A joint spare part and maintenance inspection optimisation model using the Delay-Time concept

Spare parts and maintenance are closely related logistics activities where maintenance generates the need for spare parts. When preventive maintenance is present, it may need more spare parts at one time because of the planned preventive maintenance activities. This paper considers the joint optimisation of three decision variables, e.g., the ordering quantity, ordering interval and inspection interval. The model is constructed using the well-known Delay-Time concept where the failure process is divided into a two-stage process. The objective function is the long run expected cost per unit time in terms of the three decision variables to be optimised. Here we use a block-based inspection policy where all components are inspected at the same time regardless of the ages of the components. This creates a situation that the time to failure since the immediate previous inspection is random and has to be modelled by a distribution. This time is called the forward time and a limiting but closed form of such distribution is obtained. We develop an algorithm for the optimal solution of the decision process using a combination of analytical and enumeration approaches. The model is demonstrated by a numerical example.     

一家铁路企业的成本优化

 在铁路企业生产能力过剩、铁路业主正在实施私有化以及由此导致用户关注降低成本的背景下，以地区列车的转向架为例，介绍了ADtranz戴姆勒·克莱斯勒铁路系统有限公司转向架生产部门内“设计一成本一过程”及其在上层目标成本过程链中的集成，阐述了为达到既定目标价格所采用的结构化方法和所采取的措施．     

不平顺条件下高速铁路轨道振动的解析研究

为了分析不平顺条件下高速铁路轨道结构振动,推导了移动车辆在轮对处和轨道结构在轮轨接触点处的柔度矩阵,考虑移动轴荷载和轨道不平顺,建立了移动车辆-轨道垂向耦合振动的解析模型.模型中,移动车辆考虑为弹簧和阻尼器连接的多刚体系统;有碴轨道结构模拟为连续弹性3层梁;轮轨间考虑为线性赫兹接触.算例分析了单台TGV高速动车引起的有碴轨道结构振动,得到轨道不平顺引起的动态轮轨力和轨道各部分的最大振动加速度,研究了列车速度、轨道不平顺以及轨下垫板及扣件、道床和路基等轨下基础刚度对轨道振动的影响.计算表明:随着列车速度和轨道不平顺的增加,轨道结构的振动响应不断增大;轨下基础刚度对轨枕和道床的振动影响较大,对钢轨振动的影响较小.     

Analysis of OREDA data for maintenance optimisation

This paper provides estimates for the average rate of occurrence of failures, ROCOF (“failure rate”), for critical failures when also degraded failures are present. The estimation approach is exemplified with a data set from the offshore equipment reliability database “OREDA”. The suggested modelling provides a means of predicting how maintenance tasks will affect the rate of critical failures.     

Cyclic optimisation for localisation in freeform surface inspection

Over the last decade, several papers have been published on the contribution of the Theory of Constraints (TOC) and Throughput Accounting to product mix decisions. In these papers, the quality of the TOC-based approach to generate good or even optimal solutions is assessed with very different results, particularly when compared with other product mix decision tools. It is the objective of this article to expose the often, not explicitly revealed, premises that are mandatory for generating optimal product mix decisions using a TOC-based approach. By analysing several examples, which are modifications of one and the same basic example, premises on the solution space and the objective function are examined and conclusions made, which are shown in a checklist at the end of the paper. If more than one binding multi-product constraint occurs, optimal product mix decisions can only be derived in special cases. Also, the assumption of integer solutions may lead to non-optimal TOC-based solutions. Furthermore, a correct cost allocation is mandatory, even if sometimes not easily conductible. In particular, taking into account only (all) material costs may lead to wrong decisions. Finally, it is shown that discontinuities in the objective functions deriving, for example, from price discounts, may affect the product mix decision and require special modifications of the TOC-based approach.     

Multi-objective imperfect preventive maintenance optimisation with NSGA-II

Maintenance optimisation is a multi-objective problem in nature, and it usually needs to achieve a trade-off among the conflicting objectives. In this study, a multi-objective maintenance optimisation (MOMO) model is proposed for electromechanical products, where both the soft failure and hard failure are considered, and minimal repair is performed accordingly. Imperfect preventive maintenance (IPM) is carried out during the preplanned periods, and modelled with a hybrid failure rate model and quasi-renewal coefficient. The initial IPM period and the total number of IPM periods are set as the decision variables, and a MOMO model is developed to optimise the availability and cost rate concurrently. The fast elitist non-dominated sorting genetic algorithm (NSGA-II) is applied to solve the model. A case study of wind turbine’s gearbox is provided. The results show that there are 30 optimal solutions in the MOMO’s Pareto frontier that can maximise the availability and minimise the cost rate simultaneously. Compared with the single-objective maintenance optimisation, it can provide more choices for maintenance decision, and better satisfy the resource constraints and the customer’s preference. The results of the sensitivity analysis show that the effect of age reduction factor on optimisation results is greater than that of failure rate increase factor.     

A numerical model for the cyclic deterioration of railway tracks

An elasto‐plastic material model is proposed that can be used to simulate the cyclic deterioration of railway tracks. The model describes the envelope of the irreversible, plastic material response generated during a cyclic loading process, thereby distinguishing the mechanisms of frictional sliding and volumetric compaction. The reversible response is represented by a pressure‐dependent, hypo‐elastic material law. After the numerical integration of the model is specified, the model is calibrated on laboratory experiments and employed in a finite‐element case study of the long‐term settlement behaviour of a railway track. The main features of the model are illustrated by comparing the computed response with the response obtained by in situ track measurements. Copyright © 2003 John Wiley & Sons, Ltd.     

桥梁电子化人工巡检养护管理系统

将危险性分析与量化管理理念应用于桥梁电子化人工巡检养护管理系统的总体设计中,为大型桥梁量身定制巡检养护规划,规范桥梁巡检养护流程,编制巡检养护手册,用于指导巡检养护工作的实施;针对桥梁结构的具体特点,建立专门结构的损伤库,编制电子化人工巡检养护软件,实现巡检养护管理工作的电子化。     

Maintenance schedule optimisation for a railway power supply system

High reliability is the crucial requirement in railway operation and a power supply system is one of the key components of electrified railways. The cost-effectiveness of the maintenance works is also the concern of the railway operators while the time window on trackside maintenance is often limited. Maintenance scheduling is thus essential to uphold reliability and to reduce operation cost. It is however difficult to formulate the optimal schedule to meet both reliability and maintenance cost for a railway power supply system as a whole because of its functional complexity and demanding operation conditions. Maintenance scheduling models to achieve reliability and maintenance cost are proposed in this study. Optimisation algorithms are then developed to attain the solutions of the model. The applicability of the models and efficiency of the solution algorithms are demonstrated in an example. The proposed methods are vitally important for the railway engineers and operators to assure the service quality in the increasing demands of the modern electrified railways.     

Effect of three countermeasures against the illegal crossing of railway tracks

This study was designed to investigate the effects of three countermeasures – landscaping, building a fence and prohibitive signs – on the frequency of trespassing, which in this case means crossing the track at places where it is forbidden. At each location the official route was no more than 300 m away. The main results showed that the effect of each countermeasure on the frequency of trespassing was statistically significant. Specifically, the fencing reduced trespassing by 94.6%, followed by landscaping (91.3%) and prohibitive signs (30.7%). The majority of illegal crossings were committed alone and the persons trespassing were mostly adults and men. In addition, the results demonstrated some tendencies of how the effects of the selected countermeasures can vary with the characteristics of the trespassers. The main implication of this study is that the building of physical barriers such as landscaping or fencing is recommended for reducing trespassing. However, if the required resources are not available or the site is not suitable for such measures, the use of prohibitive signs is recommended. Further, there is a need to tailor the countermeasures to the characteristics of the trespassers in order to ensure that the most appropriate countermeasures are applied.     

Development and optimisation of a maintenance policy under environmental constraints

Nowadays companies have to consider the impact of their production systems on the environment. This paper presents a condition-based maintenance policy for a production system under environmental constraints. The studied system is subject to random failures that can cause production losses and have a negative influence on the environment. Such failures cause the release of a refrigerant gas that induces a heavy penalty cost from a given threshold level. A mathematical model is developed in order to reduce environmental degradation where the average cycle cost per time unit is considered as the objective function. A numerical example and a sensitivity study are given to illustrate the proposed model.     

Optimal maintenance decisions under imperfect inspection

The process industry is increasingly making use of Risk Based Inspection (RBI) techniques to develop cost and/or safety optimal inspection plans. This paper proposes an adaptive Bayesian decision model to determine these optimal inspection plans under uncertain deterioration. It uses the gamma stochastic process to model the corrosion damage mechanism and Bayes’ theorem to update prior knowledge over the corrosion rate with imperfect wall thickness measurements. This is very important in the process industry as current non-destructive inspection techniques are not capable of measuring the exact material thickness, nor can these inspections cover the total surface area of the component. The decision model finds a periodic inspection and replacement policy, which minimizes the expected average costs per year. The failure condition is assumed to be random and depends on uncertain operation conditions and material properties. The combined deterioration and decision model is illustrated by an example using actual plant data of a pressurized steel vessel.     

Condition-based inspection scheme for condition-based maintenance

In condition-based maintenance (CBM) with periodic inspection, the item is preventively replaced if failure risk, which is calculated based on the information obtained from inspection, exceeds a pre-determined threshold. The determination of optimal replacement threshold is often based on minimisation of long-run average maintenance costs per unit time due to preventive and failure replacements. It is assumed that inspections are performed at equal time intervals and that the corresponding cost is negligible. However, in many practical situations where CBM is implemented, e.g. manufacturing processes, inspections require labours, specific test devices, and sometimes suspension of operations. Thus, when inspection cost is considerable, it is reasonable to inspect less frequently during the time the item is in healthier states, and, more frequently as time passes and/or the item degrades, namely, a condition-based inspection scheme. This paper proposes a novel two-phase approach for determination of replacement threshold and a condition-based inspection scheme for CBM. First, it takes into account failure and preventive replacement costs to determine the optimal replacement threshold assuming that inspections are performed at equal time intervals with no cost. This assumption is, then, relaxed and its consequences on total average cost are evaluated using a proposed iterative procedure to obtain a cost-effective condition-based inspection scheme. The proposed approach can be utilised in many CBM applications. For the sake of simplicity of presentation, the approach is illustrated through a simplified case study already reported by some researchers referenced in the paper.     

Optimal inspection policies with predictive and preventive maintenance

A model is proposed to study the inspection and maintenance policy of systems whose failures can be detected only by periodic tests or inspections. Using predictive techniques, the time of the system failure can be predicted for some failure modes. If the system is found failed in an inspection, a corrective maintenance action is carried out. If the system is in a good condition but the predictive test diagnoses a failure in the period until the next inspection, then the system is replaced. The cost rate function is obtained for general distribution function of the signal time of a future failure and for one specific distribution function recently proposed. An algorithm is presented to find the optimal time between inspections and predictive tests and the optimal system replacement times for an age replacement policy. Numerical experiments illustrate the model.     

Failures in inspection procedures: case studies

The most frequently used inspection methods, i.e., visual examination, dye penetrant examination, magnetic particle examination, eddy current examination, and ultrasonic examination, are, for the most part, effective and reliable. There have been instances, however, particularly in the aviation field, where the designated methods of inspection were not able to detect cracks, and as a result catastrophic failures occurred. In the aviation field, when major-accidents occur there is usually a thorough investigation to determine the cause. We are fortunate the accident reports concerning such events are made public, so that engineers can learn from the mistakes which led to an accident and take corrective action to prevent a recurrence of such accidents. The present paper discusses a number of cases wherein faulty inspection procedures resulted in accidents. These cases are based upon published reports as well as personal experiences, and will deal with the crashes which involved: a 707 freighter (visual examination), a DC-10 (dye penetrant), a small passenger plane (magnetic particle), a 737 (eddy currents), and a 747 freighter (ultrasonics).     

机床电气设备故障检测与维修

本文对机床电气故障的诊断及维修进行了理论与实践相结合的具体分析,以工厂实际工作出发,对常用电气设备的故障原因、查找方法、检修步骤进行了具体的叙述。并以Y3150型滚齿机为例,根据故障现象分析故障产生的原因,给出排除故障的方法。     

Joint optimisation of production rate and preventive maintenance in machining systems

This paper studies an integrated control strategy of production and maintenance for a machining system which produces a single type of product to meet the constant demand. Different from previous research, we assume in this study that during the production, the production rate not only influences the life of cutting tool, but also the reliability of the machine. Both the replacement of cutting tool and the preventive maintenance (PM) of machine are considered in this paper. The machine is preventively maintained at the Nth tool replacement or correctively repaired at the machine failure, whichever occurs first. PM and corrective repair may cause shortage which can be reduced by controlling inventory. There are two decision variables p and N, where p denotes the production rate and N denotes the number of cutting tool replacement before the PM is performed. An integrated model is developed to simultaneously determine the optimal production rate and PM policy that minimise the total expected cost per unit item produced. Finally, an illustrative example and sensitivity analysis are given to demonstrate the proposed model.     

Optimal replacement policy and inspection interval for condition-based maintenance

In condition-based maintenance (CBM), replacement policy is often defined as a rule for replacement or leaving an item (or a system) in operation until the next inspection, depending on monitoring results. The criterion for determining the optimal threshold for replacement, also known as optimal control limit, is to minimise the average maintenance costs per unit time due to preventive and failure replacements over a long time horizon. On the one hand, higher frequency of inspections provides more information about the condition of the system and, thus, maintenance actions are performed more effectively, namely, unnecessary preventive replacements are avoided and the number of replacements due to failure is reduced. Consequently, the cost associated to failure and preventive replacements are decreased. On the other hand, in many real cases, inspections require labour, specific test devices, and sometimes suspension of the operations and, thus, as the number of inspections increase, the inspection cost also increases. In this paper, preventive and failure replacement costs as well as inspection cost are taken into account to determine the optimal control limit and the optimal inspection interval simultaneously. The proposed approach is illustrated through a numerical example.     

混凝土试件的制作、养护、质量检查

介绍了混凝土试件的制作养护和质量检查步骤,为保证施工单位制作合格的混凝土试件,确保混凝土的工程质量符合设计要求,提供借鉴。