Optimal maintenance center inventories for fault-tolerant repairable systems

In many military and commercial contexts, complex equipment which is expected to perform very reliably is often designed to be fault-tolerant, that is, able to function although some of the parts have failed. A popular fault-tolerant design is the m-out-of-n system, where there are n identical parts, at least m of which must be functional for machine operation. Complex equipment of this type often undergoes scheduled maintenance overhauls at regular intervals during which all failed components are replaced. Failure to have replacements on hand for failed parts requires emergency measures at premium cost. When repairable parts are highly reliable and expensive, both holding and shortage costs are high. A reasonable objective is to choose initial spares inventory to minimize the sum of holding costs and expected shortage costs.We first develop a model to determine the optimal repairable parts inventory for a maintenance center servicing machines containing a single m-out-of-n system. The model is then extended to handle a related problem, finding optimal maintenance center inventories for machines containing several m-out-of-n systems of different parts, minimizing total expected costs subject to a constraint on total inventory investment.We assume that there is a fleet of machines, which experience identical workloads. There is a cycle time of T days between overhauls for an individual machine. A machine arrives at the maintenance center for overhaul each day. At the overhaul, all failed parts are removed and sent to a repair shop, from which they eventually return to the maintenance center to be used again as spares. The total number of spares undergoing repair and on hand is a constant. There are no backorders; if the number on-hand spares is insufficient to meet demand at an overhaul, a shortage penalty is assessed which depends on the number and type of spares required.While computing holding costs is straightforward, computing expected shortage costs is more complex. Expected shortage costs are dependent upon several factors, including component failure rates, the values of m and n, part repair rates, and the initial number of spares on hand. We assume that the system of interest is well specified, so that the parameters of the model are known except for the number of initial spares of each type, which are the decision variables. We model the on-hand inventory of each type of part as a Markov chain with the number of spares on hand at the end of each day as the states, under the assumptions that failure rates are constant and repair times follow independent exponential distributions. We then calculate the steady-state probabilities of stockout of various numbers of spares, as a function of the initial spares inventory. The expected shortage costs for a given type of spare may then be calculated by finding the product of the penalty cost for lacking p spares and the probability of lacking p spares and summing over all possible p values.Solutions to the problem of finding optimal initial inventory level for a machine containing a single m-out-of-n system may be found easily by enumeration. Solutions to the constrained problem where the machine contains several independent m-out-of-n systems, may be found by dynamic programming. Sensitivity analysis of costs to changes in the inventory investment constraint is clear, and computational effort is reasonable. A simple example is included to illustrate the solution method for both problems.     

An evaluation of overtime policies for a repair shop

This article presents the first attempt to develop and examine overtime policies for a repair shop environment. Overtime is used to augment repair capacity as needed to offset short-term demand fluctuations. If overtime provides sufficient additional repair capacity, it may be possible to reduce investment in spare parts inventory.The use of overtime in a repair shop requires managerial attention to several issues. In this article, the following five issues are examined: The relationship between overtime policies and spares stocking levels The timing of overtime—reactive or proactive The amount of overtime to use The level in the product structure at which overtime is most beneficial The priority scheduling and labor assignment policies usedSix overtime policies are developed that explore the above issues. These are examined using a simulation model of a hypothetical repair shop. Since the focus of the article is on overtime policies, a single labor assignment policy is used in conjunction with two priority scheduling rules.The results indicate that reactive overtime policies work well in this environment and overtime is most effective at the lowest level of the product structure, where repair times are relatively shorter. In addition, lowest level parts provide more usage flexibility to handle anticipated future failures.     

Maintenance management: Keeping up with production's changing trends and technologies

Current trends in manufacturing have led to reduced levels of work-in-process inventory and increased levels of automation and machine complexity. These changes have two important implications for maintenance management. First, machine breakdowns not only cause a loss of output from the down machine, but also result in lost production on downstream machines. This occurs because of the elimination of buffer stock between workstations. Second, the gap between the technological expertise of the operators and the technological sophistication of their machines has widened. This gap decreases the ability of operators to make adjustments and minor repairs to their own machines.These two factors cause an increase in maintenance costs associated with machine downtime and maintenance labor. Historically, the proportion of preventive and corrective maintenance has attempted to balance these costs. As buffer stock is reduced and more machines are coupled, the costs of downtime will increase, increasing the need for preventive maintenance and decreasing its cost compared to the cost of lost production.Increases in the use of computer-based planning systems have resulted in the development of computerized preventive maintenance systems that have proven to be successful in planning for maintenance labor and materials. Integrating planning methods with corrective maintenance tasks will help reduce breakdowns, make more efficient use of maintenance craft labor, and provide information for inventory control of maintenance, repair, and operating (MRO) materials.This article examines methods of reducing machine downtime costs, maintenance labor costs, and MRO inventory costs in today's changing production environment. Techniques for reducing these costs are explored in the areas of maintenance planning, maintenance methods improvement, and MRO inventory control.     

On the interpretation of cointegration in the linear–quadratic inventory model

The traditional formulation of the linear–quadratic inventory model with unit roots predicts cointegration between inventories and sales. That formulation implies that marginal production costs and the marginal benefits of inventories are both tending to ∞, and the cointegrating coefficient reflects the optimal trade-off between these competing factors. This paper suggests a reformulation of the problem in which marginal production costs and marginal inventory benefits are both stationary and in which the cointegrating coefficient is the same as the value that characterizes the target inventory level in the cost function.     

考虑役龄阈值的最佳缓冲区库存维修策略研究

预防性维护PM对于生产系统有重要的作用。由于设备维护时会造成生产停滞,影响企业的经济效益,所以在生产线上下游设备之间建立库存缓冲区,保证生产的持续进行。考虑引入设备役龄阈值,构建费用模型和维护策略模型。以设备役龄阈值和缓冲区库存量为自变量,以最小化周期内总费用为目标,获得最佳的设备役龄阈值和缓冲区库存量。最后运用Matlab仿真软件进行了算例分析,验证了模型的有效性。     

油气田生产装置检修费用控制措施分析

油气田在开发过程中的成本构成变化遵循着一定规律,随着油气田开发时间的延长,各类生产装置检修费用在操作成本中的比重越来越高,控制检修费用成为老油田降本增效的重要措施之一。文章分析了油气田生产装置检修成本上升的四个方面原因,指出了节约检修成本的六个障碍和六个措施,以期对现场生产经营管理起到一定的指导作用。     

基于边际分析法的可修复备件最优库存研究

分析了可用度和短缺数效能指标之间的关系,用短缺数代替可用度求最优库存。以备件保障经费为约束条件、短缺数最大为目标函数,建立了单基地最优库存模型。用边际分析法求解,并将库存模型应用于实例,计算工具是Matlab。进行了数值实验和理论分析,结果证明该模型的优化效果和实用性很好。     

A generalized lot sizing model for the process cycling problem

Most scheduling/lot sizing models for the single-machine problem assume that aggregate demand equals aggregate production; and that backorders are to be avoided. Where working inventories are low, the scheduler may wish to avoid short production runs and willingly incur some backorder penalties so as to increase the length of production runs and reduce setup costs per unit of time. The model proposed here identifies optimal lot sizes with respect to the backorder/setup cost relationships. Use of the model will result in an optimally balanced inventory even when aggregate inventory levels are changing.     

Improving agility and resilience of automotive spares supply chain: The additive manufacturing enabled truck model

The global supply chain disruption by the COVID-19 pandemic is difficult, if not impossible, to estimate as over 94% of the top 1000 fortune companies were badly affected. The need for building resilient supply chains to mitigate the effect of such disruptions is rising rapidly than ever before across the global business spectrum. Building resilience in the automotive spare parts (ASPs) supply chain is critically important as any disruption to automotive spares supply chain will affect the operations of the logistics sector, the backbone of global supply chains. This research work contributes to improving the resilience of the automotive spare parts supply chain by proposing a Viable Supply Chain (VSC) framework design that incorporates Additive Manufacturing (AM) enabled trucks in the automotive spares supply chain network. Based on the proposed model, conceptual case models are developed and tested with proposed AM enabled truck manufacturing closer to end customer. A heuristic approach called shortest time heuristic is also proposed to solve the routing and scheduling of an AM enabled truck to deliver customers’ orders of the spare parts through an online platform. Importantly, the study demonstrate how additive manufacturing can help the ASPs industry to switch from the existing practice of make-to-stock to a more efficient inventory management and cost saving make-to-order model while also achieving resilience and sustainability in by providing a source of spares support for discontinued models of vehicles.     

Mission impossible? The boss wants to double our inventory turns.

Despite the prolific implementation of manufacturing systems, JIT principles, Kaizen events, and cycle time reduction programs over the past few years, high inventories still plague many companies. The assumption that implementing these principles and techniques will automatically result in inventory levels that satisfy management frequently proves to be false. Events like mergers, introduction of new competition, and a dropoff in business often trigger edicts to cut inventories. The cost of inventories also extends beyond the traditional accounting measurements to include hidden operating costs that everyone should want to eliminate. This article looks at the reasons for inventories and explores strategies for reducing them.     

虚拟人机关系在越野群车加油车设计中的应用

在装备设计中,利用计算机建立的虚拟人机关系指导设计值得关注。以越野群车加油车设计为例,应用CATIA人机工程模块对该车数字模型引入虚拟人体模型,建立虚拟人机关系,通过虚拟人机关系指导相关零部件的精确设计和优化设计,并合理安排这些零部件的位置。通过虚拟人机关系的应用,使所设计的越野群车加油车在满足功能的前提下尽量符合人的生理和心理特征,便于操作和维修,达到人机的最佳匹配和统一。     

浅谈在施工现场对工程机械故障的应急维修

在工程机械使用中,机械设备一旦损坏,往往会由于缺少个别零部件而影响机械的使用和施工工期,同时,在工程机械修理过程中,也常常会因为缺少合适的工具、备品备件导致修理工作不能正常进行,因此应该能够在工程机械的使用和维修工作中,灵活处理一些看来比较棘手的问题。文章从零部件故障没有备件的应急办法、几种应急维修技巧、几种能够在工程机械修理中应急代用的日常生活用品三个方面,介绍了工程机械应急维修的方法。     

浅谈我国汽车维修业的未来发展

随着汽车技术的快速发展,日益呈现出汽车维修的高科技特征,与此同时,汽车维修理念也不断更新。汽车维修已不再是简单的零件修复,现代汽车维修是汽车销售、零件销售、资讯及售后服务四位一体紧密结合。汽车维修的新趋势是维修对象的高科技化、维修设备现代化、维修咨询网络化、维修诊断专家化、维修管理电脑化及服务对象的社会化。我国的维修业在发展的道路上面临竞争和挑战,只有在维修业的人力、信息、以及品牌化上加以关注,加强投入。才能适应汽车业发展的新形势。     

基于精益思想的配送中心作业系统研究

在精益思想的基础上,本文对传统配送中心作业系统的库存控制、订单分拣等作业环节进行了分析,指出了传统配送中心在这几个作业环节存在的优缺点.最终分析了应如何选择合适的优化方法对作业环节进行精益化.     

发电机的运行与维护浅析

发电机的正常检修及维护是实现发电机经济运行的重要保障。应根据发电机的运行工作特点,掌握发电机的运行规律,做好发电机组的日常检修和维护工作,以预防为主,尽量减少不必要的检修工作,以认真负责的态度对发电机的每一个组成部件进行日常维护,从而保证发电机的所有设备处于良好的状态。     

Pricing competition with inventory considerations in a hazard rate-prone market of durables

This paper addresses Bertrand-type pricing competition between two firms producing partially differentiated durables over a finite planning horizon. The demand for durables, characterized by increasing returns of scale to a price reduction, is led by the hazard rate. While the effect of inventories on pricing of non-durables is widely recognized, the management and marketing literature typically overlooks this effect in regard to horizontally competing firms for durables. In this paper we show that the pricing trajectory of durables may significantly alter when inventory dynamics are accounted for. In particular, the price may hike upwards before dropping; gradually grow; or even stay at the same level over the entire product life while it would only decline if inventories and related costs are disregarded. Furthermore, the well-known, optimal pricing strategy of following the pattern of sales does not necessarily confirm even for symmetric equilibria when the competing firms have either an inventory surplus or shortage.     

如何检测发动机电子控制系统的故障

发动机电子控制系统一般由传感器、控制单元、执行器三部分组成,出现故障后,自诊断系统会记录故障的大概范围,但故障部位还得靠维修人员去检测,才能排除故障,那么在查找故障部位时,维修人员怎样才能检测发动机电子控制系统的故障部位。     

联合库存管理在石油企业中的应用研究

从供应链协同管理的角度,构建了石油企业备件联合库存管理模型,该模型分为供应商与物资总库的联合和物资总库与分公司的联合两部分,使原来的分散库存变为集中库存。并采用建立网络信息系统、供需协调管理机制和直达供应的策略达到联合库存管理的目的。     

Estimating and auditing aggregate inventory levels at multiple stocking points

Relatively little attention has been given to methods for estimating and auditing distribution network inventories in the aggregate. This article shows that a simple polynomial equation whose coefficients can be determined by means of regression analysis techniques is a good, basic tool for estimating overall inventory levels and can be used to provide insight into the effectiveness of inventory control policies at different stocking points. It also serves as the basis for auditing and controlling current inventory policies on an ongoing basis. Tests of the model in a number of actual cases revealed a predictable association between the inventory policy being used and the coefficient values in the polynomial equation.     

维修备件库存的两级优化研究

针对汽车维修备件的库存储备存在配置不合理的问题,提出了一种优化方法。该方法在遗传算法的基础上,采用仿真方法建立汽车的使用可用度模型,实现备件库存配置的两级优化,并实现了在满足备件库存成本约束的前提下,使汽车的使用可用度达到最大,即修复率的最大化。给出了应用该方法的具体步骤,并通过数值分析与讨论,验证了该算法的有效性。