Chapter 7: Reliability

OVERVIEW

Reliability n ” may be relied on; trustworthiness , authenticity, consistency; infallibility, suggesting the complete absence of error, breakdown, or poor performance. In other words, when we speak of a reliable product, we usually expect such adjectives as dependable and trustworthy to apply. But to measure product reliability, we must have a more exact definition. The definition of reliability then, is: the probability that a product will perform its intended function in a satisfactory manner for a specified period of time when operating under specified conditions.

Thus, the reliability of a system expresses the length of failure-free time that can be expected from the equipment. Higher levels of reliability mean less failure of the system and consequently less downtime. To measure reliability it is necessary to:

• Relate probability to a precise definition of success or satisfactory performance

• Specify the time base or operating cycles over which such performance is to be sustained

• Specify the environmental or use conditions that will prevail

Note  

Theoretically, every product has a designed-in reliability function. This reliability function (or curve) expresses the system reliability at any point in time. As time increases the curve must drop, eventually reaching zero.