Ammar AlkhaldiReal Reliability

2 April 2016

PART 2: Begin with the end in mind

How to Measure Reliability

Use reliability to measure reliability

Sound obvious…

Reliability is probability that a product, system or service will perform its intended function adequately for a specified period of time, operating in a defined operating environment without failure.

So it’s a probability, for specific time (t).

Use reliability to measure reliability

Sound obvious…

Reliability is probability that a product, system or service will perform its intended function adequately for a specified period of time, operating in a defined operating environment without failure.

So it’s a probability, for specific time (t).

This summon the magical reliability function

𝑅(𝑡)

Reliability function in action

𝑅(𝑡) Vs. MTBF (MTxx)

Or any another

Diode MTTF = 20 YearDiode MTTF = 20 Years

Reliability function in action

USER B

So the average life of the population is 20 years, and if the Standard Deviation = 1 year then 99% of the relays will fail within 17-23 years.

Now I’ll set my strategy

USER A

So 50% of the population will fail after 20 years.

Now I’ll set my strategy

How can “taking MTBF value as it is” lead to wrong strategy

Diode MTTF = 20 YearDiode MTTF = 20 Years

Reliability function in action

USER B

So the average life of the population is 20 years, and if the Standard Deviation = 3 years then 99% of the relays will fail within 17-23 years.

Now I’ll set my strategy

USER A

So 50% of the population will fail after 20 years.

Now I’ll set my strategy

How can “taking MTBF value as it is” lead to wrong strategy

Diode MTTF = 20 Years

Reliability function in action

USER C

Will assume: A. The vendor calculate the MTTF in the right way. B. Constant failure rate (i.e. exponentially distributed).

How can “taking MTBF value as it is” lead to wrong strategy

𝐹 𝑡 = 1 − 𝑒−𝜆𝑡, MTTF= 1/𝜆 Do the math

Where 𝜆= failure rate, 𝑡 = Time

𝑅 𝑡 = 1 − 𝐹 𝑡𝑅 1 𝑌𝑒𝑎𝑟 = 1 − (0.0487) = 95.1% Reliability by the first year

𝑅 20 𝑌𝑒𝑎𝑟𝑠 = 1 − (0.632)= 36.8 % Reliability by 20 years

Diode MTTF = 20 Years

Reliability function in action

USER C

Will assume: A. The vendor calculate the MTTF in the right way. B. Constant failure rate (i.e. exponentially distributed).

How can “taking MTBF value as it is” lead to wrong strategy

𝐹 𝑡 = 1 − 𝑒−𝜆𝑡, MTTF= 1/𝜆Where 𝜆= failure rate, 𝑡 = Time

𝑅 𝑡 = 1 − 𝐹 𝑡𝑅 1 𝑌𝑒𝑎𝑟 = 1 − (0.0487) = 95.1% Reliability by the first year

𝑅 20 𝑌𝑒𝑎𝑟𝑠 = 1 − (0.632)= 36.8 % Reliability by 20 years

IS THIS WHAT EXPECTED BY 20 YEARS MTTF ???

Diode MTTF = 20 Years

Reliability function in action

USER C Now let’s come to the assumptions

Will assume:

A. The vendor calculate the MTTF in the right way. B. Constant failure rate (i.e. exponentially distributed).

A. Testing 12 diodes for 1month doesn’t reflect 1year reliability nor introducing all the failure modes, if the data provided by the vendor s not accurate then this assumption is wrong, using your own field data is better and reflecting the real running environment, ask the vendor how the ALTA was done.

How can “taking MTBF value as it is” lead to wrong strategy

Diode MTTF = 20 Years

Reliability function in action

USER C Now let’s come to the assumptions

Will assume:

A. The vendor calculate the MTTF in the right way. B. Constant failure rate (i.e. exponentially distributed).

B. For electronics components, assuming constant failure can be acceptable (in fact it’s a random failure rate) since the constant failure rate phase aka useful life phase can be long enough for the components to fail due to the random failures before it even reach the wear out phase.

What if it wasn’t constant failure rate

How can “taking MTBF value as it is” lead to wrong strategy

What if it wasn’t constant failure rate

In this turbine’s winding Example it’s clear That the exponential distribution is not fitting The failure data.

In such case we won’t be able to determine the reliability function given MTBF value alone.

We need special tools to find the 𝑅(𝑡) Credit to blog.minitab.com

Why vendor still using MTBF ?

20 years MTBF sound much better than “There is 63.2 % chance our component will fail by 20 years”.

Specially if the competitors still using MTBF furthermore customers even asking for it.

Calculating the reliability function

Begin with the end in mind

First thing we need to do before measuring reliability, is to ask this question:

Begin with the end in mind

First thing we need to do before measuring reliability, is to ask this question:

Why are we measuring the reliability ?

Begin with the end in mind

Begin with the end in mind

Does it really matter ?Can’t we just measure it now and decide later ?Actually I might use one tool for multi purposes!

Begin with the end in mind

Yes it does matter. Reliability is a little bit different, We have different tools, each have its own usage

Does it really matter ?Can’t we just measure it now and decide later ?Actually I might use one tool for multi purposes!

Begin with the end in mind

Does it really matter ?Can’t we just measure it now and decide later ?Actually I might use one tool for multi purposes!

The wrong tool will give a wrong (misleading) result.

WHY?• Preventive Maintenance• Spare parts strategies• Inspection scheduling

• Warranty • Benchmarking• Risk assessment • Risk mitigation

WHAT?• Component/System

• Repairable/Non-Repairable

• HOW?• Life data analysis (Weibull)

• Reliability Growth• Reliability Block Diagram (RBD)

• And many other tools

If you are interested how to use some of these tools, stay toned for the upcoming post.

Salam

Ammar Alkhaldi, CSSBB

How to use reliability tools

NO MTBF Blog http://nomtbf.com/

SIEMENS Background information on MTBF

http://www.weibull.com/hotwire/issue22/hottopics22.htm

http://blog.minitab.com/blog/understanding-statistics/choosing-the-right-distribution-model-for-reliability-data

References

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