Design for Reliability

Reliability

The ability of a product to perform as expected over time

Reliability as a probability

Value between 0 and 1, a numerical measure with a precise meaning

Different ways to see reliability

1. The probability that a system will perform on a given trial, the frequency of successful performances of a system in a given number of attempts

2. The probability that a product will last for a given length of time in use

Components of reliability

• time

• performance

• Standard/ prescribed operating conditions

Time as a component of reliability

Perform its function for a specified period of time

Performance as a component of reliability

related to the objective for which the product was made
(failure leads to expectations of performance of the intented function not being met)

Functional Failure

At the start of the product’s life due to manufacturing or material defects

Reliability Failure

After some period of use

Standard/ Prescribed Operating conditions as a component of reliability

The type and amount of usage and the environment in which the product is used

Inherent reliability when measuring reliability

The predicted reliability is determined by the design of the product

Achieved reliability when measuring quality

The actual reliability observed during use (normally lower than inherent)

Product life characteristics curve (bathtub curve)

Early stage failure → Useful Life → End of life failures

Early stage failure

High failure rate at the beginning of products life due to weak components, resulting in poor manufacturing or quality control procedures

• To minimize this manufacturers incorporate a “burn-in” phase at the end of the manufacturing process, where products are run for a specified amount of time, products that succeed the burn in are released for shipment

Useful Life

Low and constant failure rate, normal pattern of random failures caused by uncontrollable factors (to reduce failure rate would generally require a redesign of the product)

End of life failure

Failure rate increases as the age of the product takes over, due to the product exceeding its intended normal life expectancy

System reliability

Many systems are composed of individual components; the reliability of each component can be used to predict the reliability of the system at the design stage

Components in Series

• All components are required for the system is to work

  • Increasing the reliability of a component increases the reliability of the entire system

• Systems with a very large number of components: there is a limit to how much system reliability can be increased

• To further increase system reliability, we can use redundancy

Components in parallel

• The extra components increase the overall reliability because if the primary system or component fails, then the backup component or system can take over, and the overall system wont fail

• Parallel redundancy

Parallel Redundancy

Add backup components or an additional backup system to the existing system