Series Reliability & What Does it Mean?

The theme of this article: How to improve system reliability?

Keywords: #reliability #reliabilityanalysis #seriesreliability #systemic #reliabilityimprovement

A Story:

During World War II, a group in Germany was working under Wernher von Braun To develop the infamous V-1 missile, primarily to attack England.

But the first batch of the V-1 missile was a fiasco. The first ten missiles either exploded on the launching pad or landed 'too soon' in the English Channel.

This happened in spite of careful engineering attempts to provide high-quality parts and by paying careful attention to details.

Naturally, the German engineers weren't very happy about it. So they commissioned Robert Lusser, a mathematician, as a consultant, to explore and understand this problem.

His task was to analyze the missile system, and he quickly came up with the product probability law of series components.

His theorem, valid under special assumptions, concerns systems functioning only if all the components are functioning.

It says that the reliability of such a system is equal to the product of the reliabilities of the individual components which make up the system.

Discussion:

If a system is made up of a large number of components, the system reliability may be rather low, even though the individual components have high reliabilities. For instance, if a system is made up of say 5 components, each having high reliability of say 90% (or 0.90) then the reliability of the system turns out to be: 0.90 x 0.90 x 0.90 x 0.90 x 0.90 = 0.59 or 59%.

To get over this problem, engineers in the United States, attempted to compensate for "low system reliability" by improving the quality of the individual components, i.e. by aiming to improve the reliability of individual components. For example, improving the reliability of individual components from say 90% to say 99%. Better raw materials and better designs for the products were demanded. Naturally, higher system reliability can be obtained by this method. Considering our previous example, if the reliability of individual components improves from 90% to say 99% then the system reliability becomes 95% instead of 59%. This path is still being followed through the extensive application of "Quality Tools" with a special thrust on improving human skills in workmanship and assembly.

The Problem & the Way Forward:

But it would be immediately apparent to any experienced maintenance and reliability engineer that this path is not only tedious to follow in a real-life situation but also not practical because of the uncertainties that are involved in improving reliability in this manner.

So, what is the way out of this? The answer lies in conducting an extensive systemic (not systematic) analysis of the problems and failure modes, which was probably not carried out at that time (1950 to 2000s). This is because we simply didn't have the tools to do so.

The keyword is "systemic.'

But what does systemic mean? There are two aspects of systemic analysis. The first is to study the interactions that cause functional failures and failure modes. The second is to study the deterioration pattern of each component over time when subjected to various parameters that promote degradation and functional failures.

Conclusion:

The result of such a systemic study is to come up with appropriate maintenance strategies and critical actions that would help achieve high system reliability and system availability (as desired by the user) with the minimum effort, cost, and resources.

But conducting a systemic study is easier said than done. It is a discipline in itself.