Overview

In order to set the stage for our readers we would like to define some basic terms used in this text.

Machines are man-made concrete systems consisting of a totality of orderly arranged and functionally connected elements. A system is characterized by having a boundary to its environment. The system's connection to its environment is maintained by input and output parameters. Each system can usually be subdivided into two or more subsystems. Generally, these subdivisions may be made with a varying degree of detail depending on our overall purpose. Consider, for example, the "clutch coupling" system shown in Figure 2-1. We would usually find this "system" as an assembly within a machine. However, if we wanted to investigate the system from a functional point of view, we could dissect it into the subsystems "elastic coupling" and "clutch." These subsystems, in turn, could be broken down into system components or individual parts.

Figure 2-1: System "coupling." a h are system elements, i l are connecting elements, S is the total system, S ₁ the subsystem "elastomeric coupling," S ₂ the subsystem "clutch," E inputs, A outputs [1].

For the purpose of reliability assessments we have found the following definitions useful.

System and Mission

A system is any composite of hardware or software items that work together to perform a mission or a set of related missions. A mission is the external "goal" of a system. A function in turn is the internal...