Introduction:

Reliability allocation involves setting reliability objectives for components or subsystems in  order to meet a system reliability objective. Reliability 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 MTBF (mean  time between failures). Then a uniform allocation of the objective to all subsystems would  require each subsystem to have an MTBF of 4000 hours to meet the system objective. While  uniform allocation is easy to calculate, it is generally not the best way to allocate a reliability  objective. The "best" allocation of reliability would take into account the cost or relative  difficulty of improving the reliability of different subsystems or components.

Reliability allocation usually starts from a base of past experience and is first performed at a fairly high level. By this we mean that we are examining tradeoffs between subsystems rather than a more detailed reliability allocation which might look at tradeoffs between specific vendor components. However, this level of detail is appropriate for the first stages of design because it can guide later design work. It is not efficient to develop a detailed design and then have to redesign and reallocate reliability if the initial allocation is not achievable.

Figure 1. Reliability Allocation Results.

The left pane shows the optimal reliability level for each upgrade, while the right pane shows changes in performance and costs. In this case, the best allocation costs $10,100 to exceed the system MTBF objective of 200 hours.

Benefits:

• Provides a realistic view of  subsystem or module performance required to meet system objectives.
• Saves money by pointing out the most cost effective areas for design improvements.
• Helps avoid over designing subsystems where additional reliability gains may not significantly improve system reliability.
• Provides an excellent design management tool.

 Capabilities:

•        Reliability goal setting and optimal allocation.
•        Evaluation of new designs against reliability goals.