Monthly Archives: December 2011

Certified Reliability Engineer (CRE) Preparation Course

Jan 17 – Feb 28, 20126pm-10pm one night a week, 7 weeks

Course Instructor John Cooper, Ops A La Carte
Length: 7 weeks
Cost: $1295. We offer 25% discount for those taking via webinar or for unemployed or for students not getting reimbursed.
Location: San Jose, CA

Offered via webinar for out of town students.

Becoming certified as a Reliability Engineer (CRE) can be valuable to your employer and your career. We are offering this Exam Preparation Course. Students have found it very valuable in preparing for the exam. Even if you are not planning on taking the exam but need a good, in-depth course in Reliability Engineering, this can benefit you substantially.

Course Webpage: CRE Course by Ops A La Carte

When performing various reliability tasks, non-repairable systems or products are treated differently from repairable systems or products.  Some of the tools that are used for one type are not applicable to the other.   Obviously, at some level, repairable systems are composed of non-repairable parts.   Examples of non-repairable systems would be “one-shot” devices like light bulbs or more complex devices like pacemakers.  Examples of repairable systems are computers, automobiles, and airplanes.

 

What is unique about repairable systems?  Availability becomes a key measure of importance.  In simple terms, availability is the percentage of time that the product or system is able to perform its required functions.  When the required functions cannot be performed because a failure has occurred, the system must be repaired to restore the functionality.  This is where another measure, maintainability, impacts the system availability.  The faster the system can be repaired, the greater the availability to the customer.  For systems that require high reliability or availability, redundancy can improve the design.  However, repairable systems will benefit significantly more than non-repairable systems when using redundancy.

 

Common metrics used in measuring system types are shown in the table below.

METRIC

NON-REPAIRABLE

REPAIRABLE

Time to Failure MTTF Time to First FailureHazard Rate MTBF Time to First FailureROCOF/Failure Rate
Probability Reliability Availability(Reliability)
Maintainability N/A Maintainability Downtime
Warranty Product replacement within warranty period Part/product replacement within warranty period

The table below compares some additional areas of non-repairable systems and repairable systems.

NON-REPAIRABLE

REPAIRABLE

Discarded (recycled?) upon failure Restored to operating conditions without replacing entire system
Lifetime is random variable described by single time to failure Lifetime is age of system or total hours of operation
Group of systems – lifetime assumed independent & identically distributed (from same population) Random variables of interest are times between failure and number of failures at particular age.
Failure rate is hazard rate of a lifetime distribution – a property of time to failure Failure rate is rate of occurrence of failures (ROCOF) – a property of a sequence of failure times

 

Reliability modeling is usually more complex for repairable systems.  Often, methods like Markov models (chains) is required to adequately model repairable systems as opposed to simple series block diagram methods for non-repairable systems.

In the area of monitoring or analysis, the following table compares methods for both types of systems.

METHOD

NON-REPARIABLE

REPAIRABLE

Weibull Useful method (single failure modes only) Not used at system level
Reliability Growth –  Duane

– AMSAA

Usually not used Used during development testing
Mean Cumulative Function (MCF) Usually not used Useful method (non-parametric)
Event Series (Point Processes) HPP (For random, constant average rate events) NHPP (Parametric method) – complex

 

It is important to understand the type of system being designed and use the appropriate reliability methods and tools to match that system.  This may require some research but it’s important to use the correct methods so as not to have misleading results.

What has been your experience in doing analysis of repairable systems compared to non-repairable systems?