Services - Machine Diagnostic Technique  

Maintenance methods commonly employed in industry can be broadly categorized into three groups. In Industries, inexpensive machines and important process duplicated machines are usually run until they breakdown. Loss of production is not significant, spare machines can usually take over. This is called" RUN BREAK DOWN MAINTENANCE".

In chemical plants the product often has a very high value and because of this large unduplicated machines are sometimes "Run to Breakdown ". But in this case it is of vital importance to know what is going wrong and when the final breakdown can be expected.

This information can be obtained by analyzing vibration level trends built up from regular measurements. Knowing what is going wrong will allow the plant engineer to collect the necessary spare parts together will ahead of time, and thereby avoid a large stock of spare part. Furthermore, the maintenance personnel are better prepared and can be expected to effect a more reliable repair in a shorter time.

Where important machines are not fully duplicated or where unscheduled production stops can give large losses, maintenance operations are often performed at fixed intervals such as every 3000 operating hours or every year. These intervals are after determined statistically as the period during which the manufacturer expects less that 2 % of the machined to breakdown from being in a new or fully serviced condition. By servicing so often it is generally believed that, as will be a rare occurrence. This system is therefore called. "Preventive Maintenance ". Or rather, "Time Based Preventive Maintenance".

Let us introduce the term "Failure Rate", i.e. the percentage of machines that fail per unit of time. When we consider any single machine we have no idea how its actual failure rate is going to develop with time, therefore we must stop using the time based preventive Maintenance and instead treat each machine as individual. This method is practiced in so-called "on- condition Maintenance "where fixed interval overhauls are replaced by fixed interval measurements of the machine's running condition. The measurements can be admirable expressed in terms of vibration as previously described. This is called "condition Monitoring". The axiom of "on condition Maintenance "is that servicing is only permitted when measurements show it to be necessary. This is furthermore in agreement with most engineers' suspicion that one should not interfere with smoothly running machine.

By following the development of faults it becomes possible to extrapolate the data in order to predict when non- acceptable vibration levels will be reached and when the machine must be serviced. This is called "predictive Maintenance". It allows the maintenance engineer to plan the necessary maintenance operations well ahead in time. The ability to predict the probable period when the vibration level will exceed the maintenance limit, the red line depends to a large extent on the quality for the analysis performed on the vibration data.

Maintenance Techniques and policies

Maintenance is defined as " a combination of any actions carried out to retain an item in or restore it to an acceptable condition ". In other words, it also includes repair.

Types / classification of Maintenance activities

Condition - based maintenance is the 'maintenance carried out in response to a significant deterioration in a unit as indicated by a change in a monitored parameter of the unit condition or performance'.

It involves:

    1. DETECTION - spotting at sufficiently early stage an adverse change in the operating conditions of a component or a subsystem.
    2. DIAGNOSIS - Determination of the cause of causes of the adverse change.
    3. PROGNOSIS - Estimation of a subsequent future state of the system or component and the time scale and lastly.
    4. CURE - maintenance planning and execution.

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