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A New Look at Criticality Analysis for Machinery Lubrication

A good read for Machinery Engineers regarding machinery lubrication
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  (  A New Look at Criticality Analysis for Machinery Lubrication Jim Fitch (/Authors/Detail/1), Noria Corporation Tags: maintenance and reliability (/Meta/Tags/maintenance and reliability)For decades, reliability scholars have been stressing the importance of prioritizingnew maintenance thrusts and investments based on need. The word they like touse is "criticality." For any given machine, how critical is its reliability? What if it failedsuddenly and catastrophically? What would be the consequences - lost production, expensive repairs, fatality?Criticality is the logical starting point for all reliability initiatives.There are many different ways to enhance reliability and improve the quality of maintenance. The best options should be risk-based. After all, if it doesn’t reducerisk, why do it? Why spend an incremental dollar to enhance a machine’s reliabilityif it doesn’t yield multiple dollars in return?There’s also priority. What should be done first, second and third, and what shouldnot be done at all? How do you know which machines return big dollars for enhanced reliability, which machines return marginal dollars and which machinesreturn nothing at all?Once you understand machine criticality and a machine’s risk profile, you can worksmarter to customize improvements. For guidance, look to the Pareto principle,which states that 20 percent of the machines cause 80 percent of the reliabilityproblems. Which machines are these?In addition, consider that 20 percent of the causes of failure are responsible for 80percent of the occurrences of failure. Which causes are these? It’s about precision -precision maintenance and precision lubrication. It’s also knowing how to makewise, risk-informed choices.  I’ve written previously about the Optimum Reference State (ORS). This is theprescribed state of machine configuration, operating conditions and maintenanceactivities required to achieve and sustain specific reliability objectives. As stated,defining the ORS requires a definition of the specific reliability objectives for a givenmachine. Defining the reliability objectives demands an understanding of failuremodes and machine criticality.This reminds me of the plant manager who told me years ago that he decided thebest way to solve his lubrication problems was to put synthetic lubricants in everymachine. Do you think he got the result he sought? Does paying a premium for synthetics guarantee a premium return in machine reliability and maintenance costreduction? Do synthetics offer forgiveness for negligent and shoddy maintenance?Is this wise decision-making? Understand the Reliability-Risk Connection The probability of machine failure needs to be inversely proportional to risk. There’sno better example than commercial aviation. Because the consequences of failureare extremely high (death), the probability of failure must be equally low (extremereliability).It is the only practical means to hedge risk. Those responsible for maintenanceusually have little control over the consequences of failure (often limited only toearly detection technology). However, reliability maintainers frequently haveconsiderable control over the probability of failure. Indeed, you can use risk andcriticality to develop a master plan for lubrication-enabled machine reliability. Thiswill be the focus of this article.Let’s begin with a list of common lubrication and oil analysis decisions (all attributesof the ORS) that can be customized (optimized) by understanding failure modesand machine criticality:Lubricant selection, e.g., premium vs. economy-formulated lubricantsFiltration, including things such as filter quality, pore size, capture efficiency,location and flow rateLubricant preventive maintenance (daily PMs) and inspection strategy  Lubricant delivery method selection and use (e.g., circulating, auto-lube, mist,etc.)Oil analysis (which machines are included and which are not?)Oil sampling frequency (weekly, monthly, quarterly, never)Laboratory and test slate selectionOil analysis alarms and limits All of these decisions and activities must be within the scope of the OptimumReference State. For this reason, the importance of criticality should not be takenlightly. However, a practical means of assigning a value to criticality, customized tomachine lubrication and tribology, has largely been elusive. In fact, the fields of lubrication and tribology raise unique issues and questions related to criticality thataren’t typically addressed and aren’t common to other types of machinery. Calculating Overall Machine Criticality Overall Machine Criticality (OMC) is a risk-profile assessment that can becalculated to a single numerical value. The OMC is what you seek to know andcontrol. The lower the OMC, the lower the risk. The OMC is the multiplied productof two factors: the Machine Criticality Factor (MCF) and the Failure OccurrenceFactor (FOF).The MCF relates to the consequences of machine failure, which combines bothmission criticality and repair costs, while the FOF relates to the probability of machine failure. This probability is highly influenced by maintenance and lubricationpractices and therefore is far more controllable.    Figure 1. Machine Criticality Factor (MCF) (Relates to the consequences of machine failure) Machine Criticality Factor   A simple method for estimating the Machine Criticality Factor is shown in Figure 1.It requires an understanding of mission criticality and repair costs. While you couldcall these SWAGs (educated guesses), it is far better to guess using a logicalmethod than to apply dartboard science or do nothing at all.The MCF is scaled 1 to 10, with 10 corresponding to extreme criticality (high risk).You start by answering the question of mission criticality. Machines that areprocess-critical can accumulate huge production losses as a result of sudden andprolonged failure.Extremely high mission criticality relates to safety (injury or death). In the eventthere is minimal business interruption or safety risk, there might still be high repair costs. Although many processes have redundant systems or standby equipment inthe event of failure, these systems don’t mitigate the cost of repair, which can bemillions of dollars in some circumstances.    Figure 2. Use this table to determine the Failure Occurrence Factor, corresponding to the probability of failure. The final consideration is the current or potential use of early detection technology(predictive maintenance) to annunciate alarms of impending or precipitous failureevents. In such cases, both downtime and the cost of repair can be substantiallyreduced. Oil analysis (wear debris analysis), vibration analysis, bearing metaltemperatures, proximity probes, motor current, etc., are all technologies that canoffer real benefit in reducing the Machine Criticality Factor (see the adjusted scaleat the bottom of Figure 1, which applies only if effective early warning systems areused). Failure Occurrence Factor   As mentioned previously, the Failure Occurrence Factor relates to the probability of machine failure. This can be estimated from the machine’s failure history or statistical analysis of a group of identical machines. Machines that are inherently
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