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Product Design > Design for reliability and safety > Flashcards

Design for reliability and safety Flashcards

1
Q

Why is reliability important

A

people (customers) expect high reliability from products

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2
Q

Discuss the reliability of commercial aircraft

A

in 2017 there was no deaths from commercial air flight despite a huge number of flights occurring

The high reliability of aircraft is partly the result of reliability being designed into aircraft systems

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3
Q

Discuss the reliability of cars

A

A typical car engine lasts over 100,000 miles

Reliability has to be designed into the product

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4
Q

Give three examples of where reliability has been designed into a product

A
  1. aircraft
  2. cars
  3. washing machines
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5
Q

Defines single point failure

A

single failure leading to catastrophic failure

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6
Q

Define common mode failure

A

single effect that causes several sub systems to fail

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7
Q

Define cascade failure

A

one failure that leads to another etc

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8
Q

Define fault intolerant system

A

single failure causes the system to fail

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9
Q

Define fail safe

A

fail in a non-catastrophic way

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10
Q

Define safe life

A

period of safe operation

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11
Q

What does MTBF stand for

A

mean time between failure

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12
Q

How is mean time between failure (MTBF) calculated

A

MTBF = 1/failure rate (gamma)

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13
Q

What is the failure rate (gamma)

A

number of failures per time period t

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14
Q

At t = MTBF, the reliability = ____

and the failure = ____

A 
reliability = 0.37
failure = 0.63
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15
Q

What are the three types of failure distribution (sketch them)

A
  1. Exponential
  2. normal
  3. weibull
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16
Q

What is component reliability R(t) defined as

A

R(t) = probability of survival as a fraction

R(t) = number of surviving parts after time t/(Total number)

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17
Q

For a constant failure rate what is the equation for component reliability

A

R(t) = e ^((-gamma)t)

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18
Q

What does R(t) stand for

A

probability of component surviving after time t

19
Q

what does gamma stand for

A

failure rate (per hours)

20
Q

How do you calculate gamma (failure rate)

A

gamma = -(lnR)/t

21
Q

What does t stand for

A

time (hours)

22
Q

for a constant failure rate: R(t) = e^((-gamma)t

When t=0 R=1 meaning..

A

no possibility of failure

23
Q

for a constant failure rate: R(t) = e^((-gamma)t

When t=infinity R=0 meaning..

A

certainty of failure

24
Q

What is the equation for component failure after time t (F(t))

A

F(t) = 1 - R(t)

25
Q

Sketch the profiles of reliability and failure probabilities with respect to time

A

SKETCH

26
Q

Draw a representation of a product that consists of a number of components that must all work for the product to function.

How is the reliability of this system calculated

A

SKETCH

Rtotal = R1 x R2 x R3 x R4

27
Q

Draw a diagram for a fault intolerant system and describe how it can fail

A

failure of any component will cause failure of the system

SKETCH

28
Q

Draw a representation of a product that has a ‘one in two’ component redundancy

How is the reliability of this system calculated

A

SKETCH

Rtotal = R1 x R4 x R2+3

where R2+3 = (R2+R3)-(R2xR3)

29
Q

Draw a diagram for a fault tolerant system and describe how it can fail

A

there is some tolerance to failure of some components. The system will still function if only components 2 or 3 fail - same as ‘one in two’ redundancy

SKETCH

30
Q

Give examples of two component redundancy systems

A
  1. Two computers on an aircraft
  2. Two tape players on a hi-fi
  3. Hospital generator for power supply
31
Q

Draw a representation of a product that has a ‘one in three’ component redundancy

How is the reliability of this system calculated

A

SKETCH

Ra+b+c = 1-(1-Ra)(1-Rb)(1-Rc)

32
Q

Is it better to have four small or two big engines on an aircraft

A

Four small engines are more reliable than two big engines

But it is often more expensive to have a larger number of smaller engines

33
Q

What are the 6 steps for reliability modelling

A
  1. Identify components
  2. Calculate reliability of each component for time t from given failure rate (gamma)
  3. Formulate block diagram
  4. Calculate reliability
  5. Identify areas of low reliability
  6. Devise methods for improving reliability
34
Q

Give examples of fail safe systems

A
  1. Overheating in an electrical product causes a fuse to fail and disconnects the electrical supply
  2. A ductile failure in a bicycle wheel rim prevents catastrophic loss of structural support
  3. most modern cars are fitted with twin hydraulic brake circuits, with two mast cylinders in tandem, in case one should fail
35
Q

Give examples of design features that ensure fail safe

A
  1. Electrical fuses
  2. electrical circuit breakers
  3. kill cord on speed boats that switches acceleration off
  4. protect kevlar liners in aircraft fuel tanks to stop or reduce fuel leakage in the event of impact
  5. shields around aircraft engines to prevent/reduce debris flying out in the event of engine failure
36
Q

Give the case study of the padstow speed boat disaster as an example of the importance of a fail safe system

A

A family were on a speed boat and no one was attached to the kill cord. The kill cord turns the accelerator off if the cord is pulled from the controls, where as a car requires constant force to keep the accelerator down, a boat often has a fixed accelerator due to the motion of the boat preventing an accelerator similar to a car

The boat did a sharp turn and everyone fell off board and the engine kept going resulting in serious injuries and death

37
Q

Give the case study of the Concorde disaster

A

The concorde aircraft went over debris on the runway at take off which resulted in a large fire, stopping two engines from working and the aircraft crashing. Investigators concluded that failure (fuel leakage) would have been reduced if the aircraft had adequate shielding

38
Q

Describe how the concorde disaster could be described as a cascade failure

A

A cascade failure involves one initial failure leading to another, then another and so on with these subsequent failures getting more and more out of control

  1. an engine wear strip had not been installed or manufactured properly which led to it falling of an aircraft on the runway
  2. the concorde ran over the engine strip and burst a tyre
  3. debris from the tyre hits the fuel tank
  4. pressure wave ruptures the weakest part of the tank
  5. fuel catches fire due to hot engines
  6. two engines stop worked
  7. plane turns and pilots reduce power on the two other engines
  8. plane stalls and crashes
39
Q

What does FMECA stand for

A

failure modes effects and Criticality Analysis

40
Q

What are the measures in a FMECA table

A
  1. Occurence
  2. Severity
  3. Detectability
41
Q

What does RPN stand for and how is it calculated

A

Risk Priority Number

RPN = O x S x D

42
Q

What would an FMECA table tell you

A

It tells you what would be the worst type of failure so that it can be mitigated

43
Q

What does FTA stand for

A

Fault tree analysis

44
Q

What is a fault tree analysis (FTA) and why is it used

A

The objective of a FTA analysis is to model or record how a failure or disaster can occur based on a series of events

Events can either be combined with an AND box or specified as independent events with an OR box.

FTA is used to help understand a past failure or predict how a future failure might occur

Product Design (5 decks)

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