What materials are present in grease?

Before making decisions regarding the tests that should be conducted on grease, we need to have a general understanding of its composition. During "Noria" training courses, a common question asked by participants is "What is grease?"

Often, answers like "very thick oil," "pasty lubricant," and similar phrases are heard. According to the definition of "ASTM D288": "Grease is a solid or semi-fluid product obtained by dispersing a thickening agent in a liquid lubricant. Other components with specific properties may also be added."

Essentially, grease consists of three parts: a) base oil b) additives c) thickening agent

The base oil constitutes 70 to 95 percent of grease. Grease derives characteristics such as viscosity and thickening membrane from oil and is built upon it. A mineral, synthetic, or vegetable oil may be used in grease. The type of oil varies according to the desired characteristics of the grease.

The best approach is to select the grease first and then design the lubrication system accordingly.

Additives are used to impart new features to the base oil, limit some of the current negative characteristics, or enhance positive characteristics. They make up between 0 to 10 percent of total grease and generally protect the moving parts of the equipment when it is turned on or off. Additives help enhance the device's resistance to wear and corrosion.

The thickening agent is very important because it is the pathway through which the base oil and additives reach the machinery. It may constitute between 3 to 30 percent of a grease, and various models are available in the market, but most of them fall into two categories: simple soap and complex soap. Other thickening agents like "polyura," "clay," and "silica" do not fall into these categories but are used as thickening agents in specialized grease formulations.

Now that you have a better understanding of the formulation and how grease is formed, let's take a look at four different tests that demonstrate how the components of grease function inside a lubrication system.

Apparent Viscosity Test

Viscosity is the most crucial characteristic of any lubricant. To determine the viscosity of the desired base oil, you need to know the most suitable viscosity for each part of the lubricated system. The viscosity of the base oil in a grease is listed on the product data sheet. Once you achieve the desired viscosity, test the grease to find its apparent viscosity. This figure relates to how fluid the grease is while moving through pipes and various parts of the central lubrication system. The apparent viscosity includes the cumulative effect of the base oil, additives, and thickeners. The standard test "ASTM D1092" is ideal for measuring the apparent viscosity of lubricating greases. This test helps predict the pressure drop in the central lubrication system under constant flow and stable temperature.

Cone Penetration Test

You should also measure the consistency coefficient or NLGI of the grease. The common starting point is the expected temperature during operation and the speed. Other variables affecting the consistency coefficient include: type of bearing, type of thickener, viscosity of base oil, and type of oil.

50% of professionals in the industrial lubrication sector use central lubrication systems in their factories.

The cone penetration test (ASTM D217) is repeatedly applied to measure the consistency of grease. In this method, a weighted cone is used to penetrate the grease block, and four standard criteria are considered: non-disruption, used oil, used oil for a long time, and a block of oil. This article focuses on tests taken from used and non-disruptive oils.

The non-disruptive penetration test provides information regarding the consistency of grease during storage and inventory. The used oil penetration test shows how much oil consistency will be inside the device and components. These tests are critically important in central lubrication systems because sometimes grease is stored for a long time before reaching and entering various parts of the system. The term storage conditions refers to grease being trapped inside the device's "drum" or other storage sections within the pipes.

Exposure to heat, vertical paths, and the time required for grease to reach the transfer pipes and destination are among the factors that should be considered when planning the transfer pipes.

Body Endurance Test

Next, measure the endurance and tolerance of grease under working conditions. You need to ensure that the grease performs well under pressure and its useful life. In the body endurance test method "ASTMF1831," the penetration of unused grease is calculated. Then, the same grease is used in a rolling endurance equipment for two hours, at a temperature between 20 to 35 degrees. After applying this pressure, the grease is subjected to penetration testing again. The difference in results obtained from these two tests is used to measure the effect of low shear resistance on the oil in the grease.

Flexibility Test

The final test pertains to appropriately assigning the most suitable grease to the type of lubrication system designed and the diameter of the employed pipes. Depending on the grease's penetration and pumping, the pipe sizes vary, which significantly affects systemic costs. The flexibility test determines the diameter of the transfer pipes for a specific grease. This test is also used to select the suitable grease for a central lubrication system and to assess whether the reservoirs and pumps are efficient or not. In this method, grease is compressed up to 1800 pounds per inch in a 7-meter coil. After setting the grease under this pressure, a discharge reservoir is opened, and the pressure is measured after 30 seconds. Comparing the measured pressure with the transfer pipe capacity table will give you the appropriate pipe diameter.

Also, select the best materials when choosing the transfer pipes. Certain metals, such as copper or galvanized steel, can have severe negative effects on grease.

Keep in mind that when grease is subjected to pressure and specific conditions, a phenomenon known as "cake lock" occurs. When the mobility of the thickening agent in the grease is restricted, it can lead to blockages in the pipes or different parts of the equipment. The base oil may still flow, but the thickener does not move. Without the thickener, the base oil may not reach the necessary parts of the device, resulting in issues with lubricant shortage in the equipment. There is a possibility of adverse effects on various components of grease (base oil, additives, and thickeners) in this phenomenon, and no single component of grease alone causes an increase or decrease in the likelihood of this incident.

From the results of these three tests, you now have sufficient information for decision-making. Consider your budget for procuring a central lubrication system, but design your system focusing on the selected grease. Avoid common mistakes such as choosing inappropriate transfer pipes, applying insufficient or excessive pressure for grease transfer, and sacrificing essential materials for grease instead of grease mobility.

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