What is Grease?
Grease composition can be assumed like a sponge with fluid absorbed inside. In place of a sponge, the thickener is there that holds the base oil and additives. The base oil and additives are the major components in grease formulations and have a considerable influence on the performance of the grease in a particular application condition.
Base Oil
In general, Mineral Oils are used as the base oil for most of the greases. These mineral oil-based greases typically provide satisfactory performance in most industrial applications. But for some special application conditions like extreme temperatures such as -40º C or + 120 º C special synthetic oils are used so that grease can survive or maintain its stability.
Thickener
The thickener is a material that combines with the base oil and produces the solid to the semifluid structure. The primary type of thickener used in current grease is metallic soap such as lithium, polyurea, sodium, aluminum, and calcium.
As grease is the combination of both base oil and thickener so for high or low-temperature applications the thickness and base oil both should be able to withstand temperature and perform. For high-temperature applications complex thickeners are used due to high dropping point and load-carrying capacity. Complex thickeners are made by the combination of the conventional metallic soap and complexing agent e.g. Lithium complex, Aluminum complex, and Complex calcium sulphonate. Non-soap thickeners are also used in special applications such as high-temperature environments. Bentonite and silica aerogel are two examples of thickeners that do not melt even at high temperatures
Additives
The major function of additives is to enhance the properties of grease to perform well under extreme conditions. The most common additives are oxidation and rust inhibitors, extreme pressure, anti-wear, and friction-reducing agents.
In addition to these additives, boundary (thin-film condition) lubricants such as molybdenum disulfide (moly) or graphite may be suspended in the grease to reduce friction and wear without adverse chemical reactions to the metal surfaces during heavy loading and slow speeds.
| Type of Additive | Purpose | Compounds |
| EP additives | Protect against cold welding in case of extreme load | Sulphur & phosphorus organic compounds |
| Anti-wear additives | Reduce wear in extreme load and low-speed applications | zinc dithiophosphates, graphite or MoS2 |
| Corrosion protection additives | Protect against corrosive attack on bearing materials | phosphates, metal salts, sulphated waxes |
| Ageing inhibitors (anti-oxidants) | Delay oxidative decomposition | phenols, aromatic amines |
| Friction-reducing additives | Reduce friction under boundary and mixed friction conditions | fatty oils, oxidized waxes, graphite or MoS2 |
| Adhesion additives | Improve surface adhesion | olefin polymers |
Characteristics of Grease: Understanding grease technical data
Consistency
It is the measure of the stiffness of a grease. A proper consistency must ensure that the grease stays in the position without generating too much friction or in other words consistency is resistance to deformation by an applied force. Grease consistency depends on the type and amount of thickener used and the viscosity of its base oil.
Consistency is classified according to a scale developed by the NLGI (National Lubricating Grease Institute) which is measured based on the penetration of a cone inside a grease sample. The softer the grease, the lower the number. Greases for bearings are typically NLGI 1, 2, or 3. The test measures how deep a cone falls into a grease sample in tenths of mm (microns). ASTM D 217, ASTM D 1403, ISO 2137 methods measure penetration of new and used greases. To measure penetration, a cone of a given weight is allowed to sink into grease for five seconds at a standard temperature of 25°C (77°F). A penetration of 100 would represent a solid grease while a penetration of 450 would be semifluid. The NLGI has established consistency numbers or grade numbers, ranging from 000 to 6, corresponding to specified ranges of penetration numbers.
Initial position of the cone is at the surface of the grease and dial indicator measures the depth of penetration by cone.
Temperature range
This characteristic defines the suitable working range of the grease. A typical operating condition may have a temperature as low as -50 º C (or even lower) and may have a high temperature of 120 º C (or even higher, not only this, some operating conditions may have frequent variation in temperature going high to going low and vice a versa. So, grease type must be selected so that it can survive and perform under such extreme conditions. Generally, greases have a defined low-temperature limit (LTL) and the high-temperature performance limit (HTPL). LTL is defined as the lowest temperature at which the grease will allow the machine to be started up without difficulty. Below this limit, starvation will occur and cause a failure.
Below LTL, grease will become so vicious and hard that it will not bleed the oil and its Pumpability will be affected. Consequently, machinery operation may become difficult due to torque limitations and power requirements, for example in the case of ball bearing starting torque will be high and metal-to-metal contact will be there. As a guideline, the base oil’s pour point is considered the low-temperature limit (LTL) of a grease.
Above HTPL, the grease will degrade in an uncontrolled way by softening and bleeding, causing grease to flow away from needed areas. So that grease life cannot be determined accurately in such situations. Moreover, by its nature, grease cannot dissipate heat by convection like a circulating oil. Consequently, without the ability to transfer away heat, excessive temperatures result in accelerated oxidation or even carbonization where grease hardens or forms a crust. Effective grease lubrication depends on the grease’s consistency. The mineral oil in grease can flash, burn or evaporate at temperatures greater than 175°C.
Dropping point
The temperature at which a grease sample, when heated, will begin to flow through an opening according to DIN ISO 2176. It is important to understand that this point is considered to have limited significance for the performance of the grease as it is always far above the high-temperature performance limit (HTPL). The dropping point can also be considered as heat resistance of the grease. With the increase in the temperature, penetration is increased and consistency is reduced.
Viscosity
A measure of a fluid’s resistance to flow. For lubricants, a proper viscosity must guarantee an adequate separation between surfaces without causing too much friction. According to ISO standards, it is measured at 40 °C (105 °F), as viscosity changes with temperature. Values at 100 °C (210 °F) allow calculation of the viscosity index, e.g. how much the viscosity will decrease when the temperature rises. For a particular application, a Minimum value of viscosity is required to form the lubrication film that to sufficient to prevent metal to metal contact.
Water resistance
This is the ability of grease to withstand the effects of water with no change in its ability to lubricate. A soap/water lather may suspend the oil in the grease, forming an emulsion that can wash away or, to a lesser extent, reduce lubricity by diluting and changing grease consistency and texture.
A glass strip is coated with the grease to be tested, which is placed into a water-filled test tube. The test tube is immersed in a water bath for three hours at a specified test temperature. The change in the grease is visually evaluated and reported as a value between 0 (no change) and 3 (major change) along with the test temperature.
Oxidation stability : What is Oxidation & how it happens, Problems due to oxidation of lubricant, Measure and Control
Imaging the cooking oil being heated in the pan (utensil). Heat and oxygen will break down the organic compound (cooking oil in this case) which will result in black deposit and sludge on the utensil. This is a very quick process. Now the lubricating oil also face similar operating conditions of high temperature due to machine running and exposure to oxygen. In terms of chemistry, oxidation is the gain of oxygen and loss of hydrogen. Refer to chemical reaction below as an example. For simplicity in explanation, oxidation of methane CH4 is considered.
CH4 + 2O2 ——> CO2 + 2H2O
So, oxidation stability is the ability of grease to resist a chemical union with oxygen. The reaction of base oil of grease with oxygen leads to the formation of insoluble gum, sludges, and black deposits that cause breakage of lubrication film, Metal-to-metal contact, sluggish operation, increased wear, and reduction of clearances. Prolonged exposure to heat or high temperatures accelerates the oxidation of base oil.
In the case of cooking oil, the oxidation process is very fast because oil is directly being heated, but in the case of Machines like IC Engines, Turbines, Gearboxes, or rotating parts the oxidation process is slow and gradual (assuming heat is generated during operation only and no direct heat to oil). So during the oxidation process, there are many intermediate products that are acidic in nature.
Problems due to oxidation of lubricant
Oxidation is the most predominant reaction of a lubricant in service. It is responsible for numerous lubricant problems – including
> viscosity increase & filter plugging – caused by the formation of condensation products like varnish, sludge, and sediment
> additive depletion
> base oil breakdown
> loss in foam control
> acid number (AN) increase or the loss in base number in engine oils caused by the formation of acids
> rust formation and corrosion
Measure and Control of Oxidation
Every lubricant is designed with an oxidation controlling method. The formulation of each lubricant, therefore, contains antioxidants. These antioxidants are designed to be sacrificial, meaning they react or oxidize before the remainder of the lubricant (the base oil) to provide protection. Tests have been designed to measure oxidation reserve (the amount of protection remaining) and oxidation progress (the amount of oxidation that has occurred). Both testing methods have their advantages, and the effectiveness of these tests depends on the operation of the in-service fluid.
Pumpability
Pumpability is the ability of a grease to be pumped or pushed through a system. More practically, pumpability is the ease with which a pressurized grease can flow through lines, nozzles, and fittings of grease-dispensing systems.
Selection of right Grease
Selection of grease for a machine or application is one of the critical tasks as grease has to withstand the operation conditions. Application may be subjected to Loads, vibrations, temperature or require some safety compliance like food compatibility, biodegradability.
Production processes and raw materials greatly influence grease properties and performance. It is virtually impossible to select or compare greases based only on their composition. Therefore, performance tests are needed to provide crucial information regarding the actual properties of the grease.
end of article.
Keywords used in the post: Grease Lubrication, Grease Basics, Grease Anatomy, How grease works, Machinery Lubrication, Properties of grease, Base oil, Thickener, Extreme Pressure Additives, Functions, Application suitable for grease, Characteristics of grease, pumpability, water resistance, consistency, dropping point, oxidation stability, temperature range, high-temperature effect, lubrication management, National Lubricating Grease Institute (NLGI) , ASTM D 217, ASTM D 1403, ISO 2137, DIN ISO 2176, Dropping point, Temperature Range,