Critical machines preventive maintenance policies for group technology shops

This study develops a set of preventive maintenance policies, based on the idea of critical machines, for use in group technology shops. These policies designated certain machines in the shop as critical. When a critical machine breaks down, it is repaired and preventive maintenance is applied to a block of machines (either the remaining machines in the cell, the other machines of the same type or all of the remaining machines in the shop). Simulation of a complex group technology shop was used to compare twelve definitions of criticality and three preventive maintenance conditions. The definition of criticality was not found to be a major cause of performance differences between shops, since in many cases, the same machines were critical by any definition. The application of preventive maintenance to all machines in the shop when a critical machine had failed led to the best shop performance, indicating that the reduction in machine interference caused by doing so was more important than the unique features of group technology.     

The failure intensity process and the formulation of reliability and maintenance models

A unified approach to the formulation of failure event models is presented. This provides a common framework for the analysis of both repairable and nonrepairable items, preventive as well as corrective maintenance, and it also applies for items with dormant failures. The suggested procedure is supported by a set of graphs, thereby identifying the significance both of the inherent reliability (i.e., hazard rate) and of the maintenance/repair policy. The definition/interpretation of various failure intensity concepts is fundamental for this approach. Thus, interrelations between these intensities are reviewed, thereby also contributing to a clarification of these concepts. The most basic of these; concepts, the failure intensity process, is used in counting processes (Martingales), and is the rate of failures at time t, given the history of the item up to that time. The suggested approach is illustrated by considering some standard reliability and maintenance models.     

Identification of safety and security critical systems and activities

There are many initiatives taken to identify safety and security critical systems and activities, at different levels and in different contexts, ranging from infrastructures at the societal level to equipment on the production plant level. Different approaches are implemented to define the critical systems and activities. Some of these relate to vulnerabilities, others incorporate the probability dimension and are risk based. We also see approaches taking into account values of the decision-maker and relevant stakeholders. In this paper, we discuss the rationale for these approaches. Is vulnerability an adequate measure to be used as a basis for determining criticality? Is it meaningful to specify safety and security critical systems and activities without addressing risk? How should we take into account the limitations of the risk assessments? Should we extend the concept of criticality to also cover utility aspects? We bring new insights into the discussion by being precise on the key risk concepts—including uncertainty, probability and expected value—and considering alternative risk perspectives. A novel approach is suggested based on expected values and uncertainties in underlying phenomena and processes. Our main concern is activities with potential severe consequences and large uncertainties.     

Criticality Analysis for Maintenance Purposes: A Study for Complex In‐service Engineering Assets

The purpose of this paper is to establish a basis for a criticality analysis, considered here as a prerequisite, a first required step to review the current maintenance programs, of complex in‐service engineering assets. Review is understood as a reality check, a testing of whether the current maintenance activities are well aligned to actual business objectives and needs. This paper describes an efficient and rational working process and a model resulting in a hierarchy of assets, based on risk analysis and cost–benefit principles, which will be ranked according to their importance for the business to meet specific goals. Starting from a multicriteria analysis, the proposed model converts relevant criteria impacting equipment criticality into a single score presenting the criticality level. Although detailed implementation of techniques like Root Cause Failure Analysis and Reliability Centered Maintenance will be recommended for further optimization of the maintenance activities, the reasons why criticality analysis deserves the attention of engineers and maintenance and reliability managers are precisely explained here. A case study is presented to help the reader understand the process and to operationalize the model. Copyright © 2015 John Wiley & Sons, Ltd.     

Maintenance strategy based on a multicriterion classification of equipments

In an industrial plant, the level of maintenance provided to individual equipment is directly related to the availability that is expected from it. Thus, it is hoped that the most critical equipments will not fail or, at least, that any failure will be rapidly detected and corrected in the minimum time possible. Since resources are limited, it is necessary to determine how they should be distributed, so that no important equipment remains neglected while more resources are concentrated on the most critical items. Therefore, it is necessary to classify equipment in an objective way according to its importance. The method of multicriterion classification of critical equipments (MCCE)², which is described in the present work, allows systematic and detailed quantification of the criticality of all equipment, that is to say, it provides an evaluation of the importance that its correct operation has for the plant. To provide this information, the consequences for a company of any failure in the equipment concerned are analysed. Finally, a real case example of an urban wastewater treatment plant is described, in which the MCCE method is applied.     

Risk assessment techniques for civil aviation security

Following the 9/11 terrorists attacks in New York a strong economical effort was made to improve and adapt aviation security, both in infrastructures as in airplanes. National and international guidelines were promptly developed with the objective of creating a security management system able to supervise the identification of risks and the definition and optimization of control measures.Risk assessment techniques are thus crucial in the above process, since an incorrect risk identification and quantification can strongly affect both the security level as the investments needed to reach it.The paper proposes a set of methodologies to qualitatively and quantitatively assess the risk in the security of civil aviation and the risk assessment process based on the threats, criticality and vulnerabilities concepts, highlighting their correlation in determining the level of risk.RAMS techniques are applied to the airport security system in order to analyze the protection equipment for critical facilities located in air-side, allowing also the estimation of the importance of the security improving measures vs. their effectiveness.     

Risk awareness and criticality assessment of driving situations: a comparative study between motorcyclists and car drivers

The general framework is about risk awareness through the aspect of cognitive abilities ? to perceive and assess the criticality of a driving situation ? among motorcyclists against car drivers. To study risk awareness, some tools based on video films of road scenes have been developed. The experimental task consists in assessing the criticality of dynamic driving situations, via a Likert scale, and qualifying these situations using Osgood?s semantic differential. The results show that, on the whole, motorcyclists consider the sample of driving situations as less critical than car drivers do. The main differences observed between the two populations are presented.     

Automated retrieval and ranking of similar parts in agile manufacturing

The environment considered is an agile enterprise where manufacturing partners share product related data to come up with new, customized, and high quality products at minimal leadtimes. Given this context, this paper addresses the problem of identifying existing parts that are similar, in one or many characteristics, to a new part at the design stage. The proposed method is based on the principles of group technology (GT), and on the definition of the neighborhood of similarity of critical design attributes. A two-step procedure is proposed: (1) a search procedure, which acquires and processes the designer's search intent to retrieve similar parts; and (2) a sorting procedure, which ranks these parts in order of their similarity to the candidate part. Both procedures are based on GT codes, while the second can employ more detailed critical design information. This information is assumed available in the product databases of distributed partners, and can be generated via an existing GT design processor. The approach employs a systematic procedure to combine independent similarity indexes to a unique measure for sorting. A software system using object-oriented technology has been developed to implement the procedure.     

Impact analysis of change in component reliabilities in safety‐critical systems

A safety‐critical system or life‐critical system is a system whose failure or malfunction may result in one (or more) of the following outcomes: death or serious injury to people and loss or severe damage to equipment/property. Such systems are very common in nuclear power plants and are composed of several components, performing different functions. The criticality of these components is ranked according to the criticality of the functions they perform. Therefore, the impact of component failure on system will be different for different components. It is essential to determine the impact of failure of any component on overall system to take preventive and corrective actions. This paper proposes a technique to determine the criticality of the components for their impact on the overall system using Bayesian approach. The theoretical basis and effectiveness of the proposed technique is shown and validated on a real case study of a nuclear power plant system.     

Using the Method of Lines to determine critical conditions for thermal ignition

There are many situations in which the critical conditions for thermal ignition cannot be determined analytically. These include cases where the chemistry needs to be properly considered, where the geometry is not just the simplest and where other processes must be included. In these circumstances, numerical (or at least semi-analytical) means are used to determine critical conditions for thermal ignition. Once confronted with a numerical approach to solving a problem, it is necessary to be a little circumspect about the results and seek independent means to corroborate them. For this reason, the present paper reports on the Method of Lines to investigate a recent reactive hotspot problem which has previously been shown to display unexpected behaviour and demonstrates the use of sensitivity analysis to rigourously determine criticality in such a dissipative system.     

A literature review on selective maintenance for multi‐unit systems

Selective maintenance is regarded as a type of profit‐generating maintenance policy, playing an important role in balancing limited maintenance resources with system performance. Since 1988, increasing interest has been focused on this research area. Nevertheless, to the best of our knowledge, there is a lack of critical reviews of selective maintenance. This paper is the first systematic review focusing on this relevant topic. In this work, a definition and some specific features of selective maintenance are elaborated. Based on these features, a set of criteria that have been considered in selective maintenance optimization are summarized into 3 categories: system characteristics, maintenance characteristics, and mission profile characteristics. Based on these criteria, a comprehensive literature review on selective maintenance is undertaken. The solution approaches, as well as a general procedure for selective maintenance optimization, are discussed. Finally, some possible directions for further research are provided.     

Reliability demonstration test planning: A three dimensional consideration

Increasing customer demand for reliability, fierce market competition on time-to-market and cost, and highly reliable products are making reliability testing more challenging task. This paper presents a systematic approach for identifying critical elements (subsystems and components) of the system and deciding the types of test to be performed to demonstrate reliability. It decomposes the system into three dimensions, (i.e. physical, functional and time) and identifies critical elements in the design by allocating system level reliability to each candidate. The decomposition of system level reliability is achieved by using criticality index. The numerical value of criticality index for each candidate is derived based on the information available from failure mode and effects analysis (FMEA) document or warranty data from a prior system. It makes use of this information to develop reliability demonstration test plan for the identified (critical) failure mechanisms and physical elements. It also highlights the benefits of using prior information in order to locate critical spots in the design and in subsequent development of test plans. A case example is presented to demonstrate the proposed approach.     

Failure frequencies of non-coherent structures

Algorithms to determine the failure frequency of a non-coherent structure function are few, and they depend on the knowledge of the prime implicants. For coherent structures, there exists a well-known approach based on criticality and the Birnbaum importance measure, which is more flexible concerning the form in which the structure function is given. This concept is extended to cover non-coherent structure functions by considering two types of criticality functions and a corresponding extension of the Birnbaum importance measure. This theory is a generalization of presently existing concepts, as it includes systems described by prime implicants as well as coherent systems. For the components, renewal frequency densities are required in addition to failure frequency densities and unavailabilities. For common component models, these are given to show that everything that can be done for coherent systems can also be achieved for non-coherent ones. This includes also the method of modularization, which is addressed in a final section. The topic of multi-state components is not addressed in this paper.     

Biomimetic reliability strategies for self-healing vascular networks in engineering materials

Self-healing via a vascular network is an active research topic, with several recent publications reporting the application and optimization of these systems. This work represents the first consideration of the probable failure modes of a self-healing system as a driver for network design. The critical failure modes of a proposed self-healing system based on a vascular network were identified via a failure modes, effects and criticality analysis and compared to those of the human circulatory system. A range of engineering and biomimetic design concepts to address these critical failure modes is suggested with minimum system mass the overall design driver for high-performance systems. Plant vasculature has been mimicked to propose a segregated network to address the risk of fluid leakage. This approach could allow a network to be segregated into six separate paths with a system mass penalty of only approximately 25%. Fluid flow interconnections that mimic the anastomoses of animal vasculatures can be used within a segregated network to balance the risk of failure by leakage and blockage. These biomimetic approaches define a design space that considers the existing published literature in the context of system reliability.     

A fuzzy knowledge based method for maintenance planning in a power system

The inspection planning in electric power industry is used to assess the safety and reliability of system components and to increase the ability of failure situation identification before it actually occurs. It reflects the implications of the available information on the operational and maintenance history of the system. The output is a ranked list of components, with the most critical ones at the top, which indicates the selection of the components to be inspected.In this paper, we demonstrate the use of a fuzzy relational database model for manipulating the data required for the criticality component ranking in thermal power systems inspection planning, incorporating criteria concerning aspects of safety and reliability, economy, variable operational conditions and environmental impacts. Often, qualitative thresholds and linguistic terms are used for the component criticality analysis. Fuzzy linguistic terms for criteria definitions along with fuzzy inference mechanisms allow the exploitation of the operators' expertise.The proposed database model ensures the representation and handling of the aforementioned fuzzy information and additionally offers to the user the functionality for specifying the precision degree by which the conditions involved in a query are satisfied.In order to illustrate the behavior of the model, a case study is given using real inspection data.     

The criticality hypothesis: how local cortical networks might optimize information processing

Early theoretical and simulation work independently undertaken by Packard, Langton and Kauffman suggested that adaptability and computational power would be optimized in systems at the 'edge of chaos', at a critical point in a phase transition between total randomness and boring order. This provocative hypothesis has received much attention, but biological experiments supporting it have been relatively few. Here, we review recent experiments on networks of cortical neurons, showing that they appear to be operating near the critical point. Simulation studies capture the main features of these data and suggest that criticality may allow cortical networks to optimize information processing. These simulations lead to predictions that could be tested in the near future, possibly providing further experimental evidence for the criticality hypothesis.     

Fuzzy TOPSIS approach for failure mode,effects and criticality analysis

In this paper, an alternative multi‐attribute decision‐making approach for prioritizing failures in failure mode, effects and criticality analysis (FMECA) is presented. The technique is specifically intended to overcome some of the limitations concerning the use of the conventional US MIL‐STD‐1629A method. The approach is based on a fuzzy version of the ‘technique for order preference by similarity to ideal solution’ (TOPSIS).The use of fuzzy logic theory allows one to avoid the intrinsic difficulty encountered in assessing ‘crisp’ values in terms of the three FMECA parameters, namely chance of failure, chance of non‐detection, and severity. With the proposed approach, the definition of a knowledge base supported by several qualitative rule bases is no longer required. To solve the fundamental question of ranking the final fuzzy criticality value, a particular method of classification is adopted, allowing a fast and efficient sorting of the final outcome. An application to an important Italian domestic appliance manufacturer and a comparison with conventional FMECA are reported to demonstrate the characteristics of the proposed method. Finally, a sensitivity analysis of the fuzzy judgement weights has confirmed that the proposed approach gives a reasonable and robust final priority ranking of the different causes of failure. Copyright © 2003 John Wiley & Sons, Ltd.     

ANP/RPN: a multi criteria evaluation of the Risk Priority Number

This paper presents an advanced version of the failure mode effects and criticality analysis (FMECA), whose capabilities are enhanced; in that the criticality assessment takes into account possible interactions among the principal causes of failure. This is obtained by integrating FMECA and Analytic Network Process, a multi‐criteria decision making technique. Severity, Occurrence and Detectability are split into sub‐criteria and arranged in a hybrid (hierarchy/network) decision‐structure that, at the lowest level, contains the causes of failure. Starting from this decision‐structure, the Risk Priority Number is computed making pairwise comparisons, so that qualitative judgements and reliable quantitative data can be easily included in the analysis, without using vague and unreliable linguistic conversion tables. Pairwise comparison also facilitates the effort of the design/maintenance team, since it is easier to place comparative rather than absolute judgments, to quantify the importance of the causes of failure. In order to clarify and to make evident the rational of the final results, a graphical tool, similar to the House of Quality, is also presented. At the end of the paper, a case study, which confirms the quality of the approach and shows its capability to perform robust and comprehensive criticality analyses, is reported. Copyright © 2011 John Wiley and Sons Ltd.     

A Generalization of Gröbner Basis Algorithms to Nilpotent Group Rings

 It is well-known that for the integral group ring of a polycyclic-by-finite group several decision problems including the membership problem for right ideals are decidable. In this paper we define an effective reduction for group rings over finitely generated nilpotent groups — a subclass of polycyclic-by-finite groups. Using this reduction we present a generalization of Buchberger’s Gr?bner basis method by giving an appropriate definition of “Gr?bner bases” in this setting and by characterizing them using the concepts of saturation and s-polynomials. Our approach allows to compute such Gr?bner bases by completion based algorithms and to use these bases to solve the membership problem for right and two-sided ideals in finitely generated nilpotent group rings using Gr?bner basis algorithms and reduction. Received August 21, 1995; revised version June 3, 1996     

A Failure Mode Effect and Criticality Analysis of Conventional Milling Machine Using Fuzzy Logic: Case Study of RCM

The purpose of this paper is to select the appropriate maintenance strategies for each failure mode of functionally significant item of conventional milling machine. In order to describe the criticality analysis of conventional milling machine, this paper presents a study on reliability‐centered maintenance with fuzzy logic and its comparison with conventional method. On the basis of fuzzy logic, failure mode and effect analysis is introduced integrating with fuzzy linguistic scale method. After that, weighted Euclidean distance formula and centroid defuzzification is used for calculating risk priority number. The results indicate that based on risk priority number, value criticality ranking was decided, and appropriate maintenance strategies were suggested for each failure mode. It also reflects that a more accurate ranking can be performed by the application of fuzzy logic using linguistic rule to failure mode and effect analysis. Copyright © 2016 John Wiley & Sons, Ltd.