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Preventive maintenance: an examination of the root causes of gearbox failure

By Steve Luchetta, Director of Engineering, Philadelphia Gear Corporation


When equipment fails, often a plant manager's biggest concern is how to get the equipment running again. However, equally important to getting it back online is discovering 1) why the equipment failed, and 2) how such a failure can be prevented in the future. But often, plant managers and their employees are ill equipped to identify the cause of such problems, ultimately leading to their recurrence.


Preventive maintenance is defined as the regular performance of equipment maintenance practices in order to avoid future equipment problems. An important first step in any preventive maintenance program is learning to identify the causes of equipment failure. Once the cause of failure is determined, one can take steps to avoid the problem in the future. That is the purpose of this paper - to provide plant managers and service technicians with the knowledge to identify causes of gearbox failure (a crucial link in the power transmission chain) with the hope that it will lead them to establishing an effective preventive maintenance program of their own.

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Always important when there is potential metal-to-metal contact, effective lubrication is extremely critical to all gearboxes. Proper lubrication will help prevent both gear and bearing failure. In contrast, many gear and bearing failures result from insufficient or interrupted lubrication.


Maintaining proper lubrication necessitates following proper lubrication practices. These include using the proper lubricant, keeping oil clean and free of foreign materials, and maintaining a sufficient supply of lubricant. Because selecting a lubricant is based on so many independent factors - gear type, load type, speed, operating temperatures, input power, reduction ratio - choosing a lubricant should be left up to a gear lubrication specialist. This is especially true when you consider the technical sophistication found in gearing today, along with increased speeds and loads, and the specialized lubricants and additives now available.


Lubrication problems can cause several gear problems. Failures, like scoring and galling, are generally caused by oil film breakdown resulting in metal-to-metal contact, and high temperatures resulting in tooth surface damage. If a gear continues to operate without adequate lubrication, damage will progress until the gear's tooth profiles are degraded to the point where replacement is the only remedy. Further, abrasive wear is often the result of foreign materials present in the lubricant.


Maintenance professionals have several important tools at their disposal for diagnosing gearbox lubrication problems. One of these tools, oil analysis, plays a crucial role in not only preventing such problems, but in assessing the overall health of equipment. By analyzing particulate content and concentration in the oil, engineers are able to monitor the condition of an operating gearbox. Further analysis of the oil yields vital information concerning the condition of the lubricant used in the equipment.


Analysis of oil used for lubrication can alert engineers to possible problems within a lubrication system. Equipment that has exhibited frequent mechanical problems, or which would cause an outage if it fails - such as compressor drives - needs to be checked regularly for lubrication problems.


Further, lubrication problems can 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 indicative of lubrication problems caused either by some corrosive medium within a lubricant, or by improper lubricant additives.



Vibration - the motion of a body about a reference point caused by an undesirable mechanical force - is a key indicator in the diagnosis of machine faults. Each machine fault generates a specific vibration profile, and a single vibration measurement provides information concerning multiple components. The frequency of the vibration is determined by the machine geometry and operating speed.


By analyzing shaft vibration, engineers are able to 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. High radial peaks at 1x shaft RPM, low axial vibration at 1x shaft RPM, low harmonics of shaft RPM, and 1x RPM sinusoidal pattern in the time waveform characterize imbalance. Imbalance can cause other faults to appear. Once a structure is vibrating, any number of ancillary components can become loosened - hardware and piping are good examples.


Misalignment is the deviation from a common centerline during operation. Misalignment can occur as offset (shafts are meeting square, but not on a centerline), angular (shafts are meeting at an angle from one another), or both. Gear damage caused by misalignment is visible as a fracture originating at one end of a gear tooth, occurring on a diagonal line. Misalignment is also a common cause of broken teeth on helical and bevel gears.


Often, misalignment is the result of loose bearings, resulting in shaft deflection and, later, a possible gear tooth fracture. Part of any preventive maintenance program would include the inspection of such bearings to ensure they are running with the proper clearance and are in satisfactory condition. Checking proper adjustment is often part of such a program. Otherwise, significant gear damage is possible.


Wear is another fault determinable by vibration analysis, and can cover a broader range of gear damage - from scoring and galling, to abrasive wear, to plastic yielding.


Plastic yielding - a severe flow of surface material resulting in lip ledges at the end of gear teeth - may occur on gears subjected to heavy, continuous load, as well as gears subject to intermittent heavy loads or overload.



Preventive maintenance measures are taken not only to ensure equipment is kept running; these efforts are also made to keep equipment running at peak output levels. This often means uprating a gearbox for optimum output based on its application. By performing a detailed review of a gearbox and its application, engineers can determine the equipment's uprate potential, and in many instances, a gearbox can be uprated by upgrading rotation elements, without adversely affecting existing gearbox interface requirements.


In contrast, lack of preventive maintenance often results in the opposite. Equipment in use from different manufacturers, and maintained irregularly, often is not operating at peak levels, or worse, is used at levels exceeding maximum output recommendations. In these cases, preventive maintenance practices can be taken to calibrate machinery so that it is operating at optimum levels - before it becomes a problem.



As potentially damaging as equipment failures, environmental factors often place equipment under strains that it was not designed for - whether it is corrosion from a humid environment, or lack of maintenance due to inaccessibility.


Lack of accessibility to the cooling tower drives (due to the fact that elevation of cooling towers often exceeds 80 feet) sometimes leads to poor-to-nonexistent maintenance procedures. In addition, equipment found in moist, humid environments is more susceptible to failure due to corrosion, as moisture inside the gearbox can eventually accumulate and destroy the bearings.


Compounding the problem, safety concerns often do not allow for inspection of the units while they are running. This certainly was the case with the oil refinery mentioned earlier. Consequently, this affected the inspect/repair procedures negatively and tended to render cooling tower drive failures a mystery.



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 is often considerably more difficult. The bottom line is, most plants do not have the sophisticated equipment needed to identify shaft vibration anomalies or analyze oil samples for foreign materials. Without these resources, how then can they establish a preventive maintenance program?


Outsourcing preventive maintenance functions to an outside service provider is certainly an option. Outsourcing these duties allows plants to focus on their own core competencies, letting experts, with access to both a strong knowledge base and a wide range of necessary equipment, handle maintenance and repair duties.


When selecting a preventive maintenance provider, there are several key services that should be included in any agreement. Repair and overhaul services are a necessity, as ultimately all equipment will need maintenance at one point or another. The key is to plan that maintenance downtime so that it does not negatively affect production. Such services should cover breakdowns, scheduled maintenance, parts reconditioning, service upgrades, reverse engineering, alignment and balancing, and on- or off-site diagnostic services. Providers should be thoroughly skilled in performing gearbox failure root cause analysis. For repairs, full disassembly and cleaning, inspection and measurement, engineering evaluation, uprate recommendations, performance of repairs and spin test should all be accomplished. In the case of catastrophic equipment failure, failure analysis is an important service.


Troubleshooting is another necessary service, as part of a comprehensive preventive maintenance program. Such troubleshooting duties include engineering assistance in the identification, and swift resolution of operational problems. Other important services (mentioned earlier) include vibration and oil analysis. These two monitoring techniques are paramount in identifying equipment anomalies before they become a problem.


Any good preventive maintenance contract should include a strong warranty on all equipment serviced. Such warranties often offer many of the services already mentioned, and are ultimately a benefit to the refurbished equipment. Also, as important as any warranty is the proper installation of a gearbox*. Proper installation can assure years of trouble-free operation, provided that adequate preventive maintenance procedures are performed.



While identifying the cause of equipment failure is only the first step in establishing an overall preventive maintenance program, it is an important step. The information gathered will ultimately serve as the foundation for planning future preventive maintenance - a particular necessity when working with mission critical equipment. Such information will also help service technicians avoid making the same mistakes after initial equipment repairs. Once this information is determined, working with a service provider to establish a complete preventive maintenance program is important in maintaining equipment for future use, as well as lowering equipment lifecycle costs.




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