Fundamentals of Grease

When man first invented the wheel there was a need for lubrication, and through the ages as technology advanced lubrication has kept pace.

Origin of Grease Use

We know for a fact that as early as 1400 BC, grease made of a combination of calcium and animal fat was used to lubricate chariot wheels. However, it was not until after 1859 with the discovery of oil, that grease as we know it today made its appearance.

Today, grease are employed extensively in the industrialised world in many thousands of varied applications both simple and sophisticated. 

What is Grease?

The ASTM states that “a lubricating grease is a solid to semi-solid product of dispersion of a thickening agent in a liquid lubricant”,and adds that. “other ingredients imparting special properties may be included”.

In other words a grease consists of a thickener plus oil plus additives.

Advantages of Grease

First, grease is usually preferable when the design of the device being lubricated – such as a rolling element bearing or gear set – make it impossible to avoid oil leakage which may contaminate goods being produced or create a safety hazard.

Then, there are situations where high operating temperature precludes the use of ordinary oils. In some cases synthetic oils may be used; however, often times greases are best.

Further, grease may be used to lubricate a bearing where accessibility is difficult or the unit will operate for a long period of time between servicing.

 

The inherent consistency of grease affords good sealing action. This makes it effective in dusty areas where parts may be exposed, as in earth-moving equipment. 

The same sealing action can protect machine parts from rust because it excludes their exposure to moisture.

Finally, the methods by which grease can be applied directly to the lubrication point are often less complicated and less costly than with a fluid.

Disadvantages of Grease

Conversely, one must recognise that grease can possess certain disadvantages. Since it does not flow or circulate like oil, it is not as effective in dissipating heat.

The immobility of grease also mean that it can’t flush out contaminants as readily as oil.

Furthermore, some greases tend to undergo certain structural changes with age. Some may lose their oil content and become hard and in other conditions, a grease may become soft and lose its structure because of vibration, moisture, duration in storage, bleeding etc.

Composition of Grease

A lubricating grease is made up of three basic components –

• Soap Thickeners – any material which, in combination with the selected fluid will produce a grease structure.
• Fluid Lubricant – any fluid that has lubricating properties.
• Additives and Modifiers – these represent any additives or modifiers that are used to impart special properties or modify existing ones.

Oil	Product 2	Product 3
Mineral Synthetic	Soap Non-Soap	Oxidation Inhibitors Rust and corrosion inhibitors Extreme pressure agents Pour point depressants Lubricity or friction modifiers Dyes/pigments/fillers

Grease Thickener

A grease thickener (soap) is a compound made by a chemical reaction called saponification between an alkali and a fat.

Saponification is the reaction of alkali metal and fatty acid is fundamental to the manufacture of soap type greases.

For this chemical process to occur the alkali and fat are combined in a reaction vessel under predetermined pressure and temperature conditions. A soap is consequently formed along with water. The water can be vented off.

The grease thickener is the component which makes the difference between a grease and a liquid lubricant.

Three prerequisites of a thickener material are:

• Insoluble in the fluid being thickened.
• Finely divided.
• Largest surface area.

There are two classifications of thickeners, soap and non-soap. Soap thickeners are selected in most applications as they are more cost effective and also possess an inherent lubricity.

Soap thickened grease are classified by type of alkali metal.

Types of alkali:

• Lithium
• Calcium
• Sodium
• Aluminium
• Barium

Fat Type (Acids)

Common fats used in grease manufacture are derived by rendering the fatty material from vegetable or animal tissue. Fats (which are long chain fatty acids) can range from liquids to solid materials. 

Various types of fats rendered from beef tallow are –

• Myristic
• Palmitic
• Stearic
• Oleic

Types of Thickeners

There are four main classifications of grease manufactured today.

Fluid Lubricant Types

1. Mineral Oil

The second main component used in making a grease is the fluid.

Grease made with mineral oils provide satisfactory performance in most automotive and industrial applications.

The types of mineral oils used are usually –

• Paraffinic
• Naphthenic

Bitumen is often used for open gear type applications.

2. Synthetic Fluids

In very low or high temperature applications, or in applications where the temperature may vary over a wide range, grease made with synthetic fluids are now used to a considerable extent. While the main use of synthetic greases in aircraft application, increasing quantities are now being used in specialized industrial application.

The synthetic fluids used most commonly in grease are silicones and esters, although synthetic hydrocarbons are achieving increasing popularity. Most of these materials are used because they have –

• Superior low temperature fluidity
• Higher viscosity index
• Lower volatility
• Better oxidation stability
• A combination of properties that permits manufacture of a grease with wider temperature range capacity.

Additive and Modifiers

Additives and modifiers used most commonly in a lubricating grease are –

• Oxidation inhibitors
• Rust and corrosion inhibitors
• Extreme pressure agents
• Pour point depressants
• Lubricity or friction modifiers 
• Dyes/pigments/fillers

Most of these materials have much the same function as similar material do when added to lubricating oils.

It should be notes that additive response is often different in a grease compare to oils.

Manufacture of Grease

The manufacturer of a soap based grease involves five basic steps.

1. Saponification
2. Dehydration
3. Cutback
4. Deaeration
5. Filtering

In some manufacturing processes some of these steps may be accomplished simultaneously, while in others they are distinct and separate steps.

The process require accurate weight or volumetric measurements of all feed components, intimate mixing, rapid heating and cooling, together with milling dehydration and deaeration.

The primary requirement is a suitable vessel for making the soap. Oil is first charges into the contractor, a pressure reaction vessel, and all the soap ingredients are added. Saponification is then conducted at uniform temperature.

Once saponification is completed the remainder of oil and additives are added to complete the manufacture of the grease. This is usually done in the finishing kettle.

Grease is then pumped to an homogeniser, deaerator, and then filled into drums or bulk containers.

Grease Classifications

Simple Normal Soap Grease

A normal calcium soap grease requires stearic acid plus an alkali such as lime which results in calcium stearate with water added to provide a structure modifier.

Lubricating fluid and additives are then added to the calcium stearate thickener to obtain a normal calcium soap grease. This grease would provide a relatively low dropping point of about 80-90 Degrees Celsius (176-194 degrees Fahrenheit) and would have only fair stability overall.  

Improved Simple Normal Soap Grease

In order to improve the formulation, gain better stability and yield a higher temperature range, the type of acid is changed to 12 hydroxystearic acid which provides built-in tie water (water built onto the chemical formulation).

The hydroxyl group of 12 carbon replaces water used in the first formula example. This change of acid choice increases the dropping point of the grease to about 170 degrees Celsius (338 degrees Fahrenheit). It also increases the cost of the product.

Complex Soap Greases

 To obtain a calcium complex formulation grease, the use of two or more dissimilar fats are used plus an alkali.

Since water is also added but later boils off in the process, dropping points as high as 260 degrees Celsius (500 degrees Fahrenheit) can be realised. 

Non-Soap Thickeners

The most common type of non-soap thickeners are the clay gells.

These are produced by reacting special clays (called smectites) which are basically hydrophilic with an amine compound to make the clay organophilic.

These activated clays now possess a thin flexible platelet or lathe type structure.

With the addition of an organic polar compound a new structure similar to a house of cards is formed.

This structure or gell is held together by hydrogen bonding.

Once the gell has been formed oil and additives can be included in the same manner as for soap based greases.

Grease Characteristics

Calcium Soap Grease

Calcium greases are limited in use to moderate service temperature. They have good pump ability at lower temperatures and provide good lubrication in the presence of water.

Because calcium greases contain water as part of the formulation, the product must be stored under conditions where water cannot be lost or evaporated. Should water be lost, grease will separate into gummy soap residue and oil.

Frequent re-lubrication is important since the grease structure is only moderately stable. 

Calcium greases are older type greases and thus are somewhat primitive is many of today’s applications. Very little grease of this type is sold today.

Lithium Soap Grease

Lithium soap grease were first introduced in the 1940’s and now serves over 50% of the U.S. market.

They have the heat resistance of the soda base greases (dropping point of 175 degree Celsius (347 degrees Fahrenheit), usable temperature range 100-125 degrees Celsius (212-257 degrees Fahrenheit)).

Lithium greases have a good resistance to water wash. 

These grease have good pump ability characteristics in centralised systems but may tend towards oil separation in fairly static, constant pressure systems (grease cups, slow cycle central systems).

The improvements in grease formulations have produced an extreme pressure line of lithium greases with ecologically acceptable unleaded materials.

Lithium greases have wide temperature performance characteristics. They have good mechanical stability and hold their structure well in severe working applications.

Complex Soaps

A complex soap grease basically consists of a normal soap, such as calcium, plus a salt like calcium acetate. The soaps and salts combine in fibers which provide a thickener system of unique characteristics. Complex greases possess unusual combinations of properties not found in simple or mixed soap greases.

The most important complex soap greases are made from salts of calcium, and lithium; however, sodium, magnesium, aluminium, strontium, barium and lead have some applications.

Calcium Complex Greases

Calcium complex greases are used as multipurpose automotive and industrial greases. Examples include heavy duty construction machinery and mining equipment.

Calcium complex greases have built in extreme pressure properties, good shear stability, good corrosion and water resistance. 

Preferred in high temperature applications because they retain their consistency. They have drop points of up to 300 degrees Celsius (572 degrees Fahrenheit).    

Lithium Complex Greases

Lithium complex greases are finding increasing use as multipurpose automotive and industrial greases. 

They are especially good in high temperature high speed bearings. They have drop points between 260-300 degrees Celsius (500-572 degrees Fahrenheit). They have good water sensitivity and good shear stability.

Successful applications include bearing lubrication in paper machine and in automotive front wheels equipped with disc brakes. 

Clay – Gell Greases

Clay – gell grease have a number of outstanding properties. These include –

• good metal adhesiveness
• drop points greater than 250 degree Celsius (482 degrees Fahrenheit)
• high water resistance
• multipurpose applications
• excellent pump ability in synthetic oils

Their main drawbacks are their cost compared to soap base greases and only fair pumpability in mineral oil.  

Grease Characteristics on Application

Structure

Smooth and buttery grease generally provide better pump ability – especially at low temperature, and are more suitable for multipurpose use.

Fibrous-type greases provide for better sealing and stay-in-place capabilities, while poorer pump ability and slump ability characteristics are experienced.

Note: Any grease can be made smooth/buttery or fibrous.

Temperature

Many factors determine the upper temperature limit for satisfactory grease lubrication. In general, most grease are not harmed at moderate temperatures, but continued exposure to high temperature causes them to deteriorate.

Machine elements operating at high temperature must be lubricated more often. Just how often, i.e. hours, days, weeks or longer, depends on whether the high temperature is continuous or peak, the type of element, the load, and exposure to other conditions which may harm grease properties. 

Water Resistance

Water churn conditions normally occur in the rolling element bearings. Therefore, water resistance of a grease is important in that all grease will adsorb (physical attachment) water except sodium type greases which absorb (chemical inclusion) water. Soda greases continue to absorb water until all consistency is lost – such as a bar of soap. All other greases absorb 20-100% of their volume with water.

A good quality water resistant grease adsorbs a certain percentage of water with little change in consistency, then repels all remaining water. Some greases lose a small amount of consistency such as lithium, others such as clay types firm up when contaminated with large volumes of water.

Absorption and absorption are desirable characteristics in that they eliminate free water – thus provide good rust protection.

Water spray conditions occur when water impinges directly onto an application where grease can be washed off very easily. Grease factors which can prevent rapid washing or (displacement) of a grease in an application are:

• Texture – fibrous type grease are not washed off as rapidly as smooth/buttery type greases.
• Oil Viscosity – grease made with heavy viscosity oils do not wash off as rapidly as do greases with light viscosity oils.
• Consistency – heavy consistency grease reduces the washing effect of water.

When a grease is subjected to either water churn or spray conditions, an NLGI #2 grease is the normal recommendation.

Extreme Pressure Properties

Calcium greases have inherent EP properties since the calcium soap is considered and EP agent. All other grease types have EP properties built into them. 

Oxidation Resistance 

Oxidation resistance is provided through the use of additives and proper selection of the base oil.

Mechanical Stability

Mechanical stability is formulated into the grease at the time of manufacture. Soap type also plays an important role in that the fatty components provide varying degrees of stability.

Grease Selection Factors

Selection of a grease for a given application depends on many factors such as –

• Type of machine element
• Speed 
• Temperature
• Load
• Environment
• Method of application

For example –

• Softer grade greases are preferred for centralised lubrication system.
• Low viscosity base oils tend to favour flow properties at low temperatures.
• For gun and grease cup application, stiffer grades are preferred.
• Spray application is usually limited to very soft or semi-fluid greases.

Compatibility of Grease

In general, grease having the same soap types are considered compatibility, however there are exceptions to this rule, so the best recommendation is don’t mix greases in service unless –

• Compatibility has been checked by the laboratory.
• Element to be lubricated is completely purged of the grease being replaced.
• Previous field service experience has proven the grease in question to be compatible.
• The element being lubricated is watched closely to determine if the greases are compatible (incompatibility usually leads to the grease becoming soft and leaks away from the element and failure is due to lack of lubricant). 
• When grease incompatibility exists, it is known to be more serious at higher operating temperatures.

Grease Tests

Most of the grease tests that have been standardised, define or describe properties that are related to the performance of a grease in specific applications, or to its storage and handling characteristics.

Some tests measure physical or chemical properties, but a considerable proportion are performance type tests in actual or simulated operating mechanisms.

Direct correlation between laboratory tests and field performance is rarely possible since the tests never exactly duplicate service conditions.

For these reasons, an understanding of the intent and significance of the tests is essential for those involved with the use of lubricating grease.

Penetrometer for Grease Consistency Testing by http://www.pdm.co.th/c1/services/lubricant-testing

Consistency

Consistency is defined as the degree to which a plastic material such as a lubricating grease resists deformation under the application of force. It is therefore a characteristic of plasticity, as viscosity is a characteristic of fluidity. The consistency of a lubricating grease is not constant but varies with temperature, and may also vary as a result of the handling or mechanical working that the grease has been subjected to before measurement of its consistency. 

Consistency is reported in terms of –

• ASTM Cone Penetration
• NLGI Number
• Apparent Viscosity

Cone Penetration

In this test a double-taped cone is allowed to sink, for 5 seconds, under its own weight into a sample held at 77F (25C). The amount that the cone penetrates into the grease is measured in tenths of a millimetre and reported as the penetration of the grease. Since the cone will sink further into soft grease, high penetrations indicate a soft grease.

Penetrations are reported as Undisturbed Penetrations, Unworked Penetrations, Worked Penetrations, or Prolonged Worked Penetrations.

Undisturbed Penetrations are measured in the original container without disturbance, and are determined mainly in cases where hardening or softening in storage has been reported.

Unworked Penetrations are measured on samples transferred to the greased cup with minimum disturbance. This value may have some significance with regard to transferring grease from the original containers to application equipment.

The value normally reported is the Worked Penetration, measured after the sample has been worked 60 double strokes in the ASTM Grease Worker. It is considered to be most reliable since the amount of disturbance of the sample is controlled.

Prolonged worked penetration is discussed later under Mechanical or Shear Stability.

NLGI Grease Numbers

On the basis of worked Penetration, the National Lubricating Grease Institute (NLGI) has standardised a numerical scale for classifying the consistency of grease.

The NLGI system of consistency grading is entirely adequate for most applications where the preferred consistency of grease is specified.

Apparent Viscosity

Apparent viscosities of grease are determined by forcing samples of grease through a set of capillary tubes at predetermined flow rates. From the dimensions of the capillaries, the known flow rates, and the pressure required to force the grease through the capillaries at those flow rates, the apparent viscosity of the grease can be calculated.

Apparent viscosity is useful in predicting the handling and dispensing properties of a grease. It can be related to starting and running torques in grease lubricated mechanisms, and is also useful in predicting leakage tendencies. 

Shear Stability

The ability of a grease to resist changes in consistency during mechanical working is termed its shear stability or mechanical stability.

In prolonged working, the ASTM Grease Worker is commonly run for 10,000, 50,000, or 100,000 double strokes. After working, the sample and worker are brought back to 25C in 1.5 hr, worked 60 double strokes and the penetration measured.

The change in the worked penetration is an indication of shear stability, and is indicative of the change in consistency that a grease will undergo in service.

Dropping Point

The dropping point of a grease is the temperature at which a drop of material falls from the orifice of a test cup under prescribed test conditions. Plastic materials such as conventional soap thickened greases do not have a true melting point but have a melting range during which the material becomes progressively softer. Some greases containing thickeners other than conventional soaps may, without change in state, separate oil.

The sample is placed in the cup and heated at a uniform rate. When the first drop of material falls from the lower end of the cup, the temperature is observed and reported as the dropping point.

• The dropping point of a grease is not considered to have any bearing on service performance other than a grease cannot normally be expected to perform satisfactorily at temperatures above its dropping point.
• It does not establish the maximum usable temperature for the grease.
• Dropping point is useful in identifying a grease as a type, and for establishing and maintaining bench marks for quality control.

Oxidation Stability

Resistance to oxidation is an important characteristic of greases intended for use in rolling element bearings. Improvement in this property through the use of oxidation inhibitors has enabled the development of the so-called “packed for life” bearing.

Both the oil and the fatty constituents in a grease oxidise; the higher the temperature the faster the rate of oxidation.

The test for Oxidation Stability by the “Oxygen Bomb Method” is a static test in which the grease is placed in a set of five dishes. The dishes are then placed in a pressure vessel, or bomb, which is pressurised to 110 psi with oxygen, and placed in a bath held at 99C (210F) where it is allowed to remain for a period of time, usually 100, 200 or 500 hours. The pressure is then recorded and the amount of pressure drop reported. Pressure drops to 5 to 25 psi are usually allowed, depending on the test time and the intended use of the grease.

The results of this test are probably most indicative of the stability of thin films of a grease in extended storage, as on prelubricated bearings. 

This test is not intended for the prediction of the stability of a grease under dynamic conditions or in bulk quantities in the original containers.

Water Resistance

The ability of a grease to resist washout under conditions where water may splash or impinge directly on a bearing is an important property in many applications. Comparative results between different greases can be obtained with the “Test for the Water Washout Characteristics of Lubricating Grease”.

In this test, a ball bearing with loose shields is rotated with a jet of water impinging on it. Resistance to washout is measured by the amount of grease lost from the bearing during the test. The test is generally considered to be a useful screening test for grease that is to be used where water washing occurs.

In many cases, direct impingement of water may not be a problem, but moist atmospheres or water leakage may expose a grease to water contamination. One method of evaluating a grease for use under conditions of this type is to homogenize water into it. The grease may then be reported on the basis of the amount of water it will absorb without loss of a grease structure, or the amount of hardening or softening resulting from the admixture of a specific proportion of water.

Other Tests

There are numerous other physical tests that can be performed on greases often depending upon the specific application of the grease. Some of the more common tests are:

• Churned Grease Oil Release (CGOR)
• Roll Stability
• Wheel Bearing Leakage
• EP and Wear Prevention
• Timken OK Load
• NLGI Dispensing Test

The details and significance of these tests can be obtained from the relevant ASTM procedure.

Conclusion

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