When building a PM system, decide which Failures Are Significant
In my last post, Developing a Preventive Maintenance System - Step 1, I discussed how to make a list of the possible failures of a machine. We started by asking, what are the functions of the machine, and then how can it fail to meet those functions.
The next step is to ask: Which Of These Failures Are Significant?
Now that you have a list of possible failures, you want to decide which ones you should worry about. Some failures are so unlikely that you won’t worry about them; others have such a low consequence that their impact and cost is minor.
To make our decision, we can use risk. Risk can be defined as consequence times frequency of occurrence. By including both, we account for both the acute failures, that have a big impact but don’t occur often, and the chronic failures that individually don’t have much impact, but occur so often that the cumulative impact is large.
Consequence can be measured in dollars, downtime, lost widgets or product, or any other convenient measure, as long as you’re consistent. Frequency can be Mean Time between Failure, failure rate, or some other measure. Right now we’re just looking for a ranking.
I personally like to use dollars per hour of downtime and downtime hours per year. That gives me an annualized dollar value, easily understood by the C-suite.
Machinery history is the best way to determine how often a failure occurs and what its impact is. You do have one, don’t you?
However, we can do it without the history. I’ve had success in the past using a subjective evaluation. Make a list of the failures and ask two questions: how often does this occur and what’s the impact on production when it does. Make it up as a questionnaire. Possible answers are in Table I below. This may sound simplistic but it works.
Table I
|
Score
|
Frequency
|
Effect
|
|
1
|
1/10 yrs.
|
None
|
|
2
|
1/ yr.
|
A little
|
|
3
|
1/ month
|
Some
|
|
4
|
1/ week
|
A lot
|
|
5
|
1/ day
|
Complete
|
Now send the questionnaires to a cross section of maintenance, production and management personnel. When you get them back, average the scores for each item.
The significance of a failure is the combination of two factors: frequency and effect. By taking the score for frequency score (1 to 5) and multiplying it by the score for effect (1 to 5) you’ll get a composite score for each failure in the range of 1 to 25. Rank the list by the composite score. The higher the composite score, the greater the significance of the failure.
Now you have to make a judgment call — which failures should you worry about? Often, only a few will have a high rank and you can concentrate on them. Other times most will have a high rank. This is where your knowledge of the machine and professional judgment come into play.
The next post will look at avoiding those failures.
Key takaways:
Risk has two parts: frequency and consequence
Use risk to rank the failures
You can find risk without extensive history
Tell us what you think:
What do you use to rank failure or machine criticality?
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6 Steps to a Healthy Machine