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Posted by: Bart de Boer (bart.deboer@ultracontrols.aero )
Organization:Ultra Electronics - Controls
Date posted: Wed May 9 11:40:54 US/Eastern 2007
Subject: Calendar FR vs Operating FR
Message:
RE: PRISM Field and Predicted Comparison
David Dylis reply to “Failure per Million Hours” Tue Dec 10 13:23:10 US/Eastern 2002 My interpretation of the example: Failure rates in PRISM are presented in failures per million calendar hours and not in failures per million operating hours which adds to the confusion when a user is only interested in failure rates solely due to operation. I will answer the question with an example. If an item has a total failure rate of 1 failure/million calendar hours, the operational failure rate is 0.99 failures/million operating hours and the non-operating failure rate is .01 failures/million non-operating hours. If it is operating at a 5% duty cycle then we obtain: 1 Failures/Million Calendar Hours = (.05) x 0.99 Failures/Million Operating Hours + (0.95) x 0.01 Failures/Million Non-operating Hours Now converting Failures/Million Operating Hours (.05 Operating/Calendar) x 0.99 Failures/Million Operating Hours = 1 Failures/Million Calendar Hours - (0.95 Non-operating/Calendar) x 0.01 Failures/Million Non-operating Hours or Operational Failure Rate at a 5% duty cycle = 20.010101 Failures/Million Operating Hours. The formula then is: F/MOH = ({op F/MCH + non-op F/MCH}-[non-op duty*non-op F/MCH])/(duty*op F/MCH) Similarly, for a 50% duty cycle the failure rate would be: 1.91919 Was a 0.1 non-op F/MCH accidentally used within [ ] brackets?, The PRISM predicted failure rate is based on calendar time. The duty cycle in PRISM identifies the amount of time that an item operates (e.g., 5% of the time) during the calendar period (e.g., 1 million calendar hours). Therefore, in a given calendar period, items that operate with a higher duty cycle are actually operating a greater amount of time then those at a lower duty cycle. When a conversion from calendar to operating time is made and there is no or minimal difference between two predicted failure rates with varying duty cycles, then the item with the lower duty cycle will have a higher predicted failure rate since the predicted value is based on a lower amount of operational hours. It seems then that the PRISM FR is constant over calendar time, irrespective of how it’s used. i.e. with op F/MCH much greater than non-op F/MCH, failures will be compressed into the operational time. Thus if a unit has a 100% duty cycle (always on) it will, over 114 years (1 million calendar hours), exhibit the same number of failures as a similar unit with a 5% duty cycle (on for a total of 5.7 years) over the same 114 year period, with almost all of the failures occurring in that 5.7 year operating time (i.e. failing 20x more often per OH than the always on unit).

Replies:
Calendar FR vs Operating FR
David Dylis Thu May 10 14:33:00 US/Eastern 2007

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Posted by: Stephen Cobb
Date posted: Tue Mar 6 5:24:03 US/Eastern 2007
Subject: PRISM and One Shot Devices
Message:
I am attempting to use PRISM to predict the reliability of a missile system. This system will sit in a bunker completely dormant for years and then be checked out (Mission 1). It will then be shipped somewhere (days) and then launched where it will fly for a couple of minutes (Mission 2).
How do I get PRISM to model these missions as failures per million calendar hours is not a useful measurement for Mission 2?

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Posted by: Alek
Date posted: Fri Sep 1 7:05:01 US/Eastern 2006
Subject: Prism Duty Cycle parameter
Message:
PRISM tool has a Duty Cycle parameter selector. Does it mean that a system failure rate calculated by Prism can be obtained in operating hours if I set the Duty Cycle value equal to 100% and the Cycling Rate equal to 0?

Replies:
PRISM Duty Cycle
David Dylis Fri Sep 8 14:50:59 US/Eastern 2006

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Posted by: Nagaraj
Date posted: Thu Aug 24 8:34:37 US/Eastern 2006
Subject: RAC Data
Message:
I have recently downloaded the demo version of Prism. I have the following doubts in it
1. For the components in OTHER category (E.g. In the example system V1;VALVE, CHECK in the A1;Check/Control Valve Assy.) failure rate given for RAC Data (0.06529)differs with the Merged Failure Rate for the same from (2.072773) source Search. I have tried with the different combinations of Quality and Environment, but I couldn't get that value. What is the reason behind this? 2. In the context of above question, RAC Data Search button popped up a table in which source is listed as Fictional. Is this the case for demo version? tab?

Replies:
PRISM Demo Questions
Tammy Thu Aug 24 9:24:25 US/Eastern 2006

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Posted by: Super Wong (superwang@tyanchina.com )
Organization:TYAN
Date posted: Tue Jun 6 5:00:42 US/Eastern 2006
Subject: About MTBF calculation
Message:
1)When we get the result of failure rate,how can we get MTBF(Meantime between failures)? Which expression should we use,MTBF=10e6 /failure rate or MTBF=10e9 /failure rate ?
2)Which code does PRISM follow,MIL-HDBK-217 or Bellcore?As I know,MIL-HDBK-217 MTBF calculation bases on 10e6,Bellcore MTBF calculation bases on 10e9. Best regards Super

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MTBF Calculation
David Dylis Tue Jun 6 8:50:57 US/Eastern 2006

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