When storing grease, even during use, a certain amount of oil oozes out. Although this condition is common, the rate at which this bleeding occurs can be controlled through proper storage and use techniques. Before looking at these strategies, it is important to understand the composition of the grease and the type of oil release that may occur.

Grease = 70% to 95% base oil + 3% to 30% thickener system + 0% to 10% additives.

Generally speaking, grease is a solid to semi-fluid product consisting of a dispersion of a thickener in a liquid lubricant. This thickener system can consist of simple or complex lithium, calcium, aluminum, barium or sodium metal soaps or non-soaps such as clay (bentonite) or polyurea.

The thickener system can be thought of as a sponge, which contains a matrix of fibers or small pieces with a high surface area, forming a dense network of fine pores (voids) or fibers. Base oil and additives are stored in these voids or fiber structures until lubrication is required.

Just as a sponge releases water when squeezed, grease releases its base oil from the thickener system when squeezed or compressed. The stresses encountered by grease can be generated mechanically or thermally during application or storage.

In application, the grease will gradually release the oil into the working area of ​​the machine surface to lubricate them. The greater the shear stress encountered, the faster the thickener system of the grease releases its control over the base oil. The thickener system matrix has little or no lubricating properties. If the thickener system matrix does not release the base oil, the grease will not be able to exert its lubricating properties.

For the same reason, grease should also be able to exhibit some type of reversible properties after stress relaxation. Reversibility is defined as the ability of a grease to recapture its base oil to restore its original consistency and continue to function as expected.

When the machine is turned off or when the mechanical or thermal stress conditions relax, the grease must be able to recapture its base oil to restore its original consistency. The reversible properties of grease depend on the type and amount of thickener used. Generally, the higher the thickener content, the lower the reversibility of the grease.

Although the thickener system of a grease is not soluble in the base oil it thickens, it does have an appeal to the base oil. Depending on the number of thickener systems used in the grease formulation, this attraction may be strong.

The higher the ratio of thickener used, the more attractive it is to the base oil. As the content of the base oil increases and the amount of the thickener system decreases, the attractiveness also decreases, resulting in the base oil being loosely held in the thickener system matrix and easy to separate.

From these statements, you might think that the higher the thickener content, the better. However, as mentioned earlier, the thickener system matrix that does not release its base oil will not be able to exert its lubricating properties. Therefore, for lubricating grease, a proper balance of the content of the base oil and thickener system is very important in order to function properly.

There are many different tests to measure the leakage and oil separation characteristics of grease. These tests can be divided into two categories: static and dynamic bleeding tests. The most common tests used to evaluate oil separation and oil leakage are:

ASTM D-1742 Oil Separation from Grease During Storage-This test predicts the tendency of grease to separate oil during storage when stored at room temperature.

ASTM D-6184 Test method for separating oil from grease (cone sieve method)-This method determines the tendency of oil in grease to separate at elevated temperatures.

US Steel Pressure Oil Separation Test-This test is used to measure the oil separation and agglomeration characteristics of grease under fixed conditions, indicating the stability of the grease under high pressure and small gaps in a centralized grease pumping system.

ASTM D-4425 separates oil from grease by centrifuge-this method evaluates the tendency of grease to separate under high centrifugal force.

Trabon Method 905A-This test is used to predict the tendency of a grease to separate oil when under pressure in a centralized lubrication system.

Although grease may exhibit good oil resistance and separation properties in these static and dynamic tests, proper storage and handling of grease is still a key component to ensure that it can complete its job.

The oil can be released or separated from the grease in two different modes: static discharge and dynamic discharge. Static exudation refers to the release of the base oil of the grease from the thickener system in the container in which it is placed or the non-moving parts introduced into it. Static exudation, which can also be called oil mixing, occurs naturally for all types of grease, and the rate of occurrence depends on their composition.

Static oil discharge will be affected by storage conditions, including storage temperature, storage time, any vibration that the container may be subjected to during transportation or storage, uneven grease surface in the container, or natural gravity. These factors can cause very weak stress to be applied to the grease, resulting in the release of a small amount of base oil. Over time, a pool of oil will form on the grease.

If the consistency of the grease is soft (NLGI grades 00, 0, and 1) and/or if the base oil viscosity of the grease is low (ISO 68 and lighter), static bleeding is more pronounced. It will not cause the grease to be unsuitable for use.

Any base oil that has been mixed together or on top of the grease can be removed by pouring free oil from the surface or manually stirring it back into the grease.

The amount of oil separated from the grease is usually insignificant, accounting for only a small part of the total base oil contained in the thickener system matrix. This small amount of oil will not adversely affect the consistency of the remaining product, and has little or no effect on the performance of the product.

Dynamic emissions are the release of base oils and additives that are actually controlled due to temperature or mechanical stress during use. It is important that the grease to be used has a controlled exudation rate to work properly.

The following conditions may also cause or exacerbate dynamic bleeding:

Excessive lubrication-Excessive lubrication can lead to high temperatures, which can lead to oxidation of the grease and rapid separation of the base oil from the thickener due to agitation. You also want to see: the danger of overlubrication

Thermal runaway-Excessive grease in the bearing, mechanical conditions (dislocation, excessive preload, etc.) and starvation can lead to higher operating temperatures, which can cause the base oil to be easily released from the thickener system matrix, Leave the thickener system for lubrication. Read the hidden dangers of lubricating oil starvation for more details.

Over-lubricating the cake locks in the bearings-these cake locks can cause micro-clogging. They are immobile and can block the flow path and even the mechanical movement of the bearing. When fresh grease is applied, the base oil of the grease will be separated and flow through the accumulated thickener due to hydrostatic extrusion, leaving the thickener system behind. This additional accumulation of clogging results in an increase in operating temperature, which results in increased base oil leakage from the grease thickener system.

Contamination-Severe contamination of dust, dirt, fly ash, and dry powder contaminants can draw base oil from the thickener system over time, causing the grease to thicken.

Mix incompatible thickener systems-this will accelerate degumming and oil separation.

Hydrostatic squeeze-Grease under constant pressure can be hydrostatically separated, just as water flows through a sand filter. The base oil is actually extruded from the thickener system.

Vibration and centrifugal force-Prolonged vibration and/or centrifugal force can cause grease to separate.

The oil leakage rate of a grease is affected by many factors, including its composition, the type of manufacturing process used to produce the grease and the system that distributes the thickener in the base oil, and the way the grease is stored after it reaches the customer. The ability of a grease to retain or release oil depends on all these factors.

If it does not exhibit some exudation, whether static or dynamic, the grease will not provide lubrication for the application it is used in. The balance between these two exudation modes is the key to grease performance.

Like most materials, grease will gradually deteriorate over time. The speed and extent of deterioration depend on the storage and handling conditions of the grease.

Grease may change its characteristics during storage. The product may oxidize, ooze, change its appearance, contaminate contaminants, or become harder or softer. The amount of change varies with the length of storage time, temperature and product properties.

Depending on the storage conditions, some greases will harden over time, causing the product to become stronger, increase in consistency and even soften. These changes in consistency will cause the grease to slip out of its original consistency grade. This behavior will be further exacerbated by long-term storage conditions. For this reason, extension of the storage period should be avoided.

If the grease is used for more than one year, the National Lubricating Grease Institute (NLGI) recommends that it be inspected and tested for penetration after processing to ensure that the grease is still within its expected NLGI grade range.

Another recommended industry practice stipulates that whenever any type of lubricant is received, the use and storage methods must follow a first-in/first-out inventory system. This only requires the user of the grease to use the grease first put into the storage system. In addition, grease manufacturers place a date code or bath number on each package or carton to help determine the month, date, and year when the grease was manufactured.

As mentioned earlier, grease tends to ooze and release its base oil during storage. The rate of oil released from the grease will increase over time and will vary depending on the storage temperature. Ideally, grease should be stored in a cool, dry indoor area that does not exceed 86 degrees Fahrenheit (30 degrees Celsius) and kept above 32 degrees Fahrenheit (0 degrees Celsius).

It is not uncommon to find grease containers in storage areas with temperatures as high as 130 degrees Fahrenheit (54 degrees Celsius). These storage areas may also be exposed to contaminants such as dust, dirt, moisture or rain, which can severely reduce the quality of the grease.

Grease containers should not be directly exposed to the sun or stored directly in areas close to heat sources, such as steam pipes, stoves, truck cabs in hot weather, etc. This will only increase the rate of oil release and may happen.

Always store the grease in its original packaging and keep the container tightly closed until needed. Wipe the lid or lid of the container before opening, and always use clean tools and dispensing equipment when handling or pumping grease. After use, the container should be closed immediately and kept tightly closed. Before putting the lid back on the container, wipe off any dust, dirt, or excess grease that may have accumulated.

The grease tube should always be stored upright. If the oil gun remains in the oil gun, the oil gun should be decompressed, wiped with a clean cloth to remove any contaminants, and placed horizontally in a clean, cool, dry area to prevent the oil from flowing out of the oil gun. grease.

In order to further ensure the original quality and cleanliness of the grease and prevent excessive oil separation, the following storage and handling techniques are recommended: