Reliability Improvement

 We know from the previous units that the failure rate reflects the unreliability. It is already explained through bath-tub curve about the shape and failure characteristics for a machine at each stage of its life. The information (Failure data) obtained from test results and fields use experience exhibits one of the following three different types:

• The highest rate of failure occurs during initial use (called infant failures or toothing troubles) may be due to environmental change, previously undetected faulty sub components or damage in transportation etc. and slowly these problems vanish out registering a decreasing failure rate (reliability improvement) is exhibited.
• Random failures only now and then can be seen indicating that the machine is undergoing the useful life that registers the constant failure rate. New product design is especially concerned with the failure rate during the useful performance life (the shallow portion of the bath-tub curve).
• The wear out phase follows the useful performance life. This phase experiences the increasing failure rate and reaches highest failure rate finally becoming most unreliable.

Whatsoever may be the phase through which the machine is undergoing, it does not perform properly (as desired) unless all of its sub components function correctly. In such cases the reliabilities of individual sub components must be greater than the reliability desired for the final product. This situation exists whenever the chances of failure for each sub component are independent of one another.

For example, consider the probability of starting a bike. It may not start due to various reasons, such as slip in kick rod, the failure of spark plug, the failure of magnets system generation current for the spark plug, or the carburettor not supplying petrol to the engine, etc. Thus the probability of starting or not starting of the bike depends on the probability of the functioning or not functioning of its components.