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 exisiting ones.
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.
- Larges 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:
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 -
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 -
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
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.
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 completed 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.
Simple Normal Soap Grease
A normal calcium soap grease requires stearic acid plus an alkali such as lime which results in calcium disstearate with water added to provide a structure modifier.
Lubricatinf fluid and additives are then added to the calcium disstearate 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 improved the formulation, gain better stability and yield a higher temperature range, the type of acid is changes 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.
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 posses 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.
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 pf 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 in only moderately stable.
Calcium greases are older type greases and thus are somewhat primitive in 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 service 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.
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 posses 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
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.
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 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 adsorb 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.
Adsorbtion and absorption are sesirable 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 reduce 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 is provided through the use of additives and proper selection of the base oil.
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
- 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 know to be more serious at higher operating temperatures.
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.
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