Power Engineering, September 2008 - With commodity prices continuing to rise and budgets tightening, now more than ever preventative maintenance is integral to lengthening the lifecycle of gearboxes. When equipment fails, often the biggest concern is how to get it running again, and the premium costs associated with that. However, equally important is discovering the cause and developing an understanding of how, through preventative maintenance, future gearbox failures can be minimized.
An important first step in any preventative maintenance program is identifying the potential causes of equipment failure. By providing managers and service technicians with this knowledge, a crucial link in the power transmission chain, they can establish an effective preventative maintenance program that will ultimately lengthen the lifecycle of gearboxes and help avoid excessive repair/replacement costs.
Analyzing the Problem: Lubrication
Effective lubrication is extremely critical to all gearboxes and will help prevent gear and bearing failures. The keys to maintaining proper lubrication are using the appropriate lubricant, keeping oil clean and free of foreign materials, and maintaining a sufficient oil flow. Lubricant selection is based on many independent factors including gear type, load type, speed, operating temperatures, input power and reduction ratio, and should be left to a gear lubrication specialist. This is especially important due to the technical sophistication found in gearing today, along with increased speeds and loads, and the specialized lubricants and additives now available.
When lubrication problems do occur, they can cause failures like scoring and galling. Scoring and galling are generally caused by oil film breakdown, which results in metal-to-metal contact, and high temperatures, which cause tooth surface damage. If a gear continues to operate without adequate lubrication, the gear's tooth profiles will degrade to the point where replacement is the only remedy.
Maintenance professionals have several important tools at their disposal for diagnosing gearbox lubrication problems, the most prevalent of which is oil analysis. By routinely analyzing particulate content and concentration in the oil, engineers are able to monitor the condition of an operating gearbox and circumvent damage that may be beyond repair. Further analysis of the oil can yield vital information about the condition of the lubricant used in the equipment, alerting engineers to possible problems within the lubrication system.
Lubrication problems can also be detected by examining wear patterns on gears. Gear tooth "pitting" is characterized by a large number of very small pits, distributed evenly over the working surface of a gear. The appearance of such pitting is usually an indication of gear overload, but may also be symptomatic of lubrication problems caused either by some corrosive medium within the lubricant, or by improper lubricant additives.
Analyzing the Problem: Vibration
Vibration is another key indicator in the diagnosis of machine faults. Each machine fault generates a specific vibration profile, and a single vibration measurement can provide information concerning multiple components.
By analyzing shaft vibration, engineers can determine whether the cause of the machine fault is imbalance, misalignment, general looseness or wear, bearing defects, gear defects or some other unforeseen problem.
Imbalance is the force created by a rotating body when its center of mass is offset from its center of rotation. Imbalance can cause other faults to appear. Misalignment is the deviation from a common centerline during operation and can occur as offset (shafts meeting square, but not on a centerline), angular (shafts meeting at an angle from one another), or both. Gear damage caused by misalignment often presents itself as a fracture originating at one end of a gear tooth, occurring on a diagonal line. It is also a common cause of broken teeth on helical and bevel gears. Wear is another fault determinable by vibration analysis, and can cover a broader range of gear damage - from scoring and galling, to abrasive wear and plastic yielding. Left alone, any of these machine faults can cause enough damage to necessitate shutting down a process. The key is to analyze the vibration data on a routine basis.
Establishing a preventative maintenance program
While identifying the cause of equipment failure can sometimes be as simple as looking closely at the damage, discovering the root cause of such a problem can be considerably more difficult. Instrumenting the equipment solves only part of the problem. It is having the technical skill to interpret the data over time that has the biggest impact on reducing unplanned downtime. Unfortunately, many companies have reduced the number of in-house experts available to review such data. Without these resources, how can a preventative maintenance program be established?
Outsourcing preventative maintenance functions is certainly an option, and one that allows engineers to focus on their own core competencies, while letting experts, with access to both a strong knowledge base and a wide range of equipment, handle routine preventative maintenance and repair duties.
When selecting a preventative maintenance provider, several fundamental services should be evaluated as part of the program. Providers should be thoroughly skilled in performing gearbox failure root cause analysis, as well as have the ability to react to breakdowns, scheduled maintenance outages, parts reconditioning, service upgrades, reverse engineering, alignment and balancing and on- or off-site diagnostic services.
Troubleshooting is a significant component of a comprehensive preventative maintenance program. Such duties include engineering assistance in the identification and swift resolution of operational problems. Other important services include vibration and oil analysis, which are paramount in identifying equipment anomalies before they become a problem.
Any sound preventative maintenance contract should include a strong warranty on all equipment serviced. Such warranties often cover many of the services already mentioned, and are ultimately a value-add to the refurbished equipment.
The information gathered while analyzing the problem will ultimately serve as the foundation for planning future preventative maintenance - a particular necessity when working with critical equipment. Although a preventive maintenance program requires a sustained time and financial commitment, the investment will no doubt pay dividends if it ultimately averts a gearbox failure that requires expensive repairs or, in some catastrophic cases, outright replacement of the equipment. Preventive maintenance measures ensure that equipment will be kept running and that equipment will be kept running at peak output levels during critical production cycles.
Jules DeBaecke is the Vice President of Engineering at Philadelphia
Gear Corporation. For more information on developing a
preventative maintenance program please visit www.philagear.com/manuals_maintenance
and receive a free copy of the company's whitepaper "Preventative
Maintenance: An Examination of the Root Causes of Gearbox