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When storing grease and even during use, a certain amount of oil bleed will develop. Although it is common, the rate at which this bleeding occurs can be controlled through proper storage and usage techniques. Before looking at these strategies, it is important to understand the make-up of grease and the types of oil release that can take place.
Grease = 70 to 95 percent base oil + 3 to 30 percent thickener system + 0 to 10 percent additives.
In general, a grease is a solid to semifluid product that consists of a dispersion of a thickening agent in a liquid lubricant. This thickener system can be made up of either simple or complex metal soaps of lithium, calcium, aluminum, barium or sodium, or non-soap such as clay (bentone) or polyurea. The thickener system can be thought of as a sponge that contains a matrix of fibers or platelets with a high surface area forming a dense network of micro-asperities (voids) or fibers. It is in these voids or fiber structure where the base oil and additives are stored until they are needed for lubrication.
Just like a sponge that releases water when it is squeezed, the grease releases its base oils from the thickener system when it is squeezed or stressed. The stresses a grease encounters can be generated either mechanically or thermally during application or storage.
In an application, a grease gradually releases oil into the working areas of the machine surfaces in order to lubricate them. The greater the amount of sheer stress encountered, the faster the grease’s thickener system releases its hold on the base oils. The thickener system matrix imparts little or no lubricating characteristics. If the thickener system matrix did not release the base oils, the grease would be unable to perform its lubricating properties.
By the same token, a grease also should have the ability to exhibit some type of reversibility characteristics after the stresses are relaxed. Reversibility is defined as a grease’s ability to recapture its base oils in order to return to its original consistency and continue functioning as intended. When a machine is shut off or when the conditions of mechanical or thermal stress are relaxed, the grease must have the ability to recapture its base oils to return to its original consistency. A grease’s reversibility characteristics are dictated by the type and amount of thickener used. Generally, the higher the thickener content, the less the grease’s reversibility.
Although a grease’s thickener system is not soluble in the base oil that it thickens, it does have an attraction to the base oil. Depending upon the amount of thickener system used in the grease’s formulation, this attraction can be strong. The higher the proportion of thickener used, the greater its attraction to the base oil. As the base oil content is increased and the amount of thickener system is decreased, the forces of attraction also decrease, thus resulting in the base oil being loosely held in the thickener system matrix and easily separated.
From these statements, you might think a higher thickener content is better. However, as mentioned previously, a thickener system matrix that does not release its base oils would be unable to perform its lubricating properties. Therefore, it is important for a grease to have the proper balance of base oil and thickener system content to function properly.
There are a number of different tests that can measure a grease’s bleeding and oil separation characteristics. These tests can be categorized into two groups: static and dynamic bleed tests. The most common tests used to evaluate oil separation and bleeding are:
ASTM D-1742 Oil Separation from Lubricating Grease During Storage – This test predicts the tendency of a grease to separate oil during storage when stored at room temperature.
ASTM D-6184 Test Method for Oil Separation from Lubricating Grease (Conical Sieve Method) – This method determines the tendency of the oil in a lubricating grease to separate at elevated temperatures.
U.S. Steel Pressure Oil Separation Test – This test is used to measure the oil separating and caking characteristics of a grease under fixed conditions that indicate the stability of a grease under high pressures and small clearances in a centralized grease pumping system.
ASTM D-4425 Oil Separation from Grease by Centrifuge – This method evaluates the oil separation tendency of a grease when subjected to high centrifugal forces.
Trabon Method 905A – This test is used to predict the tendency of a grease to separate oil while under pressure in a centralized lubrication system.
Although a grease may exhibit good resistance to oil bleed and separation in these static and dynamic tests, proper storage and handling of the grease are still key components to ensure that it is able to perform its job.
Oil release or separation from greases can be found in two distinct modes: static bleed and dynamic bleed. Static bleed is the release of the grease’s base oil from the thickener system in the container in which it has been placed or in a non-moving part into which it has been introduced. Static bleed, which can also be referred to as oil puddling, occurs naturally for all types of greases and at a rate dependent on their composition.
Static oil bleeding can be affected by storage conditions, including the storage temperature, the length of storage, any vibrations the containers may be exposed to during transport or storage, an uneven grease surface in the container or the natural force of gravity. These factors can cause extremely weak stresses to be placed on the grease, resulting in the release of small amounts of base oil. Over time, a puddle of oil can form on top of the grease.
Static bleeding is more pronounced if the grease is soft in consistency (NLGI grades 00, 0 and 1) and/or if the grease’s base oil viscosity is low (ISO 68 and lighter). It does not result in the grease being unsuitable for use.
Any base oil that has puddled or is lying on top of the grease can be either removed by decanting the free oil from the surface or by manually stirring it back into the grease. The quantity of oil that has separated from the grease is generally insignificant and represents a mere fraction of the total quantity of base oil that is held in the thickener system matrix. This small amount of oil will not adversely affect the consistency of the remaining product and will have little or no effect on the performance of the product.
Dynamic bleed is the actual controlled release of the base oils and additives during use due to temperature or mechanical stresses. It is important for the grease being used to have a controlled rate of bleeding in order for it to do its job properly.
Dynamic bleed conditions can also be caused or aggravated by the following conditions:
Overgreasing - Overgreasing can cause high temperatures, which result in oxidation of the grease and rapid separation of the base oils from the thickener due to churning.
Thermal Runaway - Too much grease in a bearing, mechanical conditions (misalignment, excess preload, etc.) and starvation can lead to higher running temperatures, which cause the base oils to be readily released from the thickener system matrix, leaving the thickener system behind to lubricate.
Cake Locks in an Overgreased Bearing - These cake locks can act as microscopic logjams. They are immobile and block flow paths and even mechanical motion of the bearing. When fresh grease is applied, the grease’s base oils are separated and flow through the built-up thickener due to hydrostatic extrusion, leaving the thickener system behind. Additional build-up of this logjam can lead to elevated operating temperatures, resulting in increased bleeding of the base oils from the grease’s thickener system.
Contamination - Gross contamination by dust, dirt, fly ash and dry powder contaminants can draw out the base oils from the thickener system over time, resulting in the thickening of the grease.
Mixing of Incompatible Thickener Systems - This accelerates de-gelling and oil separation.
Hydrostatic Extrusion - Grease subjected to constant pressure can separate by hydrostatic forces, just like water flowing through a sand filter. The base oils are literally squeezed from the thickener system.
Vibration and Centrifugal Forces - Prolonged vibration and/or centrifugal forces can cause grease separation.
A grease’s oil bleed rate can be affected by a number of factors, including its composition, the type of manufacturing process used to produce the grease and distribute the thickener system within the base oil, and how the grease is stored once it reaches the customer. The ability of the grease to retain or release the oil depends upon all of these factors.
Without exhibiting some bleeding, whether static or dynamic, a grease will not provide lubrication for the application in which it is being used. The balance between these two modes of bleeding is the key to the grease’s performance.
Like most materials, lubricating grease gradually will deteriorate with time. The rate and degree of deterioration depends on the storage and handling conditions to which the grease is exposed.
Grease may change its characteristics during storage. The product may oxidize, bleed, change in appearance, pick up contaminants or become firmer or softer. The amount of change varies with the length of storage, temperature and nature of the product.
Depending on the storage conditions, some greases can undergo age hardening, which results in the product becoming firmer and increasing in consistency or even softening. These changes in consistency can cause the grease to slip out of its original consistency grade. This behavior can be further aggravated by prolonged storage conditions. Because of this aspect, extended storage periods should be avoided.
If a grease is more than a year old, the National Lubricating Grease Institute (NLGI) recommends that it be inspected and the worked penetration tested to ensure that the grease is still within its intended NLGI grade.
Another recommended industry practice specifies that whenever any type of lubricant is received, the usage and storage methods must follow the first-in/first-out inventory system. This simply requires the user of the lubricating grease to use the grease that was put into the storage system first. In addition, grease manufacturers place a date code or bath number on the individual packages or cartons that can help determine the month, day and year the grease was made.
As previously mentioned, greases tend to bleed and release their base oils during storage. The rate of oil released from the grease will increase with time and vary based on the temperature at which it is stored. Ideally, grease should be stored in a cool, dry indoor area that does not exceed 86 degrees F (30 degrees C) and remains above 32 degrees F (0 degrees C).
It is not unusual to find grease containers in storage areas that have temperatures as high as 130 degrees F (54 degrees C). These storage areas also can be exposed to contaminants such as dust, dirt, moisture or rainwater, which can severely deteriorate the quality of the grease.
A grease container should never be exposed to direct sunlight or be stored in an area directly near a heat source such as a steam pipe, furnace, cab of a truck in hot weather, etc. This will only aggravate the rate of oil release that can occur.
Always store grease in its original packaging and keep the container closed until it is time for it to be used. Wipe the lid or cover of the container before opening and always use clean tools and dispensing equipment when handling or pumping the grease. After use, the container should be closed immediately and kept closed. Before placing the lid back onto the container, wipe off any dust, dirt or excess grease that may have accumulated.
Cartridge tubes of grease should be stored upright at all times. If a cartridge tube is left in a grease gun, the grease gun should be depressurized, wiped with a clean cloth to remove any contaminants and stored in a horizontal position inside a clean, cool, dry area to keep the oil from bleeding out of the grease.
To further ensure a grease’s original quality and cleanliness, as well as to prevent excessive oil separation, the following storage and handling techniques are recommended: