TABLE OF CONTENTS Reliability apportionment encompasses the problem of assigning the correct numerical reliability to each sublevel of an item in such a manner that the overall item reliability is equal to its reliability goal. It can also be thought of as the assignment of allocation to each sublevel of the allowable levels of unreliability. Reliability goal could also be set for every part, such as screw, wiring, connector, etc.; however, this is usually not practical. In designing for reliability, goals must be assigned to every major component for which the designer is responsible. Once these sublevel reliabilities are established, the designer can select the materials, configurations, and types so that the overall reliability requirement can be achieved.
Reliability goals are usually established by either the customer or proposed by the manufacturer for customer approval. The assignment of reliability goals by the customer is usually based on studies of strategic necessities, logistic requirements, mission accomplishment, operational readiness, maintainability, safety, budgetary limitations, and other factors of intrinsic importance. Apportionment is required when the manufacturer is obliged to decide how reliable various subsystems must be in order to design an end product that will meet the specified reliability requirement. It should be noted that, as reliability approaches perfection, each advance becomes very much more expensive. Morever, the manufacturer should be able to advise the customer as to what a given reliability requirement will cost in terms of time, performance, and money.
Key reasons for reliability allocations are the following:
