Identify the causes of premature equipment failure

Creative Disassembly is an important element of gathering the data needed to identify the causes of premature equipment failure

Using Precision Maintenance saves money in repairs, reduces the need for maintenance, and gets maximum production on-time because there are fewer stoppages and slowdowns, so plant availability and productivity is maximised. Precision maintenance prevents equipment problems starting, it solves the equipment problems you have, and that means more production for less cost.

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The concept of Precision Maintenance is not new; organisations have practiced it since the 1980's; achieving outstanding production performance and maintenance cost reductions. A major factor in its successful implementation is to read the root cause failure message in the parts being replaced.
It would be rare for a machine to fail and not give some material or historical evidence of why it has failed. Unfortunately much of this, particularly the material evidence, is not looked at and some experiential opinion will be offered for the cause. Consequently many of the failures, machines and systems, repeat themselves, possibly until in desperation the consultants are brought in. So often the answers are already there.
All of us are problem solvers and, although we may be reluctant to see it as such, we are root cause analysts. Root cause analysis is seen as a different thing by different people. There are numerous methods from the quite simplistic to powerful software packages, but at the end of the day they are all about preventing a repetition of the problem being addressed.
While Root Cause Failure Analysis within a plant maintenance function is a primary focus for addressing large problems, remember that all large problems began as small ones once. We also need a process that eliminates the causes of failure; this is why Creative Disassembly is an important element of Precision Maintenance. Creative Disassmbly makes us gather the data that identifies the causes of premature failure so they can be eliminated as part of doing the maintenance work.
The collecting of information by Creative Disassembly for analysis does not start with the stripping of the machine; it begins once the need for repair is identified and advances along two fronts, the historical and the operational, or running characteristics. Further evidence is collected once the machine is stopped and before stripping.
Make Time for Creative Disassembly
A machine is overhauled or repaired because it is no longer servicable, it cannot perform the duties for which it is intended. To be confident that when the machine is returned to service it will do so reliably, it is necessary to identify the causes for the failure. All the evidence that is needed to achieve this will be present – the challenge is to obtain it and analyse it.
Where there is pressure for a machine to be returned to service with minimal delay – or sooner, there may not be adequate opportunity for this process.
The options in such a circumstance may be:

• for maintenance to negotiate with production for the time needed, bearing in mind the repair may have many of the same problems returned with it and there is a high probability of further premature failures,
• to accept a temporary repair subject to a scheduled proper repair. When stripped for the second repair the machine is likely to have some very useful evidence available, especially if not run to destruction.
• to apply additional resources aimed at gathering the evidence and analysing it as quickly as possible parallel to the repair process. It is possible to address many of the causes in this way. Others may be recorded for later correction.

In determining what to look for in Creative Disassembly keep in mind the nature of failures the machine is most likely to have suffered. For most industrial machines the problems are distributed equally between mis-alignment causes, out-of-balance causes and work quality control issues. The three creative disassembly phases of collecting evidence are;

• Prior to shutdown
• Shutdown, but prior to strip down
• Strip down

Pre-Shut Down
This is the time to gather to gather historical and background data from CMMS, operators and those who have worked on the machine previously.
There is certain data that can only be obtained whilst the machine is still in service;

• Vibration and Bearing characteristics, thermographic and oil wear debris data for diagnostic purposes. Operating conditions need to be correlated with this. This can have a considerable bearing upon identifying the defect processes that are present. There may be an opportunity to change some process variables which may give further insights to what is taking place.
• Checks for running softfoot. Each hold down bolt is eased in turn and the change in vibration observed. Note that running softfoot is different to static softfoot; it occurs because of the thermal condition and /or the dynamic forces present.
• Identify the presence of resonance in the machine, its base and supporting structure, and the pipework or other attachments.

At Shutdown, but before Strip Down
Before strip down begins there is valuable information that can be obtained;

• Where thermal growth may be an important factor for alignment considerations obtain a set of hot alignment readings. These are important not only for possible implication in RCA but for ensuring the data is used for future alignments in the cold condition,
• Look for witness marks such as cracked paint or shaft marks to indicate where there may have been relative movement taking place during operation,
• Deposited material indicating belt wear or coupling wear,
• Check for static soft foot,
• Sample lubricants prior to removal.

Strip Down

• Look for witness marks, evidence of fretting, etc
• Disassemble in clean and well lit areas
• Photograph damage if applicable
• Avoid damaging during removal
• Mark the relative locations of bearings in housings, top and side, inboard and outboard
• Inspection of bearings
• when removed, prior to cutting,
• cut the cage/retainer rather than springing it,
• cut outer race from top centre to bottom centre,
• reinspect prior to cleaning,
• filter solvents to see what is in the bearing,
• analyse bearing and ball path patterns,
• spalling patterns revealing poor fitting,
• fitted surfaces revealing fretting, out of roundness, etc.
• Gearing wear patterns - eccentricity, backlash, misalignment etc
• Pulley and Belt wear and damage patterns.

If time does not permit a proper examination of the bearings and other components prior to reassembly it is likely that the machine will return to service with the same problems still present. Ensure that these components are retained for later examination so that the problems may be recorded for future correction.
A good practice is to have a table set aside in the workshop with plastic bags and labels where removed bearings and other components may be retained for examination. The old bearing should be placed in the box of the new bearing, and labelled with machine and location, so that the Condition Monitoring technician is aware of the make of the replacement item – this is critical for diagnostic purposes.

Source: http://www.lifetime-reliability.com/free-articles/precision-maintenance/creative-disassembly-explained.html