14.2 Design for use: How long will this design work? Flashcards
What is reliability?
Reliability is the“probability that an item will perform its function under stated conditions of use and maintenance for a stated measure of a variate (time, distance, etc.).”
What is a variate?
Variate is the appropriate measure of the use of the design (e.g. time, distance).
Only under what assumption can we properly measure the reliability of a component or system?
We can properly measure the reliability of a component or system only under the assumption that it has been or will be used under some specified conditions.
What is the term for the appropriate measure of use of the design?
The appropriate measure of use of the design, called the variate, may be something other than time. The variate is not always time.
What are some examples of measures of variate?
Measures of variate include the following:
- Time
- miles (for vehicles)
- Distance
- Number of uses
- Cycles
- cycles of operation (piece of vibrating machinery
In what context must we examine reliability?
Reliability must be examined in the context of functions.
What is the British standard of failure?
The British Standard 4778 of failure is “the termination of the ability of an item to perform a required function .”
Between what two points of inquiry is it useful to distinguish in failure?
It is useful to distinguish between:
- when a system fails
- how the system fails
What is in-service failure?
In-service failure is when an item fails when in use.
What is incidental failure?
Incidental failure means that an item fails, but the consequences are not detectable until (later) some other activity takes place.
What is catastrophic failure?
Catastrophic failure is characterized by failure of some function such that the entire system in which the item is embedded fails.
What is the meaning of Mean Time Between Failures (MTBF)?
Mean Time Between Failures (MTBF) is exemplified by miles per in-service failure, or some other metric, i.e. the average functional period.
Consider the two failure distributions - two reliability probability distributions have the same mean (or average), that is, MTBFa = MTBFb, but they have very different degrees of dispersion. Which design is more reliable based on the MTBF distributions? Why?
The first design is more reliable because there are fewer failures earlier in the life of the artifact.
What can cause a system to fail in series system design?
In a chain of parts or elements, the failure of any one of which would break the chain, which in turn will cause the system to fail.
What is a good analogy to explain series system design?
Just as a chain is no stronger than its weakest link, a series system is no more reliable than its most unreliable part.
What dictates the reliability of a series system?
The reliability of the system as a whole can be no higher than that of any one of the parts, because the failure of any one part will cause the system to cease operation.
Note an example of series system design.
What is redundant system design?
Redundant System Design is a design in which some or all of the parts have backups or replacement parts that can substitute for them in the event of failure.
What must happen in order for failure to occur in redundant system design?
Each of the backup/replacement components must fail in order for the system to fail.
What are the pros and cons of series vs. redundant system design?
Series system design is less reliable and less expensive (in terms of first cost). Redundant system design is more reliable, but also more expensive.
- To design for reliability, select redundant system design.
- To minimize production cost/initial cost, select series system design.
Which is less expensive of the options below?
- Series System design
- Redundant system design
Series system design is less expensive.
Parallel systems are more expensive, since many duplicate parts or elements are included only for contingent use.
Which is more reliable of the options below?
- Series System design
- Redundant system design
Redundant system design is more reliable.
Parallel systems have advantages in terms of reliability, since all of the redundant or duplicate parts must fail in order for the system to fail.
What is another term for redundant system design?
Redundant system design is also referred to as parallel system design.
Note an example of redundant/parallel system design.
What generally causes failure?
Failure generally is caused by:
- Inadequate design
- Manufacturing defects
- Use outside of specified conditions/improper use
How do designers decide between series system design and redundant system design?
Carefully weigh the consequences of failure of a part against failure of the system, along with costs attendant to reducing the likelihood of a failure.
What does designing for maintainability require?
Designing for maintainability requires that the designer take an active role in setting goals for maintenance.
How can designers design for maintainability?
Some ways to design for maintainability include the following:
- Selecting parts that are easily accessed and repaired
- Providing redundancy so that systems can be operated while maintenance continues
- Specifying preventive or predictive maintenance procedures
- Indicating the number and type of spare parts that should be held in inventories in order to reduce downtime when systems fail
What are some considerations to bear in mind when designing for maintenance and repair?
Several considerations that designers should bear in mind when designing for maintenance and repair include:
- Fault isolation and self-diagnosis
- Part standardization and interchangeability
- Modularization and accessibility
