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Ensuring The Inspections Will Catch the Defect Before A Functional Failure Occurs

Ever wonder how some organizations make their vibration or thermographic program work, and not only work but deliver huge results to their organization?  They use a systematic approach to establishing the correct frequencies of inspection.   Establishing the correct frequencies of maintenance activities is critical to the success of any maintenance program.   Too infrequently and the organization is subjected to failures, resulting in poor operational performance.  Too frequently, and the organization is subjected to excess planned downtime and an increased probability of maintenance induced failures.

This article will continue the discussion on establishing the correct frequency in a maintenance program.  There are three different approached to use, based on the type of maintenance being performed;

• Time-Based Maintenance
• On-Condition Maintenance
• Failure Finding Maintenance

This article will focus on On-Condition Maintenance.  While establishing the frequency for Fixed Time Maintenance activities is complex and is more of science, establishing the frequency for Condition Based Maintenance inspections (or On-Condition) is a mix of science and art.

Construct the P-F Curve & Establish the P-F Interval

The first step to determining the inspection frequency for on-condition tasks is to construct the P-F curve and P-F interval. Constructing a P-F curve requires recording the results of the inspection and plotting the result versus the elapsed time.  If enough measurements are taken, a fairly consistent curve can be developed for each failure mode. Making sure that the data is gathered carefully and consistently will aid in increasing the quality of the P-F curve.   Lets use an example from RCM2;

• The tread depth on a tire is directly related to the linear distance traveled.  Based on the data collected, it is safe to say that for every 3000 miles the tire wears 1mm.  So for a tire with 12mm tread when new, a potential failure point of 3 mm and a failure point of 2mm, the P-F interval is 3,000 miles.

Now this works quite well for linear P-F curves because it is predictable.  So how do you construct a P-F curve for a non-linear failure mode?  It is a bit more complex, and a bit more of art.  Let’s use another example;

• A bearing will operate with minimal vibration under normal operations.  As a defect materializes, the vibration will increase exponentially as the defect gets worse.   While the P-F Interval will be the time (or operating cycles) from the point the defect can be detected (potential failure point) to the point it becomes a functional failure, its rate of deterioration will increase dramatically towards the end of its life.  This can be quantified just as the tire in the above example, with the right data.

With P-F curve and P-F Interval (PFI) established, the frequency can be determined.

Select the Right Frequency for Inspection

Once the P-F Interval (PFI) is established, the inspection frequency can be determined.  Thankfully it is not as complicated as establishing Fixed Time Maintenance frequencies.  To determine the inspection frequency, the formula is either PFI/3 or PFI/5.

• Standard Inspection – the frequency of inspection for most equipment should be approximately 1/3 of the P-F interval (Formula = PFI/3).  For example, a failure mode with a P-F interval of 3000 miles should be inspected every 1000 miles.
• Critical Equipment Inspection – the frequency of inspection for critical equipment should be approximately 1/5 of the P-F Interval (Formuala = PFI/5).  For example, a failure mode on a critical piece of equipment with a P-F interval of 3000 miles should be inspected every 600 miles.

Now the above works well for linear P-F curves, so how do you establish the frequency for the non-linear curves?  You use the same approach as above for the initial inspection frequency.

However, once a potential failure is detected, additional readings should be taken at progressively shorter intervals until a point is reached that a repair action must be taken. For example; the initial inspection frequency is every four weeks.  Once a defect is detected, the next inspection will be at three weeks, then two weeks and then ever week.

This is only guidelines and should be adjusted based on the method used to track and trend data, the lead time of the repair parts (if not kept on site), and how quickly the data will be analyzed, and the repair work planned.  If your planning process is poor, the frequency should be more frequent, to allow for a high chance of detection sooner.

How much thought was put into your Condition Based Maintenance inspection frequencies?  Have you broken down each failure mode trended the data and established the frequency using a systematic approach?   As with the Fixed Time Maintenance activities, you may be over or under inspecting, costing your organization reliability or money.

Remember, to find success; you must first solve the problem, then achieve the implementation of the solution, and finally sustain winning results.
I’m James Kovacevic
Eruditio, LLC
Where Education Meets Application
Follow @EruditioLLC

References;

• RCM2 by John Moubray
• Establishing Fixed Time Maintenance Frequencies