Lubrication - Grease

Grease

Grease is a semifluid to solid mixture of a fluid lubricant, a thickener, and additives.

The fluid lubricant can be petroleum (mineral) oil, synthetic oil, or vegetable oil.

Fluid Lubricant

Petroleum oils in general, naphthenic oils tend to chemically mix better with soaps and additives and form stronger structures than paraffinic oils.

Synthetic oils are higher in first cost but are effective in high-temperature and low temperature extremes. With growing environmental concerns, vegetable oils and certain synthetic oils are also being used in applications requiring nontoxic or biodegradable greases.

The base oil selected in formulating a grease should have the same characteristics as if the equipment is to be lubricated by oil. For instance, lower-viscosity base oils are used for grease applications at lower temperatures or high speeds and light loads, whereas higher-viscosity base oils are used for higher temperatures or low speed and heavy load applications

 The thickener gives grease its characteristic consistency that holds the oil in place. Common thickeners are soaps and organic or inorganic non soap thickeners.

The majority of greases on the market are composed of mineral oil blended with a soap thickener. Additives enhance performance and protect the grease and lubricated surfaces

Soap Thickeners

Soap thickener gives grease its physical character. Soap thickeners not only provide consistency to grease, they affect desired properties such as water absorption, thermal stability, ageing , additive quantity and  pumpability.

The principal ingredients in creating a soap are a fatty acid and an alkali. Fatty acids can be derived from animal fat such as beef  tallow, lard, butter, fish oil, or from vegetable fat such as olive,castor, soybean, or peanut oils.

The most common alkalies used are the hydroxides from earth metals such as aluminum, calcium, lithium, and sodium.

Soap is created when a long-carbon-chain fatty acid reacts  with the metal hydroxide. The metal is incorporated into the carbon chain and the resultant compound develops a polarity. The polar molecules form a fibrous network that holds the oil.

Thus, a somewhat rigid gel-like material “grease” is developed. Soap concentration can be varied to obtain different grease thicknesses.

Viscosity of the base oil affects thickness as well. Since soap qualities are also determined by the fatty acid from which the soap is prepared, not all greases made from soaps containing the same metals are identical. The name of the soap thickener refers to the metal (calcium, lithium, etc.)from which the soap is prepared.

The high temperatures generated by modern equipment necessitated an increase in the heat resistance of normal soap-thickened greases. As a result, “complex” soap greases were developed. The dropping point of complex grease is at least 38 °C higher than its normal soap-thickened ones, and its maximum usable temperature is around 177 °C (350 ºF).

Complex soap greases are limited to this temperature because the mineral oil can flash, evaporate, or burn above that temperature .Complex greases in general have good all-around properties and can be used in multipurpose applications. For extreme operating conditions, complex greases are often produced with solid lubricants and use more highly refined or synthetic oils.

A complex soap is formed by the reaction of a fatty acid and alkali to form a soap, and the simultaneous reaction of the alkali with a short-chain organic or inorganic acid to form a metallic salt(the complexing agent). Complex grease is made when a complex soap is formed in the presence of a base oil. Common organic acids are acetic or lactic, and common inorganic acids are carbonates or chlorides.

Additives

Surface-protecting and performance-enhancing additives that can effectively improve the overall

performance of a grease. Solid lubricants such as molybdenum disulfide and graphite are added to grease in certain applications for high temperatures (above 315 °C) and extreme high-pressure applications. Incorporating solid additives requires frequent grease changes to prevent accumulation of solids in components (and the resultant wear). Dyes that improve grease appearance and are used for identification purposes.

Mechanism of lubrication

Grease lubrication can be described as a temperature-regulated feeding device: when the lubricant film between wearing surfaces thins, the resulting heat softens the adjacent grease, which expands and releases oil to restore film thickness.

                                                Types of Greases

The most common greases are described below.

a.   Calcium grease.

  Calcium or lime grease, the first of the modern production greases, is prepared by reacting mineral oil with fats, fatty acids, a small amount of water, and calcium hydroxide (also known as hydrated lime). The water modifies the soap structure to absorb mineral oil.

Calcium grease is sensitive to elevated temperatures, due to water evaporation. which results in  structural collapses, resulting in softening and, eventually, phase separation.

Greases with soft consistencies can dehydrate at lower temperatures while greases with firm consistencies can lubricate satisfactorily to temperatures around 93 °C.

 Anhydrous calcium grease is prepared from 12-hydroxystearic acid , can be used continuously to a maximum temperature of around 110 °C .

Calcium complex grease is prepared by adding calcium acetate. This provides the grease with extreme pressure characteristics without using an additive. The maximum working temperature increases to approximately 177 °C .

Exceptions being of poor pump ability in high-pressure centralized systems, where caking and hardening sometimes occur calcium complex greases have good all-around characteristics that make them desirable multipurpose greases.

Lime grease does not emulsify in water and is excellent at resisting “wash out.” With a relatively low manufacturing cost.

b.Sodium grease.

 Sodium grease was developed for use at higher operating temperatures than the early hydrated calcium greases. Sodium grease can be used at temperatures up to 121 °C.

Sodium is sometimes mixed with other metal soaps, especially calcium, to improve water resistance, due to its solubility in water. It has better adhesive properties than calcium grease.

Usage of sodium grease is limited due to its solubility in water and cannot be used for all applications involving high temperature endurance. It is, however, still recommended for certain heavy-duty applications and well-sealed electric motors.

c.Aluminum grease.

Aluminum grease is normally clear and has a stringy texture, more so when produced from high-viscosity oils. When heated above 79 °C, this stringiness increases and produces a rubberlike substance that pulls away from metal surfaces, reducing lubrication and increasing power consumption.

       Aluminum grease has good water resistance, good adhesive properties, and inhibits rust without additives(inherent oxidation stability), but it tends to be short-lived due to operating temperatures at surfaces. relatively poor shear stability and pump ability.

Aluminum complex grease has a maximum usable temperature of almost 100 °C higher than aluminum-soap greases. It has good water-and-chemical resistance but tends to have shorter life in high-temperature, high-speed applications.

d.  Lithium grease.

lithium grease, by virtue of  smooth, buttery texture is more popularly used compared to other greases.

The normal grease contains lithium 12-hydroxystearate soap. It has a wide temperature applicability between -35 °C to 175°C . It has good shear stability and a relatively low coefficient of friction, which permits higher machine operating speeds. It has good water-resistance, not comparable to that of calcium or aluminum.

 Pump ability and resistance to oil separation are good. Additives such as rust inhibitors, Anti-oxidants and extreme pressure additives easily blendable in lithium greases.

Lithium complex grease and lithium soap grease have similar properties except the complex grease has superior thermal stability as indicated by a dropping point of 260 °C. It is generally considered close to a true multipurpose grease.

e.  Polyurea grease.

Polyurea is the most important organic nonsoap thickener. It is a low-molecular-weight organic polymer produced by reacting amines (an ammonia derivative) with isocyanates, which results in an oil-soluble chemical thickener.

 Polyurea grease has outstanding resistance to oxidation , owing to lack of metallic soaps (which tend to oxidize). It effectively lubricates over a wide temperature range of -20 to 177 °C and has long life.

Water-resistance is good to excellent, depending on the grade. It works well with many elastomer seal materials. It is used with all types of bearings but has been particularly effective in ball bearings. Its durability makes it well suited for sealed-for-life bearing applications.

Polyurea complex grease is produced when a complexing agent, most commonly calcium acetate or calcium phosphate, is incorporated into the polymer chain. In addition to the excellent properties of normal polyurea grease, these agents add inherent extreme pressure and wear protection properties that increase the multipurpose capabilities of polyurea greases.

f. Organo-clay.

 Organo-clay is the most commonly used inorganic thickener. Its thickener is a modified clay, insoluble in oil in its normal form, but through complex chemical processes, converts to platelets that attract and hold oil.

Organo-clay thickener structures are amorphous and gel-like rather than the fibrous, crystalline structures of soap thickeners. This grease has excellent heat-resistance since clay does not melt. Maximum operating temperature is limited by the evaporation temperature of its mineral oil, which is around 177 °C .

However, with frequent grease changes, this multipurpose grease can operate for short periods at temperatures up to its dropping point, which is about 260 °C

A disadvantage is that greases made with higher-viscosity oils for high thermal stability will have poor low-temperature performance.

Organo-clay grease has excellent water-resistance but requires additives for oxidation and rust resistance. Work stability is fair to good. Pumpability and resistance to oil separation are good for this buttery textured grease.

 Applications suitable for grease.

 Grease is generally used for:

(1) Machinery that runs intermittently or is in storage for an extended period of time. Because grease

remains in place, a lubricating film can instantly form.

(2) Machinery that is not easily accessible for frequent lubrication. High-quality greases can lubricate

isolated or relatively inaccessible components for extended periods of time without frequent replenishing.

These greases are also used in sealed-for-life applications such as some electrical motors and gearboxes.

(3) Machinery operating under extreme conditions such as high temperatures and pressures, shock

loads, or slow speed under heavy load. Under these circumstances, grease provides thicker film cushions

that are required to protect and adequately lubricate, whereas oil films can be too thin and can rupture.

(4) Worn components. Grease maintains thicker films in clearances enlarged by wear and can extend

the life of worn parts that were previously oil lubricated. Thicker grease films also provide noise

insulation.

Advantages of grease.

(1) Functions as a sealant to minimize leakage and to keep out contaminants. Because of its

consistency, grease acts as a sealant to prevent lubricant leakage and also to prevent entrance of corrosive

contaminants and foreign materials. It also acts to keep deteriorated seals effective (whereas an oil would

simply seep away).

(2) Easier to contain than oil. Oil lubrication can require an expensive system of circulating

equipment and complex retention devices. In comparison, grease, by virtue of its rigidity, is easily confined

with simplified, less costly retention devices.

(3) Holds solid lubricants in suspension. Finely ground solid lubricants, such as molybdenum disulfide

(moly) and graphite, are mixed with grease in high temperature service (over 315 EC [599 EF]) or in

extreme high-pressure applications. Grease holds solids in suspension while solids will settle out of oils.

(4) Fluid level does not have to be controlled and monitored.

Disadvantages of grease:

(1) Poor cooling. Due to its consistency, grease cannot dissipate heat by convection

     like a circulating oil.

(2) Resistance to motion. Grease has more resistance to motion at start-up than oil,      

     so it is not appropriate for low torque/high speed operation.

(3) More difficult to handle than oil for dispensing, draining, and refilling. Also, exact

      amounts of lubricant cannot be as easily metered.

                            Characteristics of Greases

 Apparent viscosity. Grease exhibits a resistance to motion upon startup of machinery implying a high viscosity.

However, as grease is sheared between wearing surfaces and moves faster, its resistance to flow reduces.

Its viscosity decreases as the rate of shear increases. The temperature of grease increases with increase in shear rate and apparent viscosity is the viscosity of grease defined at that temperature.

Bleeding is a condition when the liquid lubricant separates from the thickener. It is induced by high temperatures and also occurs during long storage periods.

Migration is a form of bleeding that occurs when oil in a grease moves out of the thickener network under certain circumstances.

If grease is pumped through a pipe in a centralized lubrication system, it may encounter a resistance to the flow and form a plug. The oil continues to flow, migrating out of the thickener network. As the oil separates from the grease, thickener concentration increases, and plugging gets worse.

If two different greases are in contact, the oils may migrate from one grease to the other and change the structure of the grease. Therefore, it is unwise to mix two greases.

 Syneresis is a special form of bleeding caused by shrinking or rearrangement of the structure due to physical or chemical changes in the thickener.

Consistency,

.

Consistency is its resistance to deformation by an applied force Grease consistency depends on the type , amount of thickener used and the viscosity of its base oil.

The measure of consistency is called penetration.

Penetration depends on whether the consistency has been altered by handling or working.

ASTM D 217 and D 1403 methods measure penetration of unworked and worked greases.

To measure penetration, a cone of given weight is allowed to sink into a grease for 5 seconds at a standard temperature of 25 °C. The depth, in tenths of a millimeter, to which the cone sinks into the grease is the penetration. A penetration of 100 would represent a solid grease while one of 450 would be semifluid.

Contaminants.

Greases tend to hold solid contaminants on their outer surfaces and protect lubricated surfaces from wear. If the contamination becomes excessive or eventually works its way down to the lubricated surfaces the reverse occurs -- the grease retains abrasive materials at the lubricated surface and wear occurs.

Corrosion- resistance.

 This denotes the ability of grease to protect metal parts from chemical attack. The natural resistance of a grease depends upon the thickener type. Corrosion-resistance can be enhanced by corrosion and rust inhibitors.

Dropping point.

Dropping point is the temperature at which a grease becomes fluid enough to drip. The dropping point indicates the upper temperature limit at which a grease retains its structure, not the maximum temperature at which a grease may be used. A few greases have the ability to regain their original structure after cooling down from the dropping point.

Evaporation.

The mineral oil in a grease evaporates at temperatures above 177 ºC .Excessive oil evaporation causes grease to harden due to increased thickener concentration. Therefore, higher evaporation rates require more frequent relubrication.

 Fretting wear and false brinelling.

 Fretting is friction wear of components at contact points caused by minute oscillation. The oscillation is so minute that grease is displaced from between parts but is not allowed to flow back in. Localized oxidation of wear particles results and wear accelerates. In bearings, this localized wear appears as a depression in the race caused by oscillation of the ball or roller.

The depression resembles that which occurs during Brinell hardness determination, hence the term “false

Brinelling – usually occurs during transportation and storage conditions exhibiting sound transmission.

Oxidation stability.

This is the ability of a grease to resist a chemical union with oxygen. The reaction of grease with oxygen produces insoluble gum, sludges, and lacquer-like deposits that cause sluggish operation, increased wear, and reduction of clearances. Prolonged high-temperature exposure accelerates oxidation in greases.

 Pumpability.

Pumpability is the ability of a grease to be pumped 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.

Slumpability, or feedability, is its ability to be drawn into (sucked into) a pump. Fibrous greases tend to have good feedability but poor pumpability. Buttery-textured greases tend to have good pumpability but poor feedability.

Shear stability.

Grease consistency may change as it is mechanically worked or sheared between wearing surfaces. A grease’s ability to maintain its consistency when worked is its shear stability or mechanical stability.

A grease that softens as it is worked is called thixotropic. Greases that harden when worked are called rheopectic.

High-temperature effects.

 High temperatures harm greases more than they harm oils.Disipation of heat is minimal due to lack of convection .Excessive temperatures results in an accelerated oxidation / carbonization resulting in  hardening as a crust.

 High temperatures induce softening and bleeding, causing grease to flow away from needed areas.

 The mineral oil in grease can flash, burn, or evaporate at temperatures above 177 °C .High temperatures, above 73-79 °C dehydrate certain greases such as calcium soap grease and cause structural breakdown.

The higher evaporation and dehydration rates at elevated temperatures necessitate frequent grease replacement.

Low-temperature effects

If the temperature of a grease is lowered enough, it will become so viscous that it can be classified as a hard grease. the base oil’s pour point is considered the low-temperature limit of a grease.

Texture.

 Texture is observed when a small sample of grease is pressed between thumb and index finger and slowly drawn apart.

Brittle: the grease ruptures or crumbles when compressed.

Buttery: the grease separates in short peaks with no visible fibers.

Long fiber: the grease stretches or strings out into a single bundle of fibers.

Resilient: the grease can withstand moderate compression without permanent deformation or rupture.

Short fiber: the grease shows short break-off with evidence of fibers.

Stringy: the grease stretches or strings out into long, fine threads, but with no visible evidence of fiber structure.

Water resistance.

 This is the ability of a 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.

Rusting becomes a concern if water is allowed to contact iron or steel components.