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How to Maintain the Gearbox Lubrication System

By Jules DeBaecke


Power Transmission, February 2005 — For owners and operators of gearing equipment, the ultimate goal is to achieve a return on their investment. This is done by maximizing the output, reliability and efficiency of their machinery, as well as reducing downtime and operating costs.


Continued reliability, successful operation and long life of power transmission equipment are largely dependent upon the constant supply of lubrication oil of proper quantity, quality and condition. The lifeline of the gearbox is its lubrication system, which is critical for supporting the drive under all modes of operation.


The purpose of a gearbox lubrication system is to provide an oil film at the contacting surfaces of all working components to reduce friction and wear. In addition, the oil serves to remove and dissipate heat from where it’s generated, preventing gearing component temperatures from rising to excessive levels. Other lubrication functions include the transfer and/or removal of wear particles, as well as the filtration of rust, corrosion and any other undesirable contaminants.

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Failure of the lubrication system to perform any one or more of these functions, however, may result in premature breakdown of the equipment. Understanding the role and importance of a lubrication system in the overall life of a gearbox will help you maintain an effective system. It’s critical that maintenance professionals have the tools and knowledge to properly extend the overall life of their gearbox.


Understanding Lubrication

Lubrication can be defined as the control of friction and wear between adjacent surfaces by the development of a lubricant film between them, called an elastohydrodynamic (EHD) oil film. EHD film thickness is quite small, usually less than 1.25 micrometer (0.00005 inch). Oil film thickness is significant. If the adjacent surfaces aren’t fully separated, the EHD film leaves local areas of contact between those surfaces, making them vulnerable to surface fatigue.


Viscosity is a characteristic of fluids to resist flowing freely. It’s one of the most important characteristics of a lubrication fluid. The viscosity of lubricating oils changes appreciably with temperature, and is generally stated at two temperatures: 100 degrees Fahrenheit (40 degrees Celsius) and 210 degrees Fahrenheit (100 degrees Celsius).


Viscosity is usually expressed in terms of the time required for a standard quantity of a fluid at a given temperature to flow through a standard opening.


The fatigue life of contacting components of a gearbox, such as gear teeth and bearing rollers, is determined by a complex combination of speed, load, lubricant temperature, clearance and alignment. The lubricant’s role in this interaction is determined primarily by speed, viscosity and temperature. The effect of these factors on the fatigue life of elements can be dramatically altered at higher temperatures with lower viscosity and thinner resultant oil films. The selection of the correct lubricant for any application requires a careful study of expected operational and environmental conditions.


Lubrication Maintenance

There are two types of gearbox lubrication systems currently used: splash lubrication systems and force-feed lubrication. The intent of both types of systems is to distribute oil to each component of the gearbox sufficient for lubrication and cooling of that component, yet minimizing heat generation by oil churning.


Given the integral role that the lubrication system plays in the overall life and longevity of a gearbox, it must be continually maintained. This ensures the system is functioning at peak performance. It’s important to develop a systemic method of inspection, condition verification and documentation to avoid any unexpected lubrication system failures, and ultimately, equipment damage. The following are areas of concern for maintaining a properly functioning lubrication system:


- Cleanliness. Dust, dirt, grit and wear particles in the lubricant supply must be kept to a minimum. Filters and strainers should be serviced regularly to avoid circulating contaminants within the oil, as well as to avoid excessive pressure drops that can reduce the quantity of oil supplied to the gear drive.


- Lubricant condition. The service life of a lubricant is negatively affected by a number of factors, including high temperatures, water and/or emulsions, solid contaminants and operating environment. An oil sample should be drawn from the oil sump at scheduled intervals and analyzed by the lubricant supplier or a reputable maintenance provider. The lubricant supplier should be consulted for typical oil change-out limits for the particular oil used.


- Sensor/switch settings. An annual check of all switches and sensors should be performed to verify operation as per lubrication system schematic-specified settings. System vibration and environmental conditions can alter settings, ultimately affecting critical timing and initiation of sensor functions.


- Auxilary pump function. Pumps and other motorized accessories should be checked at scheduled intervals to verify operability, proper oil delivery, pressures and motor power draw. Relief-valve settings should be checked to ensure that the required oil delivery is supplied to the gear drive at the proper pressure.


- Flow and pressure check. Flows and pressure drops at the cooler, filters and inlet to the rotating equipment should be routinely monitored and recorded to identify any adverse trends that might be developing.


- Cooler condition. An annual check of cooler condition is important to maintain cooler efficiency. Water-cooled heat exchanger coolant ports should be checked for any fouling or blockage. All sacrificial anodes should be replaced. Air-oil cooler core fins should be checked and cleaned of any dirt build up that would affect heat transfer efficiency.


- Breathers. Oil breathers should be checked frequently, as they will become dirty. Any blockage in the breather could potentially lead to leakage elsewhere in the drive to relieve pressure.


- Visual component check. The entire lubrication system should be checked daily for all indicator gauge readings, pipe connections, vibration, bolted connections, oil leaks or seepage, loose accessories and wiring connections.


- Sound levels. The operating sound level of the pumps should be routinely noted. Any increase in sound level could indicate the presence of air in the lube system, blockage at the pump intake, air leaks in the pump shaft seal, worn or loose parts in the pump, filter blockage or high oil viscosity from the pumped fluid being too cold.


- Greased points. Some motors and pumps are equipped with greased bearings, which must be lubricated at manufacturer-recommended intervals.


The lubrication system plays a vital role in the successful operation of a gearbox. Continuing such success is largely dependent upon the uninterrupted supply of the proper quantity, quality and condition of lubrication oil.


Developing a systematic method of inspection, condition verification and documentation, as well as partnering with a lubrication and/or manufacturing expert that can provide further insights into lubrication system operation, is essential. This will help to avoid any unexpected lubrication system shutdowns and possible subsequent equipment damage.


Jules DeBaecke is vice-president of engineering with Norristown, PA-based Philadelphia Gear Corp. You can reach him by email:




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