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?
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