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# How to calculate MTBF of a system based on subcomponents

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Post Number #1
6th May 2008, 04:29 PM
 JkelleyCDS Total Posts: 51
How to calculate MTBF of a system based on subcomponents

To provide better detail, our company has bid our product for a potential government contract. Their response has requested a MTBF of the system. I have the following questions?

1. If our product is a protoype, can a MTBF been performed?

2. Without official tests can MTBF be calculated?

3. If I have the MTBF data from our suppliers for subcomponents, what is the easiest way to calculate and combine these values to represent the system?

I can do the simple math of MTBF, but how can I compile these to reflect a system MTBF?

Thanks!

Post Number #2
6th May 2008, 09:18 PM
 Al Rosen Total Posts: 3,542
Re: How to calculate MTBF of a system based on subcomponents

MTBF is a statistical prediction and actual performance may never match the prediction. Find yourself a copy of mil-hdbk-217 to do the system prediction.
 Thanks to Al Rosen for your informative Post and/or Attachment!
Post Number #3
16th February 2010, 08:04 PM
 janedoe Total Posts: 25
Re: How to calculate MTBF of a system based on subcomponents

Quote:
 In Reply to Parent Post by JkelleyCDS To provide better detail, our company has bid our product for a potential government contract. Their response has requested a MTBF of the system. I have the following questions? 1. If our product is a protoype, can a MTBF been performed? 2. Without official tests can MTBF be calculated? 3. If I have the MTBF data from our suppliers for subcomponents, what is the easiest way to calculate and combine these values to represent the system? I can do the simple math of MTBF, but how can I compile these to reflect a system MTBF? Thanks!
The characteristic life parameter can not be added, from components, to determine system performance...but the reciprocal (hazard rate) can. So, add the hazard rates then determine the life parameter with this resulting number.
 Thanks to janedoe for your informative Post and/or Attachment!
Post Number #4
16th February 2010, 08:54 PM
 Miner Total Posts: 3,869
Re: How to calculate MTBF of a system based on subcomponents

Quote:
 In Reply to Parent Post by janedoe The characteristic life parameter can not be added, from components, to determine system performance...but the reciprocal (hazard rate) can. So, add the hazard rates then determine the life parameter with this resulting number.
To clarify:
• Lambda = 1 / MTBF
• Lambda System = Lambda component 1 + Lambda component 2 + Lambda component n
• MTBF System = 1 / Lambda System
 Thanks to Miner for your informative Post and/or Attachment!
Post Number #5
2nd March 2010, 05:32 AM
 danwsc Total Posts: 11
Re: How to calculate MTBF of a system based on subcomponents

1. Yes
2. Yes but they would only be Predictions (which may or may not satisfy your customer)
3. Yes but ...

Change MTBF (in hrs) to Failures In Time (number) by
Failures In Time = Time Period / MTBF
for MIL-217F, Time Period is 10^6 hrs.
for SR-332, Time Period is 10^9 hrs.

Use same formula to calculate system level MTBF.

FITs (and therefore MTBFs) are also dependent on operating and environmental conditions. So you need more than just a single MTBF from your supplier.

Regards.

Last edited by danwsc; 2nd March 2010 at 05:51 AM.
 Thank You to danwsc for your informative Post and/or Attachment!
Post Number #6
5th March 2010, 08:01 AM
 w_grunfeld Total Posts: 235
Re: How to calculate MTBF of a system based on subcomponents

All of the previous answers you got are correct, however all assume that its an electronic system for which the exponential distribution is a good assumption. If however your system includes mechanical assemblies this may no longer be correct
 Thank You to w_grunfeld for your informative Post and/or Attachment!
Post Number #7
5th March 2010, 10:28 PM
 Miner Total Posts: 3,869
Re: How to calculate MTBF of a system based on subcomponents

Quote:
 In Reply to Parent Post by w_grunfeld All of the previous answers you got are correct, however all assume that its an electronic system for which the exponential distribution is a good assumption. If however your system includes mechanical assemblies this may no longer be correct
Excellent point that we all overlooked.
Post Number #8
12th May 2010, 04:26 PM
 vetes Total Posts: 4
Re: How to calculate MTBF of a system based on subcomponents

Depending on how the reliability of each component affects the others in the system as a whole may have an effect on the expected MTBF. In this case Reliability Block Diagrams (RBD) or Fault Tree Analysis may lead to a useful predictive answer to an overall MTBF. Pardon me as I cannot yet post links. Try searching for Reliability Block Diagram on Google or Wikipedia to get an idea of what this method provides.

There are also software packages that can be used for RBD simulation, the user can create one and assign failure and repair rates to the individual components and get an answer for Availability/MTBF of the system based on a simulated amount of time such as expected life or proposed deployment of a system. Relevant applications include:

ARINC Raptor
Relex OpSim
Reliasoft BlockSim
 Thank You to vetes for your informative Post and/or Attachment!

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