Reliability Allocation

The first step in the design process is to translate the overall system reliability requirement into reliability requirements for each of the subsystems. This process is known as reliability allocation. The allocation of system reliability involves solving the basic inequality:


is the is the allocation reliability parameter for the ith subsystem

R* is the system reliability requirement parameter

f is the functional relationship between subsystem and system

For a simple series system in which the R’s represent probability of
survival for t hours, the above equation becomes

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Reliability Specification

The first step in the reliability engineering process is to specify the required reliability that the equipment/system must be designed to achieve. The essential elements of a reliability specification are:

  1. a quantitative statement of the reliability requirement.
  2. a full description of the environment in which the equipment/system will be stored, transported, operated and maintained.
  3. the time measure or mission profile.
  4. a clear definition of what constitutes failure.
  5. a description of the test procedure with accept/reject criteria that will be used to demonstrate the specified reliability.

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