SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Clay Davis
Date posted: Wed May 24 0:35:57 US/Eastern 2000
Subject: L1 to MTBF Conversion
Is there a conversion factor between a L1 number and a MTBF number? Or a generic L# and MTBF? L1 is when 1% of the population will fail. Also, is it correct to say MTBF is equivalent to L50?
Subject: Li to MTBF Conversion
Reply Posted by: Patrick T. Hetherington
Date Posted: Wed May 24 6:28:43 US/Eastern 2000
There is not a generic conversion. The figure L1 or more generally Ln is a time to failure distribtuion independent term representing the % of a population that came be expected to failure by a given time. Ln alone gives no insight as to when the n% will fail or how long the 100-n% will last, simply that n% will have failed by a given Ln time. Ln is a term generally used when speaking of non-repairables items and gives a point estimate of reliability. Non repairable items may or may not have time to failure distributions that are random(constant # of failures/unit time). MTBF is a term generally reserved for repairables systems that have times between failures randomly distibuted. In the general sense, there is no relation to Ln for a item and the MTBF for a repairable system.
For non-repairables items, the time to failure distribution can most often be approximately by the Weibull distribtuion. Other possibities include normal and log-normal distribtions. For the Weibull distribution the relationship of MTTF=thete*gamma funciton(1+1/beta)+To where beta and theta are paramters of the distribution determined by curve fitting or with software. To is interval beginning at time =0 before which no failures are expected (often = 0). MTTF represents the average of all expected times to failure of a distribtuion. This can be wrongly interpreted if that distribution of failures is not symetrical. For non-repairable units that are normally distributed (symetrical about mean), the L50 life is equal to the mean and the MTTF. For non-repairable items that have a random failure distibution, the MTTF corresponds to the L63 life ( a lot fail before the average time to failure and some last a really long time). If the opposite were the L40 life may be equal to the MTTF.