Design for Reliability and Safety Flashcards
What is single point failure?
A single failure that leads to catastrophic failure.
What is common mode failure?
A single effect that causes several sub-systems to fail.
What is a cascade failure?
One failure that leads to another.
What is a fault intolerant system?
A single failure that causes a system to fail.
What is fail-safe?
A system that is designed to fail in a non-catastrophic way.
Whats is safe-life?
A period of safe operation.
What is the equation for component reliability?
R(t) = e^-((Lambda)(t))
What is the equation for probability of component failure after time t?
F(t) = 1 - R(t)
What is a non-redundant component?
A product that consists of a number of components that must all work for the product to function.
R = R1 x R2 x R3 x R4 …
This is fault intolerant.
What is “One in Three” component redundancy?
One out of three components has to work.
Ra+b+c = 1 - (1-Ra)(1-Rb)(1-Rc)
What is the equation for when n out of m components are required to work?
R = R^(n) + (Qn-1)R^(n-1)F + (Qn-2)R^(n-2)F^(2) + … + (Qm)R^(m)F^(n-m)
where Qp is the number of permutations of p surviving out of n.
Qp = n!/[(n-p)!p!]
m = number of components required to work n = total number of components
How do you increase reliability?
Make n as large as possible.
List the steps for reliability modelling and optimisation.
- Identify components
- Calculate reliability of each component for time t given failure rate
- Formulate block diagram
- Calculate reliability
- Identify areas of low reliability
- Devise methods for improving reliability
Give some examples of fail-safe design.
- Overheating in electrical products cause the fuse to trip and disconnect electrical supply.
- Bike wheel is ductile to prevent catastrophic loss of support.
- Kill cord on speed boat that switches the accelerator off.
Explain Failure Modes Effects and Criticality Analysis (FMECA).
O = Occurence (10 = v likely) S = Severity (10 = v severe) D = Detectability (10 v hard to detect) RPN = Risk Priority Number
