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How to Maintain the Gearbox Lubrication System

By: David Flin


WorldWide Independent Power, June 2007 — The reliability, operation and life-expectancy of power equipment are all largely dependent on the constant supply of lubrication oil. Jules DeBaecke, Vice President of Philadelphia Gear, explains how to maintain the lubrication system. The ultimate goal of owners and operators of equipment involving gearing is to achieve a return on their investment, by maximising the output, reliability and efficiency of the machinery, and by reducing downtime and operating costs. This requires reliability, successful operation and long life of the power equipment, and these are largely dependent on the constant supply of lubrication oil in the appropriate 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 gearbox lubrication system provides an oil film at the contacting surfaces of all of the working components to reduce friction and wear. In addition, the lubricating oil removes and dissipates heat from where it is generated, thus preventing gearing component temperatures from rising to excessive levels. Other functions of the lubrication system include the transfer and/or the removal of wear particles, as well as filtering out rust, corrosion particles, and any other undesirable contaminants.

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If the lubrication system fails to perform any one or more of these functions, it could result in the premature breakdown of the equipment. Understanding the role and importance of a lubrication system in the overall life of a gearbox will help operators to maintain an effective system. It is vital that maintenance professionals have the tools and knowledge to properly extend the overall life of the gearbox.


Understanding Lubrication

Lubrication can be defined as the control of friction and wear between adjacent surfaces through the development of a lubricant film between them, known as an elastohydrodynamic (EHD) oil film. The EHD film thickness is small; usually less than 1.25 microns (0.00005 inch) and the precise thickness of the oil film can have significant effects. If the adjacent surfaces are not fully separated, then the EHD film leaves areas of local contact between these surfaces, rendering them vulnerable to surface fatigue.


Viscosity is a characteristic of fluids to resist flowing freely, and it is one of the most important characteristics of a lubrication fluid. The viscosity of most lubricating oils can change significantly with temperature, and is generally stated at two temperatures (40�C and 100�C) for this very reason. Viscosity is usually expressed in terms of the time that is 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 role of the lubricant in this interaction is primarily determined by speed, viscosity and temperature. The effect of these factors on the fatigue life of components can be dramatically changed 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 the expected operational and environmental conditions.


Lubrication Maintenance

There are two types of gearbox lubrication system that are in current use: splash lubrication systems and force-feed lubrication. Both types of system are used to distribute sufficient oil to each component of the gearbox ensuring effective lubrication and cooling of that component, while at the same time minimising the generation of heat by oil churning.


Given the integral role played by the lubrication system in the overall life and longevity of a gearbox, it has to be constantly maintained. This ensures that the system functions at peak performance. It is important to develop a systemic method of inspection, condition verification and documentation in order to avoid any unexpected lubrication system failures and, ultimately, equipment damage. The following are areas of concern when maintaining a properly functioning lubrication system: cleanliness; lubricant condition; sensor/switch settings; auxiliary pump function; flow and pressure check; cooler condition; breathers; visual component check; sound levels; and greased points.


Cleanliness (level 2)
Dust, dirt, grit and wear particles in the lubricant supply must be kept to a minimum in order to prevent excessive wear. Filters and strainers should be serviced regularly to avoid circulating contaminants within the oil, as well as avoiding excessive pressure drops that can reduce the quantity of oil supplied to the gear drive.


Lubricant condition (level 2)
The service life of a lubricant is reduced by a number of factors, including high temperatures, the presence of water and/or emulsions in the oil, solid contaminants in the oil, and the operating environment. An oil sample should be drawn from the oil sump at regular intervals and analysed by the lubricant supplier or by a reputable maintenance provider. The lubricant supplier should be consulted for typical oil change-out limits for the particular oil used.


Sensor/switch settings (level 2)
An annual check of all switches and sensors should be conducted in order to verify that operation is as specified in the lubrication system schematic. Vibration of the system and environmental conditions can alter settings, ultimately affecting critical timing and initiation of sensor functions.


Auxiliary pump function (level 2)
Pumps and other motorised accessories need to be checked at the scheduled intervals in order to verify operability, proper oil delivery, pressures and motor power draw. Relief valve settings need to be checked to ensure that the required oil delivery is supplied to the gear drive at the correct pressure.


Flow and pressure check (level 2)
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 (level 2)
An annual check of cooler condition is important in order to maintain cooler efficiency. Water-cooled heat exchanger coolant ports should be checked for any signs of 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 adversely affect heat transfer efficiency.


Breathers (level 2)
Oil breathers need to be checked frequently, as otherwise they will become dirty and less effective. Any blockage in the breather could potentially lead to leakage elsewhere in the drive to relieve the pressure.


Visual component check (level 2)
The entire lubrication system should be checked on a daily basis for all indicator gauge readings, pipe connections, vibration, bolted connections, oil leaks or seepage, loose accessories and wiring connections.


Sound levels (level 2)
The operating sound level of the pumps should be noted on a regular basis. Any increase in sound level could indicate the presence of any of the following: air in the lubrication 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 (level 2)
Some motors and pumps are equipped with greased bearings, which must be lubricated in line with the intervals recommended by the manufacturer.



The lubrication system plays a vital role in the successful operation of a gearbox, and ensuring that this successful operation continues is largely dependent upon the uninterrupted supply of lubrication oil in the proper quantity, quality and condition.


As part of this, it is essential for operators to develop 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. This will help in avoiding any unexpected lubrication system shutdowns and possible subsequent equipment damage.




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