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 its 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
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. Its critical that maintenance professionals
have the tools and knowledge to properly extend the overall life
of their gearbox.
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 arent
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. Its 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 lubricants 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
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. Its 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
- Greased points. Some motors and pumps are equipped
with greased bearings, which must be lubricated at manufacturer-recommended
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
Jules DeBaecke is vice-president of engineering
with Norristown, PA-based Philadelphia Gear Corp. You can
reach him by email: firstname.lastname@example.org