A survey of the research done on preventive maintenance is presented. The scope of the present survey is on the research published after the 1976 paper by Pierskalla and Voelker [98]. This article includes optimization models for repair, replacement, and inspection of systems subject to stochastic deterioration. A classification scheme is used that categorizes recent research into inspection models, minimal repair models, shock models, or miscellaneous replacement models.

A survey of preventive maintenance models for stochastically deteriorating single-unit systems

The accuracy of intermittent demand estimates

https://smartcorp.com/wp-content/uploads/2015/07/IJF_Bootstrap_paper_Smart_Software.pdf

A new approach to forecasting intermittent demand for service parts inventories

The categorization of alternative demand patterns facilitates the selection of a forecasting method and it is an essential element of many inventory control software packages. The common practice in the inventory control software industry is to arbitrarily categorize those demand patterns and then proceed to select an estimation procedure and optimize the forecast parameters. Alternatively, forecasting methods can be directly compared, based on some theoretically quantified error measure, for the purpose of establishing regions of superior performance and then define the demand patterns based on the results. It is this approach that is discussed in this paper and its application is demonstrated by considering EWMA, Croston's method and an alternative to Croston's estimator developed by the first two authors of this paper. Comparison results are based on a theoretical analysis of the mean square error due to its mathematically tractable nature. The categorization rules proposed are expressed in terms of the average inter-demand interval and the squared coefficient of variation of demand sizes. The validity of the results is tested on 3000 real-intermittent demand data series coming from the automotive industry.

On the categorization of demand patterns.

This paper investigates the gap between research and practice in spare parts management, with specific reference to durable goods addressed to private or professional customers The paper provides a critical literature review of theoretical contributions about spare parts classification and demand forecasting for stock control The discussion of ten case studies, then, allows to analyze the reasons for this gap, by addressing the limitations of models developed in literature, the role of contextual factors and the maturity in companies' spare parts management practices Four main directions for research are proposed in order to bridge the gap, namely: to develop integrated approaches to spare parts management; to define contingency-based managerial guidelines, to favor the knowledge accumulation process in companies, and to supplement theoretical models with practical relevance

Spare parts classification and demand forecasting for stock control: Investigating the gap between research and practice

A sporadic or lumpy demand pattern is characterized by large transactions separated by periods of zero demand. Such demand patterns occur frequently for items in parts and supplies inventory systems. A forecasting procedure is presented, to be used in conjunction with a base-stock (order-up-to) inventory-control policy under periodic review. The procedure determines the size and timing of replenishment orders. Although a base-stock policy calls for a replenishment order after each transaction, it is shown that a delay in placing the order can result in significant holding-cost reductions with little additional risk or cost of stockouts.

Forecasting and Inventory Control for Sporadic Demand Under Periodic Review

Today many manufacturing firms are focusing on cycle or throughput time reduction to gain a competitive advantage In many instances, cycle times at individual workstations are adversely affected by unscheduled machine breakdowns In this paper, existing closed-form approximations for the expected cycle time at a parallel machine workstation with general process and interarrival times and for the variability in departures from the workstation are used to demonstrate how the mean and variance of machine repair or downtime affects cycle times at a given workstation and downstream workstations Results from the machine interference literature are used to characterize the dependence of repair time on a base stock inventory policy used for the replenishment of spare components Expressions for the mean and approximations for the variance of repair times are obtained for machines with either a single critical component or a series of critical components when such components fail exponentially Numerical example

Spare parts inventory and cycle time reduction

Recent research on the scheduling of job shops has demonstrated the need for dispatching rules that are effective over a wide range of due-date tightness. In this paper a new dispatching rule is presented and compared with other prominent dispatching rules. The new rule employs shortest-processing-time (SPT) scheduling in a controlled manner: using due-date information to expedite jobs that are late or behind schedule, and employing a heuristic to control the scheduling of jobs with long processing times. Based on a simulation analysis, the rule is shown to perform nearly as well as the SPT rule with respect to mean job flowtime, and without the undesirable SPT side-effect of large conditional mean tardiness. The rule is also shown to be robust to changes in due-date tightness. In comparison with other well-known dispatching rules, its performance is generally superior with respect to mean flowtime, mean tardiness and conditional mean tardiness, with only the COVERT rule performing nearly as well. Unlike ...

An expediting heuristic for the shortest processing time dispatching rule

Traditional single-item inventory control policies operating under continuous review call for the immediate placement of a replenishment order each time a demand causes the inventory position to drop to or below the order point. In an environment where lead times are small relative to the average time between demands, and holding costs are large relative to other inventory costs, it may be more economical to delay ordering to achieve holding cost savings with little additional risk or cost of shortages.

This paper introduces the concept of delayed replenishment orders for expensive items subject to intermittent, unit-sized demands. The inventory system modeled is one with complete backlogging, constant lead times, arbitrary inter-demand distributions, linear carrying costs, and emergency handling of shortages at a premium cost. Results are given for the optimal delay for inter-demand distributions with increasing failure rates. Specific expressions for the optimal delay are given for several commonly used inter-demand distributions.

Replenishment Delays for Expensive Slow-Moving Items

The one-for-one (S - 1,S) inventory policy, which calls for a replenishment order after each demand equal in magnitude to the size of the demand, is often advocated for controlling the stock levels of expensive, slow-moving items. In particular, this policy has frequently been promoted for use in recoverable-item inventory systems. An important managerial question is: When is the (S - 1,S) policy optimal? Results in this article provide guidance for the selection of this policy. Conditions under which it is not economical to batch demands are developed, provided the renewal function of the renewal process of demand sizes is concave. This includes the important case of unit-sized demands. The inventory system considered is one with continuous review, constant lead times, general interarrival and discrete demand distributions, complete backlogging, and linear holding and penalty costs per unit per unit time. Examples are given for the special case when the demand process is Poisson and when the demand process is a particular compound Poisson distribution known as the stuttering Poisson distribution.

Carl R. Schultz

Papers

Forecasting and Inventory Control for Sporadic Demand Under Periodic Review

Spare parts inventory and cycle time reduction

An expediting heuristic for the shortest processing time dispatching rule

Replenishment Delays for Expensive Slow-Moving Items

On the optimality of the (S — 1,S) policy