Preventive maintenace: Some theory and its application

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1970

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The purpose of this thesis was to study different preventive maintenance models and if possible, suggest a more realistic and practical model. During the course of study, it was found that all the models discussed by different workers in this field suggest a constant time interval for subsequent preventive inspections. These models are based on the assumptions that (1) a repair job is perfect and (2) at the time of inspection a system can either be in a state of satisfactory operation or in a state of unsatisfactory operation. There are many systems where the probability of failure or the rate of change of system characteristic is less when the system is relatively new. In these cases, it will be economical to inspect the systems less frequently in the beginning. In actual practice, many times a repair job cannot be perfect. Based on these ideas, a deterministic model for preventive maintenance was developed. Later on, along similar lines a probabilistic model was formulated. In these models, after every inspection there are three possible actions i.e., (1) leave, (2) repair and (3) replace. These actions depend upon the state of the system which in turn is determined by analyzing the most important characteristic of the system. For the analysis of these models, it has been assumed that (1) a repair cost is directly proportional to the amount of change in characteristic at the time of inspection; (2) an inspection cost is inversely proportional to the amount of change in characteristic; (3) the system cannot be made flawless after completion of repair; (4) there is a constant set-up cost associated with every repair. A computer program was written for the deterministic model and trial runs were made with different values of the parameter. It was found that, in most of the cases, it is economical to increase the frequency of inspection as the operating-time of the system increases. Also, in case of the probabilistic model, an illustrative example was worked out to show how this model could be applied. The suggested models can be used for any system that consists of several components or a group of homogeneous parts by slight modification in the parameters.

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