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  #1  
Old 04-06-2009
dabombchibz dabombchibz is offline
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Default MTBF MTTR

I'm having trouble understanding the difference between MTTR and MTBF...

I am trying to model a conveyor where frequent stoppages occur on average once an hour (largely due to jams) -- no information is given on the type of distribution. And the time between belt stop and belt start for the last 50 occurrences follow an erlang distribution with a location value of 0, and a scale value 0.17, shape value of 21 and random number stream 1

Then is is okay to say (using the tool box) that:

MTBF follows an exponential distribution with scale value 60

MTTR follows erlang - with the values given above

Thanks

Lulu
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  #2  
Old 04-06-2009
Alistair Smitheman Alistair Smitheman is offline
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Lulu,

The difference is:
MTBF is the Mean Time Between Failures, this is the time that machine is running for between breakdowns.
AND
MTTR is the Mean Time To Repair, this is the time it takes to get the machine up and running again after a break down.

Hope this helps.

Alistair
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  #3  
Old 04-06-2009
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Steven Hamoen Steven Hamoen is offline
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Lulu,

You have to be carefull here. If you have a breakdown once every hour for 10 minutes it means that your MTBF is 50 minutes!!

So in your case you have to have an average of 60 minutes minus the avg of your erlang distribution
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Old 04-06-2009
dabombchibz dabombchibz is offline
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thanks for the responses...

Steve i still dont get what you mean....

Sorry
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Old 04-06-2009
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Steven Hamoen Steven Hamoen is offline
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Ok, I'll explain it one more time!

Say you have a down period (your MTTR) of 10 minutes on average. You also know that your machine goes down 1 time per hour. This means that in every 60 minutes your machine is down for 10 minutes. That means that the time between 2 down periods (from the end of the previous till the beginning of the next) is not 60 but 50 minutes.

So in this situation your MTBF = 50 and your MTTR = 10

In your situation, if you have a stop 1 time per hour and the length of the stop is an erlang distribution with an average of x minutes your MTTR is that erlang distribution and your MTBF is 60 - x

Hope this is clear enough
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Old 04-07-2009
dabombchibz dabombchibz is offline
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ahh...that makes sense. I'm terrible at reliability engineering... hehe

thankyou
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