I find the world of maintenance a very odd place to find MTBF. While it is possible, that a set of equipment or a machine may actually have a constant failure rate it is the exception rather than all that common. Assuming a constant failure rate doesn’t make it so.
Recently Ricky Smith posted a link to a copy of the 1978 Nowlan and Heape RCM Study (which is a very good read, btw). One of the elements of the study was the analysis of the failure rate patterns seen by United Airlines. A screen shot of one of the graphics shows six different patterns and they also list the percentage of events that exhibit each behavior.
Only 11% of the items in the study would benefit with preventative maintenance. Note this isn’t the lubrication or adjustment type PM, rather the replacing of parts that are soon to wear out and fail. The remaining 89% have the best strategy of replace upon failure.
I’ve seen this chart or similar charts in other books and presentations. While the numbers are interesting, they really only apply to those items the folks at United Airlines included in the study from the mid ’70s.
If you are running an airline using equipment from the 1970’s and United maintenance program, then you should be doing PM’s on about 11% of your equipment. If not, then you number if items to include will be different and most likely will be different.
Get the data, sort out the failure rate patterns and then make decisions. Simply assuming MTBF and a constant failure rate (or primarily blaming vendors!) is a pretty naive approach. Most organization keep some kind of record on when down time occurs due to equipment failure. Use that data to improve your maintenance program.
The overall RCM document is very good (how we use it sometimes certainly could be improved) and I highly recommend you including this study in your must read list. To make it easy, you can find the complete work here and it is possible to download.
Finally, check, recheck and use the data and results to create meaningful information. When I see a study like the RCM study and Exhibit 2.13 I take away there are at least six failure rate patterns and I need to find which of my equipment follows which pattern — then let’s talk about the appropriate preventative maintenance approach.