Availability is the ultimate measure of machine reliability. It tells you what percentage of scheduled time the machine is actually capable of producing parts. It is derived directly from two sub-metrics: Mean Time Between Failures (MTBF) and Mean Time To Repair (MTTR).
A CNC machining center was scheduled to run for 8,000 hours over the last year. Maintenance logs show 8 breakdown events that required a total of 40 hours to repair.
Divide 8,000 hours by 8 failures to get an MTBF of 1,000 hours. Divide the 40 hours of downtime by 8 failures to get an MTTR of 5 hours.
To find the availability: 1,000 ÷ (1,000 + 5) = 0.9950. Multiply by 100 for a final availability of 99.50%.
The math is absolute, but the data rarely is. Poor data collection practices will skew your metrics and give a false sense of security.
- MICRO-STOPSIf operators fail to log frequent 5-minute jams, your failure count is artificially low, which makes your MTBF look much better than reality.
- WAIT TIMETrue MTTR should include the time spent waiting for a technician or a spare part, not just the physical wrench-turning time.
- PREVENTIVEDo not count scheduled preventive maintenance (PM) in your breakdown failure count. MTBF is strictly a measure of unplanned failures.