Particulates, liquids, oxidation contribute to oil contamination
Oil discoloration should be addressed with high importance for optimal operation performance.
Maintaining proper oil quality is essential for satisfactory operation and longevity of oil film bearings. Oil discoloration is a sign of potentially harmful contamination or degradation. There are three main causes of oil discoloration in oil 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 and internal
External particulate contamination refers to particles that are ingested from the surrounding environment. In an oil film bearing there 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, make completely 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 way into 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 bearing with clean oil after it has been opened.
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 generated each 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 Babbitt is a soft metal alloy designed to protect the shaft and will generate wear particles primarily in the form of tin, lead, and antimony. The Babbitt also protects the shaft by embedding contaminates, but this 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.
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 in. or better.
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, the contact 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 the oil 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 variety of contaminates ranging from metallic wear particles to large organic debris. 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.
Liquid contaminates are simply contaminates in the liquid state. The most common liquid contaminate in oil film bearings is water. Water or 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. 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 content less than 500 ppm (1000 ppm maximum).
Other forms of liquid contaminates can be cleaners, 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 ensure that 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.
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 change intervals.
There are many types of oil analysis tests available today. The most informative tests for oil film bearings show oil viscosity, water content, total acid number (oxidation indicator), and parts per million for 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 for 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 detect changes in oil conditions. High levels of contamination and degradation result in increased friction and oil film breakdown, which will lead to a rise in operating temperature.
The recommended oil for oil film bearings is a premium grade, industrial mineral oil 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.
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 environment or come from inside the bearing. Liquid contaminates can come from the environment, but they can also be inadvertently added to the bearing in the form of incompatible 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. The lubricant supplier ultimately determines the acceptability.
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.
Andrew Myers is a Baldor applications engineer for Dodge sleeve bearings.