Preventive Maintenance Ratio—Is there Such a Thing as the Correct Proportion of PM Work Orders?

Is there a perfect preventive maintenance ratio of inspection checks that leads to minimizing equipment failure?

How often should Preventive Maintenance inspection work orders be done to prevent breakdowns?

Below is an email thread discussion about the Preventive Maintenance ratio of PM work orders to aim for in a maintenance program.




We are revising our maintenance KPI’s to improve focus on PM and PdM work.

I plan to measure:

1. Percent of work scheduled that is PM/PdM.

2. PM work order compliance. Percent done to schedule.

3. Percent of work orders that are a result of PM / PdM checks. Thinking this will be a good measure of PM effectiveness from perspective of they are being done, the right things are being looked at, and the data is being captured.

4. Ratio of PM /PdM to Break-in / unplanned work.

My questions to you are:

1. Do you agree with these KPIs?

2. Are there others you would recommend?

3. What would you recommend for targets? (i.e. percent of work scheduled that is PM / PdM should be 40% +/- 5% )

Thank you, Bill



Hello Bill,

Your list of preventive maintenance KPI measures are commonly used.

I am skeptical that there is a stable Preventive Maintenance ratio that automatically gets you a perfect maintenance program.

One comment is there is nothing about equipment reliability and improving your plant’s reliability. What about number of work orders that improved reliability and productivity?

A second comment is the measures are lagging indicators—they tell you about the history of your maintenance. You also need leading indicators that tell you about the preparedness and likely performance of your maintenance.

You should also be measuring workmanship quality and whether your maintenance work is causing early life failures. I’d be tracking how often the same maintenance repair recurs, or the same parts are replaced on each machine.

All the very best to you,

Mike Sondalini | LRS Consultants Global



Thanks Mike and fully agree. Working on leading indicators as well as these. Hadn’t thought of tracking repetitive repairs though, thanks for that idea!

Are you a fan of the 6:1 PM rule? It seems to make sense. I need a target for Percent of all work orders that are a result of PMs / PdM. Any thoughts?



Dear Bill,

The 6:1 PM Rule of preventive maintenance ratio to corrective work done may have seemed like a good thing to do in the 1980’s. If you didn’t have any historic parts replacement data to model the spread of the failure dates, then it would have been a place to start.

In the 6:1 Rule, the purpose of a condition inspection PM is to find degradation and rectify the situation before failure. The theory is you inspect at a frequency where you find degradation once in every six checks. Thus you get the 6:1 preventive maintenance ratio. That seems wasteful to me, as on average you are doing five preventive maintenance checks which won’t reveal anything except that all is okay to continue to the next check.

It also doesn’t fit well with the condition monitoring rule-of-thumb to set predictive maintenance (PdM) work order frequency at a quarter of the P-F window interval so you detect the potential failure point (P) with hopefully three-quarter of the P-F time period remaining before the component reaches its functional failure point (F). It means you keep doing the PdM checks at the set frequency until a P point condition is identified. The 6:1 PM Rule is meaningless in PdM strategy because inspection frequency is dependent on the P-F interval.

The 6:1 Rule applies even less if you have equipment with on-board mounted condition monitoring sensors that read condition continuously. Such equipment is permanently monitored and when the alarm arises it’s because degradation is already confirmed.

Best practice in the 21st Century is to use your physical asset management, operating, and maintenance processes to stabilize the equipment degradation rate so you “know” when a part is coming up for preventive replacement. But that takes a big mind-set and skill-set change in a company. Though there are serious operating profits to be made if you can get that level of control in your business.

To minimize the number of PM inspections you can use a simple statistical analysis of when potential failure points are reached. You won’t schedule a PM inspection until near the date of the P point. From then on you schedule inspections at a quarter of the P-F interval. You need several data points for the same component, 10 historic points is ideal, so you can determine the distribution curve from shortest to longest time between P points. Depending on the severity and consequence of a failure, and the spread of the historic P points, you set your first inspection date. If you can’t afford a breakdown, then the first inspection period is set at slightly shorter than the least historical time to a P point. Because you can’t know the exact date of a P point, you keep checking from then on at a quarter of the P-F interval. If you can accept an occasional breakdown, such as in a batch plant operation where you can make-up the lost downtime, you could extend the inspection frequency nearer to the mean time, and see how lucky you get.

In the research I also came across the 10% PM Rule were you must do PM’s within 10% of the scheduled date, . That’s probably another KPI worth tracking.