Evaluation of maintenance systems.

Ungureanu, Nicolae ; Ungureanu, Miorita ; Cotetiu, Radu 等

1. INTRODUCTION

Maintenance is defined as all actions which have the objective of retaining or restoring an item in or to a state in which it can perform its required function. The actions include the combination of all technical and corresponding administrative, managerial, and supervision actions.

The maintenance activities are influenced by many factors. The most important are presented in figure 1. According with influences of these factors in present in Romania are applied a few systems of maintenance (fig.2) (Antonescu 1988).

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The parallel between Romanian maintenance systems (Ungureanu 2003) and maintenance system applied in other countries in European Union (Smith 2008) is easy. Thus corrective maintenance correspond to discretionary maintenance where is possible to make rearrangements and modifications of construction, current functional maintenance, functional preventive maintenance and planned preventive maintenance correspond to proactive maintenance with scheduled operations. The palliative maintenance is as a matter of fact a reactive maintenance used in case of breakdowns and emergencies.

Beside this systems are used and increasing more and more the modern maintenance systems especially the predictive maintenance system in all forms: vibration analysis, infrared, ultrasonic and machine history.

2. EVALUATION OF MAINTENANCE

The biggest problem for the management is the selection of the best maintenance system for concrete situation. To solve this problem is necessary to have a set of tools to evaluate the efficiency of one or other of maintenance systems. At this moment for evaluate the efficiency of maintenance systems in Romania are used the indicators of maintainability and availability (Ungureanu, 2003):

* Mean time to repair (mean time to restore), MTTR. For

calculus of MTTR in the phase of design is used the formula:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

where

* n is number of similar components;

* [n.sin.i][[lambda].sub.i]--failure rate for same type components;

* [t'.sub.i]--average number of failures for group of elements [n.sub.i];

* [t'.sub.i]--average time estimated for discharge a failure for one element of group [n.sub.i];

* k--number of distinct group with component elements with know reability.

In the case of experiments or on base of observations collected from exploitation the MTTR can be calculating with formula:

MTTR = [t'.sub.1]+[t'.sub.2]+....+[t'.sub.r]/r = [[summation].sup.r.sub.i=1][t'.sub.r]/r (2)

where

* r is number of maintenance actions;

* [t'.sub.r]--time used for discharge the failure r.

* Availability coefficient can be calculate with formula:

[K.sub.D] = MTBF/MTBF + MTTR (3)

where

* MTBF is mean time between failure;

* MTTR--mean time to restore.

* Percentage of inactive time:

[K.sub.IN] = MTTR/MTBF + MTTR (4)

* Percentage of availability:

[K.SUB.DI] = MTTR/MTBF (5)

* Coefficient of use:

[K.sub.U] = MTBF/[T.sub.E] (6)

where [T.sub.E] is the calendar time of use, including time of effective use of machines, time for maintenance and time of interruption of use.

At the beginning of 2007 was approved a new European standard entitled "Maintenance--Key Performance Indicators". This standard use a system of indicators structured in three categories: economic, technical and organizational. (figure 3)(European Committee for Standardization 2007)(Grencik & Legat 2007)

The economic indicators are 24 divided in 4 levels and use as unit of measurement the money. In the most cases the reference for indicators is "total maintenance cost". (13 indicators from 24). For example E14 is defining as:

E14 = Preventive maintenance cost/Total maintenance cost x 100 (7)

Total maintenance cost

The technical indicators are 19 divided in 4 level and use as unit of measurement the time. Same as in the case of economic indicator we have a reference "number of failure". In the list of indicator we meet "classic" indicators as men time between failure, MTBF--reliability indicator and mean time to failure, MTTF--reliability indicator.

The organizational indicators are 34 divided in 4 level and use as measurement unit the time and in some cases number of persons.

The E.U. standards "Maintenance--Key Performance Indicators" present also the definition of indicators and a methodology to use the Key indicators. The methodology follows the logical diagram presented in figure 4.

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3. CONCLUSION

The process of evaluation of maintenance systems is not an easy job because are many factors involved and is not possible to make a pattern ready to use. Each time before evaluation is necessary to analyze the concrete situation and choose from the list of indicators the agreed indicators. In the same time is possible, in particular cases, to not find in list of EU standard or in national standards an indicator that uses the avaible data. In this case is possible to create a specific indicator, but only with condition to define coherent the terms.

The recommendation for evaluation is to choose a small number of indicators and the calculus of these indicators must respect the principles of statistics mathematic.

4. REFERENCES

Antonescu, V., Stoichitoiu, D., Elemente de teorie si culegere de probleme de fiabilitate, mentenabilitate, disponibilitate, vol I, II, (Elements of theorie and practical aspects of reliability, maintainability and availability) Institutul central pentru industria electrotehnica, Oficiul de informare documentara, Bucuresti, 1988

Grencik, J., Legat, V., Maintenance audit and benchmarking--search for evaluation criteria on global scale, Eksploatacja i niezawodnosc, Polska Akademia Nauk Oddzial w Lublinie nr 3/2007, pag 34-38.

Smith, M., Planned Maintenance Techniques, Available from: http://elsmar.com/Planned_Maintenance/index.htm, Accessed:2008-02-17

Smith R., Mobley K., Industrial Machinery Repair: Best Maintenance Practices Pocket Guide, ISBN 0-7506-7621-3, 2003, Elsevier Science

Ungureanu N., Fiabilitate si diagnoza, (Reliability and diagnosis) ,Editura Risoprint Cluj Napoca, 2003

*** "Maintenance--Key Performance Indicators" European Committee for Standardization, 2007