Richard Dwight

Imperfect inspection and replacement of a system with a defective state: A cost and reliability analysis

We consider a system with three possible states, good, defective and failed. Failures are detected as soon as they occur; the defective state, which is only revealed by inspection, does not prevent the system from fulfilling the function for which it was designed. We present a maintenance model consisting of periodic inspections to check the state of the system, in which inspections are subject to error. At a false positive inspection the system is unnecessarily replaced; at a false negative inspection a defect remains unrevealed with reliability implications for future operation. The model is illustrated with an example from the railways. In this context, we suppose that system lifetime is heterogeneous so that the time the system spends in the defective state is a random variable from a mixed distribution. We determine under what circumstances the cost of maintenance cannot be justified by its efficacy, and suggest that when there is the possibility that replacement is poorly executed (lifetime heterogeneity) the natural response to imperfect inspection of increasing the inspection frequency can be counter-productive.

Searching for real maintenance performance measures

Performance measurement remains a complex issue. This is particularly so if some absolute measure of performance is sought. A definition of performance in terms of value is restated and further developed. Reacting to this strict definition, the systems audit approach to measuring performance is developed and its use illustrated. Concludes that the absolute definition of maintenance performance, in terms of changes in value, presents difficult practical problems. Notes that a systems audit approach to performance measurement can potentially overcome some of these problems while preserving the focus on both business outcomes.

An Age-Based Inspection and Replacement Policy for Heterogeneous Components

This paper considers a hybrid maintenance policy for a single component from a heterogeneous population. The component is placed in a socket, and the component and socket together comprise the system. The s-population of components consists of two sub-populations with different failure characteristics. By supposing that a component may be in a defective but operating state, so that there exists a delay time between defect arrival and component failure, we consider a novel maintenance policy that is a hybrid of inspection and replacement policies. There are similarities in this approach with the concept of ldquoburn-inrdquo maintenance. The policies are investigated in the context of traction motor bearing failures. Under certain circumstances, particularly when the mixture parameter is large, and the distribution of lifetimes for the two component types are well separated, the hybrid policy has significant cost savings over the standard age-based replacement policy, and over the pure inspection policy. In addition to the cost metric, the mean time between operational failures of the system under the hybrid policy can be used to guide decision-making. This maintenance policy metric is calculated using simulation, and using an approximation which assumes that operational failures occur according to a Poisson process with a rate that can be calculated in a straightforward way. The simulation results show good agreement with the approximation.

On reliability criteria and the implied cost of failure for a maintained component

This paper considers age-based replacement and block replacement when reliability is also a decision criterion. We describe how specification of the operational reliability of component function determines the replacement policy, and that setting a value for the cost of failure and specifying an operational reliability requirement are equivalent. This duality then implies a simple method for checking the consistency of the cost of failure and operational reliability measures when they are set system operators and maintainers. A simple expression for the median time between operational failures for a socket subject to age-based replacement is also obtained. These ideas are considered briefly in the context of block replacement. Data from an actual case relating to the maintenance and replacement of train traction motors is used for illustration.

Choosing an optimal model for failure data analysis by graphical approach

Abstract Many models involving combination of multiple Weibull distributions, modification of Weibull distribution or extension of its modified ones, etc. have been developed to model a given set of failure data. The application of these models to modeling a given data set can be based on plotting the data on Weibull probability paper (WPP). Of them, two or more models are appropriate to model one typical shape of the fitting plot, whereas a specific model may be fit for analyzing different shapes of the plots. Hence, a problem arises, that is how to choose an optimal model for a given data set and how to model the data. The motivation of this paper is to address this issue. This paper summarizes the characteristics of Weibull-related models with more than three parameters including sectional models involving two or three Weibull distributions, competing risk model and mixed Weibull model. The models as discussed in this present paper are appropriate to model the data of which the shapes of plots on WPP can be concave, convex, S-shaped or inversely S-shaped. Then, the method for model selection is proposed, which is based on the shapes of the fitting plots. The main procedure for parameter estimation of the models is described accordingly. In addition, the range of data plots on WPP is clearly highlighted from the practical point of view. To note this is important as mathematical analysis of a model with neglecting the applicable range of the model plot will incur discrepancy or big errors in model selection and parameter estimates.

A framework for the engineering asset management system

– The role of engineering asset management (AM) system as a controlling element within organizations is not well defined or understood. The purpose of this paper is to include the role of AM in the organizational strategy making, an issue that has not received sufficient attention. The focus of the paper is on how such role is maintained by the AM system activities, relationships and mechanisms over the asset-related activities of an organization. , – As an approach, a reference framework is required that allows research of this area. By combining a number of possible views of an organizational management system a comprehensive view can be established. A review of literature was used to establish a framework identifying the AM system as an integrated part of the organizations management system. , – A framework is established that focuses on planning and controlling asset-related activities by involving a set of activities, relationships between these activities and feedback mechanisms. A system functional model is proposed integrating the established framework as part of the control of the enterprise system. , – The framework and system functional model are established on a theoretical basis and practical experience requiring applicability to be proven by further research. , – Asset managers in capital intensive organizations can utilize the framework and the system functional model in order to study their AM system, its relationships and to consider how it may be improved. , – Exploring a holistic and relatively new concept.

Asset replacement for an urban railway using a modified two-cycle replacement model

This paper considers the application of capital replacement models at Mass Transit Railway Corporation Limited (MTRCL), Hong Kong. A particular characteristic of the replacement problems considered is that costs relating to existing equipment are generally constant or increasing only slowly. Consequently, replacement is often driven by technical obsolescence, but other criteria are used for informing decisions. The applicability of traditional OR models of replacement is then problematic. We recommend the use of a modified two-cycle replacement model and compare this model to existing capital replacement models. Issues relating to the estimation of delay costs and failure consequences and their influence on the replacement decision are also considered—this is done using a fixed horizon model, which is a special case of the modified two-cycle model. Track points and escalators are used as particular examples. In addition to modelling recommendations, we discuss the management of asset replacement with emphasis on the procedures necessary to ensure that asset replacement requirements are considered appropriately and effectively. The paper treats, in particular, the procedural issues of asset replacement, and the discussion of asset replacement system methodology reflects the current practise at MTRCL, Hong Kong, and developments within that organization through collaboration with academia. The modified two-cycle replacement model is recommended by us for general replacement applications. The asset replacement procedure is presented as an exemplar for business and industry.

Calibrating Markov Chain-Based Deterioration Models for Predicting Future Conditions of Railway Bridge Elements

Existing nonlinear optimization-based algorithms for estimating Markov transition probability matrix (TPM) in bridge deterioration modeling sometimes fail to find optimum TPM values, and hence lead to invalid future condition prediction. In this study, a Metropolis-Hasting algorithm (MHA)-based Markov chain Monte Carlo (MCMC) simulation technique is proposed to overcome this limitation and calibrate the state-based Markov deterioration models (SBMDM) of railway bridge components. Factors contributing to rail bridge deterioration were identified; inspection data for 1,000 Australian railway bridges over 15 years were reviewed and filtered. The TPMs corresponding to a typical bridge element were estimated using the proposed MCMC simulation method and two other existing methods, namely, regression-based nonlinear optimization (RNO) and Bayesian maximum likelihood (BML). Network-level condition state prediction results obtained from these three approaches were validated using statistical hypothesis tests with a test data set, and performance was compared. Results show that the MCMC-based deterioration model performs better than the other two methods in terms of network-level condition prediction accuracy and capture of model uncertainties.

Field Verification of Curving Noise Mechanisms

This paper deals with field observations of curving noise from revenue service trains on a mainline corridor. A specific trackside measurement methodology has been designed to verify the influences of the main parameters: wheel angle of attack, vertical and lateral rail forces, wheel lateral position, train speed and weather conditions. This paper focuses on the role of the wheel angle of attack on curving noise generation. A detailed method of curving noise identification, including the type of curving noise and its generating wheel, is presented. It is observed that all severe squeal noise events (>100dBA) were generated from high wheel angle of attack (>10mrad), while flanging noise has no obvious relationship with wheel angle of attack. A surprising finding from this measurement exercise is that squeal noise was generated from the outer wheel/high-rail interface, which is contradictory to the conventional knowledge.

The mechanisms of curve squeal

Existing curve squeal theory is often contradicted by field observations such as the generation of squeal from the outer wheel (including wheel flange contact), squeal occurring at various wheel natural frequencies, coupled rail vibrations when squealing wheels pass, and the obvious influence of trackform on squeal occurrence and severity. This paper discusses the deficiencies of existing theory and explores an alternative mechanism based on the concept of mode coupling instability which shows a better match with field observations from some sites.