Grease and Grease Lubrication
Grease and Grease Lubrication
Grease is a semi solid lubricant. It is a mixture consisting of natural or synthetic oil base combined with thickeners and additives. It generally consists of a soap emulsified with mineral or vegetable oil.The National Lubricating Grease Institute (NLGI) defines grease as ‘a solid to semi-solid product of dispersion of a thickening agent in a liquid lubricant. Additives imparting special properties may be included.’
The requirements which grease is required to satisfy generally consist of (i) extreme pressure (EP) resistance, (ii) reduced friction, (iii) high wear resistance, (iv) high thermal stability, (v) very good rust-proofing performance, (vi) very good fluidity, and (vii) its consistency not to vary significantly even with repeated stirring. Greasing intervals can vary depending on the usage, the environment, and the conditions. The ability of any particular grease to be handled by grease pumps, grease dispensers, and other components in an automated greasing system depends on the grease viscosity (thickness). Grease is widely used in specialized lubrication applications including roller bearings and low speed gear systems.
The characteristic feature of grease is that it possesses a high initial viscosity, which upon the application of shear, drops to give the effect of an oil-lubricated bearing of around the same viscosity as the base oil used in the grease. This change in viscosity is called shear thinning. Grease is sometimes used to describe lubricating materials which are simply soft solids or high viscosity liquids, but these materials do not show the shear-thinning properties characteristic of the classical grease. Grease is normally applied to mechanisms which can be lubricated only rarely and where the lubricating oil normally does not stay in position. It also acts as sealant to prevent ingress of water and incompressible materials.
Depending on the application, grease can present several benefits over fluid lubrication. Grease provides a physical seal preventing contamination ingress, resists the washing action of water, and can stay in place in an application even in vertically mounted positions. Grease is uniquely suited for use in applications where re-lubrication is less frequent or economically unjustifiable, due to the physical configuration of the mechanism, type of motion, type of sealing, or the need for the lubricant to perform all or part of any sealing function in the prevention of lubricant loss or ingress of the contaminants. Due to their semisolid nature, grease does not provide application cooling and cleaning functions associated with the use of a fluid lubricant. With these exceptions, grease performs all other functions of a fluid lubricant. While fluid lubricants are typically preferred by design, the afore mentioned mechanical circumstances always exist in several places and thus the need for grease remains. As a result, greases are used in around 80 % of rolling elements bearings.
Grease was first used by the Egyptians on their chariot axles more than 3000 years ago. Grease from the early Egyptian or Roman eras is thought to have been prepared by combining lime with olive oil. The lime saponifies some of the tri-glyceride which comprises oil to give the calcium grease. In the middle of the 19th century, soaps were intentionally added as thickeners to oils. Over the centuries, all manner of materials have been employed as greases. For example, black slugs ‘Arion ater’ were used as axle-grease to lubricate wooden axle-trees or carts in Sweden. Lithium soap greases, the most common worldwide, were introduced in the early 1940s. Lithium complex greases were introduced in the early 1960s.
Lubricating grease is composed of liquid and solid phases and is basically comprised of three components namely (i) base oil, (ii) thickener, and (iii) additives and modifiers. The thickener defines the type of the grease. The liquid phase of grease is primarily formed by the base fluid and the solid phase is formed by a network structure of soap molecules or a dispersion of solid particles such as inorganic clays or other fillers. The solid phase thickener can consist of soap molecules with or without added polymer. The base oil in the grease is immobilized by the soap molecule network structure, resulting in a semi-solid to solid appearance. The base oil solubilizes performance additives and modifiers.
Base oil comprises the largest component of grease, representing 80 % to 97 % by weight. The choice of base fluid can be mineral oil, synthetic oil, or any other fluid which provides lubricating properties. The base oil portion of the grease performs the actual lubrication except in very slow or oscillating applications. The same rules which are applied to determine proper viscosity grade in a fluid lubricant, apply to the selection of the base oil portion of lubricating grease.
The thickener in the lubricating grease is also responsible for the thickness of the lubrication film on the working surfaces of the friction joints. The function of the thickener is to provide the gel-like network structure. Generally, the soap thickener is a metallic salt of a long-chain fatty acid, e.g. lithium 12-hydroxy stearate. The soap thickener form interlocked fibers in grease. Thickener can be any material which, in combination with the base oil, produces the solid to semi-fluid structure. Simply put, a grease thickener in combination with the base oil acts much the same way as a sponge holding water. Principal thickeners used in greases include lithium, aluminum, and calcium soaps, clay, and polyurea either alone or in combination. Lithium soap is the most common thickener in use today. Incorporating polymers into the grease can further enhance the properties of the grease such as consistency, shear stability, water resistance, adhesion, tackiness, and soap yield. Polymers such as poly-ethylene, poly-propylene, poly-iso-butylene, halogenated poly-ethylene, poly-methacrylate, and poly-urea are reported to improve the properties of greases.
Apart from the thickener, the lubricating grease microstructure can also contain solid additives (Fig 1). Their main function is to improve the tribological properties of the lubricant. They significantly influence the rheological properties of the grease. The percentage of the solid additives in the grease normally does not exceed 5 %. Additives also called sometimes modifiers in the grease, as in lubricating oil additives, impart special properties or modify the existing ones. Additives and modifiers normally used in lubricating greases are oxidation or rust inhibitors, metal powders, polymers such as poly-tetra-fluoro-ethylene (PTFE), EP additives, anti-wear agents, lubricity or friction-reducing agents (soluble or finely dispersed particles such as molybdenum di-sulphide, MoS2 and graphite), and dyes or pigments. Dyes or pigments impart colour only having no effect on the lubricating capability of the grease.
Fig 1 Types of solid additive for grease
In order to achieve the optimum tribological and rheological properties of a lubricating compound, greases containing variously dispersed thickener particles ought to be mixed. It concerns particularly the lubricants thickened with metal soaps. Too high particles dispersion of the thickener in the grease can negatively influence its lubricating properties. Such particles do not present enough capability of making spatial, three-dimensional structures resulting from the physicochemical interactions. Too long particles of the thickener cause too high an increase of the consistency of the lubricating grease and lead to its easy breaking both in the lubrication systems and the tribological pairs.
Lubricating greases are rheologically complex two-phase non-Newtonian fluids. They are chemically and physically heterogeneous. The dispersive phase is normally a mineral oil, a synthetic oil or a vegetable oil, whereas the dispersed phase is a thickener and, depending on the needs, solid additives. The particles of the thickeners can vary in their dimensions. Soaps, for example, do not normally exceed 100 micrometres in length, and their diameter is not shorter than 0.1 micrometre and not longer than 0.5 micrometre. The lithium and calcium soaps particles are normally bigger than the sodium soaps particles. The isometric aggregates of bentonite clay and mica are around 0.5 micrometre in width and 0.1 micrometres in thickness. The solid additives have similar dimensions. Due to the size of the thickener particles, the greases acquire the characteristics of a mechanically dispersed (suspension) or a colloidal system. The thickener particles size depends primarily on the process of the grease production, as well as on the conditions in the friction node. During the shearing of grease in the friction node, the size of the particles can change considerably.
Microstructure of the lubricating greases
Microstructure of the lubricating greases with soap thickeners can be compared to a sponge with the lubricating oil (Fig 2). It makes a three-dimensional, coherent network of interconnected particles (flow units), known as ‘floccules’. The oil is locked in the free spaces of the microstructure through the mechanical occlusion, the capillary phenomena as well as the molecular attraction between the thickener and the polar components of the oil. It is estimated that the amount of the oil locked in the microstructure of the soap greases can amount even to above 90 %. The soap particles making the microstructure, from the chemical viewpoint, are associated molecules, namely groups of identical molecules generated as a result of the dipol-dipol type of interaction or the hydrogen (H2) bonds. The final skeleton of the microstructure (shaped in the dispersion centre) is made in situ in the process of crystallization of the soap particles and/or through nucleation, namely making crystallite nucleuses and their further growth.
Fig 2 Typical microphotographs taken by SEM
The shape of the thickener particles and their surface topography can be different, depending on the kind of soap used and the particles’ size. If they exceed the size of 1 micrometre, their structure is rough and resembles twisted ropes. The crystallites of the colloidal size are definitely smoother and less twisted. The microscope photographs also show a clear difference in the surface structure of the lithium, sodium and calcium thickener floccules. The calcium soap particles are rougher than the particles of the lithium and sodium soaps, independently of their length. In the case of the non-organic thickeners, the grease microstructure is made of numerous individual aggregates popularly called open card-house, for example bentonite (Fig 2), mica or vermiculite. Such particles have a skeleton structure which resembles heterogeneous, fuzzy-edged, curled flakes or straight plates piled one on the top of the other.
The shape of the thickener (soap, other organic or non-organic thickeners) particles, their anisometry (in reference to their length and lateral dimensions), and their dispersivity as well as their percentage in the full volume of the grease, greatly influence the physical properties of the ultimate lubricating compound. The last two factors are critical for the easiness of the making of different energy connection types between the elements of the microstructure. Apart from providing the appropriate consistency, thickeners influence the way the lubricating grease flows, changes its shape or the type of flow resistance (pumpability) it presents. It is particularly important in the central lubrication systems where the lubricant is often transported in long conduits to particular receiving points.
Consistency of grease
Consistency is a basic property of grease. It is defined as the degree to which a plastic material resists deformation under the application of a force. In the case of lubricating greases, this is a measure of the relative hardness or softness and has some relation to flow and dispensing properties. Consistency is measured by the cone penetration method.
Consistency is normally measured by means of ‘the cone penetration test’. The results also can be affected by recent agitation or applied shear stress. To take this phenomenon into consideration, grease is generally subjected to ‘working’ (a standardized churning process) prior to measuring its penetration value. The cone penetration test is one of the most commonly performed tests on grease products. In the test, a penetrometer cone of given material, weight, and finish is dropped into the grease. The cone is allowed to sink into the grease under its own weight for 5 seconds. The depth the cone has penetrated is then read, in tenths of a millimeter on the dial. The further the cone penetrates the grease, the higher the penetration result and the softer the grease. Fig 3 shows a picture of the penetrometer, the device used to measure penetration and the principle of testing. The figure also shows the varying degrees of penetration or consistency with the left-hand picture depicting the beginning alignment of the cone, the middle picture shows the cone dropped into thicker grease, and the right-hand picture shows the cone dropped into slightly less thick grease.
Fig 3 Cone penetration method for testing grease consistency
Grease can be affected by agitation or shear stress. To determine this property, then a perforated plate can be driven through the grease via a so-called stroker for a specified number of cycles, usually 60 strokes. But for applications where shear stability is critical, 100,000 strokes are applied.
NLGI has developed a classification system for rating the consistency of greases based on the millimeters the cone penetrates the grease. NLGI has standardized a numerical scale for grease consistency based upon ASTM D 217 worked penetration ranging from 000 for semi fluid to 6 for block greases. The most common grease grade is NLGI 2 representing a smooth, buttery consistency. It is to be noted that grease consistency is related to thickener content and has no relationship to base oil viscosity. Tab 1 gives the NLGI grease grades.
|Tab 1 NLGI classification of grease grades|
|NLGI consistency grade||Penetration range (1/10 mm)||Description|
|000||445 – 475||Semi fluid|
|00||400 – 430||Semi fluid|
|0||355 – 385||Very soft|
|1||310 – 340||Soft|
|2||265 – 295||Common grease|
|3||220 – 250||Semi hard|
|4||175 – 205||Hard|
|5||130 – 160||Very hard|
|6||85 – 115||Solid|
The test methods provide different results for ‘worked’ and ‘unworked’ grease. Handling or working the grease tends to soften it a bit, resulting in a higher penetration number. Since any grease which is delivered through an automated greasing system is definitely worked, we have provided only worked values here. Data sheets, available from grease manufacturers usually provide the worked and unworked penetration values, along with the NLGI grade.
Structural stability of grease
The mechanical stability is an essential performance characteristic of the lubricating grease as it is a measure of how the grease consistency changes in service when it is subjected to mechanical stress (shear) resulting from the churning action caused by moving elements or vibrations generated by, or external to, the application. Grease softening in a bearing can eventually cause grease to leak out from the housing, requiring more maintenance and frequent grease replenishment to avoid premature failure resulting from lack of lubricant on the rolling elements. In order to have good mechanical stability, greases are developed through careful selection of the thickener composition and optimization of the manufacturing process. Mechanical stability is often measured using the prolonged worker test (e.g., 100,000 double strokes), or the roll stability test. The roll stability test subjects the grease to shearing by rotating a cylinder containing a 5 kg roller at 165 rpm (revolutions per minute) for 2 hours. The change in penetration at the end of the tests is a measure of the mechanical stability. This test produces low shearing forces just about equal to those found in the grease ‘worker’ used in the cone penetration test.
In application and use, ingress of environmental contaminants is unfortunately a common reality which often adversely affects the mechanical stability of the grease. It is important that grease not only be developed to provide excellent structural stability in a perfect state, but also in the presence of environmental contaminants such as water, process fluids, or other contaminants. This can be assessed by means of laboratory bench tests operating in a variety of conditions with presence of water.
The dropping point of grease is the temperature at which the thickener loses its ability to maintain the base oil within the thickener matrix. This can be due to the thickener melting or the oil becoming so thin that the surface tension and capillary action become insufficient to hold the oil within the thickener matrix. In the standard method used to determine the dropping point of grease, a small grease sample is placed in a cup and heated in a controlled manner in an oven-like device. When the first drop of oil falls from the lower opening of the cup, the temperature is recorded to determine the dropping point. Dropping point is a function of the thickener type. High drop points, typically higher than 240 deg C, are commonly observed for lithium complex, calcium complex, aluminum complex, polyurea and clay greases while much lower dropping points are typical of conventional lithium (180 deg C), calcium (180 deg C) and sodium (120 deg C) soaps. The dropping point is one of the determinations that characterize the thermal stability of the grease. However it is not an accurate prediction of the upper operating temperature limit for the grease which is a function of many variables such as base oil oxidation stability, additive degradation, thickener shearing, oil separation and many more. A high dropping point, while not a predictor of upper operating temperature, is an indicator of the maximum peak temperature which the grease can be subjected to for a short duration while not releasing oil excessively and hence drastically reducing the life of the grease and potentially damaging the application in the long run.
Advantages and disadvantages of grease lubrication
Advantages – Grease has a better stop-start performance. When the system shuts down, the oil drains away in case of oil lubrication, while grease remains in the component. The risk of contamination products such as food and pharmaceutical type products is reduced with the use of grease due to its resistance to flow into the products. Grease decreases dripping, splattering and leakage and reduces noise. With the use of grease the machinery tends to need less power.
Disadvantages – Grease reduces cooling and heating transfer. In case of oil lubrication, the flow of oil removes heat from the point of generation where it can be removed or dissipated. Grease tends to hold heat in place. Grease has poorer storage ability. Too long storage can lead to separation of base oil and thickener and can also result in altered properties. In case of grease lubrication, grease is usually not reaching all places in need of lubrication. Grease also cannot be used at the high speeds for which liquids lubricants are well suited.