A simplifying assumption associated with using MTTF or MTBF implies a constant hazard rate. Some assume we’re in the useful life section of the bathtub curve. Others do not understand what assumptions they are making.
Using MTTF or MTBF has many problems and as regular reader here know, we should avoid using these metrics.
By using MTTF or MTBF we also lose information. We are unable to measure or track the rate of change of our equipment or system’s failure rates (hazard rate). The simple average is just an average and does not contain the essential information we need to make decisions.
Let’s explore five different reasons the rate of change of a failure rate is important to measure and track.
1. Clues About the Nature of the Failure Mechanism
A decreasing failure rate tends to indicate manufacturing, assembly, transportation or installation type problems. An increasing failure rate tends to indicate wear out type problems.
Both patterns can occur at anytime, thus good failure analysis of units that have failed is still important.
2. Identify Necessary Spare Part Stocking Accurately
A decreasing failure rate would indicate the reduced need for spares in the future. Likewise an increasing failure rate would indicate an increased need for spares in the future.
Simply maintaining stocking levels base on previous counts of failures may over or under stock expensive or critical spares.
3. Determine Priorities for Improvements
If there are two elements of product both showing an increasing failure rate, which do you address first? The one increasing at a faster rate, right? Another consideration is the severity of consequences of a failure and magnitude of failures. Yet when all else is even the one with a higher rate of increase should receive your attention first.
4. Identify Potential Major Field Issues Sooner
When tracking field failures a common outcome of the analysis is to spot potentially epidemic or higher than expected number of fairies. By monitoring the rate of failure occurrences one can spot those increasing failure rates. An increasing failure rate, even if with just a few actual failures permits your team to investigate and understand the failure mechanism.
Finding major problems well before them become major is a good thing.
5. Effectively Conduct Predictive Maintenance or Prognostic Health Management
Using just an average failure rate provides little insight on what will actually occur tomorrow. If the goal is to repair or replace items just before they have an unacceptable chance of failing (or just before failure occurs), we need to understand that rate of change for the probability of failure.
We can use the combination of our understanding of the failure mechanisms and existing time to failrue data (or degradation measurements) to forecast when the best time for repair/replacement.
Let your data tell the story around the changing change of failure over time. It happens. It is very rare that an item or system has a constant failure rate – thus you need to monitor the changing failure rates to make better decisions, save money and time, plus improve the reliability of your system.