Lubricants

lubricant

A substance (such as grease) capable of reducing friction, heat, and wear when introduced as a film between solid surfaces to improve efficiency and reducing wear.

ex-  oil,grease,graphite.

lubrication

lubrication may be defined as " the reduction of friction and wear between the surface. this type of substances are called lubricant & this process is called lubrication.

classification of lubricant-

Solid lubricants Semisolid lubricants Liquid lubricants Gaseous lubricants

Solid Lubricants:-

A solid lubricant is basically any solid material which can be placed between two bearing surfaces and which will shear more easily under a given load than the bearing materials themselves. The coefficient of friction in dry lubrication is related to the shearing force and the bearing load. Two primary property requirements are : The material must be able to support the applied load without significant distortion, deformation or loss in strength. The coefficient of friction and the rate of wear must be acceptably low.

Advantages & disadvantages of solid lubricants are listed in Table-

Advantages and Disadvantages of solid lubricants.

Semi-Solid Lubricant:- Grease

In layman’s language Grease is A black or yellow sticky mass used in the bearings for lubrication purpose. Lubricating greases consist of lubricating oils, often of quite low viscosity, which has been thickened by means of finely dispersed solids called thickeners. It consist of base oils(75 to 95%), additives(0 to 5%) and minute thickener fibers(5 to 20%).

Semi-solid lubricant.

Base Oil:- Many different types of base oil may be used in the manufacture of grease, including petroleum (naphthenic: more popular, paraffinic) and synthetic (PAO's, esters, silicones, glycols). The viscosity of the base oil is the most significant property. A lighter, lower viscosity base oil is used to formulate low-temperature greases, while heavier, higher viscosity base oil is used to formulate high-temperature greases. Additives:- Chemical additives are added to grease in order to enhance their performance. Performance requirements, compatibility, environmental considerations, color, and cost all factor into additive selection. Solid lubricants such as graphite, MoS2, EP additives are few examples. Thickener:- The two basic types of thickeners are organic thickeners and inorganic thickeners. Organic thickeners can be either soap-based or non-soap based, while inorganic thickeners are non-soap based. Simple soaps are formed with the combination of a fatty acid or ester (of either animal or vegetable origin) with an alkali earth metal, reacted with the application of heat, pressure or agitation through a process known as saponification. The fiber structure(Fig. 4.48) provided by the metal soap determined the mechanical stability and physical properties of the finished grease. In order to take on enhanced performance characteristics, including higher dropping points, a complex agent is added to the soap thickener to convert it to a soap salt complex thickener. The greases are then referred to as "complexes“. Classification grease based on simple and complex soap thickeners is listed in Table 4.16. The most commonly economic grease is lime(calcium) base grease (max. temperature 55-800C). Soda(Sodium) base grease (max. temperature 90-1200C) is preferred over lime based grease in rolling bearings. Complex Grease:- Complex grease is similar to a regular grease except that the thickener contains two dissimilar fatty acids, one of which is the complexing agent. This imparts good high-temperature characteristics to the final product.

Synthetic lubricants-

• 
  Polyalphaolefins (PAO)

Polyalphaoleins are the most popular synthetic lubricant. PAO’s chemical structure and properties are identical to those of mineral oils.
Polyalphaoleins (synthetic hydrocarbons) is manufactured by polymerization of hydrocarbon molecules (alpha-olefins). The process occurs in the reaction of ethylene gas in presence of a metallic catalyst.

• Polyglycols (PAG)

Polyglycols are produced by oxidation of ethylene and propylene. The oxides are then polymerized resulting in the formation of polyglycol.
Polyglycols are water soluble.
Polyglycols are characterized by a very low coefficient of friction. They are also able to withstand high pressures without EP (extreme pressure) additives.

• Ester oils

Ester oils are produced by the reaction of acids and alcohols with water.
Ester oils are characterized by very good high temperature and low-temperature resistance.

• Silicones

Silicones are a group of inorganic polymers, molecules of which represent a backbone structure built from repeated chemical units (monomers) containing Si=O moieties. Two organic groups are attached to each Si=O moiety: eg. methyl+methyl ( (CH₃)₂ ), methyl+phenyl ( CH₃ + C₆H₅ ), phenyl+phenyl ( (C₆H₅)₂ ).

Gaseous lubricant

Lubrication with gas is analogous in many respects to lubrication with a liquid since the same principles of fluid-film lubrication apply. Although both gases and liquids are viscous fluids, they differ in two important particulars. The viscosity of gases is much lower and the compressibility much greater than for liquids. Film thicknesses and load capacities, therefore, are much lower with a gas such as air (see Table 1). In equipment that handles gases of various kinds, it is often desirable to lubricate the sliding surfaces with gas in order to simplify the apparatus and reduce contamination to and from the lubricant.