /1/

Figure 1: Maintenance system components /1/

2.2 Maintenance management

The management of a maintenance system is performed through a closed scheme of informational and mutual actions of the managed object and the management system, presented in Figure 2. The starting programme is defined through the given level of basis reliability – calculator output ψo (t) of the controlled object, and is adopted as the regulating organ which is deciphered as the successiveness of regulating actions coming into the executive body (EB).

EM12044-2

The regulating organ presents the sub department of the maintenance system which task is to draft decisions how, where and when to perform maintenance. The executive body includes sub departments which directly take part in the preparation and performance of maintenance. This body generates management actions (performance of maintenance) on the executive body with the intensity Xo (t). As a result of the mutual actions of accidental unrecorded factors on the technical system, the intensity output (exploitation reliability) Ψ (t) will be different from the calculator output (basis reliability). The basis scheme of maintenance management system is presented in Figure 2.

Figure 2: Scheme of maintenance management system /1/

2.3 Availability

According to the definition of system availability, it can be noticed that availability depends on various factors and includes different indicators and time categories. When talking about availability, some systems can be isolated and are different from other systems. There are systems which are required to work continually. The system availability is reduced if failures occur in the system or its working is ended on a planned basis, necessary for maintenance. The maintenance of a system enhances its availability only when the system is used occasionally or its function is not demanded. Sometimes operative requirements exceed the system possibilities. In this case, the system availability is drastically reduced.

Operative availability of a system is defined in the same way as operative efficiency. The difference between operative efficiency and operative availability is that operative availability of a system does not include the storage time and the time planned for the usage.

2.3.1 Availability according to EU standards (EN 13306; EN 15341; EN 13460)

Availability presents the ability of the elements to perform the demanded function under the given conditions and time or during the given time interval, under the condition that all necessary external sources are provided.

NOTE 1: This ability depends on combined aspects of reliability, convenience for maintenance and the possibility to provide maintenance support.

NOTE 2: Necessary external resources, different from maintenance, do not affect availability of the elements.

Equipment availability is defined as the potential time production rate for the time during which the equipment is operating, i.e. the working is not interrupted due to equipment cancellation.

Availability based on maintenance:

T 1=TOTAL OPERATING TIME

TOTAL OPERATING TIME +DOWN TIME DUE TO MAINTENANCE⋅100

(1)

EM12044-3

2.3.2 Statistical modelling of a system

Statistical modelling of a system presents methods which experimentally obtained data process by using tools of the statistical analysis. Original methods of the statistical analysis are linear statistic methods, like the Multiple Linear Regression = MLR. Afterwards, some nonlinear statistic analysis methods were developed, like Artificial Neural Networks ANNs.

One linear statistic analysis method, which is applied in modelling the system parameters, according to experimental data, is the Multiple Linear Regression MLR analysis. The MLR method defines output of linear correlation dependable variable (output values of the system) in the function of independent variables (input values of the system). In this way there is dependence of type:

X = a + bXi1+cXi2+dXi3+...+nXin

(2)

3 EXPERIMENTAL RESEARCH

3.1 The plan of the experiment

The research was carried out in the example of maintenance management system in the Coal Mines “Banovići”.

In order to realize the objective, the following was done:- The equipment and the maintenance system were defined,- The method of the existing maintenance management system was analysed,- Operative availability was analysed,- A new method of maintenance management system was suggested,- A comparative analysis of the existing and new maintenance management system,- Drawing up a method of maintenance management system on the basis of operative

availability.

3.2 The defined equipment and maintenance system

3.2.1 Excavator Marion 201

Excavator “MARION 201-M”, serial number 23317, presented in Figure 3, was set in motion on 19th May 1987 in the presence of experts of the company “MARION”. The excavator Marion 201 is the biggest excavator in the Coal Mines “Banovići”, with an electro engine, power 780 kW and a shovel capacity of 21 m³. During the probation time the excavator worked without problems and presented very good results, which corresponded with its planned capacity.

Figure 3: Excavator Marion 201

If you consider the excavator TEREX RH – 120 E as one system, which at the port has energy, material and information, and as the final output gets capacity, effectiveness and quality, then you can divide the system into some subsystems. In view of the mentioned, the technical system excavator

EM12044-4

MARION 201 consists of six subsystems: subsystem travel, subsystem central lubrication, subsystem digging, subsystem lifting, subsystem swing and subsystem work setup. The mentioned subsystems as being parts of the technical system excavator MARION 201 are presented in Figure 4.

Figure 4: The technical system excavator TEREX RH 120E with subsystems

All the given subsystems of the excavator MARION 201 are in a serial relation, which means that in case of a failure of any subsystem of the technical system excavator Marion 201, the whole system is in failure.

3.2.2 Analysis of the existing maintenance management system

The cybernetic maintenance model presented in Figure 5 presents the maintenance model of excavators in the Coal Mines “Banovići”. The whole maintenance system is made up of an operative level divided in sections which perform a monthly and yearly maintenance planning (MP). After the maintenance planning time expires, a report about the realization of the maintenance planning is drafted up (RMP). The gathering of down time data, drafting up a waiting and down time chart (WDC) and calculating the availability of all the mechanization, is performed in the technical preparation department, which belongs to the mechanical maintenance department.

The drafted waiting and down time chart presents a document available to the mechanical and electro department but not to the operative maintenance department. The working department for the maintenance of excavators performs the maintenance planning according to need for available working hours of the excavators expressed by the pit managers in the pit plans, but not according to the waiting and down time chart which is administered in the mechanical preparation department.

The maintenance in the Coal Mines “Banovići” is divided into the electro and the mechanical maintenance department. Therefore, there is not one head who would direct and coordinate all failures, either electro or mechanical failures. Unnecessary down times due to time anticoincidence of the electro and mechanical mending occur often in the Coal Mines “Banovići”.

Each department, the electro and the mechanical department, consists of a technical preparing unit which collects, processes and analyses down times.

The technical head of each assembly runs the planning, coordinating, performing and controlling of all maintenance tasks. They do not possess the information about down time types, the down time duration, or down time frequency for the past period.

Figure 5: Scheme of the existing management mode of the maintenance system (MS)

EM12044-5

3.3 The analysis of the operative availability of the excavator Marion 201

Figure 6 presents the availability of the excavator Marion 201 taken from the working and down time chart which is administered in the technical department of the mechanical maintenance in the Coal Mines “Banovići”. The presented availability is calculated by using effective working time hours of the excavator and down times administered in the working and downtime chart, including the reserves, lack of trucks, routes, etc. The calculated values of the operative availability do not present the operative availability, because they do not include the down times which are not in correlation with the down times due to maintenance.

The excavator Marion 201 had the highest availability in April 2008 (80, 7%) and August 2008 (80%). The data is presented in the working and down time chart taken from the mechanical maintenance department in the Coal Mines “Banovići”.

1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9

2005 2006 2007 2008 2009 2010

0102030405060708090

AVAILABILITY OF THE EXCAVATOR MARION 201

AVAILABILITY

Figure 6: Availability of the excavator Marion 201 according to the workingand down time chart in the Coal Mines “Banovići”

It can be seen in the ABC Analysis presented in Figure 7 that the most down times occurred due to lack of trucks (21%), waiting for the mechanical mending (19%), mechanical mending (15%), lubrication (10%) and resting (9%), which at the same time make up the group of the most influential elements (A).

Figure 7: Pareto chart down time analyses for the excavator Marion 201

EM12044-6

3.4 Some proposals for a new maintenance management system in the Coal Mines “Banovići”

According to the theory of the system and the existing maintenance management system of excavators presented in Figure 6, a new maintenance management system of excavators was presented. Its cybernetic features are presented in Figure 8. The suggested maintenance management system of excavators is based on setting boundaries to availability in (availability set) in the management unit (management programme). After the completed setup of availability boundaries, the planning of the maintenance time using the genesis of events from the working and downtime chart is performed. After having chosen a suitable maintenance system, i.e. a strategy, organization, and maintenance technology for the set availability, a monitoring of the planned maintenance time and calculating the achieved operative availability (Treal) is performed. The collection of the achieved operative availability and comparison with the set ones will be performed in the technical preparation department and according to this; together with the working unit of excavator maintenance and transport system, an influence on the strategy, organization and maintenance strategy will be exercised.

Figure 8: Scheme of the suggested maintenance management system (MS)

3.5 Comparative analysis of the existing and new maintenance management system

By analysing the existing maintenance management system presented in a scheme in Figure 6, can be noticed that maintenance is performed according to a monthly and yearly maintenance plan. An output quantitative value, according to which basis value to set managing input values for the technical system in order to keep the output values in set boundaries does not exist in the existing maintenance management mode. The separated working unit of excavator maintenance from the working and downtime chart which possess the working times and the down times for each technical system enable the insight into the genesis of the observed technical system and question the validity of the maintenance actions.

The suggested maintenance management system presented in Figure 9 resumes the maintenance management of excavators to planning the operative availability and maintaining the planned operative availability within the planned boundaries. Maintenance is performed by planning the boundaries of operative availability, according to the working and down time chart and the need for excavators and by monitoring the realised availability as the output value of the technical system. The output value of operative availability is compared within the technical preparation unit with the set operative availability and in this way the time for maintenance as the input value on the managing system is planned together with the working unit of excavator maintenance. In order to apply this management system, it is necessary to calculate operative availability on the basis of maintenance time and divide the maintenance time into different kinds, in this way locating easier bottlenecks and reducing it with adequate management actions. However, this is not the case in the existing maintenance management system.

EM12044-7

3.6 Drafting up a maintenance management model on the basis of given operative availability

3.6.1 Drafting up a maintenance management model by applying Multiple Regression Equation for the excavator Marion 201

During the observed time of 69 months, the following equation was obtained for the excavator Marion 201:

T1 = 69,1 + 0,0539 Twef - 0,132 Tm - 0,0877 Twait (3)

During the observed time the average operative availability for the excavator Marion 201 is 63%. In accordance with the need for the excavator for which are Twef=500 effective working hours (Twef) planned, the operative availability is also planned Tgiv = 75%.

The two planned services weekly of four hours down time present an optimal solution for the active maintenance time. The mentioned service includes also the preventive exchange of used parts. A planned change of the maintenance strategy suggests maintenance according to condition with parameters observation and weekly value measurements of the chosen parameters with a down time of two hours (Preventive maintenance is Tmp=50). Tmc=90 hours of down time are planned for the remaining time of the active maintenance. The total time for active maintenance is therefore T m = 140 hours. For the planned time in the regressive equation follows a waiting time of Twait = 29, 3. The distribution of time for the maintenance model excavator Marion 201 obtained from the regression analysis is presented in Figure 9.

Tdt

Twef

0 50 100 150 200 250 300 350 400 450 500

90 50 29.3

500

Distribution of time for planned availability of 75%

Twef

Tmp

Tmc

Twait

Figure 9: Distribution of time of the maintenance model excavator Marion 201 obtained by MRE

3.6.2 Drafting up a maintenance management model by applying the software “Planer” for the excavator Marion 201

Specialized software for the planned needs of the maintenance system in the Coal Mines “Banovići” was developed. This software enables the election and monitoring of parameters of the maintenance system. The software data, updated every month, enables the planning of chosen quality parameters and the set-up of a maintenance system in view of applied technologies, chosen strategies and the existing organisation for the given availability and available resources. The perspective of the software for planning the maintenance system with the given operative availability is presented in Figure 10.

Figure 10: Software “Planer” for excavator Marion 201

EM12044-8

In accordance with the need for the excavator for which are Twef=500 effective working hours planned, the operative availability is also planned Tgiv = 75%.

The two planned services weekly of four hours down time present an optimal solution for the active maintenance time. The mentioned service includes also the preventive exchange of used parts. A planned change of the maintenance strategy suggests maintenance according to condition with parameters observation and weekly value measurements of the chosen parameters with a down time of two hours (Preventive maintenance is Tmp=50). Tmc=90 hours of down time are planned for the remaining time of the active maintenance. The total time of active maintenance is therefore Tm = 140 hours. The obtained waiting time in the software “Planer” is Twait = 26,67. Distribution of time is presented in Figure 11.

Tdt

Twef

0 50 100 150 200 250 300 350 400 450 500

90 50 26.6

500

Distribution of time for planned availability of 75%

Twef

Tmp

Tmc

Twait

Figure 11: Distribution of time of the maintenance model excavator Marion 201 obtained by “Planer”

4 CONCLUSION

Long hours spent in down times due to maintenance of expensive and high productive equipment produce large expenses, either direct or indirect ones. By knowing the fact that the existing maintenance management system of excavators presents an open system without feedback, directly influencing the input values of the technical system, it can be concluded that engineering of the maintenance system of excavators is necessary and that it is necessary to adopt a maintenance system which enables the management of failures and to avoid the existing condition where equipment failures manage the maintenance system.

For a successful realization of the set maintenance management system model and the management of the set maintenance system, it is necessary to adopt a quantitative value which changes directly influence the maintenance system. The chosen operative availability as an important internal system characteristic speaks for the applied maintenance system. This adopted operative availability as the quantitative characteristic for the maintenance management model of excavators is special in the fact, that by calculating it takes into account only the working time of the excavator and the time spent on maintenance. By setting up operative availability as the target function and its changes, it is possible to define time which in its down time generated a reduced operative availability and according to this bring those managing actions to keep the operative availability on a planned level.

REFERENCES

/1./ Avdić H., Tufekčić DŽ., Terotechnology I, University of Tuzla (2007), Tuzla, BiH/2./ Adamović Ž., The Theory of the System, University of Novi Sad (2007), Serbia/3./ Todorović J., Maintainability engineering, Beograd (2006), Serbia/4./ Adamović Ž., Workbook with exercises Theory of the System, University of Novi Sad (2004),

Serbia

EM12044-9