Spare Parts Stock Research Articles

Joint optimization of level of repair analysis and spare parts stocks

In the field of after sales service logistics for capital goods, generally, METRIC type methods are used to decide where to stock spare parts in a multi-echelon repair network such that a target availability of the capital goods is achieved. These methods generate a trade-off curve of spares investment costs versus backorders. Backorders of spare parts lead to unavailability of the capital goods. Inputs in the spare parts stocking problem are decisions on (1) which components to repair upon failure and which to discard, and (2) at which locations in the repair network to perform the repairs and discards. The level of repair analysis (LORA) can be used to make such decisions in conjunction with the decisions (3) at which locations to deploy resources, such as test equipment that are required to repair, discard, or move components. Since these decisions significantly impact the spare parts investment costs, we propose to solve the LORA and spare parts stocking problems jointly. We design an algorithm that finds efficient solutions. In order for the algorithm to be exact and because of its computational complexity, we restrict ourselves to two-echelon, single-indenture problems. In a computational experiment, we show that solving the joint problem is worthwhile, since we achieve a cost reduction of over 43% at maximum (5.1% on average) compared with using a sequential approach of first solving a LORA and then the spare parts stocking problem.

Stochastic model of forecasting spare parts demand

Stochastic model of forecasting spare parts demand

Book Review: A Practical Guide to Implementing Clinical Mass Spectrometry Systems

Book Review: A Practical Guide to Implementing Clinical Mass Spectrometry Systems

Spare parts stock control for redundant systems using reliability centered maintenance data

In the classical approach to determine how many spare parts to stock, the spare parts shortage costs or the minimum fill rate is a key factor. A difficulty with this approach lies in the estimation of these shortage costs or the determination of appropriate minimum fill rates. In an attempt to overcome this problem, we propose to use the data gathered in reliability centered maintenance (RCM) studies to determine shortage costs. We discuss the benefits of this approach. At the same time, the approach gives rise to complications, as the RCM study determines downtime costs of the underlying equipment, which have a complex relation with the shortage cost for spare parts in case multiple pieces of equipment have different downtime costs. A further complication is redundancy in the equipment. We develop a framework that enables the modeling of these more complicated systems. Based on the framework, we propose an approximative, analytic method that can be used to determine minimum stock quantities in case of redundancy and multiple systems. In a quantitative study we show that the method performs well. Moreover, we show that including redundancy information in the stocking decision gives significant cost benefits.

Optimization models for critical spare parts inventories—a reliability approach

In industries characterized by heavy utilization of equipment and machinery, such as mining, oil & gas, utilities, transportation, adequate stockholding of critical spare parts becomes essential. Insufficient stocks affect overall performance of physical assets, as lack of spares may result in gross penalties, lower availability or increased operational risks. On the other hand, oversized inventories lead to inefficient use of capital and may imply severe expenditures. This paper presents various approaches for the determination of the optimal stock size, when the stock is composed of (i) non-repairable or (ii) repairable parts. The paper is focused on spares for relatively expensive, highly reliable components, rather than on fast-moving spare parts. Optimization criteria considered are minimization of costs, maximization of equipment availability, and the achievement of a desired stock reliability (probability that a spare part request will not be rejected because of the lack of spares in stock). For stock reliability, instantaneous and interval reliability calculations are considered. In addition, models directed to the estimation of the remaining life of a given stock of spare parts (at a certain stock reliability level) are introduced. The paper describes several models subject to practical industrial application, and presents case studies from utilities and mining to illustrate their use.

Modelling the spare parts stock levels and its applications in industrial systems

This paper addresses a practical decision making by developing an innovative inventory and transportation cost model to determine the optimum number of the spare parts and its location in the warehouses subject to run-in, random and usage failure inside a maintenance network. The spare parts slow moving items inventory management is a complex problem. The demand is sporadic and difficult to forecast, the utilisation is specific, the part economic value is high, the failure causes system downtime costs and normally this complexity is compensated by overestimated the part inventories. The proposed procedure can be used to determine the spare parts stock level and allows to minimise a function cost sum of aggregate inventory costs and downtime production plant costs caused by the parts failure and transportation from the storage areas to the user or machines failed. Four real case studies from different industrial application projects are illustrated in order to demonstrate how to apply the proposed model into real world applications. The results and findings from such industrial applications are found useful to engineers and managers in industry.

Practical application of a RAMS analysis for the improvement of the warranty management

Abstract This paper will be focused in one of the stages included in a reference framework proposed for the improvement of the warranty management. Particularly, the referred stage will be the RAMS analysis, and how it can help the decision-making related to some aspect of the warranty management as the acquisition of a stategical spare parts stock or how to foresee possible complaints from the customer. For that purpose, the study starts with a brief introduction where the current notion of warranty will be defined, as well as concepts like Reliability, Availabilty, Maintenability and Safety. Later on, once defined very succinctly this framework, the general characteristics of a RAMS analysis will be presented in order to be finally applied in a specific study scenario. In general, this paper aims to explore different aspects related to RAMS analysis, which can influence in the management of warranty assistances. During the process of warranty management for a specific product, many decisions and actions appear affecting aspects as the design quality, used technology, technical complexity, frequency of failures, accessibility, etc. which are important influences on, for instance, the possible expectations to extend the product warranty under reasonable costs. Therefore, this kind of analysis can provide to aftersales managers a useful decision tool that improves the efficiency on the decision-making process related to the spares parts acquisition, the control of failures among others.

REDUCING COSTS OF SPARE PARTS SUPPLY SYSTEMS VIA STATIC PRIORITIES

We study static repair priorities in a system consisting of one repair shop and one stockpoint, where spare parts of multiple, critical repairables are kept on stock to serve an installed base of technical systems. Demands for ready-for-use parts occur according to Poisson processes, and are accompanied by returns of failed parts. The demands are met from stock if possible, and otherwise they are backordered and fulfilled as soon as a ready-for-use part becomes available. Returned failed parts are immediately sent into repair. The repairables are assigned to static priority classes. The repair shop is modeled as a single-server queue, where the failed parts are served according to these priority classes. We show that under a given assignment of repairables to priority classes, optimal spare parts stock levels follow from Newsvendor equations. Next, we develop fast and effective heuristics for the assignment of repairables to priority classes. Subsequently, we compare the performance of the system under these static priorities to the case with a First-Come First-Served (FCFS) service discipline. We show that in many cases static priorities reduce total inventory holding and backordering costs by more than 40%. Finally, we analyse the effect of the number of priority classes. We show that 2 priority classes suffice to obtain 90% of the maximal savings via static priorities.

Production planning model with simultaneous production of spare parts

The paper presents, proposed by the authors, a production planning model with simultaneous production of identical components for the need of own assembly and as spare parts in machine industry plants. The model was based on literature studies and the authors’ own conclusions drawn during the research works carried out in Grundfos Pompy Ltd. The proposed production planning model has been based on the classical stock management theory and material requirement planning methods. It results in merging production management and stock management in a one coherent material flow management system. The evaluation of the proposed model was made based on its empirical verification in the environment of the machine industry plant mentioned above. The results turned out to be very promising – the value of spare parts stock was reduced by nearly 50%.

OPTIMIZING JOINT MAINTENANCE AND STOCK PROVISIONING POLICY FOR A MULTI-ECHELON SPARE PART LOGISTICS NETWORK

ABSTRACT Spare part stock management attempts to ensure that the failed equipment items can be replaced immediately to maintain a sufficient productivity level. In maintenance, the inventory policy determination for spare parts is an important issue. The age-based preventive replacement policy may seek the least total cost for spare part replacement. Considering the criticality of equipment where it is installed, demand for a certain spare part can be categorized into critical and non-critical. The stock level for critical demand can be set to reserve certain spare parts in stock for critical equipment users. This stock policy is named the inventory rationing policy. The stock policy has a significant impact on production system performance, particularly when a machine breakdown causes a large amount of production lost. This study optimizes the joint age-based preventive replacement and inventory rationing policy in a multi-echelon spare part logistics network. In this study, there are equipment users, spare part distribution center and spare part suppliers in the multi-echelon system. Inventory policy optimization for the spare part logistics network is extremely complicated. This paper adopts the scatter search based simulation-optimization method to obtain the optimal configurations with respect to maximizing the total profit of the spare parts logistics network. An implementation example of a spare part logistics system is used to demonstrate the advantages of employing the joint policy.

Closed loop two-echelon repairable item systems

In this paper we consider closed loop two-echelon repairable item systems with repair facilities both at a number of local service centers (called bases) and at a central location (the depot). The goal of the system is to maintain a number of production facilities (one at each base) in optimal operational condition. Each production facility consists of a number of identical machines which may fail incidentally. Each repair facility may be considered to be a multi-server station, while any transport from the depot to the bases is modeled as an ample server. At all bases as well as at the depot, ready-for-use spare parts (machines) are kept in stock. Once a machine in the production cell of a certain base fails, it is replaced by a ready-for-use machine from that base’s stock, if available. The failed machine is either repaired at the base or repaired at the central repair facility. In the case of local repair, the machine is added to the local spare parts stock as a ready-for-use machine after repair. If a repair at the depot is needed, the base orders a machine from the central spare parts stock to replenish its local stock, while the failed machine is added to the central stock after repair. Orders are satisfied on a first-come-first-served basis while any requirement that cannot be satisfied immediately either at the bases or at the depot is backlogged. In case of a backlog at a certain base, that base’s production cell performs worse. To determine the steady state probabilities of the system, we develop a slightly aggregated system model and propose a special near-product-form solution that provides excellent approximations of relevant performance measures. The depot repair shop is modeled as a server with state-dependent service rates, of which the parameters follow from an application of Norton’s theorem for Closed Queuing Networks. A special adaptation to a general Multi-Class Marginal Distribution Analysis (MDA) algorithm is proposed, on which the approximations are based. All relevant performance measures can be calculated with errors which are generally less than one percent, when compared to simulation results. The approximations are used to find the stock levels which maximize the availibility given a fixed configuration of machines and servers and a certain budget for storing items.

On the availability of a k-out-of-N system given limited spares and repair capacity under a condition based maintenance strategy

This paper considers a k-out-of-N system with identical, repairable components. Maintenance is initiated when the number of failed components exceeds some critical level. After a possible set-up time, all failed components are replaced by spares. A multi-server repair shop repairs the failed components. The system availability depends on the spare part stock level, the maintenance policy and the repair capacity. We present a mathematical model supporting the trade-off between these three parameters. We present both an exact and an approximate approach to analyse our model. In some numerical experiments, we provide insight on the impact of repair capacity, number of spares and preventive maintenance policy on the availability.

Initial spare parts supply of an orbital system

This article presents a method of management of the initial spare parts supply. This generic problem is of particular interest in certain systems in which the difficulty of accessibility or the life cycle duration constitute risks tied to the possible depletion of the spare parts stock. In this paper we consider the spatial context. After having commented on the particularities of the management of the spare parts supply of a space station, we propose a method based on the minimization of the risk of postponement of a maintenance operation. Finally, an application to the Columbus laboratory of the International Space Station is presented.

Spare parts management for technical systems: resupply of spare parts under limited budgets

In this paper, we study the operational availability of a complex technical system consisting of several components. The components are subject to breakdowns, and hence for each component a limited number of spare parts are held in stock. If a system's component fails and it can not be replaced immediately, due to a lack of spares, the system becomes unavailable until a new component is installed. Failed components are disposed of and hence, to keep the spare parts stock at an appropriate level, new components have to be purchased. We assume that only a limited annual budget is available for procurement, while any further procurement requires a considerable lead time. We investigate at an aggregate level what budgets are needed to attain a target availability level for the system. In addition, we develop various operational strategies for spending the annual budget during each year. Numerical results indicate that the so-called Balance Focussed strategy provides the best results in terms of system availability as a function of time.

An easily implementable hierarchical heuristic for a two-echelon spare parts distribution system

This paper addresses a two-echelon spare parts stocking and distribution system consisting of a central Distribution Center (DC) and regional facilities. Because the primary purpose for holding inventory is to provide timely repairs of customer's equipment, we set as our objective to minimize total inventory investment subject to constraints on the delay due to parts outages. We decompose the resulting problem by level and by facility. By simplifying the expressions for the delay constraint? and applying previously developed heuristics for the single-level problem [1], we are able to derive closed-form expressions for the inventory control parameters. We then develop a search algorithm (on DC fill rate) to approximate the parameters (Lagrange multipliers) in the closed-form expressions. Numerical comparisons against an analytic lower bound and, for small problsms, exact solutions show the approximation to be quite accurate. We also found that it outperforms methods currently in use by the firm that motivated this work. Finally, because it yields closed-form expressions for inventory control parameters and the parameters are only updated periodically, the policy is “easily implementable” once suitable Lagrange multipliers have been computed.

SPARTA II: Further development in an expert system for advising on stocks of spare parts

This paper is part of a continuous story of SPARTA - an expert system for advising on spare stocks for maintenance purposes. Using an item approach, SPARTA II suggests the assortment and quantities of both repairable and consumable parts of electronic systems. The domain of expertise is restricted to the parts for which the exponential distribution of the times between failures can be assumed. The output of SPARTA II includes: (1) the list of recommended stocks of spares, (2) the calculated reliability with and without the spares recommended, (3) suggestions concerning further investment in spare parts.

Gulf legacy: from factory floor to desert war

The role in the Persian Gulf War of maintenance and of contractor support of the high-tech systems used is examined. Two of the most difficult maintenance challenges were the F-15E and the Joint Stars surveillance aircraft. The measures that enabled maintenance crews to achieve a full mission-capable rate of 94% for the F-15E are described. The Joint Stars program, in which two modified Boeing 707 aircraft were outfitted with an advanced radar to detect moving targets over vast stretches of ground and provide high-resolution images of some of them, was a developmental project whose operational use was sustained by constant contractor involvement. With a war effort depending on developmental systems too new to have stocks of spare parts, rushing newly minted parts to the theater was of paramount importance. Deployment of men and material was planned, simulated, and monitored on computers. Transportation and logistics within the theater, however, were often horrendous. The Defense Logistics Agency hopes to improve the process with a laser-card-based system. >

A microcomputer expert system for advising on stocks in spare parts inventory systems

This paper describes the development and application of a logistic expert system for advising on the stocks of spare parts in inventory systems. The expert system designed, called SPARTA, is efficient and simple microcomputer application software and uses a standard programming environment and PC Plus as AI tool. SPARTA is a rule-based expert system consisting of a knowledge base and an inference engine. It combines causal knowledge and surface knowledge which allows a condensed design of the knowledge base. SPARTA's domain of expertise is restricted to the items for which the exponential distribution of times between failures can be assumed. SPARTA's output includes: (1) the list of recommended stocks, (2) the effectiveness of investment in spares, and (3) suggestions concerning further investment in spare parts.

Multicriteria ranking of spares allocations

This article deals with the problem of choosing the list of spare parts for maintenance purposes that is considered to be the best according to given performance measures. In the first step a number of stock lists, each of which is the best of good according to a single point of view, has to be generated. After that, PROMETHEE-type MCDM methods are employed to construct outranking relations and find a partial preorder or a complete preorder among generated stock lists. They help in making the rank of the stock lists from the best to the worst and in the final choice of a single recommended spare parts stock list. The procedure is efficient for a multicriteria approach to sparing problems with even thousands of part types.

Service Exchange Systems — The Stock Control of Repairable Items

In addition to stocks of spare parts for its machinery, an organization will often find it advantageous to hold stocks of repairable sub-assemblies. These can be used to shorten the repair and overhaul times of expensive machines. This paper describes extensions of standard stock control theory to the case of repairable items and also discusses some of the difficulties which can arise with service exchange systems in practice.