Chapter 6-THEORY OF LUBRICATION 



The viscosity of an oil is its tendency to re- 

 sist flow. An oil of high viscosity flows very 

 slowly. Raising the temperature of an oil lowers 

 its viscosity; lowering the temperature in- 

 creases the viscosity. The measurement of vis- 

 cosity is discussed in chapter 7 of this text. 



The viscosity index of an oil is a number in- 

 dicating the effect of temperature changes on 

 viscosity. A low viscosity index signifies a rel- 

 atively large change of viscosity with changes 

 of temperature. An oil which becomes thin at 

 high temperatures and thick at low temperatures 

 is said to have a low viscosity index; a high vis- 

 cosity index signifies that the viscosity changes 

 relatively little with changes of temperature. 



The pour point of an oil is the lowest tem- 

 perature at which the oil will barely flow from 

 a container. The pour point is closely related to 

 the viscosity of the oil. In general, an oil of high 

 viscosity will have a higher pour point than an 

 oil of low viscosity. 



The flash point of an oil is the temperature 

 at which enough vapor is given off to flash when 

 a flame or spark is applied under standard test 

 conditions. 



The fire point (higher than the flash point) of 

 an oil is the temperature at which the oil will 

 continue to burn when it is ignited. 



The auto- ignition point of an oil is the tem- 

 perature at which the flammable vapors given 

 off from the oil will burn without the application 

 of a spark or flame. 



The neutralization number of an oil is a meas- 

 ure of the acid content; it is defined as the num- 

 ber of milligrams of potassium hydroxide (KOH) 

 required to neutralize one gram of the oil. All 

 petroleum products oxidize in the presence of 

 air and heat, and the products of oxidation in- 

 clude organic acids. The acids, if present in 

 sufficient concentration, have harmful effects 

 on alloy bearings at high temperatures. The 

 presence of acids also may result in the forma- 

 tion of sludge and emulsions too stable to be 

 broken down. An increase in acidity is an indica- 

 tion that lubricating oil is deteriorating. 



The demulsibility of an oil is the ability of 

 the oil to separate cleanly from any water pres- 

 ent. Demulsibility is an important characteris- 

 tic of lubricating oils used in forced-feed lubri- 

 cation systems. 



The precipitation number of an oil is a meas- 

 ure of the amount of solids classified as asphalts 

 or carbon residue contained in the oil. The pre- 

 cipitation number is reached by diluting a known 

 quantity of oil with naphtha and separating the 



precipitate by centrifuging. The volume of the 

 separated solids equals the precipitation num- 

 ber. An oil with a high precipitation number is 

 not suitable for certain applications because it 

 may leave deposits in an engine or plug up valves 

 and pumps. 



Lubricating Greases 



Some lubricating greases are simple mix- 

 tures of soaps and lubricating oils. Others are 

 more exotic liquids such as silicones and 

 di-basic acid esters, thickened with metals or 

 inert materials to provide adequate lubrication. 

 Requirements for oxidation inhibition, corrosion 

 prevention, and extreme pressure performance 

 are met by incorporating special additives. 



Lubricating greases are supplied in three 

 grades: soft, medium, and hard. The soft greases 

 are used for high speeds and low pressures; the 

 medium greases areusedfor medium speeds and 

 medium pressures; the hard greases are used for 

 slow speeds and high pressures. 



CARE OF LUBRICATING OIL 



Lubricating oils may be kept in service for 

 long periods of time, provided the purity of the 

 oils is maintained at the required standard. The 

 simple fact is that lubricating oil does not wear 

 out.-^ although it can become unfit for use when 

 it is robbed of its lubricating properties by the 

 presence of water, sand, sludge, fine metallic 

 particles, acid, and other contaminants. 



Proper care of lubricating oil requires, then, 

 that the oil be kept as free from contamination 

 as possible and that, once contaminated, the oil 

 must be purified before it can be used again. 



PREVENTING CONTAMINATION 



Strainers or filters are used in many lubri- 

 cating systems to prevent the passage of grit, 

 scale, dirt, and other foreign matter. Duplex 

 strainers are used in lubricating systems in 

 which an uninterrupted flow of lubricating oil 

 must be maintained; the flow may be diverted 

 from one strainer basket to the other while one 

 is being cleaned. Filters may be installed di- 

 rectly in pressure lubricating systems or they 

 may be installed as bypass filters. 



The additive content of an oil may be exhausted as 

 the additive combats the special conditions for which 

 it was included in the oil; but this is a gradual process 

 and is never catastrophic. 



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