In predictive maintenance programs, strategy comes first

All rotating equipment requires optimum installation, running conditions, and maintenance in order to deliver long life at the lowest cost to the operator or machine owner. With increasing interest in proactive approaches, predictive maintenance (PdM) systems have gained significant ground as ideal methods for gathering comprehensive intelligence on machine conditions and production processes with the ultimate goal of increasing plant reliability by extending maintenance intervals and eliminating unexpected downtime.

The term predictive maintenance is used to describe a range of technologies to detect developing machinery faults at an early stage before they can become a problem, allowing for maintenance personnel to order parts in advance, schedule manpower, and plan multiple repairs during a scheduled downtime. After a repair is completed, PdM technologies can also be applied to repaired equipment to quantify repair efforts and confirm quality-repair results.

PdM systems for rotating equipment (both roller bearing and sleeve bearing machinery) can engage a full complement of sensor technologies, industrial computers, and analytical and data management software to capture, trend, and diagnose vital information on operating conditions of machinery assets, such as vibration, temperature, pressure, lubrication viscosity, and others which impact on the health of equipment.

Before a company makes an investment, however, a maintenance strategy should be in place. Decisions to apply PdM technology should further be prioritized according to the risks associated with equipment failure and the possible impact of failures on the safety of personnel, production processes, and environment. An essential assessment of equipment will serve to govern design, implementation, and management of PdM technologies in a particular operation.

Several PdM technologies have emerged as especially innovative or noteworthy. These include remote monitoring, operating deflection shape (ODS) analysis, operator-driven reliability (ODR), and lubricant analysis.

Excessive vibrations in machine housings and components can contribute significantly to the degradation of machine performance and premature operational service life. Operating deflection shapes (ODS) analysis provides a quick insight into the movements and associated problems of machinery as a function of frequency.

The ODS is measured with a machine at its normal operating condition and is intended to analyze the machine's response at a specific time or frequency. Both amplitude and phase information are collected at various locations on the structure and (with the application of analysis software) the vibrating "shape," or response of the machine, can be animated. These animations show the analyst "how" the machine is moving during normal operation and pictorially provides an immediate estimate of how much a machine is moving, where it is moving the most, and the area in which the deflection modes (flexible or rigid body) are present.

ODS data acquisition requires the technical skills inherent in universal vibration route methods used for data-logging vibration trend measurements. In the past, the measurements for defining structural deformations required a basic knowledge of modal analysis as well as an understanding of its practical applications. Now with the introduction of advanced software and hardware systems, a machine ODS survey can be accomplished by a defined route measurement sequence and use of analysis software program packages according to a structured procedure.

The procedure also requires technicians to position a reference accelerometer at an accessible machine location where vibration levels are relatively high. A second accelerometer can be designated as the roving transducer and positioned in accordance with a route sequence downloaded from the software. Triaxial accelerometers are favored to fulfill the role of roving transducers, because they can measure response in the three major planes without a need to move or reposition (promoting accuracy and saving time).

Operator-driven reliability (ODR) can extend and augment an existing maintenance program by enabling operators to become actively involved in basic maintenance activities. The ODR concept represents a framework for coordinating the activities of plant operations personnel with a company's reliability maintenance practices. Involving operators as key resources in a plant's reliability initiatives can deliver a cost-effective means to improving machine and process operation, taking advantage of the front-line people who are most often closest to critical assets and who have the ability to react quickly to changes when they occur.

Under ODR, operators perform basic maintenance activities such as cleaning, minor adjustments, lubrication, and other preventive and corrective tasks traditionally handled by the maintenance department.

Today's ODR programs use technology to the fullest, such as electronic clipboards and software that allow critical operational information to be shared easily among all plant personnel. More sophisticated systems incorporate a decision-support software system to equip an operator with immediate access to maintenance advice and proposed remedial actions.

As with any PdM process, lubrication inspection and analysis can help detect a problem and diagnose the source. Not all working lubrication inspections have to be performed in a laboratory. Many characteristics can be examined visually or otherwise easily onsite.

Clarity and water contamination can be observed in a standing sample; ferrous materials (filings, metal dust) can be detected using a magnet drawn up the side of a glass jar containing lubricant diluted with a solvent; flow and discoloration can be noted in a bull's eye sight glass; and viscosity can be monitored using simple inplant tools.

Beyond the day-to-day observations, though, lubricant analysis as a PdM activity should target at least three critical parameters:

• Machinery wear particles. Small wear debris can be measured by standard emission spectroscopy techniques. Large severe wear particles should be measured periodically, too.
• In machines where the predominant regime is hydrodynamic (full fluid film) and the wearing components are nonferrous bearing surfaces (such as with sleeve and pad bearings), rotrode filter spectroscopy (RFS) is appropriate. For machines with rolling element or steel gear component wear as the primary failure modes, the appropriate method is direct reading ferrography (DRF).
• Contamination. Selection of an analytical method for contamination will depend on the machine, lubricant, and environment. Contamination can be present in four different forms: gaseous, fluid, semi-solid, or solid. A rule of thumb is to select test methods relevant to the probability that a specific contaminant can enter the machine's lubricating system or be produced within the machine.
• Lubricant degradation. Standard analytical methods for measuring degradation include increases in viscosity or changes in alkalinity and/or acidity. When changes occur (from degradation and not contamination), the lubricant is overdue for changing, and sludge and varnish have already begun to form in the machine. These changes should be considered "condemning" (not "warning") limits.

As these few approaches suggest (and there are many more), users have a vast array of options from which to choose when deciding to embark on the PdM path. The recommended assessment by expert professionals is the best first step to keep machinery assets performing as intended.

Kevin G. Hardig is strategic accounts manager for SKF Reliability Systems. He can be contacted at 513-941-8254, or kevin.g.hardig@skf.com. Article edited by Richard L. Dunn, executive editor, 815-236-2196, or dunncomm@comcast.net.