Forum: Reliability & Maintainability Questions and Answers

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Posted by: Wu Xiangping (7815@notesmail.huawei.com.cn )
Date posted: Tue Aug 3 22:44:36 US/Eastern 1999
Subject: Reliability Prediction
Message:
We all know that the result of reliability prodiction is much higher than the true reliability level.Does anyone know the proportion between them?
Another problem is,a new product,has no reliability requirement from customer,how to decide the reliability requirement that the product should achieve? Thanks very much.

Replies: (list all replies)
Reliability Prediction
Clay W Davis Mon Aug 9 11:25:45 US/Eastern 1999
Reliability Prediction
Jon Ilseng Mon Aug 9 15:27:09 US/Eastern 1999
Reliability Prediction
Bruce Dudley Thu Aug 19 9:18:02 US/Eastern 1999
Reliability Prediction
Sumandra Ghosh Chowdhury Tue Aug 24 1:04:35 US/Eastern 1999

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Posted by: Julien NICO (ARZ_St_ELB@compuserve.com )
Organization:Aérazur - Zodiac Group
Date posted: Mon Aug 2 11:45:52 US/Eastern 1999
Subject: Reliability Prediction
Message:
I'm looking for information to calculate reliability of a component. I'm using the NPRD-95 and the FMD-97 to establish some analysis. Unfortunately, for one of our component, there is one part that we cannot find on this document. I try to determine MTBF for a frangible self sealing valve (for fuel application on an Helicopter).
Component is composed of two opposite check valve (reliability given by the NPRD) and a metallic part to maintain both check valves open. This metallic part has a predifined level of rupture and in case of crash, shall broke to allow both valves to close and separate the fuel line. I find two failure mode for the part. One, it will fail with a level of stress under the predifine level (rupture of the line without crash), the second, it will not break when needed (fuel line will not separate in case of crash). I don't know how to calculate MTBF of this metallic part and also define the percentage of occurrence of each failure mode. I will really appreciate if someone have method to study reliability of this component (and in fact, reliability of all new component)

Replies:
Reliability of Mechanical Parts
Seymour Morris Mon Aug 2 14:47:32 US/Eastern 1999

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Posted by: Joni Siipola
Date posted: Fri Jul 30 6:36:18 US/Eastern 1999
Subject: component derating
Message:
I noticed an interesting question and since the answers were not particularly helpful, I'd like to arise the question once more.
"Does anyone have any new quantifiable data on the use of component parameter derating (thermal, power etc). I am used to the usual "margin of ignorance" type arguments, package glass transition temperatures and the junction/channel type Arrhenius relationships (generally with assumed activation energies). However, is there any more recent quantifiable data available to convince a sceptical design engineer and to what extent is derating used?" Thank you for your help!

Replies:
Latest Derating Study
Seymour Morris Mon Aug 2 13:02:10 US/Eastern 1999

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Posted by: Dan Kenny (kenny_dan@mail.aera.com )
Organization:AERA
Date posted: Wed Jul 28 17:33:03 US/Eastern 1999
Subject: Ao software
Message:
Does anyone know enough of the following pieces of software to offer advantages/disadvantages?
AvSim, MEADEP, RAM Commander, RAPTOR, Relex RBD, SPAR and TIGER Much appreciated, Dan KEnny

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Posted by: John Mathis (John.Mathis1@West.Boeing.com )
Organization:Boeing
Date posted: Wed Jul 28 12:18:14 US/Eastern 1999
Subject: Weibull vs Exponential
Message:
I have some questions concerning Wiebull vs Exponential failure rate grading. According to MIL-PRF-55365, paragraph 1.1, table III and table IV a Wiebull failure rate level B (0.1% failures / 1000hours at a 90% confidence level) graded part is substitutable for a Exponential 'S' (0.001% failures / 1000hours at a 60% confidence level) part. What is the technical rational behind this? If the reliability of a wiebull 0.1% is equal to or better than an exponential 0.001% does the system level prediction or FMECA reflect the difference? Is the difference due to the confidence level of 90% vs 60%? If a system had all "S" (0.001%) parts and you substituted all of them with "B" (0.1%) parts wouldn't the system reliability be considerably worse? Can anyone explain this!

Replies:
Weibull Part Grading
Bruce Dudley Thu Aug 12 10:41:21 US/Eastern 1999

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