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IMRF Journals 82 OPTIMAL RELIABILITY SYSTEMS UNDERGOING TECHNOLOGICAL PROGRESS AND EXPOSED TO CCF's
Abstract
The reliability characteristic such as probability of survival, mean time to failure, frequency of failures and mean down time depend on the design and topological layout of the system. This paper deals with a single unit system that is operating in an environment exposed to the hazards of common cases failures, under some general assumptions the optional replacement policies were developed with the help of proposed measure of cost differences for old and new system.
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The equipment replacement model with equal replacement intervals has been studied by a number of authors. When new equipment is identical to old equipment, it has been proved that an optimum policy involves replacement at equal intervals (the equal life assumption). However, when equipment is subject to technological change, no such proof exists. Since almost all equipment is subject to technological improvement over time, and since the assumption of equal lives vastly simplifies the analysis, such a proof is highly desirable. In the absence of a proof, authors have attempted to justify the equal life assumption as a good approximation to a correct solution. For example, Smith argues that the process of discounting makes future replacements less important and with gradual technological improvement an equal life policy is close to optimum. This paper shows that under the definition of technological change employed by most authors working on equipment replacement problems, it is unnecessary to assume that an equal life policy is optimum, rather the optimality of such a policy can be proven.
In this paper the effects of technical change on the optimal lives of assets are explored. The generality of the dynamic programming approach to the problem of optimal asset life determination is contrasted with the traditional and highly restrictive "equal life" solution. Some of the possible effects of different forms of technical change on costs are examined and numerical examples of these are solved by dynamic programming to illustrate the consequences for the optimal lives of assets. We hypothesize that the likely effect of technical progress is a lengthening of the optimal replacement cycle.
The British Army currently uses a replacement policy for non-armoured vehicles which employs age-based repair limits derived by a sensible but rather arbitrary method. The paper describes how a proposed improved replacement policy was devised: one which uses extensive data to compute optimum age-based repair limits and allows these limits to be adjusted for vehicles having exceptionally high and low mileages. The feasibility of a resale type of policy for some vehicles is examined.
After a general introduction, the model is described in detail. There follows an account of how the assumptions of the model were tested. Then the results are presented and discussed, in particular the savings to be expected from implementing the improved policy. Finally, the proposals are described which have now been approved by Army staff and are to be submitted to the Treasury.
In the repair limit replacement method when an item requires repair it is first inspected and the repair cost is estimated. Repair is only then undertaken if the estimated cost is less than the "repair limit". Dynamic programming methods are used in this paper as a general approach to the problem of determining optimum repair limits. Two problems are formulated and the cases of finite and infinite planning horizons and discounted and undiscounted costs are discussed. Methods are given for allowing for equipment availability and for the introduction of new types of equipment. An improved general formulation for finite time horizon, stochastic, dynamic programming problems is developed.
A Study in optimum replacement of fork lift trucks is described, using two models. The first is related to minimum average costs per truck per year, the second uses the approach of discounted cash flow. The parameters used in these models include the purchase price, the resale value and the maintenance costs of the equipment. The effect of capital allowances for tax purposes is included.
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