Equipment Metrics for Total Productive Maintenance

Equipment Reliability Metrics

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Metrics help to focus efforts on the most critical equipment rather than reacting to the crisis de jour. They measure progress and help to adjust efforts accordingly. They are critical for identifying and resolving specific problems. Equipment metrics can be surprisingly simple. Only three data elements, collected for each machine and analyzed properly, are really necessary for most situations.

This discussion is about the metrics for machine performance. It does not include metrics of maintenance department productivity, budgeting or cost allocation. Such additional metrics are required to operate a maintenance department effectively.

All these metrics are most effective in graphical form. They are not very meaningful as individual numbers. However, in the context of past and future, trends, anomalies and patterns reveal themselves.

All of the first four metrics, the most useful, derive from three numbers. Assuming a calculation period of one week, the following questions must be answered:

How many breakdowns (failures) did we have this week?
How long did each breakdown take to repair?
How many hours were scheduled for the equipment?

Calculations and tracking can be further simplified by assuming that each machine is scheduled for about the same production (say 40 or 80 hours) and simply using one week as the time bucket.

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Useful Equipment Metrics

Name	Symbol	Description	Formula
Failure Rate	(Lambda)	Failure rate is one of the simplest and most useful metrics for machine performance. It can be approximated by using a week, month or other convenient period in place of actual operating hours. If data is accumulated on a (say) weekly basis, the only input is the number of breakdowns during that week.
Mean-Time-Between-Failure	MTBF	MTBF is also a metric for machine performance. It is the inverse of Failure Rate and is thus calculated from the same parameters. It is a meaningful metric for long periods of time but not suitable for daily or weekly monitoring. If there are no breakdowns in a given period, the MTBF for that period is mathematically "undefined."
Mean-Time-To-Repair	MTTR	Mean-Time-To-Repair is another simple yet valuable metric for industrial maintenance. It reflects both the severity of breakdowns and the efficacy of repair activities.
Availability	A	Availability is the portion or percentage of time that equipment is available for operation. It is commonly referred to as “Uptime”. Availability is another useful metric for industrial maintenance and you will want to track it along with Failure Rate and MTTR. Availability derives from the same data collected for MTTR and Failure Rate. It is easy to calculate.
Reliability	R(T)	Reliability is the probability that equipment will complete a mission of length “t” without failure. It is an exponential function. Reliability has limited use for most industrial maintenance although it is important for military and other applications.
Overall Equipment Effectiveness	OEE	Overall-Equipment-Effectiveness is a new metric that has received considerable publicity in recent years. It attempts to capture all the parameters in a single measure. However, the practical application is somewhat limited.