Benchmarks #2

Mean Time Between Stoppages (MTBS)

Machine reliability is defined as the ability to operate for long periods of time without stopping for either maintenance or repairs. Reliability is often expressed (in engineering terms) as the “Mean Time Between Failures” (MTBF). In mobile equipment management, MTBF is, in reality, mean time between shutdowns or machine stoppages, not actual “failures”. Therefore, it is more accurate to use the expression “Mean Time Between Stoppages” or MTBS to express reliability for mobile equipment.

Reliability -- the prime measure of a maintenance department’s success at scheduling repairs and preventing failures is machine reliability -- expressed as Mean Time Between Stoppages (MTBS). It is arguably the single most important measure of success because reliability has a dramatic impact upon mechanical availability and therefore upon efficiencies in both production and maintenance.

Machine reliability (the MTBS number) has a significant and variable impact upon mechanical availability. Graph #1 (above) shows that as MTBS (reliability) falls, the impact upon availability becomes more pronounced. This illustrates why MTBS must be measured and why it must be kept in the range where the influence on availability is gradual and roughly a straight line (above MTBS = 60 hours) rather than in the range of accelerating negative impact (below MTBS = 60 hours).

MTBS is a direct measure of reliability. It is calculated by the following method:

MTBS = Operated Hours / Number Of Downtime Incidents (Stops or Shutdowns)

Operational stoppages (or shutdowns) such as at shift change, lunch, or fueling are not counted. Maintenance or mechanically related stoppages – including scheduled lube service during PM (but not daily servicing or oil level checks) are counted.

Many factors can effect MTBS. The following items are two of the most significant in terms of impact upon MTBS.

¨ Condition Monitoring – the effectiveness of detecting problems. If problems or potential problems are not detected and reported, the likelihood that failures will result is increased. Effective condition monitoring must be a high priority if good equipment reliability is to be achieved.

¨ Backlog / follow-up – the use of condition monitoring information. If the “system” does not respond to the findings of condition monitoring, problems or failures can result. Symptoms and potential problems must be analyzed so priorities can be established and failures or any stoppages can be avoided.

Many mines have begun to measure MTBS and to use it as a benchmark. MTBS can be used alone and it can to help analyze mechanical availability.

As can be seen in Graph #2, when MTBS is used alone it can be graphed over time (monthly, for example) to establish a trend in machine reliability. A graph such as this, leads to the conclusion that machine reliability is falling. As was shown in Graph #1, when reliability falls, the impact upon mechanical availability can be very significant. Therefore, a downward trend in MTBS signals the need for an investigation (probably a work order analysis) as to the reasons for the decline.

... to be continued.