HO4 - L15: Introduction of Reliability Flashcards
What is reliability?
probability that a product will perform its intended function
satisfactorily for a pre-determined period of time in a given environment.
What are the key elements of reliability (PITE)?
- Probability
- Intended function
- Time
- Environment
Why has there been a growing concern about reliability over the last number of years?
- Customer expectation
- Sophistication: technical complexity
- Foreign competition
- Diagnosis and repairs in complex systems takes specialized skills/ equipment.
How do you design for reliability?
Involve design phases, design factors and methods.
What are the phases in designing phases?
- Concept
- Design and development
- Fullscale development
- Operational
- Disposal
Explain concept phase
Work with customers to develop a product in terms of ease of use, special training requirement, complexity of design and support.
• Up to 35% of the lifecycle cost of the product is determined in this period
• The choice of design characteristics have a significant impact on future costs.
Explain design and development
• Issues such as ergonomics, maintainability, safety and other design characteristics
become a “product on paper”.
• By the end of this period 90% of total life cycle cost have been determined.
Explain full scale development
- Prototype runs of final build
* Changes during this phase as very costly
Explain operational
- Use of the item in the field
* Ease of use and ease of maintenance have a significant impact on reliability.
Explain disposal
Designers are now required to take into consideration the disposal of the product
• Design for disassembly – but important that performance is not degraded.
What design factors to consider when designing for reliability?
- Cost
- environment
- human characteristics
- producibility
- maintainability
- good general design procedures.
What is cost factors?
- As quality rises, often so will the cost to produce such quality.
- Reliability engineers and designers must achieve optimum reliability
eg: Parts designed with too tight of tolerance - restricts machine utilization, increase inspection costs and require greater operator skill
What are environmental factors?
“Family” environment – the impact of components on each other
For example: does heat produced by one component harm an adjacent component?
• “System” environment – where will the product be expected to perform?
For example: dust, vibration
What is human factors?
Dangerous or critical controls should be made obvious or be protected by a cover and labelled
• Knobs, levers and controls designed for easy access – not protrude to permit accidental bumping.
• Foot control can be utilized if operator is in sitting position.
• Conditions should be appropriate
eg: colour coding, lighting, temp.
What are the factors to consider for human factors?
- age
- agility
- skill level
- hearing
- eyesight
What are the design methods?
Simplification Redundancy Derating Fail safe Producibility Maintainability Good design concepts Systems effectiveness
Explain simplification
• The smallest number of parts should be used without compromising performance,
particularly if the design of the system is a series design.
Explain redundancy
• The existence of more than one means for achieving a stated level of performance;
all paths must fail before the system will fail.
• As parallel elements are added the reliability increases because each new element provides a different route or bypass.
For example: 2 pumps in parallel one can be maintained while other one works
Explain derating
The operation of a machine at less than its rated maximum power in order to
prolong its life.
• Applied to reduce the failure rate.
• Design components with operating safety margin.
For example diesel engine rated at 1000hp may be derated by 10% for marine application
Explain fail safe
When failure to operate a product can lead to fatality or substantial financial loss, a
fail-safe type design should be adopted.
• When failure does occur, it should occur in the safest fashion
For example: fuse, thermostats
explain producibility
• Product must be designed not only for performance but also for ease of production
explain maintainability
• Designed so that weak or marginal parts can be replaced conveniently
For example bearings
Explain good design concepts
A good designer will select components and circuits that have been tried and tested
and will avoid unproven methods.
explain system effectiveness
• Individual items are assembled into sub-systems and then into systems (subassemblies into assemblies). As systems become more complex, the probability of
individual items failing becomes greater. It becomes important that the failure of a single item does not result in total system failure
