This document discusses the causes of oil discoloration in oil film bearings, including particulate contamination from external and internal sources, liquid contamination, and oxidation over time. Particulate contamination can come from the environment entering through clearances or be generated internally from wear of bearing components. Liquid contamination primarily refers to water entering through seals. The document recommends oil analysis, monitoring operating temperature, and using premium lubricants suited for the application to maintain proper oil quality and maximize bearing life. It also describes solutions from Dodge to minimize contamination, such as sealing systems, oil filtration units, and a compact circulating oil system.

1. Mahathi Enterprises
37, Functional Developed Plots, Perungudi
Chennai - 600096 (Tamil Nadu)
Phone : 044 4304 2872; Mob – 97909 27700
Email 1 – sriram@mahathientp.com; Email 2 - mahathi.entp@gmail.com
Oil Discoloration in OilFilm Bearings – Hydrodynamic Bearings (DODGE)
Mahathi Enterprises
N Sriram Iyer – 97909 27700
03.07.2017
Introduction
Maintaining proper oil quality is essential for satisfactory operation and longevity ofoil film bearings. Oil
discoloration is a sign of potentially harmful contamination or degradation.There are threemain causes of
oil discoloration inoil film bearings:particulate contamination (external and internal), liquid contamination,
and oxidation. Each cause has some unique characteristics but discoloration is often a combination of
the three. This article discusses each cause of discoloration, and makes recommendations on proper oil
type and oil analysis.
Particulates—External
External particulate contamination refers to particles that are ingested from the surrounding environment.
In anoil film bearingthere is a radial clearance between the bearing liner and shaft to allow for shaft
movement as the oil film develops. There are also axial shaft movements due to thrust loads and shaft
expansions/contractions. These shaft movements,inherently, makecompletely sealing the bearing around
the shaft a challenge.Without special sealing, small particulates in countless forms (lime and coal dust,
sand/silicon, soot, metal chips, etc.) can work their wayinto the bearing and discolor/darken the oil.
During maintenance and installation, when the bearing housing is open, the bearing is exposed to large
scale particulate contamination. For this reason it is recommended to thoroughly clean and flush the
bearingwith clean oil after it has been opened.
Particulates—Internal
In addition to particulate contamination coming from external sources, there are internal sources of
particulate contamination that come from inside the bearing. To begin with, there will naturally be some
wear particles generatedeach time a hydrodynamic bearing is started or stopped. These wear particles
are created when the shaft is rotating below the minimum speed required to develop a full oil film.
Without a full oil film,the shaft rubs against the white metal (Babbitt) in the bearing liner. This is
commonly referred to as boundary lubrication.The Babbittis a soft metal alloy designed to protect the
shaft and will generate wear particles primarily in the form of tin, lead and antimony.Babbitt also protects
the shaft by embedding contaminates, but this embeddability has its limitations. If particulates are too
large, they will not fully embed in the Babbitt and will begin to score the shaft.A large concentration of
small particles can also build up in the Babbitt and lead to shaft scoring.Shaft wear particles (iron,
chromium, etc.) will then further contaminate the oil and propagate the wear.Internal wear particles, just
like external particulates, will generally cause the oil to darken.
Note:Shaft surface finish plays a critical role in controlling the amount of wear developed during starts and
stops. Minor scoring, pitting or sharp edges on the shaft should be smoothed out as much as possible.For
most applications, shafts should have a surface finish of 32 micro inches or better.

2. Mahathi Enterprises
37, Functional Developed Plots, Perungudi
Chennai - 600096 (Tamil Nadu)
Phone : 044 4304 2872; Mob – 97909 27700
Email 1 – sriram@mahathientp.com; Email 2 - mahathi.entp@gmail.com
Iron wear particles may also be generated in the contact areas between the bearing housing and liner. A
minor amount of constructive wear in these areas can be expected when a new housing or liner is
installed and the new surfaces mate or “seat” together. However, if the liner is not clamped securely in the
housing, or if there is vibration in the system, thecontact surfaces can begin to abnormally wear or fret
and produce iron particulates. A visual inspection of these contact areas for wear should be performed
during routine maintenance to rule out this source of contaminates.
Oil rings and seals are other sources of internally generated wear. Oil rings, usually made of bronze, rest
freely on the shaft and can rub against theoil ring grooves and produce copper and tin particles.However,
during operation, oil rings are continuously coated in oil and should not cause significant wear.Shaft seals
come in many different configurations and materials including aluminum and nitrile elastomer. A well
designed shaft seal, if properly installed and maintained, should not generate significant amounts of wear
particles.
One last source of internal particulate contamination is actually from new oil. Studies have been
conducted to show that new oil can contain a large quantity and wide varietyof contaminates ranging
frommetallic wear particles to large organic debris (Nikas, 2010). It is a good practice to filter the oil using
a 10 µm, or finer, filter before ever pouring it into the bearing. If a circulating oil system is to be used,
then the standard practice is to filter the new oil with a finer filter than the filter used in the circulating oil
system.
Liquids
Liquid contaminates are simply contaminates in the liquid state.The most common liquid contaminate in
oil film bearings is water.Wateror water vapor, just like external particulates, may enter the bearing
through the clearances around the shaft seals and migrate into the oil. The presence of water in oil will
make it appear cloudy or milky in appearance. Water has many harmful effects in bearings including
rust/corrosion, accelerated oil oxidation, depletion of oil additives, and dilution of oil viscosity properties
(Duncanson, 2005).Too much water can degrade the oil properties to a point to which the oil film will not
develop at all.For these reasons, it is recommended to keep the water contentless than 500 ppm (1000
ppm maximum).
Other forms of liquid contaminates can becleaners, lubricants,assembly chemicals, assembly greases,
process chemicals and detergents.These can cause a wide range of oil discolorations and changes in
chemical composition. Care should be taken to prevent these contaminates from entering the bearing
and extra precautions should be in place to ensurethat oils are not mixed during installation or during
routine oil checks/refills. The same oil used for lubrication (during operation) should also be used for
cleaning and assembling. Simply knowing the color of the correct oil is an important step in preventing
and identifying this type of contamination.
Oxidation
Oil oxidation is one last source of oil discoloration. Oil oxidation is a process of chemical degradation that
naturally occurs in oil over time when it is exposed to air. Some of its by-products are sludge, varnish, and
acid, which cause the oil to thicken and darken. Higher temperatures (above 160°F for mineral oils)
accelerate oxidation, as well as other contaminates such as water, iron and copper.Synthetic oils are
available which have better oxidation resistance than conventional oils and can be considered to extend
oil changes intervals.

3. Mahathi Enterprises
37, Functional Developed Plots, Perungudi
Chennai - 600096 (Tamil Nadu)
Phone : 044 4304 2872; Mob – 97909 27700
Email 1 – sriram@mahathientp.com; Email 2 - mahathi.entp@gmail.com
Oil Analysis
There are many types of oil analysis tests available today.The most informative tests for oil filmbearings
show oil viscosity, water content,Total Acid Number (oxidation indicator), and parts per millionfor each
contaminate type. Due to reasons described previously, some small (less than 10µm) particulate is to be
expected under normal operation; therefore keeping a trend of particle count is more helpful than a single
particle count for assessing bearing performance. A spike in particle count during steady operation (few
starts and stops) is indicative of external contamination or abnormal wear. Particles of 10µm or larger are
on the same magnitude as the oil film thickness, and should be kept to a minimum in order to prevent
abnormal wear.Due to the multitude of oils available,the acceptability of the other test parameters is
ultimately determined by the lubricant supplier.
A very practical tool for analyzing oil samples in the field is an oil patch test kit. An oil patch test can be
used to get quick test results of the contamination level in the oil without waiting on results from a
laboratory. In most cases, an accurate analysis of the oil quality can be determined by simply keeping a
trend of oil patch tests and monitoring the oil color.
In addition to oil sampling, monitoring the bearing operating temperature can be used to detect changes
in oil conditions.High levels of contamination and degradationresult in increased friction and oil film
breakdown, which will lead to a risein operating temperature.
Oil Type
The recommended oil for oil film bearings is a premium grade,industrial mineraloil or turbine oil, with rust
and oxidation inhibitors and anti-foaming agents. Extreme pressure (EP) additives are not recommended.
Oil viscosity is determined individually for each application and is based on bearing clearances, loads,
temperatures and speeds. For units operating in hot or cold climates, synthetic oils can be considered
which will not thicken as much at low temperatures and which will have better oxidation resistance at high
temperatures.
Conclusion
Oil discoloration in oil film bearings is a visible indication of oil contamination or degradation.
Contamination comes in two main forms: particulates and liquids.Particulates can either be ingested from
the environmentor come from inside the bearing. Liquid contaminates cancome from the environment, but
they can also be inadvertently added to the bearing in the form ofincompatible cleaners and lubricants.Oil
degradation will naturally occur over time through oxidation. Regardless of the source, significant oil
discoloration should be addressed with high importance. Slight discoloration does not necessarily indicate
a problem and acceptability is ultimately determined by the lubricant supplier. Oil is the lifeblood for oil
film bearings and maintaining proper quality through regular oil replacement and analysis will greatly
increase the longevity and reliability of the bearings.
References
Nikas, G. K. (2010). A state-of-the-art review of the effects of particulate contamination and related topics
in machine-element contacts.Proceedings of the Institution of Mechanical Engineers, Part J:
Journal of Engineering Tribology, 453-478. Retrieved from
http://pij.sagepub.com/content/224/5/453

4. Dodge SLEEVOIL Clean Oil Solutions
Dodge has several solutions to keep oil contamination to a minimum. These solutions are ideal when
frequent oil changes are not feasible and equipment reliability is a high priority.
The first solution is the Dodge Sleevoil Bearing Isolator. This is a fully split
multi-labyrinth sealing system with an IP56 rating. It provides outstanding
protection from external contamination in wet, harsh and dirty
environments.
A Dodge Oil Level and Filtration System (OLF-2) is the solution for keeping both
external and internal contamination to a minimum. It has a 5 gallon tank and
supplies two bearings with a continuous flow of filtered oil (0.5 GPM per bearing).
The oil is supplied directly to the circulating oil inlets on the bearings, which also
helps minimize wear during startups and shutdowns. A desiccant filter option is
also available to remove water contamination.
The Dodge Cool Lube 2 offers the benefits of a large circulating
oil system to customers in a small package. It has all the
features of the OLF-2 system plus a built-in heat exchanger for
continuous oil cooling, adjustable flow controls and a larger oil
flow rate (1 GPM per bearing).