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How to use MTTF ?

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  • #41183

    bach huss
    Participant

    Hi all,
    I understand that MTTF is a component failure of a big system and the component is replaced. We can use minitab to estimate the MTTF.
    1. If the calculated MTTF = 300 hrs, what does it mean in layman’s term? (is it 50%Β of the component will fail when operations reach 300 hrs?)
    2. Another question, If a vendor publish it’s component MTBF to 500 hrs. Do I change all components that reached 500 hrs? (Assuming the component cost and the replacement cost is negligible).
    Thanks and warm regards.
    Bach Huss

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    #128875

    Paul Gibbons
    Participant

    BH,
    IF this helps here are some thoughts on the way IΒ use MTTF, MTBF and MTTR et al. This information is based on my own experience of using these measures and I would not say the definitions are textbook, rather, user friendly :)
    MTTF
    Mean Time To Failure (MTTF) is the time, on average, that you would expect a piece of plant to fail when it has been running. It is a simple indicator of plant reliability. Example.
    Plant runs for 500 hours. It breaks down 5 times during that period.
    MTTF = 500/5 = 100 hours.
    MTBF
    Mean Time Between Failure (MTBF) is the time, on average, that you would expect a plant to fail including time lost whilst repairs are undertaken. It is an indicator of the combined reliability and maintenance effectiveness/efficiency. Example.
    Plant runs for 500 hours, has 200 hours downtime due to 5 failures.
    MTBF = (500+200)/200= 140 hours
    MTTR
    Mean Time To Repair (MTTR) is the time, on average, that you would expect a stoppage to last including time spent waiting for maintenance engineer, diagnosis, waiting for parts, actual repair and testing. It is an indicator of maintenance effectiveness/efficiency. Note that there is another indicator that can be used here, Mean Corrective Repair Time (MCRT). MCRT is only interested in the actual repair time assuming all tools, spares and required manpower are available (efficiency). MTTR-MCRT= Waste and therefore gives you an indication of how ineffective your stores and maintenance resource strategy is.
    Example.
    As above, plant runs for 500 hours, has 200 hours downtime due to 5 failures.
    Β MTTR= 200/5= 40 hours
    Therefore MTBF = MTTF + MTTR.
    Availability
    MTTF, MTBF and MTTR can also be used to calculate the availability, or more importantly, the unavailability of the plant due to maintenance causing failures. Example.
    As above, plant runs 500 hours breaks down 5 times and 200 minutes are spent waiting repair/spares/repairing.
    Availability:
    = ((500 + 200)-200)/(500+200)
    = (700-200)/700
    = 71%
    Or
    Availability
    = MTTF/ MTBF or MTTF/(MTTF+MTTR)
    =100/140 = 71% or 100/(100+40) =71%
    Unavailability
    = 1 Β– availability
    = 29%
    Or
    = 200/ (500 +200)
    = 29%
    Or
    MTTR/ MTBF or MTTR/ (MTTF+MTTR)
    = 40/140 or 40/ (100 +40)
    = 29%
    Summary
    Β 
    As a maintenance engineer your objectives must be to ensure that the plant is available to be used as and when required by production with no compromise of quality and safety, all at minimum total cost. There are indicators that can be used to measure performance against the for-mentioned criteria. MTTF is an indicator of plant reliability and should be as long as possible. Increased MTTF can be achieved through an effective preventative and/or predictive maintenance plan. MTTR should be as short as possible and is an indicator of the combined effects of the maintenance strategy, organisation and systems used. MTTR can be reduced by having the right person, with the right tools, with the right spares, in the right place at the right time (sound familiar?). MTBF should be as short as possible and is an overall indicator of reliability and effectiveness/efficiency.
    Β 
    Hope this helps.
    Β 
    Paul

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    #128876

    BBMole
    Participant

    The view that Paul has given is correct. I would highlight that most failures are not linear using eta and beta are better estimates for MTTF, etc
    Have a look atΒ  http://www.weibull.com
    Also there are many software packages out there that can help with determining the optimum time for Preventative maintenance. One that I have use before is Winsmith. The following site let you download a test version, which from memory does not allow you to print and has some other restrcitions, it is also fairly cheap
    http://www.barringer1.com/demo_ws.htm
    If you have the failure data it is fairly simple to determine the rates.
    If you have any problems I can send you some info on it uses for specific problems.
    Hope this helps
    Β 

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    #155977

    Seymur Maharramov
    Member

    Hi,
    MTBF = (500+200)/200= 140 hours should beΒ  changed to (500+200)/5=140
    Regards,
    Seymur

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    #180740

    Rahul Phatangare
    Participant

    Hi Paul
    This is regarding MTBF calculations. I think it should be MTBF = (500 +200 ) / 5 = 140 hours

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    #189432

    Long
    Member

    Hi Paul,
    I understandΒ the relation between MTBF, MTTR and availability. It’s a very good explanation. Still I have question.
    How could IΒ calculate the relevant amount of components for the storage area?
    thx
    Roger
    Β 

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    #189491

    Paul Gibbons
    Participant

    Roger,
    I guess you need to understand the lead time to supply the spares and compare this to the number required.
    Paul

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