5.3 Reliability Allocation

In a large complex system, it is necessary to translate reliability requirements into detailed specifications for the numerous units that make up the system. The overall system reliability requirement is translated or broken down into subsystem, component or part reliabilities. This is essential in particular when different design teams, subcontractors or sub-manufacturers are involved. The appropriate values of reliability have to be included in the subsystem, component, or part specifications.

The process of apportioning reliability requirement to individual subsystems, components or parts from specified system reliability is called Reliability Allocation or Reliability Apportionment. This allocation technique varies with the complexity of the system and the role of the subsystem, components, or parts in the functioning of the system. The sensitivity of the reliability allocation at the design stage to the uncertainty of its parameters is a crucial consideration. Usually, weight, the initial cost of the system or performance characteristics are used as objective functions. If the allocated parameters for a system cannot be achieved using current technology or is not cost effective then the allocation should be reassigned. A trade-off analysis among the contradicting parameters should be made and the procedure should be repeated until an allocation is achieved that satisfies the reliability requirements.

5.3.1 Mathematical formulation

The allocation of system reliability involves solving the basic inequality:

(5.4)

Where

R* is the required system reliability

is the i ^th subsystem reliability requirement

For a series system, equation (5.4) is simplified as:

(5.5)

Theoretically this...