Benchmarks #4

Mechanical Availability and Maintenance -- As mentioned above, both the Production Department and the Maintenance Department use mechanical availability – but in different ways. For Maintenance, mechanical availability is an indicator of overall maintenance system performance and effectiveness.

Mechanical availability is a function of MTBS and MTTR. The formula showing the relationship follows:

MA% = [MTBS / (MTBS + MTTR)] x 100

(Note: Although this availability formula looks different than the one given for the Production Department, namely MA% = [Operated Hours / Operated Hours + Downtime Hours] X 100, they are the same. Substitution of the factors for MTBS and MTTR into the “Production availability formula” will yield the “Maintenance availability formula”.) Because of this mathematical relationship, if any two of the three factors are known, the third can be calculated (solved for). This means data only has to be collect for two factors instead of three. The third factor can be established mathematically instead of trough additional data collection.

In addition, when mechanical availability changes, this mathematical relationship shows which of the other two factors, MTBS or MTTR had the greatest influence upon that change. If MA, MTBS and MTTR are graphed together it is easy to determine which factor is having the greatest impact. This allows management to react appropriately to changes in availability.

Graph #5 (above) illustrates how MTBS and MTTR impact upon availability. For example, in November and December, the drop in availability can be attributed to a lowering of MTBS (reliability) with little change in MTTR. To understand what occurred in these two months we would need to look at a Pareto analysis of repair or downtime incidents (frequency) but not worry about repair duration.

In April and again in August, low availability resulted from a drop in MTBS and an increase in MTTR. To understand what took place in these two months we would have to analyze both downtime frequency and repair duration (repair turnaround time). We need to know what lowered reliability. We also need to know if MTTR increased because of inefficiency or because of efficiently done, but long duration jobs.

As the previous examples show, Mechanical Availability, MTBS and MTTR are important equipment management benchmarks. Individually they tell a mine about equipment reliability, workshop efficiency and the ability to reach production objectives. Collectively they allow management to quickly and efficiently focus upon problems and move toward resolution.

... to be continued.