What is Predictive Maintenance (PdM)?

Alan R. Bessen, P.E.

 

Predictive Maintenance Service for aggregates operations is generally comprised primarily of Vibration Analysis and Infrared Thermography.

 

Vibration Analysis is used to monitor the condition of rotating equipment.  It can detect bearing and gear faults, drive train misalignment and other mechanical fixture problems that lead to premature failures. 

Measured vibration data is compared to fault frequency data provided by the manufacturer. This makes it possible to detect specific faults such as inner or outer race spalling, cage faults, ball or roller damage in bearings and excessive wear in gears.  Equipment condition is determined by measuring the vibration amplitude and observing the change in amplitude over a period of time.  It is generally accepted that vibration amplitudes in excess of .02 inches per second at a given fault frequency indicate that a problem exists.  The higher the amplitude the more severe the fault.  Many faults are detected early in their failure cycle so multiple readings and trending of fault data helps to accurately assess the need for timely action.  If follow-up readings indicate a slow rate of deterioration, repair or replacement may be deferred until maintenance can be budgeted and scheduled or until data indicates that the probability of failure has increased.

 

Infrared Thermography is used to locate and identify heat-related faults.  It is generally used on electrical equipment such as starters, breakers, transformers and overhead power lines.  Loose connections and overloaded circuits generate excessive heat that is visually indicated with the infrared camera.  These faults are not visible with the naked eye and are difficult to find without specialized equipment.  Most faults identified by infrared thermography are corrected by tightening loose connections or replacing undersized components.

 

Predictive Maintenance Data must be collected and analyzed periodically to identify new equipment problems and to assess the rate of deterioration on previously identified faults.  The interval will vary for individual plants depending on number of components monitored, initial equipment condition, production capacity, operating schedule, number of faults identified initially and the rate at which new faults are identified.  Generally, three assessments within the first year will provide sufficient data to determine the correct survey frequency for any plant.  In aggregates operations, survey intervals typically range from two to six times per year and often include a survey one to two months prior to a scheduled shutdown to assist in planning winter or major overhaul maintenance. 

 

A Comprehensive Maintenance Program includes: 

Reactive Maintenance                        …fix it when it breaks.

Reactive maintenance is part of every maintenance system but must never be the major method of maintenance.  It increases the cost of operating and reduces productivity, creating chaos.  Reactive maintenance eventually results in large expenditures for equipment overhauls, excessive unplanned downtime and premature equipment replacement.

 

Preventive Maintenance  …maintenance / overhaul scheduled based on hours operated.

Lubrication / maintenance and replacement intervals are scheduled based on manufacturer’s recommendation or modified based on operating history or predictive maintenance assessment.

 

Predictive Maintenance   …repair / overhaul scheduled based on actual equipment condition.

Mechanical and electrical condition of all critical assets are periodically evaluated using PdM technology.  Condition reports are used to plan and schedule maintenance or repair prior to failure.  Condition reports are also used to modify or verify preventive maintenance and overhaul intervals.

 

Proactive Maintenance    …cause of failure determined and eliminated.

Root cause failure analysis is employed to treat the actual problem and not the symptom.  Identified problems are eliminated by design or specification changes.

 

Predictive Maintenance Value comes primarily from the reduction of unplanned downtime accomplished by identifying and correcting equipment problems prior to failure.  The subsequent increase in equipment availability results in increased production capacity and lower production cost.  Repairing equipment prior to catastrophic failure often limits the repair to component replacement, extends its useful life, reduces repair cost and eliminates unnecessary / early equipment replacement.  PdM data is also used to accurately schedule the overhaul or replacement interval of equipment based on current machine condition and repair history.