This technique was developed primarily as a method of allocating
reliability without repair for mechanical-electrical systems. In this method,
subsystem allocation factors are computed as a function of numerical ratings
of system intricacy, state of the art, performance time, and environmental
conditions. These ratings are estimated by the engineer on the basis of his
experience. Each rating is on a scale from 1 to 10, with values assigned as
System Intricacy. Intricacy is evaluated by
considering the probable number of parts or components making up the
system and also is judged by the assembled intricacy of these parts or
components. The least intricate system is rated at 1, and a highly
intricate system is rated at 10.
State-of-the-Art. The state of present
engineering progress in all fields is considered. The least developed
design or method is a value of 10, and the most highly developed is
assigned a value of 1.
Performance Time. The element that
operates for the entire mission time is rated 10, and the element that
operates the least time during the mission is rated at
Environment. Environmental conditions are
also rated from 10 through 1. Elements expected to experience harsh
and very severe environments during their operation are rated as 10,
and those expected to encounter the least severe environments are
rated as 1.
The ratings are assigned by the design engineer based upon his
engineering know- how and experience. They may also be determined by a group
of engineers using a voting method such as the Delphi technique.
An estimate is made of the types of parts and components likely
to be used in the new system and what effect their expected use has on their
reliability. If particular components had proven to be unreliable in a
particular environment, the environmental rating is raised.
The four ratings for each subsystem are multiplied together to
give an overall rating for the subsystem. Each subsystem rating will be
between 1 and 10. The subsystem ratings are then normalized so that their sum
The basic equations are: