Gearboxes play an important role in the everyday operations at processing and manufacturing facilities. For example, the steel and primary metal industry uses gearing components, such as precision shell pinion gearing, rolling mill drives and reducers, each engineered for large loads. In other industries, such as pulp and paper, gearing units ensure that sometimes difficult-to-maintain production machines are available to run continuously at high speeds.
For plant professionals, the ultimate goal is to achieve a return on investment (ROI) by maximizing machinery output, reliability and efficiency while minimizing downtime and operating costs. Gearing plays an important role in achieving that ROI. That's why a gearbox failure can be such a costly setback to overall plant operations.
When gearing equipment fails, the greatest concern is getting it running again. As important as getting it back online is discovering why it failed and how to prevent such a failure in the future. Oftentimes, plant managers and maintenance technicians aren't equipped to identify the root cause of such problems, which ultimately can lead to a recurrence.
An important first step in any preventive maintenance program is learning to identify the causes of gearing equipment failure. Then, one can take steps to avoid a repetition. Providing plant managers and service technicians with the knowledge to identify causes of gearbox failure allows them to establish an effective preventive maintenance program of their own.
Always important when there is potential metal-to-metal contact, effective lubrication is extremely critical to every gearbox. Proper lubrication helps prevent both gear and bearing failure. In contrast, many gear and bearing failures result from insufficient or interrupted lubrication.
Proper lubrication means following proper lubrication practices. These include using the correct lubricant, keeping oil clean and free of foreign materials, and maintaining a sufficient supply of lubricant. Because lubricant selection is based on so many independent variables , gear type, load type, speed, operating temperature, input power, reduction ratio , it's best to leave lube selection 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.
Ineffective lubrication causes several gear problems. Failures, such as scoring and galling, are generally caused by metal-to-metal contact. The tooth surface damage results from oil film breakdown and high temperatures. Continued operation without adequate lubrication degrades the gear's tooth profile 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 tools at their disposal for diagnosing gearbox lubrication problems. Oil analysis prevents problems by assessing overall equipment health. By analyzing particulate content and concentration in the oil, engineers can monitor the condition of an operating gearbox. Advanced oil analysis yields vital information about the lubricant's fitness for use.
Lube oil analysis can alert plant professionals to possible problems in the lubrication system. Equipment that exhibits frequent mechanical problems or can cause downtime if it fails needs to be checked regularly for lubrication problems.
Further, wear patterns on gears can reveal lubrication problems. Gear tooth "pitting," characterized by a large number of very small pits evenly distributed over the gear's working surface, is usually an indication of overload, but also may indicate a problem associated with lubricant choice.
By analyzing shaft vibration, one can determine whether the cause -- imbalance, misalignment, general looseness, wear, bearing defects, gear defects or some other unforeseen problem.
High radial peaks, low axial vibration, low harmonics or sinusoidal speed pattern in the time domain at shaft speed characterize imbalance, which can cause other faults to appear. Once a structure is vibrating, any number of ancillary components can loosen.
Misalignment can occur as offset (shafts are meeting square, but not on a common centerline), angular (shafts are meeting at a slight angle), or both. Misalignment can cause a fracture originating at one end of a gear tooth and propagating along 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 localized gear tooth loading and, later, a possible gear tooth fracture. A preventive maintenance program should include an inspection of bearings to ensure they have proper clearance and are in satisfactory condition. Checking proper adjustment is often part of such a program.
As well as revealing gear wear, vibration analysis can cover a broader range of 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 not only ensure equipment keeps running, but running at peak output. This often means rerating a gearbox for optimum output based on its application. A detailed review of a gearbox and its application identifies the rerate potential, and in many instances, a gearbox can be rerated by upgrading rotational elements without adversely affecting existing gearbox interface requirements.
If equipment is maintained irregularly, operated at off-peak levels, or worse, at levels exceeding maximum output recommendations, preventive maintenance can be used to calibrate machinery so it operates at optimum levels , before it becomes a problem.
Stop, look, listen
Compounding the problem, safety concerns often don't allow for inspection of the units while they're operating. Consequently, this limits inspection and repair routines, making cooling tower drive failures a mystery.
The PM program
Whereas identifying the cause of equipment failure sometimes can be as simple as looking at the damage closely, discovering the root cause of a problem is often considerably more difficult. The bottom line is that most plants don't have the sophisticated equipment needed to identify shaft vibration anomalies or analyze oil samples for foreign materials. Without these resources, it's difficult to establish a PM program.
Outsourcing preventive maintenance functions is certainly an option that allows plants to focus on core competencies, while letting experts, with access to both a strong knowledge base and the necessary equipment, handle maintenance and repair duties.
Several key services should be included in any service agreement. Repair and overhaul services are a necessity; all equipment will need maintenance sooner or later. Such services should cover breakdowns, scheduled maintenance, parts reconditioning, service upgrades, reverse engineering, alignment and balancing, and on-site as well as 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, rerate recommendations, repairs and spin test throughout the entire operating range should be part of the package. Failure analysis is another important service in the case of catastrophic failure.
Troubleshooting is another necessary part of a comprehensive preventive maintenance program. This should include assistance in identifying and resolving operational problems swiftly. Other services include vibration and oil analysis, the keystone monitoring techniques that are paramount in identifying equipment anomalies before they become a problem.
Any good PM contract should include a strong warranty. Also, as important as any warranty is proper gearbox installation to ensure years of trouble-free operation, provided that adequate preventive maintenance procedures are performed.
While identifying the cause of gearbox 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 also will 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 reducing equipment life-cycle costs.
Jules DeBaecke is vice president of engineering at Philadelphia Gear Corp. in Norristown, Pa. Reach him at (800) 766-5120.
Figures courtesy of Philadelphia Gear Corp.