HO4 - L15: Introduction of Reliability

Question 1

What is reliability?

Answer 1

probability that a product will perform its intended function
satisfactorily for a pre-determined period of time in a given environment.

Question 2

What are the key elements of reliability (PITE)?

Answer 2

1. Probability
2. Intended function
3. Time
4. Environment

Question 3

Why has there been a growing concern about reliability over the last number of years?

Answer 3

1. Customer expectation
2. Sophistication: technical complexity
3. Foreign competition
4. Diagnosis and repairs in complex systems takes specialized skills/ equipment.

Question 4

How do you design for reliability?

Answer 4

Involve design phases, design factors and methods.

Question 5

What are the phases in designing phases?

Answer 5

1. Concept
2. Design and development
3. Fullscale development
4. Operational
5. Disposal

Question 6

Explain concept phase

Answer 6

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.

Question 7

Explain design and development

Answer 7

• 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.

Question 8

Explain full scale development

Answer 8

• Prototype runs of final build

* Changes during this phase as very costly

Question 9

Explain operational

Answer 9

• Use of the item in the field

* Ease of use and ease of maintenance have a significant impact on reliability.

Question 10

Explain disposal

Answer 10

Designers are now required to take into consideration the disposal of the product
• Design for disassembly – but important that performance is not degraded.

Question 11

What design factors to consider when designing for reliability?

Answer 11

1. Cost
2. environment
3. human characteristics
4. producibility
5. maintainability
6. good general design procedures.

Question 12

What is cost factors?

Answer 12

• 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

Question 13

What are environmental factors?

Answer 13

“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

Question 14

What is human factors?

Answer 14

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.

Question 15

What are the factors to consider for human factors?

Answer 15

1. age
2. agility
3. skill level
4. hearing
5. eyesight

Question 16

What are the design methods?

Answer 16

Simplification
Redundancy
Derating
Fail safe
Producibility 
Maintainability
Good design concepts
Systems effectiveness

Question 17

Explain simplification

Answer 17

• The smallest number of parts should be used without compromising performance,
particularly if the design of the system is a series design.

Question 18

Explain redundancy

Answer 18

• 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

Question 19

Explain derating

Answer 19

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

Question 20

Explain fail safe

Answer 20

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

Question 21

explain producibility

Answer 21

• Product must be designed not only for performance but also for ease of production

Question 22

explain maintainability

Answer 22

• Designed so that weak or marginal parts can be replaced conveniently
For example bearings

Question 23

Explain good design concepts

Answer 23

A good designer will select components and circuits that have been tried and tested
and will avoid unproven methods.

Question 24

explain system effectiveness

Answer 24

• 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