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Gearbox Preventive Maintenance is a Priority

Power Engineering Magazine, November, 2002 — WHEN POWER PLANT equipment fails a plant manager's biggest concern is getting the equipment back into service as quickly as possible. However, equally important is discovering why the equipment failed, and if the type of failure can be prevented in the future.


Effective lubrication is extremely critical to all gearboxes and proper lubrication will help prevent gear and bearing failures. Many gear and bearing failures result from insufficient or interrupted lubrication. Maintaining proper lubrication includes using the proper lubricant, keeping oil clean and free of foreign materials, and maintaining a sufficient supply of lubricant. Selecting the correct lubricant will depend upon the gear type, load, speed, operating temperatures, input power and reduction ratio


Lubrication problems can cause several gear problems. Scoring and galling are generally caused by oil film breakdown resulting in metal-to-metal contact. The resulting high temperature causes tooth surface damage. Continuing to operate the gearing without adequate lubrication will eventually degrade the gear's tooth profiles to the point where replacement is the only remedy. Abrasive wear is also often the result of foreign materials present in the lubricant.

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Analyzing Problems

Maintenance professionals have several tools at their disposal for anticipating and diagnosing gearbox lubrication problems. One of these tools is oil analysis. By analyzing particulate content and concentration in the oil, engineers are able to monitor the condition of an operating gearbox and the lubricant's condition. Likewise, 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. However, it might also be indicative of lubrication problems caused by corrosive or improper lubricant additives.


All power plant equipment generates specific vibration profiles and their frequency depends upon a machine's geometry and operating speed. By analyzing shaft vibration, engineers are able to determine whether the cause of the machine fault is imbalance, misalignment, wear, bearing defects and/or gear defects. Once equipment starts vibrating, damage to other ancillary equipment including piping can occur. Misalignment and wear are other problems with gearing systems. When misalignment occurs cracks can develop at the end of a gear tooth causing helical and bevel gearing to fail.


Wear caused by vibration can cover a broader range of gear damage, from scoring and galling to abrasive wear and plastic yielding. Plastic yielding can occur on gears subjected to heavy, continuous load, as well as gears subject to intermittent heavy loads or overload.


Reducing Maintenance Costs

A lubrication problem nearly caused a catastrophic failure in a gear reducer at a coal-fired power generating station in the mid-Atlantic. Over time, lack of lubrication caused asphalt buildup on two of the plant's ball mills, clogging the gear reducer's lubrication system. This resulted in excessive wear and considerable asphalt buildup on the roots of the gear, pinion teeth and pinion drip pan. These failures were so significant that they also damaged the mill pinions and gears.



All gearing should be inspected for wear.

Removing the pinion and damaged gear and repairing and reinstalling it is a very time consuming, labor intensive and expensive process. As a result the plant looked at alternative solutions to reduce the repair cost and downtime. In consultation with Philadelphia Gear, it was determined that the gear could be reconditioning without removing it from the unit.


After refurbishing, including the recommendation of a graphite-based grease for lubrication, the mills have run problem free. The temperature profiles across the gear ranged from 144 F to 155 F and the vibration levels were reduced by 50 percent. Prior to refurbishing the temperatures ranged from 155 F to 182 F. Because of the success of refurbishing the first unit, the plant is looking at repeating the same procedure on a second unit in 2003.


Another project involved a major city in the Southeastern United States that was using gas turbine generators to supplement their power grid during times of peak usage. These units, installed nearly three decades ago, had received very little maintenance and only periodic oil refills. Because of inadequate maintenance, the rotating elements, bearings and pinions all suffered from metal fatigue and severe pitting. Ultimately the bearings in one of the units failed and the subsequent vibration destroyed the gears.


After the gearbox was removed it was sent to a Philadelphia Gear Regional Service Facility for inspection and cost/benefit analysis. The inspection revealed that repairing the gearing and bearings within the unit would cost $100,000 and take up to 20 weeks to repair. The alternative was a new gas turbine that would cost twice as much as the repair and take 30 weeks to deliver and install. While the problem was one of improper lubrication, vibration caused the most damage.


Establishing a PM Program

While identifying the cause of equipment failures can sometimes be as simple as looking closely at the damage, discovering the root cause of such a problem is considerably more difficult. In some instances plants do not have the sophisticated equipment needed to identify shaft vibration anomalies or analyze oil samples for foreign materials. However, one solution is outsourcing the preventive maintenance (PM) functions. Outsourcing PM allows plants to focus on their own core competencies, the operation of the plant.


When selecting a preventive maintenance provider, there are several key services that should be included in any agreement. Any agreement should cover breakdowns, scheduled maintenance, parts reconditioning, service upgrades, reverse engineering, alignment and balancing, and on-site or off-site diagnostic services. Providers should be thoroughly skilled in performing gearbox failure root cause analysis.


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. Such information will also help avoid making the same mistakes again.






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