Pocket Maintenance Advisor (eBook)

A Maintenance and Operation Crew best practices reference book

Mike Sondalini (Autor)

eBook Download: EPUB

2003 | 1. Auflage
85 Seiten
Bookbaby (Verlag)
9781618421517 (ISBN)

Infos

e-book contains thorough explanations of the 48 top issues in maintenance and reliability that people working in maintenance and operations must know. Every person in the maintenance department and the production department learns dozens of benefits overnight from reading this one reference book.

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It brings world class maintenance into perspective for maintenance and operations crews. Now in one book all the concepts that are necessary to grasp have been laid out in 'black and white' for maintenance and operations workers. With this book you can talk to them about the most important issues to address to achieve major improvements. It's a great resource for people who want their workers to rapidly learn, understand and appreciate the critical maintenance and reliability issues. It contains all the important information a maintenance, repair or operations crew need to have at their finger tips so that they can appreciate the importance and the methods of best practice maintenance. With 'The Pocket Maintenance Advisor' at hand they will know where they are going!

Textauszug

2.     Equipment Maintenance Best Practice.

2.1     Creative disassembly

This topic is the work of Peter Brown, Industrial Training Associates, www.itatraining.com.au

It would be rare for a machine to fail and not give some material or historical evidence of why it has failed. This material evidence must be looked at and some experiential opinion will be offered for the cause. Often the answers are already there. This process of Creative Disassembly is an important element of gathering the data that identifies the cause of premature failure.

All of us are problem solvers and, although we may be reluctant to see it as such, we are root cause analysts. At the end of the day we are all about preventing a repetition of the problem.

The collecting of information 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.

Creative Disassembly

A machine is overhauled or repaired because it is no longer serviceable, 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 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,

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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 pattern of problem distribution is much as shown in the neighbouring pie chart.

The three 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 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

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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 soft foot. Each hold down bolt is eased in turn and the change in vibration observed. Note that running soft foot is different to static soft foot; 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 pipe work 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 Root Cause Analysis 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,

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- 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 labeled with machine and location, so that the CM technician is aware of the make of the replacement item – this is critical for diagnostic purposes.

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2.2     Bolting and gaskets in flanged connections

Bolts and gaskets behave like springs and unless flange bolts and nuts are properly tensioned the flange will leak. The correct tightening torque depends on bolt diameter, material of construction, process pressure and bolt fabrication.

A bolt is not a bolt.

A bolt is not a bolt - it is a spring! When tightening a bolt you are tensioning or slackening a spring. The sketches below show how a spring can be considered to replace the bolt pulling the flanges together.

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In this article a bolt is referred to as a ‘bolt-spring’. The ‘bolt-spring’ must pull the flanges together more than the forces acting to push them apart. If the ‘bolt-spring’ is too loose, the pressure stretches the bolt and the flange leaks. To prevent the flanges separating, the bolts are preloaded (stretched). Bolt torque figures are calculated to produce a bolt stretched to at least 65% of its yield strength. Yield strength is the stress at which the bolt shank starts to deform permanently (Take it beyond this & it will snap off.).

A gasket is not a gasket

When a gasket is sandwiched between flanges it behaves both as a seal and a spring. The sketch below is the same as the previous except the gasket is now turned into a spring like material. In this article it will be referred to as a ‘gasket-spring’. A ‘gasket-spring’ will creep with time or temperature. Gasket creep is the gradual deformation (squeezing) of the gasket when sandwiched between flanges. The spring tension in the gasket slackens off. Gasket creep may require that flanges be re-tensioned periodically. It is often necessary to check the bolt manufacturer and the gasket manufacturer torque tables to select the highest torque to be used.

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To prevent a flange leak there are a few things to check. The following table will provide some guidance.

WHAT IS REQUIRED

WHAT YOU CAN DO

The bolt-spring shaft ought to be stretched to 75% - 80% of the material yield and in excess of the maximum bolt load.

Ask the bolt manufacturer for the maximum bolt torque and the number of turns from snug to achieve this or use load-indicating washers. As a last resort use a top quality tension wrench to the bolt manufacturer’s torque specification.

Bolts of sufficient tensile strength to take the process and bolting forces.

Check the bolt head forging marks against the bolt head manufacturer’s standards to insure the bolts and nuts meet the load requirements for the service.

Bolt tension must allow for cycling of loads, shock loads, shear loads and vibration.

Swap the bolt for one of higher tensile strength and pull it up tighter. Make sure all the bolts are to the same rating and...

Produkt-Details

• Erscheint lt. Verlag: 5.8.2003
• Sprache: englisch
• Themenwelt: Geisteswissenschaften
• Themenwelt: Sozialwissenschaften ► Pädagogik ► Erwachsenenbildung
• ISBN-13: 9781618421517 / 9781618421517