SECTION 1.9
Designing for Product Reliability
In June, 1957, the historic "AGREE" report, published by the Secretary of Defense revealed some remarkable facts. During the previous decade, costly failure problems were experienced on military electronic equipment and space products. Initially, it was suspected that most of the failures were due to manufacturing or inspection errors. The report revealed that design was the major problem. Fitness-for-use problems in the field showed this breakdown: 40% due to design, 30% due to manufacturing, and 30% due to field conditions (faulty maintenance and improper operation of the product).
As the problem was analyzed, the term reliability emerged. In the "AGREE" report, reliability was defined as "the probability of a product performing without failure a specified function under given conditions for a specified period of time". More simply, reliability is the chance that the product will work. If this definition is dissected, four implications become apparent:
- The quantification of reliability in terms of a probability.
- A statement defining the successful performance of our product.
- A statement defining the environment in which our product must operate.
- A statement of the required operating time between failures.
SECTION 1.9.1
The product reliability program
In order to achieve high reliability, it is necessary to develop a reliability program, or define the specific tasks required. The reliability program should emphasize more than just the design phase; manufacturing and field-usage phases should be included so that the program spans the full product life cycle.
SECTION 1.9.2
Quantifying reliability
The significant aspect of reliability is its quantification. The act of quantification makes reliability a design parameter just like determining pull force or tensile strength requirements. Quantification also helps to refine certain traditional design tasks such as selecting land widths or the amount of back taper to use.
As experience is gained in quantifying reliability, we will learn that it is best to create an index that uniquely meets the needs of those of us who will use the index. Users of the index not only include internal technical personnel, but also marketing personnel and users of the product. Examples of reliability indices and goals are shown in the Figure 1-4.
Figure of Merit |
Meaning |
Mean Time Between Repairs (MTBR) |
Mean time between repairs of a tool that is still in a repairable condition |
Repair Rate |
Number of total repairs in the tool's life |
Mean Time to Failure (MTTF) |
Mean time to failure of a tool not intended for repair |
Mean Life |
Mean value of life (life may mean wear-out or major reconditioning) |
Mean Time to First Repair (MTFR) |
Mean time to the first repair of a repairable tool |
Longevity |
Wear-out time for a tool |
Availability |
Operating time expressed as a percentage of operating and repair time |
System Effectiveness |
Extent to which the tool achieves the requirements of the user |
Probability of Success |
Same as reliability (but used for "one-shot" or non-time-oriented tools) |
B50 Life |
Life during which 50% of the tool has been used, or median life |
Repairs / 100 |
Number of repairs required per 100 operating hours |
Figure 1-4.
Examples of reliability indices
Note that all of these examples quantify reliability. Setting realistic reliability goals requires a meeting of the minds on (1) reliability as a number, (2) the environmental conditions to which the numbers apply, and (3) a definition of successful product performance. This is a difficult accomplishment! However, the act of requiring designers and users to define with precision both environmental conditions and successful product performance forces the designer to understand the design in much greater depth and the user to understand the significance of the circumstances in which the product is used. |
