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Types Of Greases

Lithium, Calcium, Soda, Aluminium,Complex and Mixed soap base Greases, Soap free Grease, PTFE, Silicone and Molybdenum modified Greases, Specialty and Synthetic Greases.

 
Basic Defination

The original Definition of ASTM of Grease was "A solid or semi solid combination of petroleum products and a soap or mixtures of soaps with or without fillers, suitable for certain types of lubrication.

Subsequent technological advances have led to the use of synthetic lubricating fluids on the one hand and non-soap thickeners on the other hand.

Current - Modified Definition of Grease is "A solid or semi solid product of dispersion of thickening agent in liquid lubricant. Other ingredients imparting special properties may be included.

Most greases are in fact is a combination of petroleum oils and metallic soaps.

 

Grease is a lubricant of higher initial viscosity than oil, consisting originally of a calcium, sodium or lithium soap jelly emulsified with mineral oil.

Greases are typically used in areas where a continuous supply of oil cannot be retained, such as open bearings or gears.

Grease is a fine dispersion of an oil-insoluble thickening agent - usually soap in a fluid lubricant which is generally mineral lubricating oil. The soap is made up of fatty acid, tallow or vegetable oil saponified with alkali which can be hydrated lime, caustic soda, lithium hydroxide or aluminum hydroxide. The lubricating oil component is refined base oil-naphthenic, of medium viscosity index, or cylinder oil stock. Structurally grease is a water-in-oil emulsion. Its appearance is smooth, mostly translucent, soft or hard.

Properties essential for performance of grease are structural stability, lubricating quality, low and high temperature performance (which are provided by the selected lube oil base stock), where as properties such as water resistance, high temperature quality, resistance to break down through continuous use and ability of grease to stay in place are provided by the soap.

Additives e.g. Graphite, modified clay, asphalt, oxidation and corrosion inhibitors, extreme pressure additives molybdenum disulphide etc. are used to impart specific properties as required by end application.

Factors to be considered when selecting greases are the type of grease, which in turn depends on operating temperatures, water resistance, oxidation stability etc. The second factor, no less important, are the grease's characteristics, including viscosity and consistency.

 
Basic Types Of Grease  

SOAP TYPES GREASES

NON SOAP THICKNER GREASES

Metallic soaps + Lubricating Oils + Additives for imparting Special properties

Thickening Agent + Lubricating Fluid (Synthetic oils) + Additives for imparting Special properties

 

The value of plastic lubricants has been recognized since very early times when animal fats were used for Axles and the like.

In general any lubricating fluid can be gelled by means of a suitable thickness to form a lubricating Grease. In practice mineral oils are almost exclusively used. For soap type Greases mineral oils of relatively low viscosity index 60 are preferred since the greater solvency of the more aromatic oils for soaps makes manufacture easier and gives greases with lower soap contents and with better properties.

Synthetic Fluids: Esters and silicones are used, only in greases for special applications, where cost is secondary to performance to meet low temperature and high temperature requirements; - 75C to + 320C.

The Soaps used are the metallic salts of long chain fatty acids derived from wide range of natural fatty materials. Soaps are mainly produced by specifying the fat or fatty acid with appropriate alkali in the presence of oil.

In some cases pre-formed soaps are used, such as aluminium stearate, lithium sterate etc.

Non Soap Thickners may be inorganic e.g. silica or clay or organic with a very high melting point . e.g. terephthalamates, arylureas (these may be either pre-formed or formed in situ) and the dye stuffs, indanthrene and phthalscyanine. They need to be oleophilic and this characteristic may be imparted either in the thickner formulation (i.e. by including a long chain hydrocarbon chain or surface esterification ) or by coating the particle surfaces with organic cations. In the latter case about 75 percent of the particle surface is covered, which gives the desired effect without affecting inter particle forces too much. Pre-treated clays (i.e. coated and dried) need aids to facilitate dispersion in oil. These dispersion aids are low molecular weight polar organic compounds such as methanol, acetone, diacetone Alcohol and propylene Carbonate.

A satisfactory Grease for a given application is expected to:

  1. Provide adequate lubrication to reduce friction and to prevent harmful wear of bearing components
  2. Protect against corrosion
  3. Act as a seal to prevent entry of dirt and water.
  4. Resist leakage dripping or undesirable throw off from the lubricated surfaces.
  5. Resit objectionable change in structure or consistency with mechanical working (in the bearing) during prolonged services.
  6. Not stiffen excessively to cause undue resistance to motion in cold weather.
  7. Have suitable physical characteristics for the method of application.
  8. Be compatible with elastomer seals and other materials of construction in the lubricated portion of the mechanism.
  9. Tolerate some deqree of contamination, such as moisture, without loss of significant characteristics.

Soap Greases consist of dispersions of soap fibres in oil in concentrations mostly in the range 5 to 22%. By weight sometimes solid block greases contain 50 to 40% of soap.

The thickeners in non-soap greases are usually particulate rather than fibrous in nature and in this case the structure is maintained by 3 dimensional linkages due to very short range inter particle forces (Vander waals forces). These linkages are readily broken and re-formed.

 
Types Of Greases

The metallic radical of the soap largely determines the characteristics of the greases the fatty radical having a secondary effect. Greases are therefore classified in terms of the metal they contain.

 
Calcium Soap Greases Sodium Soap Greases

The conventional types ( excluding the complex soap greases have a smooth buttery texture and are water resistant, with dropping points around 90- 100 C. They are water stabilized, that is , the water is present in the soap crystals as water of crystallization. The optimum amount of water varies considerably with the type of formulation, fatty material and mineral oil used, but is usually within the range of 0.5 1.3% by wt. at high temperatures the water is gradually lost and the soap structure is weakened. Consequently, calcium soap Greases are restricted to use at fairly low maximum temperatures about 50 60C. They all turn fluid after exposure to high temperatures and may separate into the oil and soap phases. Only those which contain other stabilizers besides water ( e.g. wool grease) will regain their structure on cooling.

These are usually more or less fibrous in structure/ texture depending mainly on the nature of the fatty material, high unsaturation yielding very fibrous greases. They have high dropping points usually not less than 150C and sometimes as high as 200C and are useful for relatively high temperature service. Owing to the solubility of the soap in water they are not water resistant.

 
Lithium Soap Greases Aluminium Soap Greases

These greases first appeared during -the World War II and were made from Lithium Stearate preformed soap. Now a-days lithium hydroxy stearate greases made by saponification in situ from hydrogenated castor oil predominate. Depending on the composition Lithium greases are smooth or slightly grainy in appearance. They have the highest dropping point about 190C of the conventional greases and the highest service temperatures. They are water resistant, mechanically stable, and can be made with a greater variety of types of oil than other greases. Their versatility and wide operational scope especially in high-speed service has led to their use as Multipurpose greases to the displacement of earlier types of more specialized greases.

Aluminium Greases have an attractive translucent smooth and polished appearance. The dropping points are low about 90C and the greases tend to become rubbery at high temperatures. They are almost invariably made from high viscosity oils and often incorporate polymers. Such products are water Resistant; stringy, and adhesive, and find application as chassis and gear lubricants. They are not recommended for rolling bearings.

 
Mixed Soap Greases Complex Soap Graeses

A variety of mixture is used, the commonest being Sodium-Calcium and the greases are generally manufactured by saponifying the fatty materials with mixed alkalies derived from different metals. One of the soaps usually predominates and determines the general character of the grease, while the other modifies the structure in some way. This results for example, in changes in texture and improved mechanical stability.

The normal soaps can be complexed with various inorganic salts, usually short-chain aliphatic compounds. The commonest type is a calcium soap calcium acetate complex although many other combinations have been patented. The greases are water resistant and have very high dropping points, in the range 200 -300C. With EP additives incorporated they are encountered as steel mill greases. New greases have recently been introduced for multi-purpose.

 
Non- Soap Greases

There are two main types, these intended for general industrial use and those for specialized applications. The former include greases thickened with silica and clay ( Bentonite) and organic derivatives,such as terephthalamates, diamido dicarbonyl and aryl substituted ureas. High temperatures, mechanical stability, water resistant etc. suggest that they are multipurpose greases with a wider scope than soap greases. However, they have not made any great impact on the market. The type used for specialized applications includes greases made from the dye stuffs indanthrene and phthalocyanine which are generally combined with synthetic fluids, such as diesters and silicones and hence are very expensive. One such product is claimed to operate between - 75C and + 320C.

 
Factors
Factors to be considered when selecting greases include
 
 
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