Reliability is associated with unexpected failures and understanding why failures occur helps improve reliability. Failures can be caused by design flaws, overstressing, wear-out, variation, wrong specifications, misuse, or operating outside intended environments. Measuring reliability is important for safety, competitiveness, profits, costs, reputation, and involves defining failure modes, operating environments, and reliability requirements. The bath tub curve represents reliability over time with high failure rates early on, low steady rates later, and increasing rates again over the lifetime. Common distributions used to model reliability include exponential, Weibull, and lognormal distributions.

1. Quality Control and
Reliability Engineering
Mr.S.Gokul, Assistant Professor
Sri Eshwar College of Engineering
Department of Mechanical Engineering
Basic Concepts

2. Sri Eshwar College of Engineering
Department of Mechanical Engineering Page 2
Reliability: Reliability is associated with unexpected failures of products or services and
understanding why these failures occur is key to improving reliability.
Various source of Failure:
The product is not fit for purpose or more specifically the design is inherently incapable.
The item may be overstressed in some way.
Failures can be caused by wear-out
Failures might be caused by variation.
Wrong specifications may cause failures.
Misuse of the item may cause failure.
Items are designed for a specific operating environment and if they are then used outside
this environment then failure can occur
Reliability importance:
Safety
Competitiveness
Profit margins
Cost of repair and maintenance
Delays further up supply chain
Reputation
Good will
Measuring reliability:
Many customers will produce a statement of the reliability requirements that is included
in the specification of the product. This statement should include the following:

3. Sri Eshwar College of Engineering
Department of Mechanical Engineering Page 3
• The definition of failure related to the product’s function and should cover all failure
modes relevant to the function;
• A full description of the environments in which the product will be stored, transported,
operated and maintained;
• A statement of the reliability requirement
Bath tub curve: The bath-tub curve is a representation of the reliability performance of
components or non-repaired items
Distributions:
Exponential Distribution
Weibull Distribution
Lognormal Distribution