Reliability Allocation Overview
Reliability allocation involves setting reliability objectives for subsystems and assemblies to meet a system reliability goal. The reasons for reliability allocation are:
• They force the designer to understand and develop the relationships between assembly, subsystem, and system reliabilities.
• They allow the designer to consider reliability in a framework that incorporates other issues in the design process, such as cost, physical dimensions, and weight.
The allocation should occur in the initial stages of design or prior to designing major system upgrades. The simplest method for allocating reliability is to distribute the objective uniformly among all subsystems. For example, suppose a system with 20 subsystems has a reliability objective of 200 hours for the MTBF (mean time between failures). To meet the system objective, a uniform allocation of the objective to all subsystems would require each to have 4000 hours for the MTBF.
While uniform allocation is easy to calculate, it is generally not the best way to allocate a reliability objective. The best allocation of reliability takes into account the cost or relative difficulty of improving the reliability of different subsystems and assemblies.
Reliability allocation usually starts from a base of past experience and is first performed at a fairly high level. It typically examines tradeoffs between components rather than looking at tradeoffs between specific vendor components. This level of detail is appropriate for the first stages of design because it can guide later design work. Developing a detailed design that then has to be redone is inefficient if the initial allocation is not achievable.