Machinery Lubrication magazine published by Noria Corporation
Issue link: https://www.e-digitaleditions.com/i/120548
AS I SEE IT • Oil sampling frequency (weekly, monthly, quarterly, never) • Laboratory and test slate selection • Oil analysis alarms and limits All of these decisions and activities must be within the scope of the Optimum Reference State. For this reason, the importance of criticality should not be taken lightly. However, a practical means of assigning a value to criticality, customized to machine lubrication and tribology, has largely been elusive. In fact, the fields of lubrication and tribology raise unique issues and questions related to criticality that aren'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 be calculated to a single numerical value. The OMC is what you seek to know and control. The lower the OMC, the lower the risk. The OMC is the multiplied product of two factors: the Machine Criticality Factor (MCF) and the Failure Occurrence Factor (FOF). The MCF relates to the consequences of machine failure, which combines both mission criticality and repair costs, while the FOF relates to the probability of machine failure. This probability is highly influenced by maintenance and lubrication practices and therefore is far more controllable. Machine Criticality Factor A simple method for estimating the Machine Criticality Factor is shown in Figure 1. It requires an understanding of mission criticality Figure 3. An example of a pre-ORS Reliability Elements Quotient. 4| March - April 2013 | www.machinerylubrication.com and repair costs. While you could call these SWAGs (educated guesses), it is far better to guess using a logical method 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 are process-critical can accumulate huge production losses as a result of sudden and prolonged failure. Extremely high mission criticality relates to safety (injury or death). In the event there is minimal business interruption or safety risk, there might still be high repair costs. Although many processes have redundant systems or standby equipment in the event of failure, these systems don't mitigate the cost of repair, which can be millions of dollars in some circumstances. The final consideration is the current or potential use of early detection technology (predictive maintenance) to annunciate alarms of impending or precipitous failure events. In such cases, both downtime and the cost of repair can be substantially reduced. Oil analysis (wear debris analysis), vibration analysis, bearing metal temperatures, proximity probes, motor current, etc., are all technologies that can offer real benefit in reducing the Machine Criticality Factor (see the adjusted scale at the bottom of Figure 1, which applies only if effective early warning systems are used). 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 prone to failure