O. P. Gandhi

Modelling and evaluation of mean time to repair at product design stage based on contextual criteria

Time to repair of a mechanical system is a function of its maintainability and external influences in the plant. Repairability means the ease of maintenance. In this paper, repairability features of the mechanical system are identified, and are modelled in terms of repairability digraph. The nodes in the digraph represent the repairability features and the edges represent the degree of influence among these. An equivalent matrix representation of the digraph is developed to define the system repairability function (SRF). Repairability correction factor (RCF) is defined as a ratio of the actual to the ideal values of SRF. A high value of RCF indicates that the system support structure is closer to the ideal state, which implies that the mean time to repair (MTTR) is same as that estimated by the designer. The presented model is illustrated with an example. The results show that the MTTR of mechanical equipment, based on contextual criteria, can be predicted, by extrapolating the design value of MTTR. The proposed methodology is capable of evaluating the realistic MTTR based on plant influencing factors and is expected to aid the designers in selecting the best design and will also help the maintenance planners.

A life cycle cost based approach of O&M support for mechanical systems

Performance of mechanical systems over their life cycle is the main concern of users. Original equipment manufacturers (OEMs) of such systems are made to deliver customized products with documented support to meet the objective. The product support is of various kinds, one of these is operation and maintenance (O&M) support. Cost incurred for each support action adds to the system life cycle cost (LCC) and therefore, a trade-off between O&M support, operational objectives and design characteristics is required to optimize the LCC. In this paper, important issues are identified for O&M support of mechanical systems and steps are proposed. Mathematical models are developed to assess the support objectives i.e. LCC and operational availability of the system. The LCC takes into accounts the acquisition cost and discounted sum of support activity cost, which consist of operating cost, inspection, corrective, preventive, overhaul and logistic cost. The proposed methodology has been implemented on a real life problem, in which the OEM provides O&M support to their compressors, installed at compressed natural gas workstation in national capital region, India. The results shows that a vital saving can be made in system’s LCC, when the support has been optimized in context to preventive maintenance, viz: age based at system level, while group maintenance based on optimum replacement interval and components’ scale parameter of their failure distribution. The sensitivity analysis validated the robustness of the solution methodology and the obtained results. This work will be helpful to OEMs, customers, academician, researchers, industrialists, and other concerned persons, in understanding the importance, severity and benefits obtained by the application and implementation of the O&M support.

Equipment redesign feasibility through maintenance-work-order records using fuzzy cognitive maps

Initiation of maintenance-work-order (MWO) by maintenance–planning unit drives maintenance activities, as it authorizes this work to be carried out. The MWO contains information on resource requirements besides specifying the time-frame for work completion. On conclusion of the maintenance actions, it is experienced that these and other maintenance parameters vary appreciably from their envisaged values. These deviations and the status of dismantled equipment are recorded in the MWO. The objective of this paper is to identify the parameters, which have a potential for design-change. It demonstrates the use of fuzzy cognitive maps to extract the desired knowledge from the MWO records. The study concluded that the MWOs, which recorded a high degree of cognitive values for surface/material failure, deviation in equipment settings and the extent of repairs carried out on the equipment do have a high degree of potential for redesign. The analysis also concluded that a high degree of time to maintain and quantum ofspares used may not be critical for immediate design modifications. This will help to identify the MWOs, which should be sent to the designer for redesign of the equipment.

A conceptual framework for analysing, improving and optimising supportability of mechanical systems

Supportability for a user is the ability of the support provider to execute all the required activities for upkeep of the system in the most effective, efficient and timely manner throughout the system’s operating life. From the support provider’s perspective, design, maintenance services and logistics along with appropriate contract help in delivering the required support in a profitable manner. However, from the user’s point of view, apart from uninterrupted availability of the product, the life cycle cost (LCC) associated with the product is of immense importance. Hence, a tradeoff between the supportability and the corresponding LCC is to be achieved. Although a vast amount of literature is available on the subject, yet an integrated approach that considers all parameters affecting supportability and LCC in obtaining an optimal solution needs to be developed. This paper presents a framework for analysing supportability along with a proposed methodology that can help in arriving at the optimal solution.

Maintenance cost minimization of manufacturing systems using PSO under reliability constraint

Profit is a main objective of production units, and minimization of its maintenance cost will help to achieve it. But with no compromise with plant machinery reliability constraints. Manufacturing systems do require maintenance, but deferring it due to production constraints may increase its requirements, resulting in the increased cost. The experience shows that carrying out of the preventive maintenance (PM) helps in avoidance of catastrophic failures, but this needs to be optimized. An optimal PM interval will minimize maintenance cost, while maintaining their lower bound reliability. Most existing optimization models do not address this issue especially in the context of real life industries. Particle swarm optimization is an effective meta-heuristic technique and is extensively applied to find optimal solution for various engineering problems. In this paper, this is proposed for maintenance cost minimization of manufacturing systems under reliability constraint. An example is illustrated to demonstrate it.

Ontological modeling of spatial shaft-position knowledge for steam turbine rotor

Large business enterprises rely heavily on their human capital. The human capital consists of domain knowledge, experience, analytical and creativity levels. It is recognized that, creativity levels and analytical skills are traits of individuals. All these need to be captured by the use of suitable knowledge capturing techniques, particularly due to exponential knowledge growth in all domains of technology. Ontological modeling has the potential to capture such knowledge, and organize it in a structured format based on the concepts and their mutual relations. It can capture the analytical and creative rationale of design and troubleshooting, besides incorporating specific design sub-routines for tasks. This paper discusses an ontology based model, which stores knowledge for the positioning-system of steam turbine rotor. The efficacy of the model has been demonstrated in capturing this domain knowledge, which can be used by the design and the maintenance personnel and thereby, increase their efficiency.

Supportability scenario-based contract alternatives for operating life of a mechanical system

Supportability for a user is the ability of the manufacturer to execute all the support activities that are required for the upkeep of the system, in the most effective, efficient and timely manner throughout the operating life of the product, whenever and wherever needed. In this work, the various issues of supportability of mechanical systems have been studied. The case study is conducted on a compressor manufacturer (Burckhardt), which provides support to their end products that are installed at compressed natural gas stations in National Capital Region (NCR). Both the Burckhardt and its customer have aggressive plans of expanding their operations to new locations in future. As a first step, it is necessary to have a mechanism to generate and evaluate various contract alternatives (CAs) with an objective of minimising the costs incurred to various players in the contract and at the same time, ensuring the minimum service level performance of the system. The methodology presented in this paper will be helpful to Burckhardt and/or any original equipment manufacturer in general, for selecting supportability-based CAs. This study will also be helpful to researchers, engineers, maintenance professionals and other persons concerned, to understand the issues involved in supportability-based CAs.

Annual maintenance budget estimation for a plant system

Purpose – The purpose of this paper is to propose a methodology to evaluate the annual maintenance budget (AMB) for a plant system as a percentage of its asset replacement value (ARV). Design/methodology/approach – Variables influencing the budget for the plant system are identified and modeled in terms of a plant maintenance Budget digraph. The nodes in the digraph represent the function of budget influencing variables and edges represent the degree of influence among these. The plant maintenance budget function is derived from the equivalent matrix of the plant maintenance budget digraph and is used to evaluate the AMB for the plant system as a percentage of its ARV. The presented model is illustrated with an example. Findings – The proposed methodology enables the maintenance managers and practicing engineers to evaluate the AMB for maintenance tasks based on plant specific variables that can vary from one plant to another, without resorting to general rules-of-thumb method of budgeting or to expert ju...

Supportability perspectives: current practices and potential area for future research

Supportability is meant to enhance efficiency and effectiveness of the system through support elements, including design and logistics at the lowest possible cost. Elements of the support are installation, documentation, training, maintenance, etc., which play an important role in proper functioning of the system. The paper outlines two research issues, which identify the dimensions of supportability and approaches for addressing the supportability, and based on this, the literature review is carried out. The paper classified the literature, identified the gaps, and suggested potential directions for future research, on the basis of which a vital area of research has been proposed. It is hoped that this work will help designers, practicing engineers and researchers involved in the area, to carry out further research.

Assessment of failures in automobiles due to maintenance errors

Maintenance errors in automobiles can lead to premature failures or even accidents. This is attributed to system design, contextual factors prevailing at the garages and human elements performing the maintenance tasks. Use of two well known diagrammatic procedures; event tree diagram and Fishbone diagram, are applied to understand failure initiation of maintenance errors and its effects, and identifying causes and sub-causes attributed to these respectively. This is followed by use of soft computing technique of fuzzy cognitive map to model the interrelationship among maintenance error causes and asses the failure due to these. This is demonstrated through a case study. The proposed methodology helps in better understanding of maintenance errors and the failures attributed to these. Moreover, it is capable of handling high degree of uncertainty and variability associated with the maintenance errors. It provides a ranking of maintenance errors, which can guide automobile service organizations in providing quality and reliability of service, including feedback to designers and practicing engineers in minimizing maintenance errors.