System-level requirements are not usually sufficient to scope
the design effort. For example, a requirement that a truck have an MTBF of
1000 hours doesn’t help the designers of the transmission, engine, and other
components. How reliable must these components be? Consequently, the
requirement process for “complex” products usually involves allocating the
reliability requirements to lower levels. When a product contains “few” parts,
the allocation of product requirements may not be necessary or cost-effective.
Functional complexity, parts counts, and challenge to the state-of-the-art are
some considerations in a typical allocation process. In some cases, the
process is iterative, requiring several attempts to satisfy all requirements.
In other cases, the requirements can't be satisfied (components are needed
with unattainable levels of reliability) and trade-off discussions with the
customer may be required.
The allocation of system reliability involves solving the basic
is the allocation reliability parameter for the ith
||is the system reliability requirement
||is the functional relationship between
subsystem and system reliability|
For a simple series system in which the ’s represent probability of
survival for t hours, Eq. (6.1) becomes:
Theoretically, Eq. (6.2) has
an infinite number of solutions, assuming no restrictions on
the allocation. The problem is to establish a procedure that yields a
unique or limited number of solutions by which consistent and reasonable
reliabilities may be allocated. For example, the allocated reliability for a
simple subsystem of demonstrated high reliability should be greater than for a
complex subsystem whose observed reliability has always been low.
The allocation process is
approximate. The reliability parameters apportioned to the subsystems are used
as guidelines to determine design feasibility. If the allocated reliability
for a specific subsystem cannot be achieved at the current state of
technology, then the system design must be modified and the allocations
reassigned. This procedure is repeated until an allocation is achieved that
satisfies the system level requirement, within all constraints, and results in
subsystems that can be designed within the state of the art.
In the event that it is found that,
even with reallocation, some of the individual subsystem requirements cannot
be met within the current state of the art, the designer must use one or any
number of the following approaches (assuming that they are not mutually
exclusive) in order to achieve the desired reliability:
Find more reliable component parts to use.
||Simplify the design by using
fewer component parts, if this is possible without degrading
||Apply component derating
techniques to reduce the failure rates below the averages.|
||Use redundancy for those
cases where (1), (2) and (3) do not
It should be noted that the allocation process can, in turn, be
performed at each of the lower levels of the system hierarchy, e.g.,
equipment, module, component.
This section will discuss six different approaches to
reliability allocation. These approaches differ in complexity, depending upon
the amount of subsystem definition available and the degree of rigor desired
to be employed. References  through
 contain a more detailed treatment of allocation methods, as well as a
number of more complex