Moving to Condition Based Maintenance

Wednesday, July 30th, 2014

As outlined in the April issue of Plant Engineering, “in the beginning there was preventative maintenance (PM). And PMs were good. Every week, month or year, a task was put into place that would improve the longevity of the machine, reduce the likelihood of a failure, and result in fewer money-grabbing breakdowns. Sounds great, right? But those regular, scheduled PMs come with a price tag of their own. Each time a task is performed, such as a filter changed, a pump or motor overhauled, or bearings lubricated, money is spent on replacement components, man hours, and often done during the available production time to perform the actions.

Predictive maintenance, (PdM) is a step up on the maintenance pyramid from preventive maintenance toward improved efficiency. Within the PdM strategy is the practice of condition-based maintenance (CBM). The concept here is to look, listen and feel the machines to know what needs to have service work and when. By doing so you will keep your finger on the pulse of the equipment and allow the maintenance staff to make sound judgments on when and what needs to be done.”

It goes on to say in the article Preventative Maintenance vs. Predictive Maintenance, “Condition monitoring, also known as predictive maintenance (PdM), is the application of condition-based monitoring technologies, statistical process control or equipment performance for the purpose of early detection and elimination of equipment defects that could lead to unplanned downtime or unnecessary expenditures. And generally speaking, you must conduct this while the equipment is in normal operation, with little to no process interruption. The purpose of these tools (vibration analysis, infrared thermography, motor circuit analysis, etc.) is to find defects not possibly found through previously available inspections methods, specifically while the machine is in normal operation.

Taking advantage of the available technology lets you accurately assess the condition of parts and the presence of defects heretofore impossible to detect. An example of the advantage these tools have in the area of quantitative inspections or sensory inspections is the use of vibration analysis to determine the presence of a defect on a rolling element bearing. Previously, mechanics and millwrights relied on “lift checks” to determine the amount of clearance in a bearing.

Unfortunately, this technique is only valid for bearing defects that resulted in the removal of material from the raceways of the bearing; this bearing would be pretty bad off to have thousandths of inches of play in it. Sub-surface fatigue is easily seen with vibration analysis and at this point in the failure propagation has resulted in no removal of material from the raceways. This is the most common example of the advantages of condition-monitoring technologies.

A failure modes, effects and criticality analysis (FMECA) can help you determine which inspection techniques should be applied, how often and with what degree of redundancy. Remember, the trick is to balance risk with rigor. How much risk you are willing to take with a given failure mode coupled with how much you are willing to pay for the inspection determines the appropriate strategy.”

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